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Formation, reactions and spectra of some group V free-radicals Yee, Kim Kuo 1967

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The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY B.A.Sc, Un i v e r s i t y of Toronto THURSDAY, NOVEMBER '23, 1967, AT 3:30 P.M. IN ROOM 225, CHEMISTRY BUILDING COMMITTEE IN CHARGE External Examiner: B.A. Thrush Department of Physical Chemistry Un i v e r s i t y of Cambridge Cambridge, England of KIM KUO YEE Chairman: I. MeT. Cowan D.C. Walker G.B. Porter .F. Aubke J.R. Scheffer C A . McDowell F.W. Dalby Research Supervisor: N. Basco FORMATION, REACTIONS AND SPECTRA OF SOME GROUP V FREE RADICALS ABSTRACT Metastable, e l e c t r o n i c a l l y excited atoms 2 0 2 0 ( D , P ) of phosphorous, arsenic and antimony have been observed following the isothermal f l a s h photolysis of Group VA hydrides and t r i h a l i d e s . Several mechanisms for the production of these atoms are discussed. The decay of the excited atoms was observed to be ra p i d due to reactions with the transients produced i n the f l a s h . photolysis of the parent compounds. Flash photolysis of the Group VA hydrides and t r i c h l o r i d e s have yielded new e l e c t r o n i c absorption spectra of the AsH and AsH 2, SbH and SbH 2, PCI, AsCl, and SbCl f r e e - r a d i c a l s . V i b r a t i o n a l analysis on some of these spectra have been c a r r i e d out. ' In the isothermal f l a s h photolysis of cyano-gen with Group VA hydrides, i t i s proposed that the following reactions of the cyanogen r a d i c a l occur r a p i d l y : CN + AH 3 - i - AH^ + HCN (1) CN + AH2 AH + HCN (2) CN + AH -*-ACN + H (A=N,P,As) (3) -*- HCN + A* (A=P,As,Sb) (4) -^-HACN (A=N,P) (5) where A, unless s p e c i f i e d , i s N, P, As or Sb and * denotes e l e c t r o n i c e x c i t a t i o n . Three of the new e l e c -t r o n i c absorption spectra observed are t e n t a t i v e l y a t t r i b u t e d to the PCN, HPCN and AsCN f r e e - r a d i c a l s produced i n reactions(3) and (5). The assignment of the other spectra i s discussed. Reactions of GN with NF, PCI and AsCl, corresponding to (3), have also been shown to occur r a p i d l y . The reactions of the cyanogen r a d i c a l i n the f l a s h photolysis of cyanogen with nitrous oxide, oxygen, water, and methyl isocyanate have also been studied and discussed. PUBLICATIONS N, Basco and K.K,, Yee, Spectra of Arsenic Hydride Radicals, Spectroscopy L e t t e r s , i n press. No Basco and K.K. Yee, Spectra of Antimony Hydride Radicals, Spectroscopy L e t t e r s , i n press. N. Basco and K.K. Yee, Spectrum of the PCI Free-Radical, Chem Comm., i n press. N. Basco and K.K. Yee, Formation of Metastable Atoms of Phosphorous, Arsenic and Antimony by Flash Photolysis, Nature, i n press. N. Basco and K.K. Yee, Spectrum of Antimony Chloride Radical, Spectroscopy L e t t e r s , i n press. No Basco and K.K. Yee, Spectrum of the AsCl Free-Radical, Chem. Comm.,, i n press. GRADUATE STUDIES F i e l d of Study: Physical Chemistry Topics i n Physical Chemistry Advanced Theoretical Chemistry J.AoR. Coop W.C. Lr. R.F. Snide? D.P. Chor Spectroscopy and Molecular Structure K.B. Harvey A.' Bree Chemical Ki n e t i c s Seminar i n Special Topic Seminar i n Chemistry Topics i n Inorganic Chemistry Topics i n Organic Chemistry Related Studies: Linear Algebra E.A. Ogryzlo N„ Basco D.C. Walker G.B. Porter D.C, Walker N. Basco B.A. Dunell W.R. Cullen R.C. Thompson N.L. Paddock H.C. Clark J.T. Kwon D.E. McGreer L.D. H a l l F. McCapra B. Chang FORMATION, REACTIONS AND SPECTRA OF SOME GROUP V FREE-RADICALS by KIM KUO YEE B.A.Sc, University of Toronto, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA October, 1967 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t t he U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag ree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rpo se s may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n -t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h Co l umb ia Vancouve r 8, Canada Depar tment o f i i ABSTRACT ? 0 2 0 ' " Metastable, e l e c t r o n i c a l l y excited atoms (•'D , P ) of phosphorus,; > arsenic and antimony have been observed following the isothermal f l a s h photolysis of Group VA hydrides and t r i h a l i d e s . Several mechanisms for the production of these atoms are discussed. The decay of the excited atoms was observed to be rapid due to reactions with the transients produced i n the flash photolysis of the parent compounds. Flash photolysis of the Group VA hydrides and t r i c h l o r i d e s i h a § ; yielded new electronic absorption spectra of the AsH and Asr^, SbH and SbH^, PCI, AsCl, and SbCl free-radicals. Vibrational analysis on some of these spectra have been carried out. In the isothermal f l a s h photolysis of cyanogen with Group VA hydrides, i t i s proposed that the following reactions of the cyanogen rad i c a l occur rapidly: CN + AH_ -»• AH + HCN ( 1 ) 3 2 CN + AH2 AH + HCN (2) CN + AH -»• ACN + H (A=N,P,As) (3) HCN + A* (A=P,As, Sb) (4) + HACN (A=N,P) (5) where A, unless speci f i e d , i s N, P, As or Sb and * denotes electronic ex c i t a t i o n . . Three of the new electronic absorption spectra observed are tentatively attributed to the PCN, HPCN and AsCN free-radicals produced i n reactions (3) and (5). The assignment of the other spectra i s discussed. Reactions of CN with NF, PCI and AsCl, corresponding to (3), have also been shown to occur rapidly. The reactions of the cyanogen r a d i c a l i n the flash photolysis of cyanogen with nitrous oxide, oxygen, water, and methyl isocyanate have also been studied and discussed. i i i TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i TABLE OF CONTENTS i i i LIST OF FIGURES v LIST OF TABLES v i i ACKNOWLEDGMENTS ix INTRODUCTION A. Flash Photolysis and Kinetic Spectroscopy . . . . . 1 B. Study of Atomic Reactions by Kinetic Spectroscopy 2 C. Study of Reactions of the Cyanogen Radical by Kinetic Spectroscopy 3 D. Aims and Scope of this Investigation 4 THEORY A. Atomic Spectroscopy 5 B. Molecular Spectroscopy 5 EXPERIMENTAL A. Description of the Flash Photolysis Apparatus 7 B. Photography and Plate Photometry 10 C. Reagents 11 D. Preparation of Chemical Mixtures 15 RESULTS A. Atomic Systems of Phosphorus-, Arsenic and Antimony 17 B. Flash Photolysis of Group VA Hydrides and Halides 31 C. Flash Photolysis of Cyanogen with Group VA Hydrides and Halides 33 D. Flash Photolysis of Cyanogen with Oxygen Compounds 41 iv Page E. Assignment and Analysis of New Electronic Absorption Spectra 1. Spectra from Group VA Hydrides 45 2. Spectra from Group VA Halides 55 3. Spectra from Cyanogen Systems 65 F. Bond Dissociation Energies 76 DISCUSSION A. Formation and Reactions of Excited Atoms of Phosphorus., Arsenic and Antimony in the Flash Photolysis of Group VA Hydrides and Halides 77 B. Reactions of the Cyanogen Radical 1. Cyanogen/Group VA Hydrides and Trichlorides 92 2. Cyanogen/Dinitrogen Tetrafluoride 108 3. Cyanogen/Oxygen 112 4. Cyanogen/Nitrous Oxide 112 5. Cyanogen/Water 115 6. Cyanogen/Methyl Isocyanate 117 CONCLUSIONS 120 SUGGESTIONS FOR FURTHER WORK 121 BIBLIOGRAPHY 122 APPENDIX 127 V LIST OF FIGURES Page 1. Schematic Diagram of Flash Photolysis Apparatus 8 2. Schematic Diagram of Vacuum System 12 3. Schematic Diagram of De-oxygenation Column 13 4. Decay of P(3p 3 2P°) in PH3/C2N2/N2 23 5. Decay of As(4p 3 2P°) in AsH3/N2 23 6. Decay of Sb(5p 3: 4S°, 2D°, 2P°) in SbH 3/N 2 25 7. Decay of Sb(5p3 2P°) in a) SbH,/N2 26 b) SbH3/C2N2/N2 26 8. Fluorescence of Sb Atoms in SbH3/N2 30 9. Kinetic Behaviour of the Transients in NH 3/C 2N 2/N 2 36 10. Kinetic Behaviour of the Transients in N2 F4/ C2 N2/ N2 37 11. Kinetic Behaviour of the Transients in PH3/C2N2/N2 38 12. Kinetic Behaviour of the Transients in PH 3/C 2N 2/N 2 39 13. Kinetic Behaviour of the AsCN Free-Radical in AsH 3/C 2N 2 /N 2 . . 39 14. Kinetic Behaviour of the Transients in N20/C2N2/N2 43 15. a) Flash Photolysis of CI-LNC0/N2 (5.0/245 mm Hg) 44 b) Kinetic Behaviour of the Transients in CH3NCO/C2N2/N2 . . . 44 16a. Absorption Spectrum of the 0-0 Band of AsH 47 16b. Absorption Spectrum of the AsH2 Free-Radical 47 17. Absorption Spectrum of the 0-0 Band of SbH 50 18. Absorption Spectrum of the SbH2 Free-Radical 50 19. Absorption Spectrum of the SbCl Free-Radical . 58 20. Absorption Spectrum of the PCI Free-Radical 61 21. Absorption Spectrum of the AsCl Free-Radical 61 22. Absorption Spectrum of the HNCN? Free-Radical in NH3/C2N2/N2 . 69 23. Absorption Spectrum of the PCN Free-Radical in PH,/C 9N 2/N 2 . . 69 v i Page 24. Effect of Total Pressure on Decay of P(3p 3 2 P ? / 2 ) in PH3/N2 ' 83 25. Effect of P g b H on Decay of Sb(5p3 2 V ° s / 2 ^ i n s b H3/ N2 8 4 26. Effect of P A s C 1 on Decay of A s(4p 5 2P°) in AsCl 3/N 2 85 27. Effect of P g b H on Decay of Sb(5p3 2P]y 2) in SbH3/N2 86 28. Comparison of Decay of Sb(5p3 2D5? / 2) and Sb(5p3 2P? / 7) in SbH3/N2 l/. 89 29. Effect of P g b H on the Growth and Decay of Sb(5p3 4 S 3 / 2 ) in SbH3/N2 . . 3 90 30. Decay of P(3p 3 2P°) in PHj/N and in PH3/C2N2/N2 95 31. Decay of As(4p 3 2P°) in AsH3/N2 and in AsH3/C2N2/N2 96 32. Effect of P on Concentration of Transient Species in NH 3/C 2N 2/N 2 N H? 98 33. Effect of Pp„ on Concentration of Transient Species in PH3/C9N2/N0 "? 99Q •2^ 2/^ 2 34. Concentration of Transient Species as a function of Time in NH3/C2N2/N2 101 35. Concentration of Transient Species as a Function of Time in NH3/C2N2/N2 . . . . . , 102 36. Concentration of Transient Species as a Function of Time in PH3/C2N2/N2 106 37. Concentration of Transient Species as a Function of Time in AsH3/C2N2/N2 107 38. Concentration of Transient Species as a Function of Time in N 2F 4/C 2N 2/N 2 110 39. Concentration of Transient Species as a Function of Time in N20/C2N2/N2 114 40. Concentration of Transient Species as a Function of Time in H20/C2N2/N2 116 41. Concentration of Transient Species as a Function of Time in CH3NCO/C2N2/N2 119 v i i LIST OF TABLES Page I. Summary of Low-lying Atomic States of N,P,As and Sb 18 II. List of P and As Atomic Transitions Observed in Absorption . . 19 III. List of Sb Atomic Transitions Observed in Absorption 20 IV. Formation of Excited Sb Atoms by CN Radical Reactions in SbH3/C2N2/N2 (List of Atomic Lines with Increased Intensity). . 27 V. Atomic Fluorescence . 29 VI. Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of Group VA Hydrides and Halides32 VII. Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of C2N2/Group VA Hydrides. . . 34 VIII. Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of C2N2/Group VA Halides . . . 35 IX. Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of Some Oxygen Compounds . . . 42 X. Summary of Transient Molecular Species Observed in the Flash Photolysis of C2N2/0xygen Compounds 42 XI. Band Heads of AsH ( 3n(a)- 3E") 46 XII. Band Heads of the 3II(a)- 3E~ Transition of SbH 49 XIII. Q-Heads of the SbH2 Spectrum 49 XIV. Band Heads of the P2(C-X) Spectrum 53 XV. Band Heads of the As 2 (A«-X) Spectrum 54 XVI. Band Heads of the Sb2(F«-X Spectrum 55 XVII. Band Heads of the SbCl Spectrum 57 XVIII. Constants for the Two Electronic States of SbCl 57 XIX. Band Heads of the PCI Spectrum 60 XX. Constants for the Two Electronic States of PCI 60 XXI. Band Heads of the AsCl Spectrum 63 XXII. a) Values for the Grourid Electronic State of Group VA Diatomic Halides 64 v i i i Bage X X I I I . Wavelengths of Spectrum A (HNCN?) 68 XXIV. Wavelengths of Lines of the 3014 A Band of PCN 71 XXV. Q-Heads of PCN( 3n(a)- 3I") 71 XXVI. Wavelength of HPON Bands 71 XXVII. Wavelengths of AsCN ( 3 n(a)- 3Z~) 73 XXVIII. Band Heads of the NCN? Spectrum 75 XXIX. Band Heads of NCN (B 3 I > X 3E") Spectrum 75 XXX. Wavelengths of FNCN? Bands 75 XXXI. ' Band Heads of the NCPCN? Spectrum 75 XXXII. Approximate Bond Dissociation Energies of NCN, PCN and AsCN . . 76 ACKNOWLEDGMENTS I am indebted to Dr. N. Basco for his interest, encouragement and guidance throughout the course of this investigation. I am grateful to Dr. G.B. Porter for the use of his equipment and f a c i l i t i e s , and to a l l the people who have helped in one way or the other. I wish to thank, in particular, Mr. John Mcintosh and Misses Kathy Greening and Jacqueline Cowley. - 1-INTRODUCTION A. Flash Photolysis and Kinetic Spectroscopy Flash photolysis is a particular photochemical technique whereby a large amount of stored energy is discharged to give high intensity radiation in a very short time. Short-lived species or transients are therefore produced in quantities sufficient for detection in the far-infrared to the vacuum-ultraviolet spectral range. If spectroscopic methods are used to follow the kinetic behaviour of the transients after the flash photolysis the technique is known as kinetic spectroscopy. The method of flash photolysis can be applied to the study of fast chemical reactionsiin the gas phase under approximately isothermal or approximately adiabatic conditions. Isothermal conditions for gaseous systems can be obtained by the addition of excess inert gas which increases the thermal capacity of the system and serves as coolant. Adiabatic conditions are used for combustion and explosion studies. Excellent articles on the technique of flash photolysis and kinetic spectroscopy and i t s applications 1 2 to the study of fast chemical reactions are given by Norrish * , Norrish and Thrush"*, and Porter 4. In addition to the study of rapid processes, such as radical-radical reactions, the application of flash photolysis makes possible the actual observation and identification of the transient absorption spectra of atoms or free radicals. Indeed, the electronic absorption spectra of many transients have been discovered with this technique 5' 6. High-resolution studies of the rotational fine structure of these spectra have yielded valuable information about the geometrical structures of the molecules concerned. - 2 -B. Study of Atomic Reactions by Kinetic Spectroscopy Flash photolysis provides a very convenient method for studying atomic reactions. This method has been used, for example, by several groups of w o r k e r s ^ " t o study the recombination of iodine atoms in the presence of various inert gases, by measuring the reappearance of the iodine absorption. In spite of the convenience of kinetic spectroscopy, very l i t t l e work has been done on the study of atomic processes in which the atoms can be observed directly by spectroscopy. Part of the d i f f i c u l t y , of course, is that most of the atoms involved in the study of atomic reactions are spectroscopically inaccessible to this technique. In fact, very few atoms have been observed in absorption at room temperature. The problem then is to select the appropriate atoms which can be studied by kinetic spectroscopy. Recently, kinetic spectroscopy studies of the deactivation of gaseous atoms have been carried out on mercury, selenium, 12 1 % iodine, and bromine. Callear and Norrish , and Callear and Williams have investigated the spin-orbit relaxation of electronically excited Hg from the 6 3 P j state to the metastable t r i p l e t state of 6 3PQ by flash photolyzing mercury vapour in the presence of an inert gas. Callear and Tyerman*4 have measured the spin-orbit relaxation rates of Se(3Pg) in various gases by flash photolyzing CSe2 in an excess of inert gas. Donovan and Husain have studied the spin-orbit relaxation of excited, metastable 1(5 ^ 1/2) by flash photolyzing some simple iodides^»*^, and excited, metastable Br(4 ^y flash photolyzing some simple bromides*?. At present there is very l i t t l e information on the formation and chemical reactions of metastable, electronically excited, gaseous atoms (particularly the group VA elements) at room temperature. The need for such investigations i s apparent. - 3 -C. Study of Reactions of the Cyanogen Radical by Kinetic Spectroscopy Reactions of the CN radical can be conveniently investigated by kinetic spectroscopy because of i t s well known electronic transitions, 2 + 2 + i s ° particularly the violet system (B £ -X £ ) at 3883 A. However, relatively few reactions of the CN radical have been studied by this method. The rates of disappearance of CN radicals in cyanogen, cyanogen chloride, hydrogen, oxygen, and hydrocarbons have been measured by Paul 19 20 and Dalby , the rate of recombination was determined by Basco et a l . , 21 the rate of the reaction with n i t r i c oxide by Basco and Norrish , and the 22 rate of the reaction with oxygen by Basco . The reaction of CN radical 23 with oxygen was also investigated by Morrow and McGrath . There i s , however, very l i t t l e data available on the fundamental processes involved in the reactions of the cyanogen radical. The study of reactions of the cyanogen radical by kinetic spectroscopy is of particular importance to electronic spectroscopy of polyatomic molecules, a f i e l d which has become increasingly active in the past ten to fifteen years. The absorption spectra of CN-containing free-radicals such as CCN, NCN, OCN, SCN, HCCN, and HNCN have been observed and analyzed or are in the process of being analyzed 5 , 6. Although the carriers of these spectra were produced in the flash photolysis of various compounds, the mechanisms for their formation are not understood. It is clear that some, i f not a l l , of these transients can be formed from reactions involving the CN radical. Thus new transients can be created for the study of the elucidation of molecular structure as well as the elucidation of chemical reaction mechanisms. The study of reactions of the cyanogen radical by kinetic spectroscopy may also be important to astrophysics. It is well known that spectroscopic - 4 -observations of the extraterrestrial regions such as st e l l a r atmospheres and interstellar space have revealed the presence of large number of molecules****24, among them CN, NH, and OH. With the presence of these transients i t is possible, i f not probable, that some radical-radical reactions do occur leading to the formation of new molecules. Therefore, a knowledge of the reactions of these molecules may become astrophysically significant. D. Aims and Scope of This Investigation The aims of this investigation are: f i r s t l y , to study the Group VA hydrides and halides by flash photolysis and kinetic spectroscopy; secondly, to elucidate the nature of the reactions of the cyanogen radical in the gas phase at room temperature, and thirdly, to assign and analyze, i f possible, the electronic absorption spectra of new transients resulting from the above studies. The formation and chemical reactions of metastable, electronically excited, gaseous atoms of phosphorous., arsenic and antimony arising from the isothermal flash photolysis of Group VA hydrides and halides are investigated. The kinetics and reactions of the cyanogen radical with the Group VA hydrides and halides, and some other compounds are also investigated. New electronic absorption spectra from the Group VA hydrides and halides as well as from the reactions of the cyanogen radical are assigned and analyzed whenever possible. Complete spectroscopic analysis of the rotational fine structure of these spectra is beyond the scope of this investigation. - 5 -THEORY A. Atomic Spectroscopy General information on atomic spectroscopy can be found in the books 25 26 27 by Herzberg , Kuhn , and Candler . The theory of atomic spectra is 28 given by Condon and Shortley . Atomic energy levels required for the calculations of the atomic transitions of phosphorous, arsenic and 29 ° antimony are taken from Moore . The conversion from wavelength A unit in air to wave number (cm - 1) in vacuo, or vice versa, i s given by the Table of Wave Numbers of the National Bureau of Standards of the U.S. Government30. B. Molecular Spectroscopy General information on molecular spectroscopy can be found in the 7 1 T O books by Dixon , and King . Comprehensive treatment on the theory and analysis of molecular spectra is given by Herzberg^* 1 8* 3 3. For purposes of identification of known molecular spectra the tabulations by Pearse and Gaydon 3 4 is useful. In subsequent discussions a l l relevent theories and analysis of molecular spectra are those covered in Herzberg*s books. For convenience the necessary formulas for the determination of vibrational constants are 18 taken from Herzberg and are given below. Determination of Vibrational Constants of Diatomic Molecules Neglecting cubic terms, or in the absence of a fourth band, the f i r s t difference of successive absorption bands (in cm"1) is given by A Gv+l/2 = G(y+l) - G(v) = uve - 2o)eXe - 26)- X v CD - 6 -and the second difference is given by A \*1 " A Gv-3/2 - A G v - l / 2 " - 2 a )e Xe < 2 ) where G(v) = OJ6(V+1/2) - a i eX e(v+l/2) 2 + (3) gives the vibrational term values of the anharmonic oscillator, v is the vibrational quantum number, o j e and weXe are the vibrational constants at the equilibrium position. weXe is a measure of the anharmonicity of the oscillator. The electronic term value T g (in cm~^ ) at the equilibrium position of the anharmonic oscillator can be calculated by using the relationship T e - = v(0-0) + G"(0) - G'(O) (4) where v(0-0) is the frequency of the transition between the zeroth vibrational level of the lower electronic state and the zeroth vibrational level of the upper electronic state, G(0) is the vibrational term value. The prime and double prime denote the upper and lower electronic state, respectively. TQ for the ground electronic state is zero. - 7 -EXPERIMENTAL A. Description of the Flash Photolysis Apparatus Comprehensive discussion on the experimental technique of flash 4 photolysis has been given by Porter . Therefore, i t is only necessary to describe the general features of the flash photolysis apparatus used in this investigation, which i s essentially that used by Basco and Norrish J . In principle the apparatus is quite simple. A schematic diagram of the electrical arrangement for synchronized flash photolysis and kinetic spectroscopy i s shown in Figure 1. The apparatus consists essentially of a reaction vessel, a photolysis lamp, a spectroscopic (analysis) lamp, a spectrograph, an electronic time delay unit, as well as some electronic equipment for charging and discharging the condensers. The photolysis lamp consists of a quartz tube 50 cm in length and about 1 cm in diameter, with tungsten electrodes sealed in at each end, and f i l l e d with an inert gas (Ar) to about 60 mm Hg pressure. The reaction vessel is also made of quartz and is the same length and diameter as the photolysis lamp. The photolysis lamp and the reaction vessel are enclosed in a metal cylinder lined on it s inner surface with aluminum f o i l which acts as a reflector. The spectroscopic lamp is constructed similarly to "the photolysis lamp but i t i s so designed so as to send a light beam along the axis of the reaction vessel on to the spectroscopic s l i t . A Hilger quartz spectrograph, model E742, was used. The aim of flash photolysis as used in this investigation is to discharge a l l the stored electrical energy in as short a time as possible so that rapid chemical reactions can be studied. The amount of energy discharged by the photolysis or spectroscopic lamp is given by E = 1/2 CV2 joules (6) where C is the capacitance in pF (microFarad) and V is the charged voltage Figure 1. Schematic Diagram of Flash Photolysis Apparatus. i 00 A. Condenser (10 KV, 33.3 uF) ; B. Condenser (10KV, 2 yF) ; C . T e s l a c o i l t r i g g e r ; D.-..Electronic delay u n i t ; E. Photolysis lamp (quartz) with tungsten electrodes; F. Spectroscopic lamp (quartz, c a p i l l i a r y ) with tungsten electrodes; G. Electromagnetic pick-up signal to delay 1 u n i t ; FT. Condensing lens; I. Reaction vessel (quartz); J . Brass casing l i n e d with aluminum f o i l f o r r e f l e c t i o n ; K. Spectrograph; L. Connection to vacuum system. - 9 -in KV (kilovolt). The voltage used for the spectroscopic lamp was 9.7 KV and unless indicated otherwise the voltage used for the photolysis lamp was 8.0 KV for a l l the experiments. Therefore, from equation (6) the energy dissipated by the photolysis lamp with a 33.3 uF capacitor and the spectro-scopic lamp with a 2.0 UF capacitor i s 1066 and 94 joules, respectively. The time to peak intensity for the photolysis lamp discharge i s 6 usec. (10~^ sec.) or a flash duration measured by the "half-peak" intensity of 13 usec. as seen from the discharge curve shown in Figure 25. Similarly, the time to peak intensity for the spectroscopic lamp discharge is 2.8 usec. or a flash duration measured by the "half-peak" intensity of 8 M sec. A typical experimental procedure for the arrangement shown in Figure 1 i s as follows. The capacitors for the photolysis and spectroscopic lamps are charged and the reactants with or without inert gas (N2) are introduced into the reaction vessel from the vacuum system. The photolysis lamp is triggered manually and discharges the stored energy, producing excited chemical species or transients in the reaction vessel. The electromagnetic signal resulting from the photolysis lamp discharge goes to the electronic delay unit which triggers the spectroscopic lamp. Thus the discharge of the spectroscopic lamp is synchronized. The light beam from the spectroscopic lamp, focussed by a lens, travels along the axis of the reaction vessel to the spectroscopic s l i t and f a l l s on a photographic plate mounted on the spectrograph. Thus a series o of absorption spectra of transients over a range of wavelength (2150-6700 A) may be obtained over a period of time by selecting the appropriate delays. The photographic plate is then developed (see next section). The selection of spectroscopic s l i t width, dictated by the working spectral range and type of photographic plate used, along with the measurement of the intensity of absorption bands and atomic lines by plate photometry are discussed in the next section. - 10 -B. Photography and Plate Photometry 1. Photography Ilford photographic plates were used for a l l experiments, HP3 for the o o spectral range 2200 - 6700 A and Q3 for the range below 2200 A. Appropriate o spectroscopic s l i t width was 0.05 mm for the region 2200 - 2600 A, 0.03 mm o o for the region 2600 - 2900 A, and 0.02 mm for the region 2900 - 6700 A. Below 2200 A* a s l i t width of 0.05 mm was used with "multiple flashes", that i s , repeated exposure using a fresh sample for each repetition until the desireable ?plate density i s obtained. A l l photographic plates were developed as follows. The plate is developed in a Kodak D19 solution for 5 minutes with continuous agitation. Then the plate i s placed in a water stop-bath containing 3% acetic acid for 1 minute and f i n a l l y the plate i s fixed in an Edwal industraFIX solution for 2 minutes. The plate is washed and then dried. The photographic prints shown in the next chapter were made using a standard photographic enlarger and Agfa BH1 print paper, the development of which is the same as for the plates except Kodak Dektol developing solution was used. 2. Plate Photometry The measurement of the intensity of absorption bands and atomic lines was made using a Joyce-Loebel double-beam recording micr<*densitometer, model MK III C. No absolute measurements of concentration were possible because of the unavailability of extinction coefficients for the observed transients. C. Reagents Purification of a l l chemical reagents condensable at liquid nitrogen temperature was carried out in the P20,. purification unit on the standard vacuum system shown in Figure 2. This purification unit was designed so that trap-to-trap d i s t i l l a t i o n in vacuo can be done with the dehydrating agent ?2®5 by-passed. Normal operating vacuum is ^ 10"^ mm Hg pressure with the use of a two-stage mercury diffusion pump. Nitrogen, used as the inert gas throughout this investigation, was de-oxygenated because the cyanogen radical reacts readily with small 22 amounts of oxygen . Purified Grade tank nitrogen gas from Liquid Air (Canada) Company was passed through a de-oxygenation column operated at 450-500°C, a ^2®$ (Baker S Adamson reagent) trap, a liquid nitrogen trap, and then collected in the vacuum system. The de-oxygenation unit, shown in Figure 3, consists of a 40 mm O.D. pyrex glass column 130 cm long and packed with copper-coated asbestos fibre. Nichrome wire was used for heating and a Chromel-Alumel thermocouple was used for temperature measurement. The preparation of copper-coated asbestos fibre was as follows. The asbestos fibre i s mixed in a concentrated solution of CuSO^ heated at 70-80°C with enough 10% NH^ OH solution added to give a deep blue color. A stoichiomietric amount of NaOH solution required for the formation of Cu(OH)2 is then added to this mixture. The resulting slurry is thoroughly mixed, then drained and washed with water. The fibre coated with Cu(0H)2 is then placed in an oven at 100°C for several hours for preliminary drying, after which i t is heated at ^ 500°C overnight to eliminate the H_0 from CufOH)^. Reduction of CuO to Cu is done by passing hydrogen through the packed column heated to ^ 300°C. The HO formed from the reaction CuO + H-Figure 2 . Schematic Diagram of Vacuum System. Gas Inlet =8c 6 O O 0 O O '-''Ii ' <i> U,i» U ( j i U ( ! , Trap S i l i c o n O i l Bubbler LT S p i r a l Gauge Hg Manometer with S i l i s o n O i l Topping To Reaction Vessel HE E 3 P^O^ P u r i f i c a t i o n Unit 3$ To To Photolysis Spectroscopic Lamp Lamp Two-Stage Hg Diffu s i o n Pump Trap tt •a: By-Pass A Trap Pump -13-Figure 3. Schematic Diagram of De-oxygenation Column. To P' 0 Unit and Vacuum System Nichrome Wire for Heating Cu Wire Glass Wool Asbestos Sheet for Insulation 130 cm Pyrex Glass Tubing (40 mm O.D.) Packed with Copper-Coated Asbestos Fibre Pyrex Glass Tubing 6 mm ^Chromel-Alumel Thermocouple B34 Cone £ Socket Glass Wool Powerstat •N^ from Tank B14 Cone $ Socket - 14 -i s trapped outside the column. Finally, the de-oxygenation column is connected to the vacuum system and the nitrogen tank. Argon, used for the photolysis and spectroscopic lamps, was obtained in a cylinder from Matheson Company (research grade). It was used without further purification. Hydrogen and Oxygen from Liquid Air (Canada) Company were collected in the vacuum system through a liquid nitrogen trap. Cyanogen was prepared by reacting potassium cyanide with copper (II) s u l f a t e 3 6 . KCN and CuS04.5H20 from B.D.H. Laboratory Chemicals (Analar grade) were used. After preparation the cyanogen was thoroughly degassed and purified by d i s t i l l a t i o n from a dry-ice/methanol (-78°C) trap to a liqaid nitrogen trap over ^2^5' ^ n e m a t e r i a l remaining in the - 78°C trap was discarded. The cyanogen collected in the liquid nitrogen trap was then placed in a -78°C bath and the top third of the material was eliminated by pumping. Finally, the purified cyanogen was collected in a storage, globe from the -78°C trap. Methyl Isocyanate (CH^NCO) from K 5 K Laboratories were degassed and purified by trap-to-trap d i s t i l l a t i o n over ^2®$' Arsine (AsH^) and Stibine (SbH^) were prepared by the methods described 37 by Gunn et a l . . Essentially, arsenic (III) oxide and potassium antimony tartrate are reduced by potassium hydroborate giving arsine and stibine, respectively. Arsenic (III) oxide analytical reagent from Mallinckrodt Chemical Works, potassium antimony tartrate from B.D.H. Laboratory-Chemicals (Analar grade), and potassium hydroborate from Metal Hydrides Incorporated were used. The f i n a l purification of the products is the same as that of cyanogen. Because stibine decomposes on prolonged standing at room temperature i t was stored at liquid nitrogen temperature when not being - 15 -used. The infrared spectra- of arsine and stibine showed no impurities. Ammonia and Phosphine were obtained from Matheson Company in lecture bottles. Purification of phosphine is essentially that of cyanogen. Ammonia was purified as phosphine but the ^2^5 w a s by-passed. Dinitrogen tetraf luoride (N^F^), research grade, obtained in a cylinder from Air Products and Chemicals, Inc., was degassed and purified by trap-to-trap d i s t i l l a t i o n over p2°s' Nitrous Oxide obtained from Matheson Company in a lecture bottle was degassed and purified by trap-to-trap d i s t i l l a t i o n over ^2^5' Group VA Trihalides: Reagent grade phosphorous trichloride and tribromide, and arsenic trichloride from Baker 5 Adamson, arsenic tribromide from K § K Laboratories, and antimony trichloride (CP.) from May § Baker were degassed and purified by trap-to-trap d i s t i l l a t i o n over ^2^S' D i s t i l l e d Water, and Deuterium Oxide from General Dynamics Corporation were thoroughly degassed by trap-to-trap d i s t i l l a t i o n . Diethyl Ketone from Matheson, Coleman § Bell was degassed and purified by trap-to-trap d i s t i l l a t i o n . D. Preparation of Chemical Mixtures Chemical mixtures to be flash photolyzed were prepared at least 4 hours before use to ensure homogeneity. Stock mixtures were made up in 2.5 l i t e r storage globes on the vacuum system (Figure 2) by f i r s t introducing the calculated pressures of reactants and then topped with purified nitrogen (inert gas) to give a required total pressure. Pressures 5 to 90 mm were measured using a calibrated spiral gauge, >90 mm were measured using a mercury manometer with the top of the mercury columns covered with silicon o i l to prevent chemical reactions between the reactants and the mercury. For - 16 -pressures <5 mm measurements were made using pre-determined expansion factors so that a higher pressure in a smaller volume can be expanded into a larger volume. - 17 -RESULTS In this chapter are presented the results of the atomic studies of phosphorous, arsenic and antimony, the flash photolysis of Group VA hydrides and halides, cyanogen with Group VA hydrides and halides, cyanogen with some oxygen compounds, assignment and analysis of new electronic absorption spectra, and some bond dissociation energies. Photographic prints of transients observed in the flash photolysis and kinetic spectroscopy studies, as well as some conclusions drawn for the assignment and analysis of the new electronic spectra are also presented in this chapter. Detailed discussion on the kinetics and mechanisms of chemical reactions, however, are presented in the next chapter. A. Atomic Systems of Phosphorous, Arsenic and Antimony Metastable, electronically excited atoms of phosphoroisiarsenic and antimony were observed in the flash photolysis of phosphine, arsine and stibine with nitrogen as inert gas. These atoms were also observed when phosphorus, trichloride and tribromide, arsenic trichloride and tribromide, antimony trichloride, as well as the mixtures of cyanogen/trihalide and cyanogen/hydride were flash photolyzed. Table I summarizes the metastable atomic states of phosphorous, arsenic and antimony. The observed atomic transitions are identified from calculations using 29 the energy levels given by Moore and using the Table of Wave numbers of the National Bureau of Standards for conversion from cm - 1 in vacuo to o A in ai r . The notation for atomic states used here is the same as that given by Moore. The atomic transitions observed in absorption and the calculated wave-lengths for the phosphorus^ and arsenic atoms are listed in Table II, and for - 18 -TABLE I Summary of Low-lying Atomic States of N, P, As and Sb State . Level Atom Config. Desig. J cm - 1 Kcal/ mole 2s 2 2p 3 2p 3 2D° 3/2 5/2 19231 19223 55 55 N I M n 2P0 1/2 3/2 28840 82.5 3s 2 3p 3 3p 3 2D° 3/2 5/2 11362 11377 32.5 32.5 P I ti II 2 p0 1/2 3/2 18722 18748 54 54 ' 4s 2 4p 3 4p 3 2D° 3/2 5/2 10593 10915 30 31 As I ti II 2p0 1/2 3/2 18186 18648 52 53 5s 2 5p 3 5p 3 4S0 3/2 0 0 Sb I " „ 2D0 3/2 5/2 8512 9854 24 28 I I I I 2p0 1/2 16396 47 3/2 18465 53 - 19 -TABLE II List of P and As Atomic Transitions Observed in Absorption o Atom Transition X a i r ^ A ^ 4s P I As I 2 p i / 2 - ~ -*n3 2n0 3p Dy2 2149.11 2 p 3/2 it 2135.47 2p «-*3/2 2n0 D5/2 2136.20 4s 2 p i/2 +• 3p 3 2 p?/2 2553.28 2 p3/2 -«- ti 2534.01 2 p i/2 2554.93 2p P3/2 2535.65 4s« 2D 3/2 5^ 2 2°3/2 5/2 -«-•«-M/2 2p0 3/2 2152.95 2154.08 5s 2 p 3/2 -«- 4p 3 2288.12 5s 2 p l/2 4P3 2860.44 2 p3/2 2745.00 2 p l/2 2P0 P3/2 2898.71 2P H3/2 •*- tt 2780.22 5s' 2°3/2 -«-3p0 F l/2 2344.03 2 D5/2 -<- 2p0 , *3/2 2370.77 2°3/2 I I 2369.67 - 20 -TABLE III List of Sb Atomic Transitions Observed in Absorption Atom Transition * a i r ^ 6s 4 P 1 / 2 «- 5p 3 4S§ / 2 2311.47 4P 3 /2 + " 2175.81 6s 2 P 1 / 2 + 5p 3 2D§ / 2 2598.05 2 P 3 / 2 " 2445.51 2 p3/2 * 2°5/2 2 5 2 8 ' 5 2 5/2 «- " 2293.44 3/2 «- " 2288.98 5/2 «- " 2208.45 6 s 4 p l / 2 2°3/2 2877.92 4 P 3 / 2 «- " 2670.64 4 p3/2 2°5/2 2769.95 Sb I 4 P 5 / 2 *• " 2598.08 6 s' 2 d5/2 *" 2°5/2 2179.19 6s 2 p 1 / 2 «- 5 p3 2 pQ / 2 3267.51 2P «- " 3029.83 Of £• 3/2 «- " 2692.25 2 p3/2 * 2 p?/2 3 2 3 2 ' 5 2 3/2 «- 2851.11 5/2 +• " 2727.23 6s 4 P 1 / 2 «- 2 P j / 2 3722.79 4 P T / 0 <- " 3383.15 3/2 '3/2 ^  '3/2 4P,,„ «- 2P° 3637.83 - 21 -TABLE III (Continued) List of Atomic Transitions Observed in Absorption o Atom Transition X 0-(A) 6s» 2 D 3 / 2 <- 5p 3 2 p 0 / 2 2574.06 1/2 <- " 2480.44 3/2 <r " 2478.32 2°3/2 * 2 f >3/ 2 2 7 1 8 . - 9 0 2 D 5 / 2 «- " 2682.76 3/2 <- " 2652.60 1/2 «- " 2614.73 3/2 <- " 2612.31 3/2 *• 2?\/2 2201.32 7s 2 P 1 / 2 <- 2 P 0 / 2 2329.10 5/2 " 2315.89 3/2 " 2306.40 5/2 +- " 2270.08 7s 4 P 1 / 2 «- 2 P j / 2 2426.35 1/2 «- " 2395.21 3/2 H 2360.50 1/2 «- " 2306.46 3/2 <- " 2262.51 4 p l / 2 «" 2 p 3 / 2 2554.64 4 p3/2 * " 2 3 5 2 ' 2 1 3/2 «- " 2481.74 5/2 " 2474.57 1/2 " 2422.13 5/2 <- " 2383.64 3/2 •*• " 2373.67 Sb I - 22 -the antimony atoms are listed in Table III. It can be seen from Tables I, II and III that highly excited metastable and 2P^ atoms, correspond-ing up to 54 Kcal/mole, are produced in the phosphorois-, arsenic and antimony systems. For antimony, being a heavy atom, intersystem combinations involving the doublet and quartet systems of terms were observed as shown in Table III. The selection rule AS=0 which prohibits intercombinations s t i l l holds, however, for the phosphorous and arsenic atoms while the selection rule AJ=0,±1 (J=0T^- J=0), the Laporte rule, and the selection rule AL=0,tl hold for every case. In Table III there are a number of upper atomic states represented only by the quantum number J which is given as a numerical fraction, for example, the 5/2-<-2Du transition o of the antimony atom at 2293.44 A. These J-represented atomic states and their energy levels are given by Moore and they are the states with unknown electronic configurations. The formation and reactions of metastable, electronically excited atoms in the phosphor<us;arsenic and antimony systems, studied over a wide range of pressures of the parent compounds, wi l l be discussed in the next chapter. However, photographic prints showing the kinetic behaviour of the phosphorous, arsenic and antimony atoms in some of the chemical systems studied are given below. Figure 4 shows the kinetic behaviour of P(3p 3 2P°) in the PH3/C2N2/N2 system where the atomic decay is rapid, visible only up to ^25 usee. This rapid decay, due to chemical reaction, is characteristic of a l l the phosphorous- systems investigated. 3 2 0 Figure 5 shows the kinetic behaviour of As(4p P ) in the AsCl 3/N 2 system where the decay of the arsenic atom, like phosphorus., i s rapid. Figure 4 Decay o f P(3p 3 2 P ° ) i n P H 3 / C 2 N 2 / N 2 (0.2/50/200 mm Hg). Figure 5 Decay of A s ( 4 p 3 2 P ° ) i n AsH,, /N 2 (0.20/250 mm Hg). 2860 A I 2780 2745 2 p0 p l / 2 2 P 0 *3/2 Blank Before 0 usee, delay 2.4 5.2 - 24 -The metastable arsenic atoms were found to decay rapidly in a l l other arsenic systems. Figure 6 shows the time dependence of the metastable and ground state atoms of antimony. The decay of the excited atoms, although significantly slower than those of phosphorus: and arsenic, i s much faster than that expected for metastable atoms. That this rapid decay i s due to chemical reaction w i l l be discussed in the next chapter. It can be seen that plate saturation from the absorption of the ground state atom is clearly evident, a fact which suggests that there is a relatively large concentration of 4so produced in the system. Atoms from other antimony systems investigated behave essentially the same as that shown in Figure 6. The intensity of the ground state atomic line at ^1 x 10"4 mm Hg of stibine was observed to be comparable to those of the excited atoms at %1 x lCT* mm Hg of the parent compound. Figures 7(a) and 7(b) show the decay of antimony atoms in the SbH,/N and SbHj/C2N2/N2 systems, respectively. Comparison of (a) and (b) clearly shows that the intensity of some atomic lines is increased while others are decreased in the SbH3/C2N2/N2 system relative to that of the SbH3/N2 system. Since the pressure of stibine as well as other experimental conditions are the same for the two different systems, there is l i t t l e doubt that some excited antimony atoms are produced by chemical reactions of the CN radical. It is interesting that a l l the lines with increased 2 0 intensity are the transitions involving specifically the m e t a s t a b l e atom and these are given in Table IV. No increase in line intensity was observed for the phosphine and arsine systems. Although both the and metastable states were observed in absorption, i t is possible that one state may be populated from the other. Figure 6. Decay of Sb(5p 3: 4S°, 2D°, 2P 0) in SbH3/N2 (1.25xl0"2/250 mm Hg). o 2374 A 2311 Figure 7. Decay of Sb(5p 3 2P°) in (a) SbH3/N2 (0.20/250 mm Hg). (b) SbH3/C2N2/N2 (0.20/40/210 mm Hg) 3268 3030 A a) b) Blank I Before I 0 usec. delay Before I 0 usee, delay 5.2 Sb2(LH-X) - 27 -TABLE IV Formation of Excited Sb Atoms by CN Radical Reaction in SbHg/Cg^/N.; (List of Atomic Lines with Increased Intensity) Transition 3722.79 3383.15 3267.51 3029.83 2692.25 2574.06 2480.44 2478.32 2426.35 2395.21 2360.50 2306.46 2262.51 6s 4P 1/2 ^ F3/2 5P3 2pn / 2 6s 2P 1/2 r3/2 3/2 6s' ZD 7s 3/2 1/2 3/2 4P *l/2 1/2 3/2 1/2 3/2 - 28 -To investigate this possibility experiments were carried out to observe the atomic fluorescence by flash photolyzing the chemical systems without the use of the spectroscopic lamp. Multiple flashes C M ) , with a fresh sample each time, were used. Studies of atomic fluorescence on the systems PH3/N2, AsH3/N2, SbH3/N2, and SbH3/C2N2/N2 were carried out and the observed transitions are listed in Table V. The atomic fluorescence involves both the 2D° and 2P° metastable states in the AsH3/N2 and SbH3/N2 systems, 2D° in the SbH3/C2N2/N2 system, and 2P° in the PH3/N2 system where investigation of the fluorescence involving the ZD atom at 2150 A is experimentally d i f f i c u l t . The fluorescence involving P( 2P°), though slightly weaker than the arsenic atoms, is very weak relative to the antimony atoms. Comparison of the atomic fluorescence in SbH3H2 and Sblij/C^^/H shows that a l l fluorescing lines are the same except the transition involving the e 2 P j / 2 which i s absent in SbH3/C2N2/N2. This can be easily explained in view of the specific formation of the 2 P j / 2 a t o m b y t n e r e a c t l o n s o f the CN radical as shown in Figure 7 and by the fact that absorption atomic lines were observed in the fluorescence studies as shown in Figure 8. This implies, therefore, from Table V that there i s a line reversal, that i s , process 2 in the transitions 6s 2 p 1 / 2 Y. ' 5 P 3 2 p i / 2 P r e d o m i n a t e s because of the formation of 2 P ^ 2 b y t h e C N r a d i c a l following the photolysis. In the SbCl 3/N 2 system, no increase in the atomic line intensity was observed. Fluorescence involving the ground state 4 S U was not observed in the antimony systems. This is consistent with the relatively large population of the ground state atom. - 29 -TABLE V Atomic Fluorescence System Atom Transition PH3/N2 P I 2535.65 4s 2 P  F3/2 + 3p 3 2p0 3/2 AsH3/N2 As I 2898.71 5s 2 p l / 2 -*• 4p 3 2P0 F3/2 2860.44 2 p *l/2 ->- 2 P0, P l / 2 2780.22 2P F3/2 2p0 F3/2 2288.12 2P F3/2 2n0 5/2 SbH3/N2 Sb I 3267.51 * 6s 2 p  F l / 2 + 5p3 2P0 F l / 2 SbH3/C2N2/N2 2877.92 4P P l / 2 •+ 2n0 D3/2 2769.95 4P F3/2 •+ 2 n0 U5/2 2670.64 4p F3/2 -y 2 D0 3/2 2598.08 4P P5/2 -+ 2n0 U5/2 2598.05 2P F l / 2 - V 2 D0 3/2 2445.51 2P 3/2 2n° D3/2 * Transition not observed in the SbH /C?N /N9 system - 31 -B. Flash Photolysis of Group VA Hydrides and Halides Ammonia, phosphine, arsine, stibine, dinitrogen tetrafluoride, phosphorus , trichloride and tribromide, arsenic trichloride and tribromide, and antimony trichloride with or without inert gas were studied by flash photolysis and kinetic spectroscopy. In addition to the observed atoms reported in section A of this chapter, the photolysis of these compounds has yielded many transient molecular species, some new and some already known. These observed transients are summarized in Table VI, with the new molecular species to be assigned and analyzed, whenever possible, in section E of this chapter. Photographic prints showing the electronic absorption spectra of these new transients w i l l be given. The new transients observed are PCI, AsH, ASH2, AsCl, SbH and SbH2 along with a new electronic system of SbCl. Table VI and the subsequent tables in sections C and D of this chapter require some explanation. The molecular species observed in each chemical system in the experimental spectral region XX 2150-6700 are given with their approximate band positions together with the corresponding electronic transitions, whenever possible. This spectral information for each different species is given only once throughout a l l these tables. Standard spectro-scopic notation i s used and the electronic state designation i s that of Herzberg 6»* 8. Where revision of electronic state designations occurs due to recent findings, those from the current literature are used. The electronic state designation for polyatomic molecules are differentiated from the group-theoretical classification of electronic states by the symbol ^ above the designation, for example, A ^A^. New transient molecular species with a query (?) beside them are those of uncertain assignment as discussed in section E of this chapter. References relevent and useful to this investigation are given for the molecular species and their electronic transitions with the literature survey done up to August, 1967. - 32 -TABLE VI Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of Group VA Hydrides and Halides System Species Approx. Position of Bands (A) Electronic Transition Ref. NH3 NH2(X 2B X) NH(X 3 E ~ ) 4500-5200 3360 X \ -A 3 n A *• * X 2*1 3 E " 6 18,38 PH2(XU \ ) 3850-5420 X \ - 2 B l 6 P H 3 PH(X 3 E ~ ) 3300-3400 A 3 n i <- X 3 E " 18,39 P 2(X V ) 1 g 2150-2400 X h+ lg 40 AsH2 3880-5025 41,42 AsH3 AsH(X 3 E " ) 2900-3400 A 3 n . <-i X 3 E - 41,42 As 2(X 2200-2450 A h+ <-u X ln 18 SbH2 4600-5000 41 SbH3 SbH(X 3T) 3300-3600 A ^  <- X 3 E " 41 Sb2(X h*g) 2200-2300 F «- XX g 18 2800-3200 D «- X 18 N2 F4 NF2(X* 2 B i) 2600 continuous absorption A «-i X 6 P C I 3 PC1(X) p~(x h+) 1 s. 2300-2500 41 PBr 3 P 2(X AsCl 3 AsCl(X) As 2(X 2400-2500 41 AsBr 3 As 2(X l E p SbCl 3 SbCl(X) 2260-2400 e- ; X 18,41,43 - 33 -C. Flash Photolysis of Cyanogen with Group VA Hydrides and Halides Mixtures of cyanogen with ammonia, phosphine, arsine, stibine, dinitrogen tetrafluoride, phosphorus; trichloride, arsenic trichloride, and antimony trichloride were flash photolyzed in the presence of excess inert gas (^) • The observed transient molecular species are summarized in Tables VII and VIII. Detailed discussion on the kinetics and mechanisms of the reactions of the cyanogen radical in these systems will be presented in the next chapter while the assignment of the electronic absorption spectra of the new transients w i l l be done in section E of this chapter. Some photographic prints showing the kinetic behaviour of some of the observed transients, however, are given below. Figure 9 shows the kinetic behaviour of the CN, NH, and NCN free-radicals in the NH.J/C2N2/N2 system. The new spectrum attributed to HNCN?is shown o at 3171 A. It can be seen qualitatively that the CN decays rapidly and o the time dependence of the new spectrum of HNCN and the 3440 A band of HNCN observed and analyzed by Herzberg and Warsop44 is very similar. Figure 10 shows the kinetic behaviour of the CN and NCN free-radicals in the N2F4/C2N2/N2 system. Also shown are the two unassigned spectra observed in this system, one labelled NCN?and the other FNCN? These spectra are so labelled for reasons to be discussed in section E of this chapter and also in the next chapter. The new spectrum attributed to HNCN, as expected, was not observed in the ^ F ^ ^ N ^ / 1 ^ system (Tables VII and VIII). It is interesting that the CF2 free-radical was also observed in this system although CF was not observed. Figure 11 shows the kinetic behaviour of the CN, PH, and PCN free-radicals and Figure 12 of the P- free-radical as well as the new species - 34 -TABLE VII Summary of Absorption Spectra of Transient Molecular Species Observed  in the Flash Photolysis of C2N?/Group VA Hydrides Approx. Pogition Electronic System Species 6£; Bands (A) Transition Ref. NH 2(£ 2B!) NH(X 3E~) CN(X 2t) 3600-4400 B 2 I + •«- X 2 E + 18,20 HNCN (X° 2 A " ) 3440 A 2 A1 -<- * 2 A " 44,6 NH3/C2N2 HNCN? NCN(£ 3 E g) 2890-3180 3290 2550-2810 -«-x h x E i 41 45,6 46 NCN(lAg) 3327 -«- \ 47,48 NCN? 3640-4270 41,23 PH2(X \ ) PH(X 3E") P 2(X UJ) CN(X 2£*) PH3/C2N2 HPCN PCN(X 6IT) 3140-3380 2845-3060 A 3 n X 3E-41 41 NCPCN? 2380-2460 41 AsH2 AsH(X 3£") AsH 3/C 2N 2 As 2(X lT.+) CN(X 2 £ + ) AsCN(X 3E~) 2815-2920 A 3IT <- X 3E" 41 SbH2 SbH(X 3£") SbH /C N Sb (X l E + ) 3 ^ 2 2 g CN(X 2E +) - 35 -TABLE VIII Summary of Absorption Spectra of Transient Molecular Species Observed  in the Flash Photolysis of C?N9/Group VA Halides System Species Approx. Position of Bands (A) Electronic Transition Ref. N 2F 4/C 2N 2 NF2(X 2 B l) CN(X 2E +) NCN# 3E", X A G ) NCN? CF2(X \ ) FNCN? 2350-2650 3200-3650 A iBi X 1Ai 49,86, 41 PC13/C2N2 PC1(X) CN(X 2E +) PCN(X 3E-) NCPCN? AsCl 3/C 2N 2 AsCl(X) CN(X 2E +) AsCNQC 3E~) SbCl 3/C 2N 2 SbCl(X) CN(X 2E +) Figure 9. K i n e t i c Behaviour o f the Trans ients in N H 3 / C 2 N 2 / N 2 (0.20/40/210 mm Hg) 3590 3440 3290 3171 3072 A 1 HNCN? Blank Before 0 usee, delay 1.9 3.8 5.2 7.5 10.3 16.7 24.2 48 100 201 361 530 867 CN,Av=+l Figure 10. K i n e t i c Behaviour o f the Trans ients i n N 2 F 4 / C 2 N 2 / N 2 (2.5/40/208 mm Hg). Figure 11. Kinetic Behaviour of the Transients in PH /C2N2/N? (0.20/50/200 mm Hg). Figure 12. Figure 13. Ki n e t i c Behaviour of the Transients i n PH 3/C 2N 2/N 2 (0.20/50/200 mm Hg) . Ki n e t i c Behaviour of the AsCN Free-Radical i n AsH 3/C 2N 2/N 2 (0.20/40/210 mm Hg). 2451 A 2388 2367 2914 A 2863 2818 Blank Before 0 usee, delay 1.9 3.8 5.2 0-7 1-7 0-6 Blank Before 0 usee, delay 1.9 3.8 5.2 7.5 10.3 16.7 24.2 48 100 201 361 630 867 1400 A f t e r AsCN - 40 -labelled NCPCN?in the PH3/C2N2/N2 system. The new species is so labelled for reasons to be discussed in section E of this chapter and also in the next chapter. It can be seen that of the two new transients, PCN rises to a maximum at ^3jisec. and lasts for ^ 150 usec. while NCPCNtfrises very slowly and lasts for >1.68 millisec. The t r i p l e t splitting of PCN, like that of NCN at ^ 3290 A in Figures 9 and 10, and the doublet structure of NCPCN?are clearly evident. The two new transients were also observed in the PC13/C2N2/N2 system (Table VIII). Figure 13 shows the kinetic behaviour of the new transient AsCN in the AsH3/C2N2/N2 system. The rapid rise of AsCN to a maximum, and the tr i p l e t structure, are clearly evident just as for the NCN and PCN free-radicals. The same spectrum of AsCN was observed in the AsCl 3/C 2N 2/N 2 system(Table VIII). No new transient molecular species involving the CN component, other than the metastable atoms reported in section A (Figure 7) of this chapter were observed in the SbH3/C2N2/N system, or in the SbCl 3/C 2N 2/N 2 system. - 41 -D. Flash Photolysis of Cyanogen with Oxygen Compounds Mixtures of cyanogen with water, nitrous oxide, methyl isocyanate, and oxygen were investigated by isothermal flash photolysis. The last system, O2/C2N2/N2, has been studied by Basco 2 2. Observed transient molecular species in the flash photolysis of the chemical systems without and with the cyanogen are summarized in Tables IX and X, respectively. Discussions on the kinetics and reactions of the cyanogen radical in these systems w i l l be presented in the next chapter while photographic prints showing the kinetic behaviour of some of the observed transients are given below. Figure 14 shows the time dependence of the NCN ( i lA^, 3E^free-radicals in the N20/C2N2/N2 system. Of a l l systems where NCN appeared the nitrous 1 ^ x oxide system produced the strongest NCN ( Ag, X ^E") spectra. It i s interesting to note that the stable molecule NO was produced in the flash photolysis of N20 with or without inert gas (Table IX). Figure 15 shows the kinetic behaviour of the HNCN, NH, NCN, HNCN? and NCO free-radicals in the CH3NCO/C2N2/N2 system where the time dependence of HNCN and HNCN?, just as in the NH3/C2N2/N2 system (Figure 9), is very similar. The NH and NCO transients observed in the flash photolysis of methyl isocyanate serve as a convenient means of identifying the same species in the CHjNCO/C^/^ system. The kinetic behaviour of the transients observed in the H2O/C2N2/N2 system, except for the absence of the CH3 and the presence of the OH free-radical i s similar to that of the methyl isocyanate system. - 42 -TABLE I X Summary of Absorption Spectra of Transient Molecular Species Observed in the Flash Photolysis of Some Oxygen Compounds System Species Approx. Pogition of Bands (A) Electronic Transition Ref. H 2 0 O H ( X 2 n i ) 2850, 3070 A 2E+ «- x 2 n . i 18 N 2 0 N O ( X 2 n) 2260, 2370 A 2T* «• x 2 n 18 C H 3 N C O NCOCX 2 n i ) 3900-4410 2600-3200 A <v B 2 Z + <-2 n i «-x 2 n i X 2Tli 50,6 51,6 C H 3 ( X 2A2') 2150-2170 <v B ^ O It X 2A 2 52,6 N H ( X h~) TABLE X Summary of Transient Molecular Species Observed in the Flash Photolysis of C?N?/0xygen Compounds System Species System Species C N ( X 2 t) N 0 ( X 2 n ) NC0(X 2IL-) CN(X V ) 2 / C 2 N 2 * , i N 2 0 / C 2 N 2 N C N ( X A G ) NC0(X 2 n A ) N C N ? . N C N ( X 3 ^ g , 1 ^ ) 0H(X 2H i) N C N ? CNfX 2 f ) CH 3 (X ^ A 2 ) N H ( X 3 r ) NCOCX" 2 n . ) 2 0 / G 2 N 2 NC0(X^ 2 1^ ) NH(X 3 E - ) N C N ( X 3 l g , 1 A g ) C H 3NC0 / C 2 N 2 C N ( X 2 £ + ) H N C N ( X 2 A " ) N C N ( X 3 ^ ; , 1 A ) g» gj HNCN? HNCN(X 2 A " ) NCN? H N C N ? NCN? * Corresponding deuterated species observed in D20/C2N2. - 4 3 -Figure 14 K i n e t i c Behaviour o f the Transients i n N - O / C ^ W N U (40/40/170 mm Hg). 3327 3290 A 1 NCN Blank Before Fe Arc 4.8 u s e c . delay 12.6 19.5 45 96 188 410 685 975 NCN(3E~) Figure 15. a) Flash Photolysis of CH3NCO/N2 (5.0/245 mm Hg). b) Kinetic Behaviour of the Transients in CH3NC0/C2N2/N2 (5.0/40/205 mm Hg). 3440 3327 3290 3171 3072 A a) b) HNCN NCN(3E-) HNCN? 2.4 usee, delay Blank Before 0 u sec. delay 2.4 - 45 -E. Assignment and Analysis of New Electronic Absorption Spectra In this section are presented the assignment and analysis of the new electronic absorption spectra observed in the flash photolysis of Group VA hydrides and halides, and cyanogen with the Group VA hydrides and halides. Kinetic evidence required to support the spectral assignments w i l l be given only b r i e f l y . The detailed discussion on the reaction kinetics and mechanisms w i l l be presented in the next chapter. 1. Spectra from Group VA Hydrides Spectra of Arsenic Hydride Radicals Dixon et a l . 4 2 have obtained the absorption spectra of the AsH and AsH2 free-radicals in the flash photolysis of arsine. The 1-0 and 2-0 3 3 bands of the 11(a)- z transition of AsH have been observed in this investigation in addition to the 0-0 band reported by Dixon et a l . Therefore, (De and u)gXe can now be calculated. Arsine (20 mm Hg) alone or with an excess of inert gas (N2) was flash photolyzed at an energy of 'vlOOO J. The electronic absorption spectra were recorded at 1.9 usee, delay. The band heads of the 3n(a)- 3£" transition of AsH are given in Table XI. The 0-0 band is reproduced in Figure 16a. From the three observed band heads the ooe and UgXg values for the A state of AsH were calculated to be 1842 cm~* and 31 cm"*, respectively, using equations (1) and (2) in the chapter on theory. A l l the AsH2 bands reported by Dixon et a l . have been observed in this laboratory (Figure 16b). An additional band (v2=7) was observed, whose Q-head f i t s the equation AsH2 v = 19,907.0 + 858.6 v 2 ;-3.4 v 2 - 2 cm"1 given by Dixon and his co-workers. - 46 -Attempts to observe the 2-0 band of the A 3IL-X 3 E~ system of NH and PH were unsuccessful. The AG1(1/2) values for NH and PH have been reported by Dixon 3 8 and Legay 3 9, respectively. TABLE XI Band Heads of AsH ( 3n(a)- 3£~) in cm"1 Band Transition v v a c Av A G v + l / 2 a A 2 ( V + 1 3 n 0 - 3 E ~ 30546 0-0 3 m- 3E~ 29959 3n 2- 3E" 29386 586 573 1780 3no- 3E" (32332) 1-0 3n,- 3E- 31739 -62 (593) '1 3TI 2- 3E~ 31155 3 n 0 - 3 E ~ (34038) 2-0 3 n i - 3 E ~ 33457 3 n 2 - 3 E ~ (32885) 584 1718 (581) (572) For 3 r i j - 3 E - component only. Values within parentheses are less accurate, probably ±15 cm" , owing to the weakness of the observed bands. Figure 16a. Absorpt ion Spectrum of the 0-0 Band of AsH (AsH^/N -20/230 mm Hg). 3399 A 3329 3271 Before Fe Arc • 3 n 2 - h - - V J1o ~1 Figure 16b. Absorpt ion Spectrum of the A s H 2 F r e e - R a d i c a l ( A S H 3 / N 2 : 2 0 / 2 3 0 mm Hg). o 4442 A 4294 4143 - 48 -Spectra of Antimony Hydride Radicals New absorption spectra attributed to the SbH and SbH2 free-radicals have been observed in the flash photolysis of stibine (SbH.j). Stibine alone or with an excess of inert gas (N2) was flash photolyzed at an energy of ^ 800 J. The optimum pressure of stibine used for the SbH2 spectrum was 20 mm Hg and for the SbH spectrum was 6 mm Hg, each mixed with 250 mm Hg of nitrogen. o The group of red-degraded bands near 3600 A (Table XII) are attributed to the three sub-bands of the 0-0 band of a n(a)- £" transition of SbH by analogy with NH38, PH 3 9, and AsH 4 2. The 0-0 band i s reproduced in Figure 17. The larger t r i p l e t splitting for SbH, as compared with the t r i p l e t splitting of ^580 cm"1 for AsH, is expected for the heavier molecule. Of the three sub-bands of SbH only the 3Hn-3E~ component has rotational structure, the other two are diffuse. The diffuseness of the sub-bands suggests that there i s a possible low-lying dissociative electronic state. The 1-0 band of SbH ( 3Il- 3E~) was not observed probably due to the strong overlapping of the Sb2(D-«-X 1 E + ) 1 8 bands. The second group of bands observed in the flash photolysis of stibine consists of a long progression of violet-degraded bands with prominent Q-heads and very complex rotational structure. Seven bands were observed (Table XIII), the three strongest of which are reproduced in Figure 18. This progression i s tentatively assigned to the upper electronic state bending vibration of the SbH2 radical by analogy with NH 2 5 3, PH 2 5 4, and AsH 2 4 2, a l l of which have very similar bands^. The Q-heads of the SbH2 progression can be represented by the equation v = 19438 + 698.0 v* - 2.6 v' 2 ± 3 cm'1 - 49 -where is the upper state vibrational quantum number. The most intense Q-head is at v' = 3. TABLE XII Band Heads of the 3n(a)- 3£' Transition of SbH Sub-bands X a i r ( A ) ,.vvac(cm ) Av(cm-i) 3 n 0- 3Z~ 3379.9 29578 1707 3 n 1 - 3 E _ 3586.9 27871 1639 3 I U - 3 £ " 3811.1 26232 TABLE XIII Q-Heads of the SbH2 Spectrum v' V2 X . (A) air*- J ^ a c t o f 1 ) AG(cm_1) 0 5143.1 19438 693 1 4966.1 20131 691 2 4801.2 20822 689 3 4647.4 21511 680 4 4505.1 22191 672 5 4372.7 22863 666 6 4248.9 23529 Figure 17. Absorption Spectrum of the 0-0 Band of SbH (SbH,/N2:6/250 mm Hg). 3821 A 3581 I 3384 I Sb 1 1 1 SbH — — zp0 2p0 V\I2 F3/2 M/2 Before Fe Arc 1.9 v sec. delay 3»8 3 n 2 - 3 r 3 n 1 - 3 E * 3 n 0 - 3 z -Figure 18. Absorption Spectrum of the SbH2 Free-Radical (SbH3/N2:20/250 mm Hg) 4647 4495 4860 A i 3 I - 51 -Emission Spectrum of P?(C - X *E*) A weak absorption spectrum consisting of many apparently violet-degraded bands was observed in the region XX2320-2870 following the flash photolysis of PH3/N2 (1.5/50 mm Hg). These bands were not observed at lower pressure of phosphine (or PCI3 and PB^) and higher total pressure. Of the ten known singlet band systems of P 2 in the region XX1220-3500 given by Creutzberg 4 u, a l l are red-degraded. Therefore, i t could be easily assumed that the observed spectrum does not belong to any one of these systems. However, from the fluorescence studies in the PH3/N2 system, i t was observed that the same bands corresponding to the apparently violet-degraded bands were observed in emission, only now they are red-degraded. These bands were found to be those of the P2(€-X) system^ 0, and the observed violet-degrading was due to the combination of the absorption and emission bands. The observed band heads (v"-19) along with some values reported by Herzberg 5 7 and some values calculated using the vibrational constants reported by Creutzberg are given in Table XIV} Because of the weak spectrum, the measured band heads are not very accurate compared to those reported or calculated. The observation of the P2 emission bands involving high v" levels may be significant. In the study of flash photolysis of phosphine, Norrish and Oldershaw 5 8 have proposed the formation of vibrationally excited P 2 (v"-7) by the reaction PH + PH P2* + H 2 (1) In view of the observed P 2 emission bands, the formation of P2* must now be considered inconclusive. It is possible that vibrationally excited P 2 could - 52 -be populated by the "optical pumping" process P 2(X) + hv + P 2(C,v=0,l,2 ) P 2(C,v= 0,l , 2 , . . . . ) -=|[v P 2(X,v=0,l,2 ) This excitation process has been shown to occur in the flash photolysis of NO**9 and of cyanogen and cyanogen h a l i d e s 2 0 . Another possibility for the formation of vibrationally excited P 2 is given by the reaction P + PH P2* + H because of the presence of excited, and most probably ground state, phosphorus atoms. This reaction w i l l be discussed in the next chapter. At present, there is no evidence to determine which of the possible processes (or a combination of the processes) is responsible for vibrational excitation. No emission bands of As 2 and Sb 2 were observed in the flash photolysis of arsine and stibine, respectively. - 53 -TABLE XIV Band Heads of the P0(C-X) Spectrum v' v" A a i r ( A ) v v a c(obs.) v Vac(Herzberg)* v v a c ( c a l c . ) * 4 19 2863.2 34916 34916.9 34910 3 18 2845.5 35133 35130.2 5 19 2827.1 35362 35361 4 18 2809.2 35587 35584.6 3 17 2792.2 35803 35797 5 18 2774.6 36031 36032.6 4 17 2757.1 36259 36258.9 3 16 2739.8 36488 36486.2 5 17 2723.4 36708 36703 4 16 2706.4 36939 36933 6 17 2690.2 37161 37156.2 3 15 2689.2 37175 37172.6 5 16 2673.6 37392 37384 2 14 2672.2 37411 37411.4 4 15 2657.7 37615 37613 1 13 2655.5 37646 37639.4 6 16 2642.2 37836 37837.5 37830 3 14 2640.1 37866 37865.1 2 13 2623.8 38101 38109.9 1 12 2607.0 38347 38345.8 6 15 2595.4 38518 38522.3 3 13 2593.0 38554 38557 8 16 2582.0 38718 38718.1 5 14 2578.5 38771 38769.7 2 12 2575.6 38814 38814.2 4 13 2562.1 39019 39017.7 1 11 2559.7 39055 39055.2 0 10 2544.0 39:296 39302.5 39295 7 14 2520.3 39666 39658.4 1 10 2514.0 39765 39772.2 3 11 2500.8 39975 39979.3 0 9 2497.9 40022 40022.3 40018 4 11 2472.8 40428 40433.3 1 9 4269.2 40487 40495.8 3 10 2456.9 40689 40698.0 40689 0 8 2453.7 40742 40747 0 7 2409.6 41488 41489.6 5 10 2403.1 41600 41597.3 2 8 2398.1 41687 41694.6 41681 7 11 2393.3 41771 41770.8 4 9 2387.9 41865 41873.5 41876 6 10 2377.6 42046 42046.2 3 8 2372.1 42144 42141 0 6 2367.4 42228 42227.3 2 7 2356.2 42428 42426U9 4 8 2346.7 42600 42601.4 6 9 2337.7 42764 42768.3 0 5 2326.0 42979 42975.3 *Band origins (band heads %1 cm - 1 from origins). - 54 -Absorption Spectrum of As 9(A 1U+ +- X 1 E p 1 8 The red-degraded band spectrum of A S 2 , with v"-3, was observed in the isothermal flash photolysis of arsine and of arsenic trichloride and tribromide. The observed band heads, which have been reported by Almy and Kinzer^ 0, are given in Table XV. TABLE XV Band Heads of the As? (A<-X) Spectrum o V v» X a i r ( A ) 5 6 5 7 6 7 8 9 9 8 10 9 11 10 12 14 11 13 12 14 13 14 15 16 17 3 3 2 3 2 2 2 2 1 0 1 0 1 0 1 2 0 1 0 1 0 0 0 0 0 2480.7 2464.7 2455.0 2449.9 2439.4 2424.9 2410.6 2395.9 2371.7 2362.1 2357.1 2347.7 2342.8 2333.8 2328.9 2324.8 2319.7 2315.4 2306.0 2301.9 2292.7 2279.6 2266.7 2254.0 2241.6 - 55 -Absorption Spectra of Sb 2 Two known electronic systems of Sb 2 were observed in the isothermal flash photolysis of stibine. These red-degraded band spectra were not observed with antimony trichloride. The band heads of the F«-X system 1 8* 6 1, with v"*7, are given in Table XVI. About eighty bands of the extensive D«-X system 1 8' 6 2 were also observed with v"-7. TABLE XVI Band Heads of the Sb? (F-«-X 1z^) Spectrum V' v" X a i r(A) 2 7 2304.9 2 6 2290.9 2 5 2277.2 0 3 2272.2 2 4 2263.3 0 2 2258.5 6 1 2244.9 2 1 2222 ;.8 2 0 2209.4 2. Spectra from Group VA Halides  Spectrum of the SbCl iree- iRadical Ferguson and Hudes 1 8» 4 3 have observed two red-degraded emission band spectra attributed to the free-radical antimony monochloride (SbCl) in the regionXX 4200-5600 by introducing SbClj vapour into a stream of active nitrogen. No other bands appeared in the range X 2000 to X 7000. A group of diffuse violet-o degraded bands near 2320 A attributed to a new system of SbCl (C-«-X) has been observed in absorption following the isothermal flash photolysis of SbCl^. Approximately 0.1 mm (vapour pressure at room temperature) of SbClj with an excess of inert gas (N2) was flash photolyzed at an energy of 1000 J. The band heads and the vibrational assignment are given in Table XVII. Part of the SbCl spectrum is shown in Figure 19. From the observed band heads the values of w., to X for the ground e» e e - 56 -electronic state (X) and the values of we, weXe, and T g for the upper electronic state (C) of SbCl were calculated using equations (1) to (4) in the chapter on theory. These constants are summarized in Table XVIII. 18 The upper electronic state i s labelled C following Herzberg's designation of A, B, and X for the three unassigned electronic states involved in the emission spectra observed by Ferguson and Hudes. Though the band at 43,107 cm - 1 is the strongest of the v"=0 progression, the one;at 43,549 cm"1 i s of comparable intensity. The assignment of v'=0,l,2 for these bands then follows from the fact that no other bands of this progression were observed to longer wavelength. The assignment given involving the presence of vibrationally excited levels of the ground state at room temperature, requires some justification. The observation of bands with v"=l and 2 can easily be accounted for in view of the low value of u i e " and the apparently favourable Franck-Condon factors for the Av=2 transitions. However, this explanation is inadequate to explain the observed transitions from v"=3 and (particularly) v"=4 and i t is clear that, i f the assignment i s correct, the system is not in equilibrium. Convincing support for this view comes from the time dependence of the intensities of the bands. Those from v"=0,l,2 increase to a maximum with the photoflash from the shortest delay while the intensities of the bands from v"=4 are greatest at the shortest delay, (Figure 19). The formation of vibrationally excited SbCl can be explained in several ways of which the most likely are by "optical pumping" and by direct formation in the photolysis of SbC^, i.e. SbCl(X,v=0,l) + hv SbCl (C,v=0,1,2) - 57 -SbCl(C) SbCl(X,v»0,l,2,3...) (1) -nv and SbCl 2 + ihv -»• SbCl(X,v=0,l,2,3...) + Cl (2) Process (1) has been shown to occur in the flash photolysis of NO 5 9 and of 20 cyanogen and cyanogen halides and i s the most l i k e l y explanation. Examples of process (2) are also known, perhaps the most s t r i k i n g being the photolysis of the n i t r o s y l h a l i d e s 6 3 where at least h a l f the excess energy of the photon can appear in vibrati o n a l excitation of NO. TABLE XVII Band Heads of the SbCl Spectrum (cm - 1) Band v v a c ( o b s . ) AC v +i/2 A z G y + 1 v v a c ( c a l c . ) 2-0 43986 437 1-0 43549 -5 442 0-0 43107 365 0-1 42742 -3 362 0-2 42380 (350) 0-3 (42030) 1-3 (42452j 42458 1-4 (42116) 42105 (425) 2-4 42541 42544 (439) 3-4 (42980) 42975 Values within parentheses are less accurate, probably ±20 cm - 1, because of the weakness of the observed bands. TABLE XVIII Constants for the two Electronic States of SbCl (cm-*) State T e ue u e x e C 43068 447 5/2 X 0 368(369.0) 3/2(0.92) Values within parentheses are from Herzberg 1-4 0-2 2-4 0-1 3-4 0-0 1-0 SbCl - 59 -Spectrum of the PCI Free-Radical o A group of diffuse violet-degraded bands near 2420 A attributed to the PCI free-radical has been observed in absorption following the isothermal flash photolysis of PCl^. The spectrum, similar to that of SbCl, has a simple vibrational strucure and was not observed when PH or PBr„ were 3 3 flash photolyzed. 0.2 mm Hg pressure of PCI? with an excess of inert gas (N2) was flash photolyzed at an energy of M.000 J. The only other spectrum observed in the region XX 2200-6600 was that of P 2(C 1 E U +• X JEg, v"^7) 4 0. The band heads and the vibrational assignment are given in Table XIX. Part of the PCI spectrum is shown in Figure 20. From the observed band heads the values of u e, u eX e, and T e for the two electronic states' °: of PCI were calculated and are summarized in Table XX. Though the band at 41333 cm"* is the strongest of the v"=0 progression, those at 42065 cm"1 and 42743 cm - 1 are clearly observable. The assignment of v'=0,l,2 for these bands then follows from the fact that no other bands of this progression were observed to longer wavelength. Although PCI could be produced directly in the primary process by elimination of C l 2 , i t is more li k e l y to arise by secondary photolysis or disproportionation of PC12. Analogous mechanisms have been proposed for NCI, NH, and PH in the flash photolysis of NC1 3 6 4, NH3 6 5, and PH 3 5 8, respectively. Any of these mechanisms, in addition to,^optical pumping" could account for the detection of vibrationally excited levels in the ground state of PCI at room temperature. However, because of the rapid decay i t was not possible to prove that PCI was in vibrational disequilibrium as was the case for SbCl. - 60 -This rapid decay of PCI and the rapid rise of i s , at least in part, by the exothermic reaction 2PC1 -»- P 2 + C l 2 (1) and possibly P + PCI -»• P 2 + Cl (2) Reaction (1) is analogous to that for PH 5 8 and provides independent evidence for the presence of PCI in the system. Reaction (2) wi l l be discus in the next chapter. TABLE XIX Band Heads of the PCI Spectrum (cm - 1) Band v v a c(obs.) A G V + 1 / 2 A 2 G y + 1 v v a c ( c a l c . ) 2- 0 1-0 0-0 0-1 0- 2 3- 1 4- 1 1- 3 42743 42065 41333 40763 40200 (42817) (43371) 40378 678 732 570 563 -54 -7 42797 43367 40376 Values within parentheses are less accurate, probably ±20 cm"1, because of the weakness and diffuseness of the observed bands. TABLE XX Constants for the two Electronic States of PCI (cm-1) State we xe A X 41234 0 786 577 27 7/2 Figure 20. Absorption Spectrum of the PCI Free-Radical (PC1,/N~ :0.2/250 mm Hg).. - 62 -Spectrum of the AsCl Free-Radical Ferguson and Hudes 4 3 have attempted unsuccessfully to observe a spectrum of AsCl in the region XX 2000- 7000 by introducing AsClj vapour into a stream of active nitrogen. A group of diffuse violet-degraded bands near 2450 A attributed to the AsCl free-radical has been observed in absorption following the isothermal flash photolysis of AsCl^. The spectrum, similar to that of SbCl, has a simple vibrational structure and was not observed when AsH-j or AsBr3 were flash photolyzed. 0.2 mm Hg pressure of AsCl? with an excess of inert gas (N2) was flash photolyzed at an energy of 'v-lOOO J. The only other spectrum observed in the region XX 2200-6600 was :that of As 9(A lZ+ + X 1E* v"-3) 1 8. c u g The band heads and the vibrational assignment are given in Table XXI. Part of the AsCl spectrum is shown in Figure 21. From the observed band heads the values of mQ and coeXe for the ground electronic state of AsCl were calculated to be 443 cm"1 and 2 cm"1, respectively. Though the band at 40865 cm - 1 is the stronger of the v"=0 progression, the band at 41385 cm"1 i s clearly observable. The assignment of v'=0,l for these bands then follows from the fact that no other bands of this progression were observed to longer wavelength. The possible processes which may lead to the formation of vibrationally excited AsCl in the ground state are analogous to those discussed for PCI. Like PCI, because of the rapid decay i t was not possible to prove that AsCl was in vibrational disequilibrium as was the case for SbCl. This rapid decay of AsCl and the rapid rise of As 2 i s , at least in part, by the exothermic reaction 2AsCl •+ As 2 + C l 2 (1) - 63 -and possibly As + AsCl •+ As 2 + Cl (2) Reaction (1) is analogous to that for PH 5 8 and provides independent evidence for the presence of AsCl in the system. Reaction (2) w i l l be discussed in the next chapter. TABLE XXI Band Heads of the AsCl Spectrum (cm - 1) v2f:n Band v y a c A G ^ 1 / 2 Gy+1 AG'd/2) 1-0 41385 0-0 40865 439 0-1 40426 -4 520 435 0-2 39991 Comparison of the Diatomic Halides of Group VA It would be of interest to compare the available spectroscopic data, particularly of the ground electronic state, of the diatomic halides of Group VA in view of the analysis of the SbCl, PCI and AsCl spectra carried out in this section. Since the normal atoms arising from the ground electronic state of SbCl are Sb(4S°) and C1( 2P°) the possible molecular states are Z~ and II, each being a t r i p l e t or a quintet. The ground electronic state 3r* has been assigned to the free-radicals NF 6 6' 6 7, NCI 6 8, NBr 6 9, and PF 7 0, a l l of which have the same number of outer electrons. If analogy holds then the ground electronic state of SbCl, as well as PCI and AsCl, should be The pjg values for the ground electronic state are known for a number of diatomic halides of Group VA. Those for the molecules mentioned above and 18 some values taken from Herzberg in addition to the values determined in this investigation are given in Table XXII. Comparison of the diatomic - 64 -halides in Table XXII generally shows, as expected, that for any one halide or any one element of Group VA the coe value decreases with increasing number of electrons and mass of a molecule. The u> value of 368 cm - 1 e for SbCl determined in this investigation is in good agreement with that from Herzberg while the values for the PCI and AsCl are reasonable compared to the other diatomic chlorides and compared to the value for PF. Therefore, i t i s reasonable to assume that the assignment and analysis of the SbCl, PCI and AsCl spectra are essentially correct, although the uie values are necessarily less accurate than the others given in Table XXII. Attempts to observe the PBr and AsBr free-radicals in the region XX2200-6700 were unsuccessful, probably because these absorption bands are extremely weak and diffuse. TABLE XXII a) Values for the Ground Electronic State of e 1 Group VA Diatomic Halides (cm - 1) N P As Sb Bi F 1141.37 846.75 - 614.2 620 Cl 827.0 577 443 ^ g * 0 308.0 Br 691.75 - - - 209.34 I - - - - 163.9 - 65 -3. Spectra from Cyanogen Systems New Spectra Observed in the C?N,,/NH?, HoO, CHgNCO Systems The spectra of NCN (3^)23,45,46,71^ C^N ( X A ) 4 7 ' 4 8 and HNCN ( 2A") 6' 4 4 3 have been observed in absorption together with NH ( E) following the isothermal flash photolysis of mixtures of cyanogen with ammonia, water or methyl isocyanate. Seven absorption bands involving at least two unknown transients were also seen. A mixture of NH3/C2N2/N2 (0.2/40/210 mm Hg), flash photolyzed at an energy of 'vlOOO J, produced the strongest spectrum of HNCN and of four of the absorption bands (spectrum A). The other three bands (spectrum B), together with rather stronger NCN spectra were also obtained from mixtures of cyanogen with oxygen, nitrous oxide or dinitrogen tetrafluoride, but for these mixtures spectrum A was not observed. For a l l systems containing oxygen NCO30*^1 was produced and OH was present with the water mixture. The optimum delay time was ^ 2 usee, and a l l spectra decayed rapidly. Spectrum A consists of four sharp, regularly spaced bands near 3171, o 3072, 2980, and 2896 A with sub-bands visible in the f i r s t three of these o (Table XXIII, Figures 9 and 22). The 3171 A band is the strongest and there is a progressive decrease in intensity, in sharpness and in the regularity of the sub-band spacing towards lower wavelength. In each band, the strongest feature i s the central apparently red-degraded component (R) with the sub-bands to the violet rather stronger than those to the red. The weakness of the spectrum with H20 and D20 compared to NH^  allowed accurate measurement o of only three sub-bands of the 3171 A band for the deuterated species and these are also listed in Table XXIII. The sub-band structure is particularly evident in the 3171 A band with a regular spacing of ^24 cm - 1. Like the 3440 A band of HNCN which has a similar, though more widely spaced ("v40 cm"1). - 66 -sub-band structure, the spacing of the sub-bands is halved on deuteration. ° o The 3171 A band appears to be the same as the 3170 A band mentioned by Herzberg and T r a v i s 4 5 and the other three bands have been observed by Herzberg, Shoosmith, Travis and Kroto.* o Spectrum B consists of three red-degraded bands at 4264, 3924 and 3639 A (Table XXVIII) of which the f i r s t two have been previously reported 2 2* 2 3. While the presence of a l l the observed species can be explained, a f u l l discussion of the reactions involved will be given in the next chapter. Briefly, then, the CN radical, produced in the photolysis of C2N2, reacts rapidly and successively with NH^ and producing NH and this radical is also produced in the photolysis of NH^6^. The essential reactions are then CN(X 2E) + NH(X 3E) •+ H( 2S) + NCN(X 3E', 1A ) •*• HNCN(X ZA") With water, CN produces OH which in turn reacts with CN to produce NH" or NCO. The reaction of CN with oxygen'' also produces NCO and NCN is produced in the reactions of CN with NCO23. While the definite assignment of spectra A and B must await complete spectroscopic analysis, the following considerations are relevant to an interim identification. Spectrum B is believed to be three bands of one species on the grounds that the bands occur together with similar relative intensities and time dependance and because of the similarity in the two spacings (2028 and 1996 cm"1). The species 3 can only contain C and N and is not NCN ( E) since, under some conditions, i t s decay is faster (Figures 38-41). Though i t s rate of decay is similar * It was pointed out by the referee of a submitted paper in the J. Chem. Phys. that spectrum A is the same as that observed by Herzberg et a l . (unpublished observations). - 67 -to that of NCN(*A), there is a small difference in their relative intensities in different chemical systems (Figures 40 and 41). Since the known spectra of CNN72, CCN 7 3 and CNC 7 4 were not observed, i t is concluded, that the species responsible for spectrum B is either another metastable state of NCN or a more complicated combination of the elements N and C. Spectrum B is henceforth referred as NCN? o The species responsible for the 3171 A band of spectrum A evidently contains no elements other than C and N except for at least one H atom. The other three bands seem to beldng^to the same species for the reasons given in connection with N C N ? , but this i s not proven. The species i s produced by or following the reaction of CN with NH. If i t is a primary product, i t is either HNNC or a metastable state of HNCN (see discussion). However, secondary reactions of the products could produce several other species and further speculation i s not warranted. Spectrum A i s hence-forth referred as HNCN? - 68 -TABLE XXIII Wavelengths of Spectrum A (HNCN?) X . (A) a i r v J v v a c ( c m - 1 ) A G ^ ( c m ) Deuterated A o air ^ J "vac' A a i r ( A ) V V 3 r ( e m " ) 3175.9 31478 3173.6 31501 3170.8R 31529 3170.3 31534 3168.4 31553 3166.1 31576 3165.6 31581 3078.7 32472 3075.4 32507 3072.7 32535 3072.0 R 32543 3070.8 32555 3066.3 32603 3062.0 32649 2981.5 33530 2980.3 R 33544 2974.7 33607 2895.5 R 34526 3172.5 3171.2 3170.1 31512 31525 31536 1014 1001 982 R, Band component degraded to red. Figure 22. Absorption Spectrum of the HNCN? Free-Radical in NH./C-N /N2(0.20/40/210 mm Hg). o 3440 A 3327 3290 3171 Figure 23. Absorption Spectrum of the PCN Free-Radical in PH3/C2N2/N2 (0.30/80/170 mm Hg). 0 3222 A 3100 3014 PH(l-O) HPCN PCN - 70 -Spectra of the PCN and HPCN Free-Radicals The formation of the NCN and HNCN free-radicals in the flash photolysis of NH3/C2N2/N2 mixtures has been discussed in the previous spectral assignment. Since similar reactions would be expected to occur between CN and PH, the flash photolysis of PH3/C2N2/N2 mixtures has been studied and the two new absorption spectra attributed to the PCN and HPCN free-radicals have been observed. A mixture of PH3/C2N2/N2 (0.2/50/200 mm Hg pressure) produced the strongest PCN spectrum and was flash photolyzed at an energy of ^ 1000 J . The PCN spectrum rises to a maximum intensity of ^ 3 u sec. and is vi s i b l e for ^150 usee (Figure 11). Similar behaviour was observed for the HPCN spectrum, the overall intensity of which was increased by using 0.3/80/170 mm Hg pressure of PH3/C2N2/N2. The f i r s t spectrum consists of two bands with prominent t r i p l e t Q-heads, o o the strongest band near 3014 A and the other near 2857 A (Tables XXIV and XXV, Figure 23) and is assigned to PCN. The absence of hydrogen is proved by the fact that the same spectrum is obtained from PCI3/C2N2/N2 mixtures and the structure PCN is preferred to that of CNP by i t s mode of formation and by analogy with the NH3/C2N2/N2 system where only NCN is observed. The sub-bands near 3014 A are attributed to the 000-000 band of a 3 n ( a ) - 3 E ~ transition of the linear PCN by analogy with NCN (A 3 n u «- X 3 E ~ ) 4 5 . It is o l i k e l y that the 2857 A band is due to vibrational excitation in the or v 3 mode of the upper electronic state in view of the relatively large separation between the two bands (^1830 cm"1). The larger t r i p l e t splitting for PCN (^ 104 cm"1), as compared with the t r i p l e t splitting of M0 cm"1 for NCN, is expected for the heavier molecule because of increased spin-orbit interaction. - 71 -TABLE XXIV Wavelengths of Lines of the 3014 A Band of PCN TABLE XXV Q-Heads of PCN ( 3n(a)- 3E -) *airCA) v (cm - 1) vac Band Sub-band x a i r ( A D f Ay (cm-1) 3056.4 32709 3n-- 3-z~ 3024.0 33059 3040.4 32881 0 - 7 ? 106 3030.6 32987 3014 A J>v 3014.4 33165 3029.4 33000 i 102 3024.0 33059 V 3 r " 3005.1 33267 3020.2 3019.1 33101 33113 3 n 3 E- 2865.5 34888 3018.2 33123 2857 A 3n i - 3 z -106 3014.4 33165 2856.8 34994 3008.6 33228 103 3005.1 33267 3V3E" 2848.4 35097 3002.5 33296 3000.4 33319 2999.6 33328 TABLE XXVI 2998.2 33344 2996.9 33358 Wavelength of HPCN Bands 2994.4 33386 o 2993.0 2990.4 33402 33431 System A a i r(A) v v a c ( c m Av(cm" 2989.3 33443 3377 29604 2980.4 33543 114 2966.2 33703 a 3364 3354 29718 29807 89 3187 31368 139 b 3173 31507 109 3162 31616 91 3153 31707 101 3143 31808 - 72 -The second spectrum consists of two weak and diffuse multiple sub-o o band systems* one near 3360 A and the other near 3160 A (Table XXVJ), the o appearance of which is very similar to the perpendicular band of the 3440 A r\, J ^ 2 A A system of HNCN (A A* «- X 'A") analyzed by Herzberg and Warsop . The carrier of the second spectrum is tentatively assigned to HPCN by analogy with HNCN and because the spectrum was absent in the flash photolysis of PC13/C2N2/N2 mixture. Spectrum of the AsCN Free-Radical A new absorption spectrum attributed to the AsCN free-radical has been observed in the isothermal flash photolysis of mixtures of cyanogen with arsine or arsenic trichloride. A mixture of AsH, (or AsCl 3 ) / C 2N 2/N 2 (0.2/40/210 mm Hg pressure) was flash photolyzed at an energy of MOOO J. The AsCN band rises to maximum intensity at ^ 3 usee, and lasts for ^ 150 usee. (Figure 13} The spectrum (Table XXVII i), i s attributed to the three sub-bands of 3 3 -the 000-000 band of a n(a)- E transition of the linear AsCN by analogy with NCN (A 3n «- X 3 E ' ) 4 5 . The larger t r i p l e t splitting for AsCN (^550 cm*1), as compared with the tr i p l e t splitting of M0 cm"1 for NCN and ^ 104 cm"1 for PCN is expected for the heavier molecule because of increased spin-orbit interaction. The formation of AsCN can be explained by a mechanism, similar to that proposed for NCN and PCN, involving the reactions CN(X 2 E ) + AsH(X 3 E ) •+ H(2S) + AsCN(X 3 E ) CN(X 2 E ) + AsCl + C1(2P) + AsCN(X 3 E ) - 73 -From its mode of formation and the analogous reaction of CN with NH the structure AsCN is preferred to that of CNAs. The spectrum of the HAsCN free-radical which like HNCN and HPCN might also be expected to result from the combination of the radicals was not observed. TABLE XXVII Wavelengths of AsCN (3ft(a)-3£~) Sub-band X a i r(A) v ^ f c m " 1 ) 2916.6 34277 2914.2 34305 3 n 2 - 3 E ~ 2913.4 34314 2910.1 34353 2869.0 34845 2868.1 34856 3 n 1 - 3 E - 2863.4 34913 2862.6 34923 2861.4 34938 2824.5 35394 2823.4 35408 3n 0- 3E- 2820.9 35439 2817.5 35482 - 74 -Other Spectra of Free-Radicals A red-degraded system of bands in the region XX2550-2810 has been observed when mixtures of cyanogen with oxygen,.nitrous oxide, water, methyl isocyanate, dinitrogen tetrafluoride or ammonia were flash photolyzed. The identity of the carrier of this spectrum is uncertain. However, this spectrum is similar to that of NCN (S 3~~ x' 3E") observed by Milligan et a l . in the photolysis of cyanogen azide in a solid nitrogen matrix. Although the observed band heads differ from those reported, the band spacings are comparable (Table XXIX). The shift of these band heads (^ 230 cm"1), as compared with the shift of ^150 cm"1 for the known NCN o band at 3290 A, is probably due to the matrix-molecule interaction. There i s no reason, however, to expect the same matrix shift for the two band systems of NCN. A new absorption spectrum observed in the isothermal flash photolysis of N2F4/C2N2/N2 is shown in Figure 10, and the wavelengths of the bands are given in Table XXX. There is no spectroscopic evidence for the assignment of this spectrum and i t remains, therefore, necessarily unassigned. However, there are reasons to believe that this spectrum may be due to FNCN, the possible formation of which is discussed in the next chapter. The isothermal flash photolysis of mixtures of cyanogen with phosphine or phosphorus.' trichloride has yielded a new absorption spectrum with red-o degraded, close double-headed bands at 2388 and 2451 A in addition to that of PCN. That this new transient is not PCN can be seen from Figures 11, 12 and 36 where the rise of the PCN is very much faster. This new spectrum (Table XXXI) is labelled NCPCN? or P(CN)2?, the formation of which wi l l be discussed in the next chapter. o Transient continuous absorption spectra below 2400 A were observed in the isothermal flash photolysis of cyanogen with the trichlorides but not with - 75 -TABLE XXVIII Band Heads of the NCN? Spectrum air (A) vvac( c n r- 1 ;) AG (cm - 1) TABLE XXX Wavelengths of FNCN? Bands X, air o (A) v rem"*") vac • " ' 4263.8 3924.4 3639.2 23447 25474 27471 2027 1997 a i r (A) 3617.7 3551.3 3453.1 3347.9 27634 28151 28951 29861 TABLE XXIX Band Heads of NCN (B 3E~ X 3E") Spectrum x a i r ( A ) * v v a c(cm _ 1) AvCcm"1) 3009.8 3004.8 2917.2 2914.0 2831.0 33215 33270 34269 34307 35313 2807.8 35605 1063 2827.0 35363 2732.0 36592 2749.6 36358 2726.4 36668 1097 2746.1 36404 2667.2 37481 2672.4 37408 2654.2 37665 1032 2669.2 37453 2586.5 38651 2601.3 38431 2583.4 38697 2598.5 38472 *Band heads from Milligan et a l . 4 6 TABLE XXXI 1054 1044 1045 1050 1023 Band Heads of the NCPCN? Spectrum air (A) v v a c ( c n r l ) AG^cm- 1) 2451.5 2451.0 H 23882.7 2388.2 H 40779 40787 41851 41860 1073 - 76 -the hydrides. It was not experimentally possible to distinguish these spectra. F. Bond Dissociation Energies Although the bond dissociation energies derived from the present investigation can only be approximate and in fact, only the lower limit, they may be of interest because of the newly discovered molecules. In the previous section the proposed mechanism for the formation of the NCN, PCN and AsCN free-radicals is where A i s N, P or As and X is H or a halogen. If this mechanism is correct, then from Appendix I the bond dissociation energies for the ground electronic state of NCN, PCN and AsCN are >85, >71 and >67 Kcal/mole, respectively. The value of -100 Kcal/mole for NCN has been given by Setser and Thrush 7 5/Table XXXII, Appendix I). It is possible to calculate the bond dissociation energies for the diatomic chloride of phosphorctfs:, arsenic and antimony using the Birge-Sponer Extrapolation method 18. However, in view of the small and inaccurate ueXe values the calculated bond dissociation energies may not be meaningful and therefore were not determined. CN + AX -»- ACN + X TABLE XXXII Approximate Bond Dissociation Energies of NCN, PCN and AsCN (Kcal/mole) Compound § Bond Dissociation Energy N-CN >85 P-CN >71 As-CN >67 Sb-CN - 77 -DISCUSSION A. Formation and Reactions of Excited Atoms of Phosphorous, Arsenic and  Antimony in the Flash Photolysis of Group VA Hydrides and Halides The production of PH2, PH and vibrationally excited P 2 in the flash c o photolysis of phosphine has been explained by the mechanism3 PH, + hv + PH2 + H (1) H + PH3 + H 2 + PH2 (2) 2PH2 * PH3 + PH (3) 2PH + P* + H 2 (4) The reaction H + PH2 -»• H 2 + PH (5) was considered less l i k e l y . The same mechanism may be applied to the flash photolysis of ammonia65 and i t seems likely that similar reactions could occur for arsine and stibine. The observation of the spectra of AsH^AsH and As 2, and SbH2, SbH and Sb* in the flash photolysis of arsine and stibine (Table VI) is consistent with this view. Generalizing then, the possible mechanism for the flash photolysis of these hydrides i s AH3 + hv -> AH2 + H (6) H + AH3 + H 2 + AH2 (7) H + AH 2 ->- H2 + AH (8) 2AH2 -+ AH3 + AH (9) 2 AH + A* + H 2 (10) where A denotes the N, P, As or Sb atom. Similarly, the mechanism for the flash photolysis of the trihalides is AX3 + hv -> AX2 + X (11) 2AX2 + AX3 + AX (12) 2AX -> A* + X2 (13) where X is Cl or Br and A for reaction (13) is P or As. The reactions - 78 -corresponding to (7) and (8) are endothermic, at least f o r A = P, As or Sb, for the t r i h a l i d e s (Appendix I ) . This mechanism i s supported by the observat ion o f NC1 2 and N C I 6 4 , PCI and P* , AsCl and As* , and SbCl (Table VI) i n the f l a s h photo ly s i s o f the appropriate h a l i d e s . That exc i ted Sb 2 i s produced from s t i b i n e , whereas no S b 2 was observed with S b C l ^ , i s in accord with the probable exothermici ty of the corresponding react ion f o r S b C l . The formation of ground state (A 4 S ° ) atoms can r e a d i l y be explained by a l o g i c a l extension of the above mechanims. The poss ib l e react ions in the product ion of atoms are AH + H * A + H 2 (14) AH + AH -*• A + A H 2 (15) AH + AH 2 -»- A + A H 3 (16) for the h y d r i d e s , and AX + AX + A + AX 2 (17) AX + AX2 A + A X 3 (18) f o r the h a l i d e s . The r e l a t i v e l y large concentrat ion of ground state Sb observed i n the f l a s h photo ly s i s o f s t i b i n e or antimony t r i c h l o r i d e suggests that the above processes are re spons ib l e , to a large extent , f o r the formation of A ( 4 S ° ) . The ground states of P and As were not observed, however, because t h e i r t r a n s i t i o n s occur in the vacuum u l t r a v i o l e t . Reaction (14) i s s u f f i c i e n t l y exothermic to produce A ( 2 D ° ) as can be seen from Table I and Appendix I . No exc i t ed n i trogen atom, however, can be formed by (14). To account f o r the observed exc i t ed atoms o f phosphoruss, arsen ic and antimony i n a l l the appropriate systems by t h i s mechanism, the AX or AX^ r a d i c a l s (and perhaps AH or A H 2 also) must be v i b r a t i o n a l l y or e l e c t r o n i c a l l y e x c i t e d . It i s pos s ib l e that a s u f f i c i e n t concentrat ion of the v i b r a t i o n a l l y or e l e c t r o n i c a l l y exc i t ed r a d i c a l s could be produced by termolecular - 79 -recombination and reactions (14) to (18) then follow. Atom excitation by energy transfer from these (or other) excited species is also possible. Some of the possible processes involving termolecular recombination and energy transfer or secondary photolysis are given below. Representing the ground state A( 4S°) by A, the metastable state 2D° or 2P° by A*, the third body by M, and excitation by *, the processes are: 2A + M •+ A* + M (19) A 2(el.) + A-*- A* + A 2 (20) k*2 + hv A* + A (21) Generalizing (19) and (20) the recombination and energy transfer processes are then given by * Rx + R2 + RjR2 + M (22) RXR2* + A -»• A* + RjR2 (23) R^ftand R2 are free-radicals or ground state atoms and RjR 2* is the vibrationally or electronically excited recombination product, the electronic state of which depends on the spin conservation rule and the excitation energy above the ground electronic state. The intensity of the observed atomic line for the ground state produced from 5 x 10~4 mm Hg of stibine i s comparable to that of the excited atoms produced from 0.2 mm. Therefore, i f $10% of the stibine is photolyzed then the maximum concentration of A* required for observation is <3 x 10~9 mole/liter since the transition p r o b a b i l i t i e s 7 6 of the atomic transitions are comparable. The actual concentration required is lik e l y to be much lower. Taking 0.2 mm of stibine and for the same % photolysis, the free-radical concentration i s of the order of 10"6 mole/liter. With a rate constant of 10 1 1 l i t e r 2 mole"2 sec. - 1 and a total pressure of 250 mm Hg, the rate of formationoof the excited molecules by recombination is of the order of 10~9 mole/liter/nsec. The - 80 -recombination is likely to be the rate limiting step assuming reasonable rate constants for reactions (14) to (18) or for energy transfer and this then is the upper limit for the rate of formation of A* which is sufficient to account for their observation. Alternatively, excited AX(AH) and AX2(AH2) radicals could be produced by primary or secondary photolysis as follows: AX3(AH3) + hv •* AX2(AH*) + X(H) (24) •*• AX* (AH*) + X 2(H 2) (25) AX2(AH2) + hv AX* (AH*) + X(H) (26) The possible formation of electronically excited NH(a*A,b *z+) by elimination of molecular hydrogen in the quartz ultraviolet photolysis of ammonia is known77. Kley and Welge 7 8 have proposed the formation of excited singlet PH by elimination of hydrogen in the flash photolysis of phosphine in the quartz ultraviolet. They postulated the existence of excited PH radicals to account for the intensity increase in the transition involving the ground state PH when different inert gases are added. There was, however, no direct evidence of excited PH and the intensity increase may have been due to pressure broadening, a phenomenon observed frequently in this investigation. It is interesting to note that although various workers have studied the flash photolysis of phosphine 5 4» 5 8» 7 8 and a r s i n e 4 2 , none of these investigators have observed the excited phosphorus or arsenic atoms. One possible explanation for this is that the flash photolysis apparatus used in this investigation is perhaps more eff i c i e n t , that i s , higher percentage photodecomposition of the parent compound for the same flash energy. A logical extension of the processes (24) to (26) is the direct production of excited atoms by secondary photolysis AX2(AH2) + hv •+ A* + X 2(H 2) (27) AX(AH) + hv + A* + X(H) (28) - 81 -I f s u f f i c i e n t concentration of the e l e c t r o n i c a l l y excited radicals i s produced then another p o s s i b i l i t y for the formation of excited atoms i s given by the spin-allowed processes A + AX*(el.) ->• AX + A* (29) A + AH*(el.) -* AH + A* (30) For antimony, direct population of Sb* from the ground state by secondary absorption i s possible. The observation of r e l a t i v e l y large concentration of the ground state atom, the doublet-quartet t r a n s i t i o n s , and the atomic fluorescence connecting the doublet-quartet terms (Tables I I I and V) suggests that Sb* can be populated by 4 S0 _ 4 p ^ 2 D0 o r 2 p0 ( 3 1 ) In f a c t , t h i s process for antimony has been i n v e s t i g a t e d 7 9 . A s i m i l a r scheme of atom excitation for phosphorous; a n ci arsenic i s u n l i k e l y , however, since the doublet-quartet tra n s i t i o n s accessible in the experimental spectral range weTe not observed, as expected. Whichever mechanism f o r the formation of metastable, e l e c t r o n i c a l l y excited atoms i s adopted, i t seems probable that only one excited state need be accounted f o r since the other can be produced by A(2D°) A( 2P) J±=^r A(2P°) (32) Evidence for t h i s i s provided by the observation of fluorescence from the 2P state to both metastable states (Table V). U n t i l the mechanism of excited atom production i s more f u l l y understood and the concentration of atoms measured, the extent to which the mechanisms for the flash photolysis of the compounds AH^ and AX^ need to be modified remains uncertain. However, in view of the P 2 emission observed in the phosphine system, two additional processes for the production of A|(vib.) must now be considered, i . e . A + AH -> A* (vib.) (33) -82-and A 2 + hv -»• A* (el.) followed by A*(el.) A*(vib.) (34) The observed rapid decay of A* is independent of the total pressure of the inert gas. This pressure independence for the decay of P( 2P°) in the PH3/N2 system is shown in Figure 24. In this and subsequent figures, the relative concentrations of the species (in arbitrary units) are plotted assuming Beer's law is satisfied. If not, then concentration should be interpreted as the change in plate density. The dependence of the rate of decay for Sb(?D°) on the pressure of stibine is shown in Figure 25 which also gives the photolysis lamp discharge curve. It can be seen that the rapid reaction of the excited antimony atom takes place during the period of the photoflash. This is also observed for excited phosphorus- and arsenic atoms as shown in Figures 24 and 26, respectively. Outside the photoflash period, however, the rate of decay of the excited atoms is very similar over a wide range of pressures as shown for SbH.j/N2 in Figure 27. Similar effects are observed for the phosphorcus- and arsenic compounds. It can be interpreted, therefore, that the excited atoms react with the transients produced by the photolysis, but not, to any significant extent, with the parent compound. At low pressures of the parent compound the concentration of A* f i r s t rises to a maximum and then decays rapidly (Figures 25 and 27). At higher pressures of the parent compound, however, the concentration of the intermediates is increased and the rate of removal of A* is such that no maximum is formed. The possible reactions of A* with the radicals produced in the flash photolysis of AH, and AX? are: A* + AH2(AX2) 2AH(AX) (35) -> A 2 + H 2(X 2) (36) A* + AH(AX) + A 2 + H(X) (37) - 83 -Figure 24. Effect of Total Pressure on Decay J 8 1 . L. 5 10 15 20 Delay Time, psec. - 84 -Figure 25. Ef f e c t of P g b H on Decay of Sb(5p 3 2 D ° / 2 ) i n SbH 3/N 2. Total Pressure = 250 mm Hg Q 1 x l O " 1 mm SbH 3 £ ± 3.13 x 10" 3 mm SbH 3 # Photolysis lamp discharg J_ 10 20 30 40 Delay Time, ysec. 50 60 70 - 85 -Figure 26. Effect of P A s C ^ on Decay of As C4p 3 2 P ° ) i n AsCl 3/N 2. Figure 27. 3 2 0 Ef f e c t of Pgkj_[ o n Decay of Sb(5p ^1/2} i n SbH 3/N 2. 60, 4-> •H c X u •H c o •H c <D O C o u 50 40 Total pressure = 250 mm Hg O 1 x 10" • 1.25 x 10" 3.13 x 10" A -4 7.81 x 10 mm 30 20 10 00 ON 20 40 60 80 100 120 140 160 180 Delay Time, usee. - 87 -A l l these reactions are spin-allowed and highly exothermic. There is no evidence for deciding which of these reactions is responsible for the removal of A* since both AH and AH2, or AX and probably AX2 are present in the flash photolysis, although AX2 was not observed spectroscopically. The rate constant for the reaction A* and the sum of a l l the radicals A* + ER & Products (38) can be estimated by the following assumptions: (a) there is no significant formation of A* after the maximum of i t s concentration vs time curve, (b) the concentration of the radicals >> that of A* so that reaction (38) is f i r s t order and (c) the sum of the concentration of radicals is constant during the h a l f - l i f e of A* and equal to the % decomposition of the parent compound*. Then from the h a l f - l i f e (tjy 2) °f A* the rate constant for reaction (38) i s given by k = In 2 1(39) tl/2y &0 where a^ is the concentration of the parent compound and y is the fraction of a Q decomposed to form R. For P(2P]*/2) i n P H3/ N2 CO.20/50 mm Hg) from Figure 24 k -v 1 x 10 1 0 l i t e r mole"1 sec." 1 (40) For As( 2p0 / 2) in AsCl 3/N 2 (1.25 x 10"2/250 mm Hg) from Figure 26 kA s* % 9 x 1 q 1 0 l i t e r m o l e - 1 sec." 1 (41) s y and for Sb(2p0 ) in SbH /N (3.13 x 10"3/250 mm Hg) from Figure 27 k ^ 1 x 10 1 1 l i t e r mole - 1 sec." 1 (42) Sb* — -Therefore, i f y is 10%, then the smallest value of k^* is 1 x 10 1 1 l i t e r mole - 1 sec. - 1. This value, in view of the above assumptions, is the minimum and the true value is probably higher. However, a high pre-exponential factor - 88 -12 - 1 1 A = 7 x 10 l i t e r mole sec. for the reaction H + C0 2 = CO + OH (43) 80 is known . If the energy of activation is ^ 0 for reaction (38), then k^* may be reasonable. A comparison of the decay of the 2P° a n d 2D° atoms of antimony is given in Figure 28. A slight i n i t i a l difference in the time behaviour is seen and a similar effect is observed for the arsenic atoms. It is not clear whether this reflects a difference in the rate of formation or the rate of decay. The ground state antimony atom cannot react according to (35) and (36) because of endothermicity and the spin rule, respectively. Reaction (36) is spin-allowed for the ground state atom i f Sb^ is an excited t r i p l e t . Reaction (37) is possible for the ground state atom, however, since i t is spin-allowed and exothermic (Appendix I), although i t may be endothermic for SbCl. It is possible that for stibine the reaction Sb(4S°) + SbH Sb 2 + H (44) predominates at the early stages while atom recombination may be important after ^100 usee, at which time the SbH radical has decayed. That reaction (44) does not remove a l l the ground state atoms in the early reaction period is seen in Figure 29. The decay of 4S° atoms after the disappearance of SbH i s , at least i n part, due to atom recombination. At low pressures of stibine the long l i f e time of 4S° is consistent with the calculated 0 , 7 s e c " -4 for SbH.j/N2 (1.95 x 10 mm Hg) assuming a recombination third order rate constant of 1 x IO 1 1 l i t e r 2 mole"2 sec." 1 and 10% of the parent compound is decomposed to form 4 S ° . At higher pressures of stibine there is a sharp drop in the concentration of 4S^ as can be seen from Figures 29 and 6 where this sharp drop coincides with the appearance of a continuum. This continuum " 89 -Figure 28. Comparison of Decay of Sb(5p 3 2Dy2) and Sb(5p 3 2 V ° 3 / 2 ) i n SbH 3/N 2 > 30 X u rt U +-> • H u rt c o •H +-> rt ?H +->• c <D O C o u 25 20 15 10 g^pj = 1 x 10 mm Hg Total 250 mm _L 10 15 20 Delay Time, ysec. 25 30 35 Delay Time, ysec. - 91 -is most likely to be Sb^, analogous to the P^ continuum observed in the flash 58 photolysis of phosphine (Figure 4) and probably As^ as shown in Figure 5. It i s possible, therefore, that the sudden drop in the concentration of may be due to the reactions Sb + Sb 2 + M Sb, + M (45) Sb + Sb 3 + M Sb 4 + M (46) and the atom recombination reaction is insignificant. - 92 -B. Reactions of the Cyanogen Radical 1. Cyanogen/Group VA Hydrides and Trichlorides Generalized mechanisms to account for the observed transient molecular species in the flash photolysis of Group VA halides and trihalides have been given in the preceeding section. The observation and assignment of the new free-radicals produced from the flash photolysis of cyanogen with these hydrides and halides have been discussed in the last chapter. To explain the formation of theses new molecular species, the following generalized mechanisms, in addition to those for the hydrides and trichlorides , are proposed: C 2N 2 + hv -> 2CN (47) CN + AH3 ->- HCN + AH2 (48) CN + AH2 ->- HCN + AH (49) CN + AH -»- H + ACN (50) -*- HCN + A (51) M -> HACN (52) CN + ACN + M -* • A(CN)2 (53) = N, P, As or Sb), and C 2N 2 + hv -»- 2CN (47) CN + AC13 -y C1CN + AC12 (54) CN + AC12 -V C1CN + AC1 (55) CN + AC1 -> Cl + ACN (56) •+ C1CN + A (57) M -V C1ACN (58) t = P , As or Sb). (50) and (56) do occur is supported by the observation AsCN free- radicals in the appropriate systems. The - 93 -analogous SbCN free-radical was not observed, however, in the flash photolysis of cyanogen with stibine or SbCl^. The failure to observe this radical may be due to i t s weak absorption spectrum or the fact that i t may absorb outside the experimental spectral range. A more likely explanation, however, is that the bond dissociation energy of SbCN is less than that of SbH or SbCl. Therefore, reactions (50) and (56) would be endothermic for the antimony systems. A general progressive decrease in bond strength from NCN to SbCN, as shown in Table XXXIL is expected. It is possible that ACN could be produced by the reaction CN + AH2 -> ACN + H 2 (59) but the corresponding reaction for the chlorides is endothermic (Appendix I). The possibility that ACN is produced by CN + A2 ACN + A (60) can be eliminated because this reaction i s endothermic, certainly for A = N and P, and most likely As as well. The formation of ACN by the termolecular recombination process CN + A+ M +ACN + M (61) 9 9 2 1 is not very l i k e l y . The rate constant of 5.6 x 10 liter** mole sec. at 20°C reported by Campbell and Thrush8^* for the reaction N + CN + N2+ NCN + N 2 (62) appears to be too low to account for the rapid appearance of ACN shown in the figures below. Also, i f SbCN ^absorbs ; within the experimental spectral range, then the absence of this species even though relatively large concentrations of CN and Sb are present in the SbH3/C2N2 and SbCl.j/C2N2 systems certainly supports this view. 'Evidence for reaction (51) is given by the hydride systems. In the flash photolysis of cyanogen with phosphine, arsine or stibine, the excited - 94 -atoms decay ^Slower than those in the flash photolysis of the hydrides alone. This is shown for phosphine and arsine in Figure 30 and 31, respectively. The slower decay of A* in the presence of cyanogen radicals, clearly evident in the arsine systems, suggests that A* is possibly produced by (51), in addition to the formation of excited atoms with the hydrides alone. Another possible explanation for the slower decay is that the radicals reacting with A* are being removed by the CN radical. The exothermicity of (51) is sufficient for atom excitation (A = P, As or Sb) although reaction (57) can only produce ground state atoms. A direct and striking example of (51) is given by the flash photolysis of stibine systems where the intensity of i 0 the atomic lines arising from the Z P s t a t e ^ s increased in the presence of the cyanogen radicals although the intensity of the other lines is clearly decreased. This is shown in Figure 7 and Table IV. The observed intensity increase in these atomic lines suggests that the reaction CN + SbH -»• HCN + Sb( 2P° / 2) (63) is predominant. Reaction (52) is supported by the observation of HNCN and is consistent with the formation of HPlCN. Reaction (58) is introduced by analogy with (52) to account for the transient continuous spectra (reported in the last chapter) which could be due to the C1ACN free-radicals. Reaction (53) is introduced to explain the observation of P(CN)^? in the flash photolysis of cyanogen with phosphine or PCl^. The formation of this species w i l l be discussed in the cyanogen/phosphine system. There is very strong evidence for reactions (48) and (49), and probably (54) and (55). The reaction for CN with ammonia has, in fact, been studied 75 by Setser and Thrush . In the flash photolysis of cyanogen with the hydrides, the intensity of the A 2 (P 2, As 2 and Sb2) absorption bands is greater and - 95 -Figure 30. Decay of P(3p 3 2P°) in PH3/N2 . and in PH /C2N2/N . « 1 ! 1 I | 0 5 10 1 5 - 2 0 25 Delay Time, ysec. Figure 31. Decay of As(4p 3 2P°) in AsH3/N2 and in AsH3/C2N2/N2. Delay Time, psec. - 97 -rises to a maximum at a faster rate in the presence of cyanogen radicals than when the hydrides are flashed alone. This is clearly shown in Figure 7 for the stibine systems. If ^ is produced by (10), then i t s intensity increase implies a greater concentration of AH and AH^ which, in turn, react to give a larger concentration of in the cyanogen systems. Additional evidence for the reactions of CN with AH^ and AH2 is given by the variation of the concentration of the transient species as a function of the pressure of AH3 as shown in Figures 32 and 33. Figures 32 and 33 show that there i s an optimum pressure of ammonia and phosphine, respectively, for the new molecular species formed by the reactions of the cyanogen radicals. Similar effects are observed with arsine. It can be seen from Figure 32 that at high pressures of ammonia the concentration of the CN and CN-containing radicals is low. This probably means that most of the CN radicals are consumed according to reactions (48) and (49) with very l i t t l e CN le f t for reactions (50) and (52). On the other hand, at very low pressures of ammonia the concentration of the CN-containing species is low because of the small amount of NH available for reactions (50) to (52). Analogous arguements can be applied for the phosphine and arsine systems. If the mechanism involving reactions (48) to (53) is correct, then for every molecule of AH^ three, and possibly four CN radicals are required. Therefore, a very rough approximation on the percentage decomposition of cyanogen can be obtained. For the optimum pressure condition in AH3/C2N2/N2 (0.20/40/210 mm Hg) and assuming CN reacts with a l l AH7, each of which requires four CN's, then the decomposition of cyanogen is ^1%. It is d i f f i c u l t to ascertain whether this figure is the lower or upper limit since not a l l the CN radicals, and probably not a l l the AH are being consumed. - 98 -P , mm Hg P , mm Hg - 100 -For the chlorides there is less evidence for reactions (54) and (55) since the change in the concentration of A2 is masked by a transient o continuous spectrum at %2400 A downward. Because the long wavelength limit o of the continuous spectrum of C1CN is -\-2270 A at high pressures and long 82 path length , i t is doubtful that the C1CN could have been observed in this investigation even i f reactions (54), (55) and (57) do occur. The rise and decay of the transient molecular species observed in the isothermal flash photolysis of cyanogen with ammonia are shown in Figures 34 and 35. That this rapid rise and decay occurs during the period of the photolysis flash is seen from the photolysis lamp discharge curve given in Figure 25. It can be shown that the rate constants required to explain the rapid formation of the HNCN and NCN radicals are reasonable. For these and subsequent calculations, a 1% decomposition of cyanogen is assumed. This assumption, based on 0.4% decomposition of ^ 2^2 r e P o r t e d b v Basco , is not too far from being correct in view of the generally more efficient flash apparatus used in this investigation. Also, this assumption is consistent, to a certain extent, with the previous discussion on the decomposition of cyanogen. For the mixture NH^/^^/^ (0.20/40/210 mm Hg) , the rate constant fo r the reaction CN + NH k Products (64) can be estimated on the following assumptions: (a) the concentration of CN >> that of NH at very short times ( f i r s t 10 psec.); (b) NH decays entirely by reaction with CN and (c) NH is no longer produced after 5 ysec. delay. Then from the half l i f e of the NH decay t j y 2 = 8 x l u"^ sec. (Figure 34) 9 -1 the second order rate constant k is calculated to be 2 x 10 l i t e r mole sec. For the third order reaction CN + NH + N, ->• Product (65) Concentration ( a r b i t r a r y units) - TOT -Delay Time, usec. - 103 -the rate constant i s 2 x 10 1 1 l i t e r 2 mole - 2 sec." 1. These values are the lower limits because most probably NH is s t i l l being produced at such short delay time. The rate constant for reaction (64) is reasonable in view of the values of 4.6 and 5.5 x 109 l i t e r mole - 1 sec." 1 for the reaction of 99 19 cyanogen with oxygen determined by Basco" and by Paul and Dalby , respectively. The rate constant for reaction (65) can be favourably compared with the value 11 9 —2 1 9 1 of 1 x 10 liter'' mole sec. - 1 determined by Basco and Norrish for the reaction CN + NO + N 2 + ONCN + N 2 (66) Although HNCN is most likely to be produced by reaction (52) there i s , however, another poss i b i l i t y for i t s formation which must be considered. This is given by the reaction NCN + NH -v HNCN + N (67) Evidence that (67) is not lik e l y to occur is given by the fact that NCN does not abstract a hydrogen atom from H 2 < This could be explained i f the dissociation energy of H-NCN is less than that of H2 but greater than that of NH. However, the HNCN radical was absent although NCN was observed 8 3 strongly in the presence of the C 2H 5 radicals produced in the flash photo-lysis of the N 2F 4/(C 2H 5) 2C0/C 2N 2/N 2 mixture (see discussion of the N 2F 4 system below). Since the dissociation energy of CH2CH2-H is considerably less than that of NH, i t is not lik e l y that HNCN is formed by reaction (67). The formation of ACN and HACN by reactions (50) and (52), respectively, has already been discussed. Arising from the reaction of CN with AH numerous different reaction products are possible. In the ammonia system some of the more likely products are formed by the following spin-allowed reactions: CN(X 2E) + NH(X 3£) -»• H(2S) + NCN(X 3E, 1A) (68) - 104 -9 t M 9 II CN(X ZZ) + NH(X 5E) HNCN(X A ), HNCN? (69) HCN(£ 1l) + N( 4S°) (70) •* CNH + N( 4S°) (71) H» CNN + H( 2S) (72) 1 CNNH (73) -»• CH(X 2n) + N 2(X 4 ) (74) The observation of the products from (68) and (69), and the absence of the CNN radical have been discussed in the last chapter. Reaction (70), a special case of (49), should occur. No spectrum which could be attributed to the CNH radical was observed in the ammonia, phosphine, arsine and stibine systems, in each of which reaction (71) i s equally possible. There is no direct evidence for reaction (74) to occur although i t is exothermic. The absence of the CH spectrum suggests that i f (74) occurs, then i t must be to a very small extent since the oscillator strength (f-value of CH(A 2A + X 2JI at 4315 A) is comparable to that of NH(A 3 n + X 3 ! - ) and 0H(A 2 E + «- X 2n) as given by Herzberg (p. 386) 1 8, or CH is being removed too rapidly to build up a sufficient concentration to be observed. The latter explanation is possible because CH may react rapidly with species such as NH3, NH2, etc. The analogous reactions of CF w i l l be discussed in the N2F4/C2N2/N2 system. If sufficient concentration of CH is present in the ammonia system and i f CH reacts with CN, then products formed by this reaction should be observed. The formation of these products depends, of course, on the spin conservation rule and the exothermicity of the reaction. That the known spectra of CCN' , CNC and HCCN were not observed i s consistent with the above explanations. The species (HNCN?) responsible for the new absorption spectrum reported in the last chapter is produced by or following the reaction of CN with NH. If i t is a primary product, i t i s either HNNC (or CNNH) - 105 -from reaction (73) or a metastable state of HNCN. However, secondary reactions of the products from reactions (68) to (74) could produce several other species and further speculation i s not warranted. The available spectroscopic and kinetic evidence used for the tentative assignment of the new spectrum to that of the HNCN? radical have been discussed in the last chapter. The similarity in pressure and time dependence of the HNCN radical and the carrier of the new spectrum can be seen from Figures 32 and 34, respectively. The rise and decay of the transient molecular species observed in the isothermal flash photolysis of cyanogen with phosphine are shown in Figure 36. It was not possible to photometer the HPCN spectrum because of i t s weakness. The rapid formation of the PCN and HPCN radicals can be explained just as for the NCN and HNCN radicals. There i s , however, a new transient with band o heads at 2451 and 2388 A produced possibly by reaction (53) in the phosphine or phosphorus>" trichloride system, which requires explanation. Unlike that of the PCN and HPCN radical, this species rises and decays relatively slowly as seen from Figure 36, and there is l i t t l e doubt that i t is a different radical. The fact that this species is produced in both the hydride and chloride systems means, most probably, that i t i s a product of the PCN radical with i t s e l f or with the CN radical, although a combination of the CN or PCN radical with the radical cannot be formally ignored. The existence of 84 the P(CN) 3 molecule , the analogue of PClj, certainly favours the assign-ment of the new spectrum to the NCPCN radical. In the absence of spectroscopic analysis, however, no conclusive assignment can be made. If the new species is P(CN)2» then i t is most lik e l y to be formed by the reaction CN + PCN + N •> P(CN) + N (75) 2 2 0 30 40 50 60 70 80 90 100 Delay Time, usee. o Delay.Time, ysec. - 108 -Because the identity of the new species is completely unknown, an estimation of the rate constant for reaction (75) may not be meaningful. The rapid formation of the AsCN radical in the arsine system, as shown in Figure 37, can be explained just as for the NCN radical. That no spectrum which could be attributed to the HAsCN radical was observed is under-standable in view of the extremely weak spectrum of the HPCN radical in the phosphine system. 2. Cyanogen/Dinitrogen Tetrafluoride The equilibrium between dinitrogen tetrafluoride (tetrafluorohydrazine) 85 and the nitrogen difluoride (difluoramino) free-radical is well known , with the constant Kp = 8.8 x 10~7 atm. or 6.7 x 10"4 mm at 25°C. The fact that the equilibrium N 2F 4 •* 2NF2 (76) exists implies that the NF radical cannot be formed by the reaction of the NF 2 radicals, but i t can, perhaps, be produced by primary or secondary photolysis of these radicals. The only spectrum observed following the flash photolysis of N 2F 4, with or without inert gas (N 2), was that of the NF2 radical. This weak o continuous spectrum at 2600 A, however, was not observed before the photolysis because of the unfavourable Kp and low pressure of N2F^ used (2.5 - 10 mm Hg). No absorption spectrum which could be attributed to the NF radical was observed in the region 2200-6700 A. Two forbidden electronic transitions involving the ground electronic state of this species have 66 67 ° recently been observed in emission ' above 5000 A. The molecular species observed in the isothermal flash photolysis of cyanogen with an equilibrium mixture of N^F^ and NF^ are summarized in - 109 -Table VIII, and the time dependence of some of these species is shown in Figure 38. The NCN radical in this system can only be produced by the reaction of CN with NF since the reaction CN + NF 2 NCN + F 2 (77) i s most lik e l y endothermic. The presence of the NF radical and the absence of i t s spectrum suggests that the allowed electronic transition may occur o below the lower experimental wavelength limit of 2200 A. Another possible explanation is that the NF radical may be present at too low a concentration (or the f-value of the allowed transition i s too low) to be observed. The formation of the NCN radical can be explained by the following mechanism: N 2F 4/NF 2 + hv NF 2 (78) C 2N 2 + hv 2CN (47) CN + NF 2 -»- FCN + NF (79) CN(X 2 E ) + NF(X 3 E)H-F( 2 P) + NCN(X 3Z,V) (80) Although the bond dissociation energy of FCN is uncertain, i t can be safely assumed to be greater than 85 Kcal/mole from comparison with those of BrCN and C1CN given in Appendix I. Therefore, both reactions (79) and (80) are exothermic. There is no direct evidence, in the absence of spectroscopic observation, that the NF radical is being produced by photolysis of NF 2» rather than by reaction (79). In view of the similarity between reaction (80) and the reaction of CN with AH or AC1 discussed previously, i t is possible, i f not probable, that the reactions CN + NF -> FCN + N (81) + FNCN (82) do occur. - 110 -Figure 38. Concentration of Transient Species as a Function of Time i n N 2 F 4 / C 2 N 2 / N 2 ( 2 - 5 / 4 0 / 2 0 8 m m H § ) • 90 80 70 60 50 40 30 20 10 20 40 60 80 100 120 Delay Time, ysec. - I l l -The observation of the CF2 spectrum ' ~ in the flash photolysis of cyanogen with an equilibrium mixture of N and NF2 requires some explanation. The intensity of the \>2 progression of this spectrum, though weak, was observed to rise in hundreds of ysec. The weil known A £"-X II oq no system of the CF radical observed by various workers was not detected in this system. This spectrum was only observed, however, in discharge systems or at a very high temperature (2400°K) where appreciable quantity of the CF radical may have been produced. Therefore, in view of the weak CF2 spectrum and the small oscillator strength of the CF (A-X) system reported by Harrington et a l . 9 3 , the absence of the CF spectrum in this system is reasonable. Another possible reason for the absence of the CF spectrum is that the CF radical reacts rapidly with other species. The formation of the CF2 radical can be explained by the mechanism CN + NF -• N 2 + CF (83) CF + NF -»• CF2 + N (84) CF + NF2 -*• CF2 + NF (85) CF + N 2F 4 ->• CF2 + N 2F 3 (86) A l l these reactions are spin-allowed and exothermic. The observation of CF2 certainly suggests that (83) does occur in this system and hence by analogy the corresponding reaction with NH is possible. Because of the complexity of the reaction system, the assignment of the new spectrum (labelled FNCN?) reported in the last chapter i s completely uncertain. The presence of the CF radical means that numerous products from the reactions involving the CN, NF, NF2, CF and CF2 radicals are possible. However, the observation of the HNCN radical produced from reaction (69) suggests that the carrier of the new spectrum could be FNCN produced by reaction (82). - 112 -As seen from Figure 38, the rise of NCN and FNCN? is very rapid, just as for the transients observed in the ammonia/cyanogen system shown in Figures 34 and 35. The decay of the NCN radical, however, is significantly slower in the dinitrogen tetrafluoride/cyanogen system. It can also be seen from Figure 38 that the NCN (*£) decays faster than the NCN(X 3E) while the rate of decay for NCN? is comparable to that of NCN^A). The spectrum of NCN? and i t s possible assignment have already been discussed in the last chapter. 3. Cyanogen/Oxygen 99 This system has been studied by Basco , and the reactions of the cyanogen radical with oxygen have also been discussed by Morrow and McGrath . The observed species are summarized in Table X. Briefly, then, the essential reactions are CN + 02 NCO + 0 (87) CN + NCO -»• NCN(X 3E, 1A) + CO (88) -> NCN? The reaction CN + P 2 •* CO + NO (89) was found to occur to a small extent. 4. Cyanogen/Nitrous Oxide 2 + 7 The spectrum of NO (A E «- X II) was observed when nitrous oxide, with or without nitrogen as inert gas, was flash photolyzed (Table IX). From the correlation between the electronic states of a polyatomic molecule and those of the separated atoms or groups of atoms given in Herzberg (p. 283) 6, the ground state N20 molecule cannot dissociate into 1 3 ground state products. Therefore, the products N 2 + 0 ( E + P) or - 113 -N + NO ( 4S + 2n) are not possible. It can be seen that these products also violate the spin rule. Thus, in the quartz ultraviolet region, the 77 most probably process (McNesby and Okabe, p. 185) is N20(X 1 E + ) + hv N 2( 1E*) + 0( 1D) or ( JS) (90) The consequent formation of NO is then 0(1D) or (*S) + N20 -+ 2N0 (91) In the isothermal flash photolysis of cyanogen with nitrous oxide, the observation of the NCO and NCN radicals (Figure 39) can be explained by the mechanism C 2N 2 + hv -> 2CN (47) 0( XD) or ( XS) + C2N2-»- NCO + CN (92) CN + N20 NCO + N 2 (93) CN + NCO NCN(X 3E, 1A) + CO (88) The reaction CN + N20 NCN + NO (94) is not likely because i t is probably endothermic. Although the reaction 0(LD) or ( 1S) + N20 ^ N 2 + 0 2 (95) is spin-forbidden for the ground state O2, i t is energetically possible and spin-allowed for excited 0 2(a *A,b *£). Therefore, i f this occurs with a rate constant comparable to that of the formation of the NCN radical in (88), then the reaction CN + 0 2 -v NCO + 0 (96) is an additional source of the NCO radical, which in turn, reacts to form NCN. Reaction (92) is known to occur and has been studied by Morrow and 23 McGrath . There is no evidence, however, to determine the relative importance of reactions (93) and (95). Of a l l the systems where the NCN and NCN? radicals were observed, the cyanogen/nitrous oxide yielded the strongest spectra. This is due, probably, to the increased concentration of the CN radical produced - 114 -o 00 o "vO. e • H H -<4H O c o • H 4-1 O * c / N nj s s </) o c/> 7—I CD • • H o CTI O t o <D PH o <D C/D U v ' •P bO C ' CM • H 0) 2 PH • H . * ^ (/) CM c 2 nj CM u -E -o <4-i CM O. 2 C C o • H • H 4-> 4-> e CD CJ C o • U o o CM o o o 00 o o •3-o CL) CD 6 • H 03 I—I CD Q O C M o 00 o o o IS) o o t o o CM (sixun Axvxzxqx-e) uoxq.Bj;q.uaouo3 - 115 -by (92) and the various reactions producing NCO which subsequently lead to the formation of NCN? and NCN. Like the cyanogen/^F^ system, the time dependence of NCN(3~) differs from that of NCN(*A) and NCN? as shown in Figure 39. If the small apparent difference in the kinetic behaviour of NCN(*A) and NCN? is significant, then obviously NCN? is not NCN(*A) but could possibly be another singlet state of NCN. 5. Cyanogen/Water The transient species observed in the isothermal flash photolysis of cyanogen with water are summarized in Table X, while the time-dependence of some of these species is shown in Figure 40. That this system is very complex is seen from the large number of observed radicals. The formation of these species can be explained by the mechanism H20 + hv -V H + OH (97) C 2N 2 + hv -> 2CN (47) CN + H20 ->- HCN + OH (98) CN + OH -y NCO + H (99) -y NH + CO (100) CN + NCO -y NCN# 3S,1&) + CO (88) CN + NH -y NCN(X 1E, 1A) + H (68) M ->• HNCN (69) A l l these reactions are spin-allowed and exothermic. The reaction H + NCO -»- NH + CO (101) is most probably exothermic and therefore could occur. Reaction (98) should occur in view of the hydrogen abstraction by the cyanogen radical in the Group VA hydride systems previously discussed. Reaction (100) has been Concentration ( a r b i t r a r y units) - 911 -- 117 -22 previously proposed and reactions (88), (68) and (69) have already been 94 discussed. The combination of OH radicals giving H 0 is known , although 2 1 the exothermic reaction OH + OH -> 0 2 + H2 (102) i s possible. In view of the low concentration of OH this reaction can be neglected. The above mechanism, of course, also applies to the C2N2/D2O system which was used to study the isotope shift in the HNCN? spectrum discussed in the last chapter. 6. Cyanogen/Methyl Isocyanate The primary photodissociation of methyl isocyanate is not f u l l y under-stood. The flash photolysis of this compound has yielded the NCO, CH3 and NH radicals (Table IX). It is reasonable to assume that the f i r s t two of these radicals are produced by CH3NC0 + hv •*• CH3 + NCO (103) The formation of the NH radical, however, cannot be explained easily. It is conceivable that another primary process CH3NC0 + hv -»• CH3N* + CO (104) could occur, and consequently the NH radical could be produced by the dissociation of the electronically excited free-radical (CH3N* 5 CH2NH*)->- CH2 + NH (105) There is no evidence or justification for process (105) although isomerization is well known. Another, perhaps more likely, possibility for the production 6 of NH is given by the secondary dissociation process (Herzberg, p. 484) CH3 + hv -v CH2 + H (106) followed by H + NCO NH + CO (101) - 118 -18 6 Unfortunately, CO and CH^ absorb in the vacuum ultraviolet, and therefore, these processes cannot be supported. The transient species observed in the isothermal flash photolysis of cyanogen with methyl isocyanate are summarized in Table X. The time-dependence of some of these species is shown in Figure 41. The formation of the observed radicals can be explained by the mechanism (CH3NCO) + hv -4- CH3 + NCO + NH (107) C 2N 2 + hv 2CN (47) CN + NCO -> NCN(X 3Z, 1A) + CO (88) CN + NH -»• NCN(X £, A) + H (68) + HNCN (69) A l l these processes have already been discussed. Undoubtedly, side reactions involving the CN radical in this and the other systems investigated do occur. Consideration is given, however, only to those reactions which are relevant to the production of the observed species. Comparison of Figures 40 and 41 shows that the ratio of the concentration (in arbitrary units) of NCN? to that of NCN(1A) is significantly different. This is consistent with the view that NCN? may not be NCN(1A) but could possibly be another singlet state of NCN as has been discussed in the C2N2/N2O system. Concentration ( a r b i t r a r y units) o — r o o -p. o On O O o 3 -rt n TO a: c Z CD o 4*. \ t—' n • o n \ o z 3 o o CD 3 <—\ r+ On i-j • P O rt \ H --P» o O 3 O 0"l TO o H i H P ' 3 (/> H -CD 3 r+ CO CD O H -CD !/> P c / ) C 3 o o 3 o H) H H * 3 CD - 6 1 1 -- 120 -CONCLUSIONS Metastable, electronically excited atoms (2D°,2P°) of phosphorus-, arsenic and antimony are produced in the isothermal flash photolysis of phosphine, arsine, stibine, phosphorois trichloride and tribromide, arsenic trichloride and tribromide, and antimony trichloride. Several mechanism for the production of these atoms have been proposed. The rapid decay of the excited atoms is due to chemical reactions with the radicals produced in the flash photolysis of the parent compounds. 42 The AsH and AsH2 , SbH and SbH2, PCI, AsCl, and SbCl free-radicals are produced in the flash photolysis of the appropriate hydrides and t r i -chlorides. New electronic absorption spectra of these transients are observed and vibrational analysis on some of these spectra have been carried out. The following reactions of the CN radical occur rapidly: AH2 + HCN (1) AH + HCN (2) ACN + H (A=N,P,As) (3) HCN + A* (A=P,As,Sb) (4) HACN (A=N,P) (5) where A, unless specified, i s N,P, As or Sb. Reactions of CN with NF, PCI and AsCl, corresponding to (3 ) , also occur rapidly. New electronic absorption spectra tentatively attributed to the PCN, HPCN and AsCN free-radicals are observed in the isothermal flash photolysis of cyanogen with the Group VA hydrides and trichlorides. CN + AH 3 + CN + AH2 -y CN + AH + ->-M - 121 -SUGGESTIONS FOR FURTHER WORK The study of the formation of electronically excited atoms of phosphorus i arsenic and antimony is by no means complete, and indeed, i t i s just the beginning. In the flash photolysis of phosphine, arsine and stibine, the re act ion AH + H + A + H2 (1) is sufficiently exothermic to produce A(2D^). It would be interesting to extend the experimental spectral range to the vacuum ultraviolet so that atom excitation, i f any, in the flash photolysis of ammonia can be investi-gated. For ammonia, excited nitrogen atoms cannot be produced by reaction (1). Extension of the spectral range, of course, also allows the observation of the ground state atoms of phosphorous and arsenic. A knowledge of the concentration of the excited and ground state atoms would certainly help to elucidate the proposed mechanisms for atom excitation. Unlike the excited atoms, the antimony and presumably the phosphorus and arsenic ground state atoms decay slowly. Therefore, chemical reactions involving these atoms with compounds such as 0 2, C l 2 , etc., can be investigated. The new absorption spectrum (FNCN?) observed in the flash photolysis of cyanogen with an equilibrium mixture of N and NF2 should be investigated. 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Venkateswarlu, Phys. Rev., 77, 676 (1950) 89. E.B. Andrews and R.F. Barrow, Proc. Phys. Soc, 6£, 481 (1951) 90. D.E. Mann, H.P. Broida and B.E. Squires, J. Chem. Phys., 22_, 348 (1954) 91. J.L. Margrave and K. Wieland, J. Chem. Phys., 21_, 1552 (1953) 92. B.A. Thrush and J.J. Zwolenik, Trans. Faraday Soc, 59, 582 (1963) 93. J.A. Harriington, A.P. Modica and D.R. Libby, J. Chem. Phys., 44, 3380 (1966) 94. M.C. Chen and H.A. Taylor, J. Chem. Phys., 27, 857 (1957) - 126 -95. T.L. Cottrell, The Strength of Chemical Bonds, 2nd ed., Butterworths Scientific Pulbications, London (1958) 96. Handbook of Chemistry and Physics, 47th ed., The Chemical Rubber Co., Cleveland (1966-67) 97. V. Kondratyev, The Structure of Atoms and Molecules, p. 505, Dover Publications, Inc., New York (1965) 98. J.G. Calvert and J.N. Pitts Jr., Photochemistry, John Wiley and Sons, Inc., New York (1966) 99. Vedeneyev, Gurvich, Konrat'yev, Medvedev and Frankevich, Bond Energies, Ionization Potentials, and Electron A f f i n i t i e s , Edward Arnold Ltd., London (1966) 100. S.C. Woo and T.K. Liu, J. Chem. Phys., 3, 544 (1935) 127 APPENDIX I  BOND DISSOCIATION ENERGIES (Kcal/mole) NOTES: a) The most recent bond dissociation energies of the molecules considered in this investigation have been abstracted from several sources (see references). The bond dissociation energy for the process RX ->• R + X is given by D(R-X) = AH|(R) + AH°(X) - AH°(RX) where H£ is the heat of formation from the elements in their standard states (gas: ideal gas at 1 atm., with a heat content equal to that of the real gas at the same temperature). DQ and D2gg are the bond dissociation energies at 0°K and 298.2°K, respectively. b) AH° is the heat of dissociation. c) E is the mean bond dissociation energy and is given by E(R-X) = l(RX n)^ — heat of atomization Compound DQ E u298 Ref. & Bond or A I J ° M298 As- 91 95 92 96 As-Br in AsBr3 58 95 : . As-Cl in AsCl 3 70 95 As-H in AsH, ^59 95 66.8 37 Br 2 46.08 96 Br-CN 75 9 7 CF -vl06 95 114 98 107 96 109 99 CF-F 1120 6 118 98 - 128 -Compound Dn E D 0 Q a Ref. e n J V £.\) O p o r f A H i 5 Bond " 298 Q '298 CH 80 95 CH-H 127 98 108 96 *97 6 CH2CH2-H 38 98 CH3-NC0 <111 100 C l 2 57.87 96 Cl-CN 85 97 CN 188 95 195 99 174 • 96 CO 256.7 96 D-CN -D2 106.0 99 D-OD 121.4 99 F-CN -F 2 37.72 96 H-CN 129 96 130 98 H2 104.18 96 H-OH 119 96 NC-CN 145 96 N-CN :'• -100 75 N-CO 64±20 50,51 NC-0 ^140 50,51 N-F 61 99 62.6 96 NF-F 80.5 99 - 129 -Compound D- E ^289 ^e^" NF2-NF2 21 19.9 99 96 NH 84 86 99 96 NH-H 91 99 NH2-H 105.4 103 99 96 N2 226.8 96 N-NO 115.1 99 NN-0 40 96 NO 153.2 149.7 99 96 OD 103.72 99 OH 102.4 96 °2 118.86 96 P-Br in PBr 3 63 95 P-Cl in PC13 78 95 PCI 68.8 99 PC1-C1 78 99 PC12-C1 82 99 P-H in PH3 ^77 95 PH 71 99 PH-H 79 99 PH2-H 84.5 99 P2 116.7 96 Sb-Cl in SbCl 3 74 95 SbH in SbH3 60.9 37 Sb7 71.5 96 

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