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A study of the Infra-red spectra of some reactive species Ogilvie, John Franklin 1961

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A STUDY OF THE INFRA-RED SPECTRA OF SOME REACTIVE SPECIES by John Franklin Ogilvie B. Sc., University of B r i t i s h Columbia, 1959 A thesis submitted i n p a r t i a l f u l f i l m e n t of the requirements f o r the degree of Master of Science i n the Department of Chemistry We accept t h i s thesis as conforming to the required standard. The University of B r i t i s h Columbia July, 1961 In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia, Vancouver 8 , Canada. Date - I -Acknowledgment I should l i k e to thank Dr. K. B. Harvey for his counsel and generous assistance during t h i s study. The help of Mr. Denis Gilson i n obtaining the nuclear magnetic resonance spectra, and of Mr. R. Muehlchen i n the construction and maintenance of apparatus and i n the supplying of l i q u i d helium, i s also appreciated. i i i ABSTRACT Although there have been recorded many spectra of dispersions of ireactiv/e and unreactw.e molecules i n i n e r t matrices, there has been no r e a l attempt to explain quantitatively the nature of the forces and interactions of these matrices that act on the molecular vibrations of the trapped species. In the present study the i n f r a - r e d spectra of formaldehyde and water i n s o l i d argon and nitrogen matrices and of s o l i d formaldehyde are used as a basis fo r a discussion and analysis of the conditions that p r e v a i l i n such matrices. Isotope e f f e c t s , molecular association, i n t e r -molecular coupling, rotation, the e f f e c t of trapping i n d i f f e r e n t l a t t i c e positions, and matrix-gas frequency s h i f t s are considered i n the Interpretation of the ob-served spectra. CONTENTS Page Acknowleclgment i Abstract i i L i s t of Tables I i i L i s t of Figures l v Chapter I. Introduction 1 I - l The Problem 1 1-2 Methods of inv e s t i g a t i o n 3 Chapter I I . Experimental I I - l Chemicals h I I - 2 Apparatus 7 II - 3 Procedure 7 Chapter I I I . Results: Treatment and Discussion 17 I I I - l Results 17 I I I - 2 The isotope e f f e c t 21 I I I - 3 Molecular association 23 III-4 Lattice mode combinations 26 III-5 Reaction with the matrix 27 I I I - 6 Rotation i n s o l i d formaldehyde 28 I I I - 7 Rotation of water and formaldehyde i n matrix s i t e s 30 III-8 Miscellaneous notes 37 Appendix. The Structure and Symmetry of Formaldehyde 38 Bibliography 40 i i i LIST OF TABLES 1 Experimental c o n d i t i o n s o f i l l u s t r a t e d s p e c t r a 2 Frequencies and i n t e n s i t i e s f o r formaldehyde 3 Frequencies and i n t e n s i t i e s f o r water k Force constants and c a l c u l a t e d f r e q u e n c i e s f o r formaldehyde 5 F r e q u e n c i e s , frequency s h i f t s and i n t e n s i t i e s f o r water a t 4°K. 6 Character t a b l e f o r C2 V LIST OP FIGURES On or folloitfing page 1 Paraformaldehyde spectrum 5 2 Block plan of deposition system 9 3 Spectra of formaldehyde: mixing and warmup 11 k Spectra of formaldehyde: V£, and + v 2 12 5 Spectra of formaldehyde: v-j, v 2 and 2 v 2 13 6 Spectra of formaldehyde: v 2 + v 5» v4>» v l a n d v 3 + v 5 7 Spectra of water i n an argon matrix 15 8 N. m. r . spectra of formaldehyde and polyoxymethylene 16 9 N. m. r . diagrams of formaldehyde 29 10 The structure and normal vibrations of formaldehyde and water 38 1 CHAPTER I. INTRODUCTION I - l The Problem Two reactive species were Investigated i n the experimental part of t h i s research. Because part of the inquiry as o r i g i n a l l y proposed was concerned with the formyl r a d i c a l , formaldehyde, a by-product of some reactions which produce formyl, was studied. Preliminary spectra proved so i n t e r e s t i n g that i t was r e a l i z e d there was much to explain of the nature of the conditions e x i s t i n g In a s o l i d matrix. Infra-red spectra of formaldehyde being such as to make a r o t a t i o n hypothesis worth considering, the nuclear magnetic resonance spectrum of formaldehyde and the i n f r a - r e d spectrum of water i n a matrix were studied. An n. m. r.-spectrum can often Indicate, by the width of the observed absorption l i n e , the presence of c e r t a i n types of molecular motion (17). The water molecule has been the subject of several low temperature investigations, including some (1, 2, 3) i n which molecular r o t a t i o n i n the l a t t i c e of an i n e r t matrix was proposed to explain the observed spectra. Thus the obtaining of -spectra of water by employing an experimental technic s i m i l a r to that of the formaldehyde t r i a l s could provide information concerning the e f f i c i e n c y of I s o l a t i o n by the technic, and the p l a u s i b i l i t y of r o t a t i o n as an explanation of the formaldehyde spectra. Both the formaldehyde and water molecules are reactive, the former to polymerisation and the l a t t e r to a hydrogen-bonded condensed state association. Other common properties of these molecules include the same symmetry point group, C 2 v , the same nuclear spin s t a t i s t i c a l weight factors, and well investigated 2 gas phase v i b r a t i o n - r o t a t i o n s p e c t r a . These two molecules are t h e r e f o r e r e l a t i v e l y c l o s e l y r e l a t e d f o r the purposes o f t h i s i n v e s t i g a t i o n . The method used to s t a b i l i s e these r e a c t i v e substances i s the matrix i s o l a t i o n t e c h n i c , which, i n v a r i o u s m o d i f i c a t i o n s and a p p l i c a t i o n s to the s p e c t r o s c o p i c study of r e a c t i v e chemical s p e c i e s , has proved a u s e f u l experimental a d v a n c e - ( 2 1 T h e method c o n s i s t s e s s e n t i a l l y o f forming and f r e e z i n g - t h e s p e c i e s of i n t e r e s t (E) w i t h a t r a n s p a r e n t , I n e r t m a t r i x (M) a t a temper-ature s u f f i c i e n t l y low (probably l e s s than t h i r t y -to f i f t y per cent (4-) o f the m e l t i n g - p o i n t o f the matrix m a t e r i a l ) - t h a t d i f f u s i o n of the s p e c i e s S does not a p p r e c i a b l y occur. Such substances as the r a r e gases, homopolar diato m i c molecules (such as oxygen and n i t r o g e n ) , and p e r h a p s , - f o r s p e c i a l purposes, i n f r a - r e d a b s o r b i n g molecules l i k e carbon monoxide and carbon t e t r a c h l o r i d e , ' a r e s u i t a b l e as matrices f o r I n f r a - r e d s p e c t r o s c o p i c s t u d i e s . The m a t r i x r a t i o M/R can be s u f f i c i e n t l y g r e a t A t h a t the spectrum of R i s not complicated by ;the e f f e c t s o f a s s o c i a t i o n of R^ I n the ease t h a t the l a t t e r c o n d i t i o n a p p l i e s , and due to the-low tem-pe r a t u r e s i n v o l v e d , observed I n f r a - r e d bands a r e - c o n s i d e r a b l y sharpened r e l a t i v e to band widths observed f o r pure s o l i d and l i q u i d substances a t s i m i l a r and h i g h e r temperaturesv =--The con-d i t i o n s i n h e r e n t in the matrix t e c h n i c are s i m i l a r - t h e r e f o r e to those of an i d e a l gas, with the a d d i t i o n a l advantage t h a t r o t a t i o n a l e f f e c t s , i f any, w i l l be s m a l l , due to the s m a l l number of r o t a t i o n a l l e v e l s a p p r e c i a b l y populated a t the low temperature. Thus s p e c t r o s c o p i c s t u d i e s of r e a c t i v e or s t r o n g l y i n t e r a c t i n g s p e c i e s may be c a r r i e d out without the c o m p l i c a t i o n 3 of overlapping r o t a t i o n a l bands, as are prevalent i n gas phase spectra. 1-2 Methods of Investigation The methods of in v e s t i g a t i o n of t h i s problem were the following: (a) the i n f r a - r e d spectra of both water and formaldehyde were recorded i n the gaseous and s o l i d matrix states, and, for formaldehyde, i n the s o l i d state; (b) the proton magnetic resonance spectrum of formaldehyde was obtained to check the p o s s i b i l i t y of rotation; (c) a valence force p o t e n t i a l function was assumed i n order to calculate i s o t o p i c frequency s h i f t s f o r the fundamental v i b r a t i o n modes of formaldehyde; (d) the influence of association of active species Wqs examined, and (e) the ef f e c t s of some kind of molecular r o t a t i o n and of various trapping s i t e s were considered. h CHAPTER I I . EXPERIMENTAL I I - l Chemicals The formaldehyde gas used i n these experiments xvas pre-pared by heating paraformaldehyde (polyoxymethylene). The l a t t e r was i n turn prepared i n a l l cases by p r e c i p i t a t i n g the polymer from Baker and Adamson reagent grade formalin solution, In some cases simply by- allowing the formalin solution to evaporate, and In other, cases by making the solution, f i l t e r e d clear, s l i g h t l y a l k a line with sodium hydroxide, then-collecting and washing the new p r e c i p i t a t e that appeared a f t e r - a day. A l l p r e c i p i t a t e s were then dried over anhydrous calcium chloride In a vacuum desiccator f o r a few days, then stored i n another desiccator under atmospheric pressure u n t i l used. The melting points of a base-precipitated sample and an ordinary p r e c i p i t a t e were respectively 1?0-171°C. and 175-176°C., compared with 180-181°C. and 1^3-l45°C. f o r commercial technical and chemically pure paraformaldehyde samples. The i n f r a - r e d spectra-of the polymers i n potassium bromide p e l l e t s were recorded on a Perkin-Elmer Infracord spectrophotometer. The base-preclpltated samples gave spectra as i l l u s t r a t e d i n figure 1, very s i m i l a r to that ( i n reference 7) of Y, a Eu-polyoxymethylene, esp e c i a l l y i n that the 3500 cmr 1 0-H stretching band was of much less i n t e n s i t y than the 2900 cm!1 C-H stretching band. In the simply p r e c i p i t a t e d sample and the commercial chemically pure sample, the 3500 cmT1 and 2900 cmT1 bands were of si m i l a r i n t e n s i t y , whereas f o r the commercial technical sample the 3500 cmT1 band was much more intense than the other. In the spectra of both commercial 4000 3000 0.0 2000 1500 CM-i 1000 900 800 700 JO L U < £.30 o £-40 ^ 50 .60 .70 | 1.0 o o M l 1,1.1 M i l l 111 I 1 -! 1 1 1 1 1 1 1 1 ! ! H — V i ., i , . . J — , t—1 Li I 1 I I I ! 1 1 1 1..L ! : : i J 1, : I 1 > Y1 " ^ ^ ^ u w V r \ \ \ \ — 1 : • J 1 1 IN FRAC:C >RD-~P- v '137 -1280 7 8 9 10 11 WAVELENGTH (MICRONS) 12 13 14 15 SPECTRUM NO. ORIG!N PREPARE IEGEND REMARKS SAMPLE 1. P A L L E T S'LAN.k' KS.,- P t L L E T yALk-AU - PREdPITATEP PURITY 7. SAMPLE PARAPCflMALPErfYCte PHASE S ^ I D DATE THICKNESS OPERATOR co d Ifl I — m 3 JTHP ppok'iKi.Pi MFP rpippnp AT]OK; (sjnn\.'./A! K' r o M K t 6 samples, there was a band at 1040 cmT 1* lacked by both types of prepared sample. The 1040 cmT1 band i s a t t r i b u t e d to the C-0 stretching mode of the methoxyl group. The positions of hydroxyl and methoxyl groups i n polymeric molecules of formaldehyde are the ends of the polymer chain. According to Walker (22) c y c l i c polymers do not depolymerize on vapori-sation; since the gas and s o l i d state spectra obtained In th i s work are (by comparison with 12 and 14) d e f i n i t e l y due to monomeric formaldehyde, and since none of the four types of polymer tested are v i s i b l y soluble i n acetone or water, (as are c y c l i c polymers)the polymers used i n t h i s research must be l i n e a r . The non-existence of a band at 1040 cmT1 and the small r e l a t i v e i n t e n s i t y of the 3500 cmT1 band therefore lead to the conclusion that the prepared polymers consist of a f a i r l y large number of monomer units, probably greater than one hundred. The 3500 cmT1 band may also be att r i b u t e d to absorbed or adsorbed water. The presence of a band, very^weak for a l l samples tested, at 1650 cmT1, the H-O-H bending mode frequency, supports t h i s suggestion; because, however, t h i s band i s ex-tremely weak, the concentration of such water molecules must be very small. Prom the preceding analysis, the base-precipitated polymer was deemed to be of s u f f i c i e n t purity for spectroscopic usage, and was accordingly employed for the low temperature spectra-of formaldehyde. Por the i n f r a - r e d work, the polymer was outgassed by warming under vacuum, then heated to give the desired vapour pressure of formaldehyde. The l a t t e r gas was mixed, I f required, 7 with a matrix gas. The water used i n the low temperature experiments was deionlsed water once d i s t i l l e d ; the l i q u i d was thoroughly outgassed before being mixed with the matrix gases. The argon was Matheson regular grade, 99.998$ pure. The nitrogen was Matheson p r e p u r i f i e d grade, greater than 99.996$ pure. Mass spectra of samples of these gases, handled i n the same manner as the i n f r a - r e d samples, revealed that possible impurities were present i n much smaller concentrations than the most d i l u t e matrix sample prepared In t h i s work. II-2 Apparatus The i n f r a - r e d spectrophotometer used i n the low temperature work was a Perkln-Elmer single-beam, double-pass Instrument, model 112G, with a potassium bromide sixty-degree fore-prism and a seventy-five l i n e s per millimeter grating blazed f o r twelve microns i n the f i r s t order, and with thermocouple and lead sulphide detectors. The low temperature c e l l was of the Deurig-Mador type ( 5 ) , having rotatable Inner chamber, and equipped with caesium Iodide windows. Attached to -the metal of the base of the inner chamber of the dewar, below the deposition window, was a Au-Co:Ag-Au thermocouple. The dewar and a u x i l i a r y vacuum rack were s p e c i a l l y constructed for the present work. The n. m. r , spectrometer used i n part of t h i s study was a Varian Associates V4200/4300B Instrument operating at forty megacycles per second and 9395 gauss for proton resonance. II - 3 Procedure The 112G spectrophotometer was c a l i b r a t e d by the use of 8 atmospheric water vibration-rotation bands, and some of the formaldehyde vibration-rotation bands (the formaldehyde gas being heated i n a 9-cm. c e l l with sodium chloride windows). For the nuclear magnetic resonance spectra, the gas from the alkal i precipitate was further purified by being passed suc-cessively through three cold fingers immersed i n ice baths at -5°C. and - 1 2°C, and a dry ice—acetone bath below -55°C., be-fore condensing i n an n. m. r. sample tube partially immersed i n liquid nitrogen. This d i s t i l l a t i o n was effected at a pressure below 1 mm. mercury. After the n. m. r. spectrum of the formal-dehyde thus prepared had been recorded, the sample was allowed to warm from 77°K. (heat being evolved) to room temperature, and another spectrum was recorded. The samples for the infra-red experiments were prepared from the component gases (the latter themselves being prepared as noted i n section II-l) by mixing i n large bulbs. That the homogeneous mixing of gases Is not a t r i v i a l consideration Is illustrated by the spectra 1 and 2a i n figure 3; the actual preparation of mixtures consisted of evaporating about 1 mm. mercury of the more dilute component into an evacuated 4 - l i t r e bulb followed by 20 or 30 cm. mercury of the other component. Spectra 1 and 2a were recorded following the deposition of the gas mixture four and twenty-four hours respectively after preparation. These spectra, i n comparison with spectrum 5c (from a mixture prepared eighteen days before deposition, a l l mixtures being similarly prepared and deposited), indicate, by the intensity of the 2830 cmT1 band (that band found i n solid 9 formaldehyde) r e l a t i v e to the lower frequency adjacent bands of the i s o l a t e d molecules, that mixed samples must be c a r e f u l l y prepared i n order to ensure homogeneity. mixing needle thermocouple deposition window bulb 7 valve > gauge inside dewar Figure 2. Flow plan f o r the deposition of gases After mixing, the samples were deposited on the i n t e r n a l window (maintained at or near 4°K. or 77°K. by means of l i q u i d helium or nitrogen), the pressure of the gas flowing^the needle valve being measured by a thermocouple gauge. After a suitable deposition period, Infra-red spectra of the deposited materials were recorded. In the cases of the matrix deposits at 4°K., a d d i t i o n a l spectra were recorded as the temperature increased a f t e r the exhausting of the l i q u i d helium. The spectra derived from some t r i a l s follow, with a summary of conditions i n table I. Table I: Summary of experimental conditions for i l l u s t r a t e d spectra. Matrix Spectrum Absorbing No. Species Infra-red Spectra Matrix Ratio M/R Temp. °K. Deposition Pressure (Microns) V Nuclear magnetic resonance spectra Deposition Period (Minutes 1 CH20 Ar 300 350 60 2a CHoO Ar 300 300 60 b 10 est. 0 20 est. 3a CH2O none 77 100 10 b none 75 15 c Ar 300 k 200 150 ka CH20 none 77 100 10 b none k 75 15 c Ar 300 200 150 5a CHpO none 77 100 10 b mat none 75 15 c Ar 300 k 200 150 6 a l H 20 Ar 300 k 300 120 a l l H 20 ? 35 est. b H 20 Ar 300 300 120 7a CH20 none 77 unknown b polyoxymethylene none 295 unknown H O (est. i s an abbreviation for estimated) 11 Figure 3 . The C-H stretching frequencies of formaldehyde 2900 cmTl to 2775 cmT1 The formaldehyde i s i n each case p a r t i a l l y i s o l a t e d i n an argon matrix, M/R = 3 ° 0 . Spectra 1 and 2a were deposited at 350 and 300 microns, k and 2k hours respectively a f t e r preparation. Spectra 2b and 2c show the e f f e c t of warmup on spectrum 2 a . FIGrURE 3. To FOLLOW PAGE (1 12 F i g u r e 4. I n f r a - r e d a b s o r p t i o n s p e c t r a o f formaldehyde i n condensed phases. Spectrum 3a i s of s o l i d formaldehyde a t 77°K.; 3b, a t 4°K. Spectrum 3c i s o f formaldehyde i n an argon matrix, M/B = 300, d e p o s i t e d a t 200u f o r 150 minutes. The v i b r a t i o n s concerned are v£ about 1170cmT1, VJJ around 1250 cmT1 and a combination band v i + V £ or V £ + v/j,. No a b s o r p t i o n between 4450 and 4650cmT1 was observed i n the matrix t r i a l s when i s o l a t i o n was s u c c e s s f u l as i n d i c a t e d by oth e r bands. The frequency s c a l e o f F i g u r e 4 i s d i s c o n t i n u o u s , the re g i o n s shown l y i n g between I I 8 5 and 1150 cmT 1, 1260 and 1230 cmT 1, and 4-575 and 4500 cmT 1 FIGURE +. TO FOLLOW PAGE U 1 3 Figure 5. Infra-red absorption spectra of formaldehyde i n condensed phases. Spectra 4a and 4b are of s o l i d formaldehyde at 77°K. and 4°K. Spectrum.4c Is of formaldehyde in an argon matrix, M/fi = 3 0 0 , deposited at 200u f o r 150 minutes. The vibrations concerned are the fundamentals V3, v 2 and an overtone 2 v 2 . The f r e q -uency scale of figure 5 i s discontinuous, the regions shown l y i n g between 1515 and 1485 cmT1, 1750 and 1700 cmT1, and 3 ^ 2 0 and 3 3 8 0 cmT1 Ik Figure 6 . Infra-red absorption spectra of formaldehyde i n condensed phases. Spectra $a. and 5b are of s o l i d formaldehyde at 77°K. and k°K. Spectrum 5c i s of formaldehyde i n an argon matrix, M/B = 300, deposited at 200u f o r 150 minutes. The vibrations concerned are the combinations v 2 + and V3 + v^ with the funda-mentals v^ and V ]_ between them. The frequency scale i s continuous and runs from 3025 cmT1 to 2700 cmT1 FIGURE . TO F O L L O W P A G E |* 15 7. Infra-red absorption spectra of water i n an argon matrix. Spectra 6a and 6b are of water i n an argon matrix at 4 ° K . , M/R = 300, deposited at 300u f o r 120 minutes. The s o l i d l i n e s of 6a and 6b represent the spectra observed at 4°K. i n the 0-H stretching and H-O-H bending regions, 3800 to 3350 cmT1 and 1680 to 1565 cmT1 respectively. The broken l i n e of spectrum 6a i s an absorption band traced during the same experiment as the other spectra of Figure 7. when the temperature had r i s e n to about 40°K. , and most of the argon had been pumped o f f . 16 8. Nuclear magnetic resonance absorption spectra of formaldehyde and polyoxymethylene. Spectrum 7a represents the derivative curve of the absorption obtained when paraformaldehyde was heated and the gas passed into a n . m. r . sample tube at 77°K. Spectrum 7b resulted when the formaldehyde sample was allowed to warm to room temperature. F I G U R E 8 . T O F O L L O W PACaE" I € 17 CHAPTER I I I . RESULTS: TREATMENT AND DISCUSSION I I I - l Experimental r e s u l t s I f one expected only a single sharp "band for each normal mode of e i t h e r molecule i n the s o l i d and matrix states, then the observed spectra would appear quite complex. Possible explanations f o r these complex spectra include the following: the isotope e f f e c t , molecular association, l a t t i c e mode coupling combinations, reaction with the matrix material, r o t a t i o n either i n a matrix or i n a molecular c r y s t a l , and the trapping i n d i f f e r e n t s i t e s In a matrix. Each of these topics w i l l be discussed i n turn as i t applies to each molecule. • : The matrix spectra can be used to chock s o l i d phase assignments. The preceding statement i m p l i e s L a knowledge of gas phase assignments. Since these are well known for water, and formaldehyde, the case of frequency s h i f t s from the gas phase to the s o l i d and matrix phases must also be considered. Tables 3 and 4 give the i n f r a - r e d absorption frequencies and i n t e n s i t i e s 1 f o r formaldehyde and water observed i n t h i s study, together with the calculated and observed frequencies of other work. 1 The abbreviations f o r i n t e n s i t y i n the following tables follow the frequencies which are i n cmT1 These abbreviations have the following meanings: v, very; w, weak; m, medium; s, strong, and sh, shoulder. Other abbreviations used i n these tables are: Ass., assignment; H., Herzberg (see reference 23, page 3 0 0 ) ; N., Nielsen (see reference 14); S. B . , Schneider and Bernstein (see reference 12) ; P., Plmentel (see reference 1 ) ; C. M., Catalano and M i l l l g a n (see reference 2 ) ; G., Glasel (see reference 3), and, w (prior to a frequency), the band appeared and disappeared during warmup. The l e t t e r s with numerical subscripts 18 following the assignments i n table 3 indicate the symmetry species of that mode, and are taken from reference 12. The symmetry of formaldehyde i s discussed i n appendix 1. The l e t t e r s and numbers below the headings i n table 3 indicate the experimental conditions. The headings on the l a s t two columns si g n i f y the frequency s h i f t s between matrix and gas, and between s o l i d and matrix ( O ^ O A I V * * * - ^ . Table 1. The frequencies and i n t e n s i t i e s observed i n various spectra of formaldehyde. Por meanings of abbreviations, see the footnote on page 17. Ass. v± (ax) v 2 (a x) v 3 (a x) V i ) , (bx) v 5 (b x) H . gas 2780s 1744vs 1503s 2874vs 1280s v 0 (b 2) 1167s v l + v 2 V2+V4 4590w N. gas ,2766.4 1746.0 1500.6 2843.4 1247.4 2 V 2 ( A X ) 2 v 3 ( A 1 ) 2973s 2 V 5 ( A X ) ? 208lw v2+v5(B1) 3003.3 S.B. 7 7 ° 2834s 1712s 1491s 2890s 1247m 1163.5 1177m 34l4w 2960m 2997 2729w 7 7 u 2829vs 1711sh 1720vs l490sh I495vs 1506sh 2885s This 4 ° 2829vs 1715vs l494vs 2885s work Ar N 2 2796vs 2799s 2800.5m 2808w 2809.5m W2817 w2823 1732m 1738s 1742vs 1498m 4 V-3 +30 1736.5m -4 1739m 1239.5w 124lw 1246s 1244sh 1250vs 1246s 1250vs II67.5vw 1172w 1174m 2862s 2871w 2880m 1245w 1247.5vw ll64w 1495 (broad) 2865m - 3 +19 -2 +4 1177s 3402w 1174.5m 1167.5m 1176sh 1174vw 3404w 2991vs 2727m 4535vw 4539sh 4562w 2993m 2727w 4545vw 4562vw 2996m 27l8w 2721sh 272 Ow 4500vw 453Ovw -7 +33 -27 +23 +5 -7 - 3 - 9 J - 1 20 Table 3. The frequencies and i n t e n s i t i e s observed i n the matrix spectra of water. The matrices are argon except i n the case of Pimentel (P.) whose experiments employed nitrogen at 20°K. as a matrix. P. C M . G. This work 1 5 5 ^ 1 5 7 2 1 5 7 4 m 1 5 9 3 1 5 9 3 vs 1 6 0 0 1 6 0 2 m 1 6 0 8 1 6 0 8 s 1 6 1 5 1 6 1 0 s 1 6 2 0 1 6 2 0 1 6 2 4 vs 1 6 3 3 1 6 3 8 1 6 3 8 m 1 6 5 5 1 6 6 3 w 3 3 5 5 3 4 3 5 3 5 1 0 3 6 2 7 3 6 9 1 3 7 2 5 3730 i 20 3772 ± 20 3508 3636 3689 3724 3748 3768 3376 w 3396 vw 3416 vw 3510 m 3574 vs 3633.5 w 365I vw 3699 s 3708 vs 3725 m 3757.5 m 3777 m 21 III-2 The Isotope e f f e c t The Isotope e f f e c t was Investigated by applying a valence-force p o t e n t i a l function to the formaldehyde molecule. (The f r a c t i o n of is o t o p i c molecules i n natural H 3 ° i s t o ° small fo r i n f r a - r e d detection, whereas for formaldehyde the carbon-13 i s o t o p i c species i s abundant to the extent of one per cent of natural carbon: e a s i l y detected, by the Perkin-Elmer 112G spectrophotometer, i n atmospheric carbon dioxide.) The p o t e n t i a l function assumed for formaldehyde was of the following form: 2V = k ^ r - ^ 2 + k 2 U r 2 2 + *r 3 2) + k 3 ( r 2 ^ i ) 2 + k ^ r ^ ) (*22 +**jZ) + ^ U r ^ U r g + ^ 3 ) + k^r-^) (A 4" ) 2 where V i s the po t e n t i a l energy, Ar^ i s a small increment i n the length of the i ' t h bond, i s a small increment i n the i ' t h bond angle, <^T l s a small increment i n the angle between the C-0 bond and the plane containing the methylene group, kj_ i s a the force constant representing the r e s i s -tance to a change from the equilibrium nuclear positions, and k]_2 i s an i n t e r a c t i o n constant connecting the C-H and C-0 stretching modes. Table k gives the force constants and the calculated frequencies fo r C l Z E 1 2 0 1 ^ C^H^O 1 6, and C 1 2H 2 2O l 6, together with the f r e -quency s h i f t s f o r the isotopic carbon molecules and the l i s t of frequencies f o r d 2-formaldehyde from Herzberg (23) , page 300. The v^ force constant and frequencies were calculated from the equation I I , 219 i n Herzberg (23), page 180. The V2j, and v^ force constants and frequencies were calculated by the Wilson 2 2 FG matrix method (24), the secular equation being factored by means of Internal symmetry co-ordinates (see appendix I ) . Table 4. Calculated force constants and frequencies for i s o t o p i c formaldehyde molecules, (c f . r e f e r e n c e 15). V i k. c C ^ H 1 20]6 cWjjOj 6 4 ^ . C 1 2 H 2 2 0 1 6 H. Cii) (md./S.) (cm. ) (cm. ) (cmT 1) (cm.-1) (cm.) v^ k 2 = 4.2757 2843.4 2831.2 -12 .2 2121 2160 V5 = O.6536 1247.5 1238.8 - 8.7 97^ 990 v6 k f = 0.9918 1136.6 1151.8 -11.8 930 938 Agreement between calculated and observed frequencies for d 2 -formaldehyde i s probably better than i s apparent because the values stated by Herzberg are not zero l i n e s , but re f e r simply to prominent features of the bands. That these features can be misleading i s exemplified i n a comparison of Herzberg 1s and Nielsen's sets of frequencies f o r C 1 2 H 1 2 0 1 ^ In table 2 . Referring to table 2 and to spectrum 3a ( i n figure 4) , one can see weak absorption bands at 1239.5 and 1167.5 cmT1, T irvtenft s h i f t s of - 10.5 and - 9 . 5 cmT respectively from the most adjacent A bands. Since the predicted s h i f t s are - 8 . 7 and -11.8 cmT1, these weak bands may be assigned to C H 2 0 . Conversely, because there are bands of approximately the correct r e l a t i v e Intensity and frequency s h i f t i n the s o l i d state, the po t e n t i a l function u t i l i z e d to predict the frequency shifts-must f i t the molecule w e l l . The frequencies calculated for formaldehyde-C1-*, i f they 13 were observed i n pure formaldehyde-C , would not-necessarily be the frequencies observed i n a mixed sample of isot o p i c species. The introduction of atoms may perturb the sel e c t i o n rules which would otherwise permit absorption for 23 out-of-plane bending only i n c e r t a i n phase r e l a t i o n s . Because the difference from the calculated s h i f t f o r v^, the out-of-plane bending vibration, i s small, the angle between the planes of neighbouring molecules l s indicated to be close to a r i g h t angle, since the dipole i n t e r a c t i o n of neighbour molecules i s a function of the cosine of the angle between these -planes (20). The agreement of calculated and observed frequencies f o r 13-formaldehyde-C isprobably very good. No weak s a t e l l i t e bands have been d e f i n i t e l y observed f o r the other fundamentals i n the spectrum of s o l i d formaldehyde. No s a t e l l i t e bands of the correct r e l a t i v e Intensity have been seen In the matrix spectra, but the in t e n s i t y of the most intense bands there has not been very great, c h i e f l y due to the small amount of absorbing molecule 2 deposited . III - 3 Molecular association Some of the observed bands i n the matrix spectra may be due to association of formaldehyde or water molecules. Pour experimental checks of thi s p o s s i b i l i t y are available: v a r i a t i o n of matrix r a t i o , v a r i a t i o n of deposition rate, warmup observa-tions, and band width. By varying greatly the matrix r a t i o , one can, as Pimentel et a l . (1) have done with water i n nitrogen, assign c e r t a i n bands to associated species and others to non-associated species. This technic was not employed i n t h i s work. A r e l a t i v e l y large deposition rate would be expected to warm the surface of the deposition window to such a temperature that d i f f u s i o n i n the matrix becomes appreciable. Deposition rates 2. The amounts of formaldehyde and water actually deposited i n matrices probably never exceeded forty mlcromoles. 24 were maintained as small as possible, and were otherwise not varied i n these experiments. The C-H and 0-H stretching bands were continuously scanned during the warmup, i . e., as the sample was allowed to warm aft e r the l i q u i d helium i n the dewar was exhausted. D i f f u s i o n becoming more rapid as the matrix slowly warms, bands a t t r i -butable to associated species should increase i n - i n t e n s i t y at the expense of bands of monomeric species. The bands of associated species should l i e i n frequency between the corres-ponding bands of the well i s o l a t e d case and the t o t a l l y associated (molecular c r y s t a l ) case. This e f f e c t was- observed i n both the cases of formaldehyde and water, and i s i l l u s t r a t e d i n spectra 2a , 2b and 2c of figure 3 . Here the three bands at 2796, 2800.5 and 2809.5 cmT 1 decrease i n i n t e n s i t y while the bands at 2817 and 2823 cmT1 increase. Just before the matrix material was pumped o f f , the l a t t e r bands were seen to decrease and to disappear, and a single band at 2829 cmT 1.rose and re-mained. The sequence was i r r e v e r s i b l e : the addition of more l i q u i d helium to the dewar simply froze the deposit at whatever stage I t had reached before recooling. Likewise -for i^ater, of the bands between 3480 and 3800 cmT 1, a l l decreased, apparently uniformly, with the exception of the 3510 cmT 1 band, which increased i n i n t e n s i t y . A s i m i l a r e f f e c t was noticed also, namely a s l i g h t , perhaps doubtful decrease of int e n s i t y of some intense bands on standing before warmup commenced. Morrow (25) and Glas e l (3) have also observed this phenomenom, to a much greater extent i n t h e i r work than In t h i s . However, i n these 2 5 experiments, no corresponding increases of other "bands were observed, perhaps because there was some evaporation of the deposit when i t was bathed? i n the r a d i a t i o n from the spectro-meter source. That absorption bands of substances dispersed i n s o l i d i n e r t matrices were very sharp, compared even to the bands of the pure s o l i d substance at the same temperature, was obvious early i n thi s research. The fac t that the absorption l i n e s of the matrix-suspended material were usually of similar width to vib r a t i o n - r o t a t i o n l i n e s of atmospheric gases, made d i f f i c u l t the fin d i n g of the former bands when they were i n a region of the spectrum overlapped by atmospheric water vapour and carbon dioxide bands. Because one would expect that c r y s t a l f i e l d and weak intermolecular bonding e f f e c t s would broaden absorption bands, r e l a t i v e l y broad bands, such as that at 3 5 1 0 - 1 - 1 cm. for water i n argon and that at 2 8 1 7 cm. for formaldehyde i n both argon and nitrogen, may e a s i l y be i d e n t i f i e d with some associated species of these molecules. A comparison of spectra 1 and 2 c i n figure 3 indicates that non-mixing of the sample causes poor i s o l a t i o n since the e f f e c t i v e matrix r a t i o Was much greater than i t was planned to be. Schneider and Bernstein have discussed ( 1 2 ) the molecular association and i n f r a - r e d spectrum of s o l i d formaldehyde; they speculate that the i n t e r a c t i o n between neighbours i n planes perpendicular (also with the molecular C 2 axes m«+M«Hy j per-pendicular) to one another would explain the observed spectral s h i f t s . They also suggest that two c r y s t a l l i n e forms may e x i s t . 26 In t h i s work only one spectrum was ever observed and was obtained consistently i n many t r i a l s . The bands of the f o r -maldehyde deposited i n ; a matrix and remaining a f t e r the matrix had evaporated were always of the same frequency as otherwise but were much broader than f o r pure formaldehyde freshly deposited. No change i n frequency or width was ob-served when pure formaldehyde was.allowed to stand for a day at 7 7°K. The spectra for the region 1 1 0 0 to 1 3 0 0 cm"1 ob-tained i n thi s work d i f f e r from the spectra of the same region observed by Schneider and Bernstein; i f the lower r e s o l u t i o n of t h e i r instrument i s considered, then the thin, f i l m of the i r work i s probably closest to the conditions encountered i n these experiments. I l l - k Lattice mode combinations Combinations of i n t e r n a l vibrations with l a t t i c e vibrations may account for the structure observed i n many of the funda-mentals of formaldehyde and water i n the molecular-crystal and i n e r t matrix cases. In the Raman spectra of some c r y s t a l s , l i n e s very close ( 0 to 1 5 0 cmT 1 ) to the e x c i t i n g l i n e have been observed, and, according to Herzberg ( 2 3 ) , are interpreted as due to vibrations of the l a t t i c e . These l a t t i c e -modes may be tor s i o n a l or t r a n s l a t l o n a l l a t t i c e vibrations, of which the former are much more common ( 2 6 ) . Because the frequencies of the V 5 and v^ bands i n s o l i d formaldehyde are not very d i f f e r e n t from the gas phase zero l i n e frequencies, and because the s p l i t t i n g of these bands i n the s o l i d phase Is very small, the frequencies of l a t t i c e vibrations probably must also be small. 27 The l a t t i c e v i b r a t i o n frequencies observed f o r s o l i d benzene (28) at 77°K. are 81, 101 and 124 cmT1, of an order of magnitude larger than for formaldehyde. Another possible source (27) of s p l i t t i n g of nondegenerate v i b r a t i o n a l modes i s intormolooular ooupllng or c o r r e l a t i o n f i e l d s p l i t t i n g . The other p r i n c i p a l source of v i b r a t i o n a l s p l i t t i n g s i s of degenerate molecular modes, s p l i t by the decreasing symmetry of the l o c a l f i e l d of a molecule upon i t s condensation, but because there are no degenerate modes f o r C^y molecules, t h i s e f f e c t i s not applicable to formaldehyde or to water. Ob-viously intermolecular coupling i s ne g l i g i b l e i n the good I s o l a t i o n provided i n an i n e r t gas matrix, and probably the l a t t i c e vibrations can also be neglected i n thi s case. I l l - 5 Reaction with the matrix Reactions of the formaldehyde•and water molecules with the matrix materials might be postulated as a cause- of the multiple absorption bands observed i n the regions of the fundamental frequencies. In t h i s case one would expect that the s o l i d phase frequencies should be sh i f t e d from the gas phase zero l i n e . The frequencies of the 0-H stretching modes i n water are very sensitive to the environment of the molecule,5 such interactions normally leading to a decrease i n frequency and a broadening of absorption bands. A l l the frequencies observed i n the matrix phase spectra as sharp l i n e s , are close to the corresponding fundamentals of the gas phase, i n d i c a t i n g that interactions between the protons and matrix materials are small, and, for formaldehyde, d i f f e r l i t t l e from the nitrogen matrix to the 28 argon matrix. During warmup i n an water experiment, a broad band was observed around 3 6 O O cmT1 (broken l i n e i n figure 7, spectrum 6a) a f t e r most of the argon had been pumped o f f the deposition window. This band may be due to an argon hydrate, or, more probably, to some c r y s t a l l i n e form of i c e , or even, perhaps, to some hydrate of the very t h i n f i l m of formaldehyde polymer remaining on the window from a previous formaldehyde experiment. Although i n no previous study on ice-has there been observed a band around 3600 cmT 1 , probably no-other workers have attempted to form ice or, f o r that matter, argon hydrates, by similar procedures. At the low temperature (less than 70°K.) reaction of the water with the polyoxymethylene would not be expected. Moreover, to d i f f e r e n t i a t e between an argon hydrate and an ice by means of the observed frequency Is not possible, because both species are expected to s h i f t the 0-H stretching vibrations to lower frequencies, even lower than the observed s h i f t , due to hydrogen bonding. I I I - 6 Rotation i n s o l i d formaldehyde Since only a few r o t a t i o n a l l e v e l s are appreciably populated at 77°K. or 4°K., r o t a t i o n within the l a t t i c e of formaldehyde molecular crystals was' considered i n order to explain the structure of the v i b r a t i o n bands. There are many fewer l e v e l s populated at 4°K. than at 77°K., whereas there are more l i n e s observed at the lower temperature due to the sharpening. Since nuclear resonance spectroscopy i s capable of detecting some types of molecular motion, the n. m. r . spectrum of formaldehyde was studied. 29 Por formaldehyde at 77°K. the proton p a i r would be expected to give a 'doublet' signal such as I of figure 9 , of which the recorded derivative of the absorption would be si m i l a r to I I . The t h e o r e t i c a l l i n e shape for a proton pair i n molecules of a p o l y c r y s t a l l l n e material i s given by III of figure 9 . The s p l i t t i n g «<., equal to 3 MT~^ where l s the proton magnetic moment and r the internuclear distance, corresponds roughly (but not exactly due to intermolecular broadening of the actual resonance l i n e ) to the distance between A and B i n I I . Ro-t a t i o n occurring about the axis perpendicular to the i n t e r -nuclear vector decreases the doublet s p l i t t i n g to 3/2/^r"*^. FIGURE % N.M.R. LIME SHAPES For the dimensions of formaldehyde as given i n appendix I, -3 n o -3 3 J i r = 5 . 2 2 gauss for the case o f A r o t a t i o n , or 3 / 2/(r = 2.61 gauss i f there i s r o t a t i o n . That the quality of the trace was very poor was probably due to the small amount of formaldehyde, and consequently the low concentration of protons actually i n the space of the spectrometer receiving c o i l . The experimental value of the doublet s p l i t t i n g i s 8.5 ± 0.5 gauss (average of four t r i a l s : 8 . 3 , 8 . 8 , 7.8 and 9.1). (The experimental doublet s p l i t t i n g indicates a proton separation of 1.71 compared to 1.89 f o r the given geometry.) The p o s s i b i l i t y of r o t a t i o n i s there-fore eliminated. 30 For the formaldehyde polymer, a smaller receiving c o i l was used. Consequently a better trace was obtained. Since the spectrum (7b i n figure 8) i s quite d i f f e r e n t from the previous (7a), the sample at 77°K. was probably monomeric formaldehyde. References 17 and 19 were consulted during the consideration of the proton, magnetic resonance spectra of formaldehyde and polyoxymethylene. The n. m. r . spectrum of the a l k a l i - p r e c i p i t a t e d para-formaldehyde was also recorded; the spectrum i s q u a l i t a t i v e l y s i m i l a r to the broader band of figure 8, spectrum 7b. However i n t h i s case there i s no central strong l i n e , but the l i n e width i s very close, 11.5 gauss, to the previous l i n e width which was 10.8 gauss f o r the broad l i n e (compared to 2.9 gauss for the narrow central l i n e ) . III-"? Rotation of water and formaldehyde i n matrix s i t e s To explain the m u l t i p l i c i t y of sharp l i n e s observed i n the spectrum of water trapped i n i n e r t gas matrices, various workers (1, 2, 3) have suggested that the water molecule may be executing some kind of r o t a t i o n within the l a t t i c e of the i n e r t gas matrix. One of the reasons for such a proposal i s the fa c t that the spectrum of water i n nitrogen i s very much simpler than that observed for rare gas matrices, the reasoning being that, because the sizes of the trapping s i t e s i n nitrogen are considerably smaller than for instance i n argon, and because the non-spherical symmetry of nitrogen molecules would tend to prevent rotation, r o t a t i o n can therefore occur i n some rare gas 31 matrices, depending on the size of the atoms, "but not i n n i t r o -gen. Shurvell (29) has observed that the spectrum In the D-O-D bending region of heavy water consists of two sharp l i n e s i n nitrogen, but of seven sharp bands i n each of argon and krypton. The temperature dependence of in t e n s i t y (2) of the water bands i n argon also suggested that they may have arisen from v i b r a t i o n -r o t a t i o n i n t e r a c t i o n . A p a r a l l e l set of observations has been made f o r formalde-hyde In argon and nitrogen, i n which only two l i n e s were observed f o r v i and v 2 i n nitrogen as contrasted with three each In argon (see table 2 ) . I f water molecules urere f r e e l y r o t a t i n g i n the rare gas matrices, what spectrum should one expect to observe? The number of possible absorption t r a n s i t i o n s i s dependent on the number of populated l e v e l s . For the ammlne r a d i c a l (NH2) a ^ 4.2°K. i n thermal equilibrium ( 3 0 ) , 99.9 per cent of the mole-cules are i n the 0$ r o t a t i o n a l l e v e l , 0.09 per cent i n l ^ and 0.002 per cent i n 1£ . Possibly a sim i l a r d i s t r i b u t i o n exists for water at 4.2°K., since the r o t a t i o n a l l e v e l s are of comparable spacing. The s e l e c t i o n rules that apply are A J = 0 , ± 1; J = 0 i i - > J = 0, and the par i t y of T changes f o r V 3 , but remains f i x e d for v i and v 2 (16, 18). Therefore one would expect tra n s i t i o n s from only the 0 Q l e v e l , three l i n e s i n a l l , one per normal mode, at 1635, 3693 and 3779 cmT1 I f there were a non-equilibrium population according to the p r o h i b i t i o n of symmetric l e v e l s not combining with a n t i -3 2 symmetric l e v e l s , but otherwise an equilibrium population of each of the ortho and para modifications, then the leve l s would be populated such that seventy-five per cent of the mole-cules were In the (para) state and twenty-five per cent i n the 1!L]_ state. The calculated l i n e s i n t h i s case are 1x> Tx* for v^: 0* 1-1 3 7 7 9 , 3 7 3 2 , 2 - 2 3 8 0 1 , 2+2 l i i l 3 8 6 3 , Tx> 08 i» V lo l'+l l i i i 2 1 l l i l i and f o r v l f T , S 1 q 0 o 3 6 9 3 , 1 6 3 5 , 3 6 7 5 , 1618, 3 7 1 1 , 1 6 5 3 . Thus four bands are expected f o r v^ and three each for v-^  and v 2 . In a comparison of the spacing between the calculated and observed bands, not one but two sets, each having d i f f e r e n t gas-matrix frequency s h i f t s , are observed. These data are l i s t e d In table 5. Table 5 . Frequencies, frequency s h i f t s and i n t e n s i t i e s f o r water at 4°K. v i 3 6 7 5 s 3 6 3 4 -41 " " 3 6 5 1 -42 1 5 7 4 -43 1 5 9 3 -42 v 3 gas 3 6 9 3 w 3 7 1 1 ra-s 3 6 9 9 -12 1 6 1 7 s 1 6 0 8 - 9 1 6 3 5 w 1 6 2 4 - 1 1 1 6 5 3 m-s 1 6 3 8 - 1 5 3 7 3 2 m 3 7 0 8 -24 373 w 3 7 5 7 - 2 2 3 8 0 1 vs 3 7 7 7 -24 3 8 6 4 vw 1 6 1 0 - 4 3 Of the observed lines not accounted f o r i n the above table are 1 6 0 2 and 3 7 2 5 cm?! which have been observed for water i n nitrogen ( 1 ) at 20°K. as 1 6 0 0 and 3 7 2 5 cmT1 The remainder but one of the bands, at 3 5 1 0 , 3 4 l 6 , 3 3 9 6 , 3 3 7 6 and 1 6 6 3 cmT1, can be assigned, by t h e i r width and distance from the zero l i n e s , 33 to associated species. The very intense narrow l i n e at 357^ cmT1 Is not explainable by the preceding analysis; s h i f t s of more than 100 cmT1 have to be r a t i o n a l i s e d i n order to f i t the observed sharp l i n e s to the calculated gas phase frequencies for B 20, but thi s presumably H 2 0 l i n e lacks any adjacent sharp li n e s that can be used to form a set sim i l a r to others of table 5» The l i n e acts normally on warmup, decreasing i n phase with the other water l i n e s , i s not due to causes i n the deposition window, and was produced equally strongly, r e l a t i v e to the other water l i n e s , i n both experimental t r i a l s of water in.argon. The i n t e n s i t y of a l i n e can be calculated (16) and i s proportional to the product of the li n e strength (or p r o b a b i l i t y of r o t a t i o n a l t r a n s i t i o n J^/ - J^" ) and the Boltzmann factor g exp(- E"/kT^ (where g-is the nuclear spin s t a t i s t i c a l weight factor, i ( 2 i + 1) f o r para states (even values of c) and ( i + l ) ( 2 i + 1) f o r ortho states (odd values o f f ) , 1 being the nuclear spin of the i d e n t i c a l n u c l e i , where E" i s the r o t a t i o n a l term value of the ground l e v e l Jj-„ , k the Boltzmann constant and T the absolute temperature). The absolute i n t e n s i t y i s pro-po r t i o n a l to the number of molecules i n the absorbing path i n the ground vlbrationless state, the l i n e width, the s l i t width, the frequency, and a factor representing the e f f e c t i v e change of dipole moment i n the v i b r a t i o n a l t r a n s i t i o n . -'In scans of the same small spectral region i n one experimental t r i a l , a l l the forementioned factors should be approximately constant, so that only r e l a t i v e i n t e n s i t i e s are relevant f o r the purpose of comparison. Jk The r e l a t i v e i n t e n s i t i e s calculated ( 3 D for water at 4°K. with the room temperature ortho-para r a t i o but otherwise i n a normal thermal d i s t r i b u t i o n are as indicated In table 5 a f t e r the calculated frequencies. Comparing the observed i n -t e n s i t i e s , frequencies and s h i f t s for the symmetric vibrations, one can see that the experimental re s u l t s are quite consistent i n that the l i n e s at 1 6 2 4 , 1593 and 3 6 5 1 cmT1 are the most Intense of the respective groups, i n disagreement with the calculated i n t e n s i t y pattern, and i n that the s h i f t s are similar f o r the corresponding sets of frequencies of each vi b r a t i o n . The observed i n t e n s i t i e s for vj t r a n s i t i o n s are less at variance with the calculated i n t e n s i t i e s i f one assumes a C o r i o i l s i n t e r a c t i o n , a perturbation of a r o t a t i o n - v i b r a t i o n l e v e l of v i by a c l o s e - l y i n g one of v-^ J such a perburbation of a p a i r of l e v e l s of the same quantum number J and the same-total symmetry properties would r e s u l t i n a displacement, perhaps small, of the frequencies and much higher i n t e n s i t y ( 1 6 ) f o r the v^ t r a n s i t i o n . As ; seen i n the i n t e r a c t i n g bands, 3&99 and 3 7 0 8 cmT1, are not completely resolved, each l i n e tending to appear more intense than i t r e a l l y Is. An examination Of the spectra obtained by Shurvell ( 2 9 ) of heavy water (DgO) i n argon, krypton and nitrogen reveals si m i l a r trends, i n that the observed frequencies are consistent with the calculated, allowing for matrix s h i f t s , but that the observed i n t e n s i t i e s , while self-consistent, are i n disagreement with the calculated. I f three d i f f e r e n t trapping s i t e s , the s u b s t i t u t i o n a l s i t e 35 and the octahedral and tetrahedral i n t e r s t i t i a l s i t e s , were available to a water molecule, and i f the water molecule were able to execute f a i r l y free r o t a t i o n i n two of the s i t e s but not i n the t h i r d , the observed spectral frequencies would be explained. I f the molecules were i n a normal thermal d i s t r i -bution of r o t a t i o n a l l e v e l s except that the ortho-para r a t i o i s of some value between those of 4°K. and room temperature, then the observed i n t e n s i t i e s would be explained. (Glasel (3) has noticed that his l i n e corresponding to 3757 cmT1 of thi s work has decreased more rapidly than the other monomer li n e s at 20°K., in d i c a t i n g a slow ortho-para conversion.) The set of r o t a t i o n a l l i n e s of lesser s h i f t from the gas phase frequencies would originate from molecules In the largest trapping s i t e s and consequently of leas^fc i n t e r a c t i o n with the matrix—probably the s u b s t i t u t i o n a l l a t t i c e s i t e s . The single l i n e s observed i n both argon and nitrogen matrices- would be pure v i b r a t i o n a l t r a n s i t i o n s of non-rotating molecules i n the sma l l e s t — t e t r a h e d r a l i n t e r s t i t i a l — s i t e s . The other set of lin e s would then be due to molecules r o t a t i n g i n the octahedral i n t e r s t i t i a l s i t e s . Not a l l observed l i n e s are accounted f o r by these hypotheses, nor a l l calculated t r a n s i t i o n s observed, but the recorded spectra are best explained by such hypotheses. Other hypotheses to inter p r e t the water spectra are a non-equilibrium population of r o t a t i o n a l l e v e l s , v i o l a t i o n of normal s e l e c t i o n rules due to d i s t o r t i o n of the molecule i n the l a t t i c e or l a t t i c e interactions, and a combination of the forementioned hypotheses. None of these explain the observed 36 spectra as well as the preceding argument; however, that strong l i n e s are observed near the zero l i n e s for both H^ O and LV,0 i n a l l matrices t r i e d hints that s t i l l another theory may be needed. The spectra of formaldehyde i n argon should also be capable of s i m i l a r i n t e r p r e t a t i o n i f r o t a t i o n i s occurring. Since however t h i s molecule i s larger than water, r o t a t i o n i s not as l i k e l y , and pure v i b r a t i o n a l t r a n s i t i o n s may then be expected. The formaldehyde molecules could s t i l l be trapped - i n - d i f f e r e n t l a t t i c e s i t e s , and the v i b r a t i o n a l transitions-thus r e s u l t i n g would d i f f e r s l i g h t l y i n frequency, depending-ort trapping s i t e . The presence of three l i n e s at the C-H and C-0 stretching f r e -quencies indicates that three possible s i t e s are available In argon, but two l i n e s , two s i t e s i n nitrogen. The experimental i n t e n s i t i e s indicate that the su b s t i t u t i o n a l l a t t i c e s i t e s are much more favoured by formaldehyde molecules 3for-occupancy than i n t e r s t i t i a l s i t e s . The gas-matrix s h i f t s of frequency are small for the fundamentals of formaldehyde except for the C-H stretching frequencies, i n which cases the s h i f t s are of +30 and +19 cmT1 f o r v^ _ and v^ i n argon, and 33 and 22 cmT1 res-pectively i n nitrogen. The s h i f t s here are sim i l a r to those observed for NH 2 (30) but are of opposite sign to those for water (see above, table 5)« These s h i f t s may be due to d i e l e c -t r i c e f f e c t s (13) i n which case a l l s h i f t s would-be expected to tend to decreasing frequency) or to some more s p e c i f i c electronic e f f e c t , perhaps a dipole-induced dipole Interaction. In the l a t t e r case the average s h i f t would be expected to Increase with Increasing atomic number of i n e r t gas, an observed-effect (29); 3 7 the magnitude and d i r e c t i o n of the s h i f t would require very s p e c i f i c c a l c u l a t i o n . Attempts to explain the formaldehyde matrix spectra by considering both free and hindered r o t a t i o n have not been generally successful ( 3 1 ) . I l l - g Miscellaneous notes. Despite Nielsen's assurances to Herzberg ( 2 3 ) , no band at 2081 cmT1 was observed i n th i s work i n the gas phase spectrum of formaldehyde. This band may be due to glyoxal. The accuracy of frequencies (given i n t h i s study i n r e c i p r o c a l centimetres corrected for vacuum) i s ± 2 cmT1 38 APPENDIX. The structure and Symmetry of Formaldehyde and Water I The structures The structure assumed f o r water Is a symmetrical, non-linear molecule. The symmetry point group Is therefore C 2 v, and the three unequal moments of I n e r t i a make the water molecule an asymmetric top for the purpose of r o t a t i o n a l analysis. The structure and' dimensions of formaldehyde assumed i n this work are as indicated i n figure 10. The. values shown were taken from (11). They are considered to be the best available because they' are calculated from accurate- physical measurements, electron d i f f r a c t i o n date (10) f o r the carbon-oxygen distance, and microwave, spectra (9) and u l t r a v i o l e t spectra (8) for the moments of i n e r t i a . The l a t t e r two c a l c u l -ations are i n good agreement with each other and the r o t a t i o n a l calculations of ( l l ) i II The symmetry of formaldehyde The formaldehyde molecule belongs to the 0^v point group and the elements of t h i s group are E, C 2, o~ and 0~ / . • The plane of the molecule i s the (x,z) plane with the C 2 axis i n the z-d i r e c t i o n . The character table for t h i s group Is table 6. Table 6. Character table for C 2 y . E C 2 ft CT~' n Infra-red A, 1 1 1 1 T „ 4 3 Ao 1 1 -1 -1 B| 1 0 B-, 1 - 1 1 -1 T x; R 4 2 B 2 1 - 1 - 1 1 T y; B£ 3 1 X r 1 2 - 2 4 2 Xp i n t . 6 0 4 2 Since 3n, - 6 = 6 fundamentals are possible (inhere n i s the F I G U R E 10 . T O F O L L O W P A G E 3 8 0 S T R U C T U R E OF F O R M A L &6HVPE NORMAL MODES OF W A T E R 39 number of atoms i n a non-linear molecule), a l l the fundamentals are infra-red active. The internal co-ordinates chosen for the symmetry analysis i n section III-2 were as follows: H l = R 2 = A r 2 R3 = Ar^ V = rl<* ^ 5 = ^ r 2 ( K R 6 = r 3 ^ The symmetry co-ordinates used to factor the secular deter-minant xsere as follows: Qx = R 2 Q? = 1 (R2 + R q) *3 = ^ - < R 2 " E 3 ) C,5 = ^1 (R6 + R 5 ) Since the six internal co-ordinates were a l l i n the plane of the molecule, and since there are only five in-plane vibrations, there i s one redundant co-ordinate, of species A . '4o BIBLIOGRAPHY 1 Van T h l e l , Becker and Pimentel, J . Chem. Phys. 22, 486 ( 1 9 5 7 ) 2 Catalano and M i l l i g a n , J . Chem. Phys. 2 0 , 45 ( 1 9 5 9 ) 3 Glasel, J . Chem. Phys. 22» 2 5 2 ( i 9 6 0 ) 4 Pimentel, J. A. C. S. 80, 62 ( 1 9 5 8 ) 5 Deurig and Mador, Rev. S c i . In s t r . 22, 421 ( 1 9 5 2 ) 6 Behringer, J . Chem. Phys. 2_2, 5 3 7 ( 1 9 5 8 ) 7 Philpotts, Evans and Sheppard, Trans. Par. Soc. £1, 1 0 5 1 ( 1 9 5 5 ) 8 Dieke and Kistiakowsky, Phys. Rev. 4£, 4 1 9 3 ^ ) 9 Lawrance and Strandberg, Phys. Rev. 82, 3 6 3 ( 1 9 5 1 ) 10 LuValle and Schomaker, J . A. C. S. 6 l , 2 5 0 8 ( 1 9 3 9 ) 1 1 Davidson, Stolcheff and Bernstein, J . Chem. Phys. 2 2 , 2 8 9 , ( 1 9 5 4 ) 12 Schneider and Bernstein, Trans. Par. Soc. $2, 13 ( 1 9 5 6 ) 13 Becker and Pimentel, J . Chem. Phys. 2 5 , 224 ( 1 9 5 6 ) 14 Blau and Nielsen, J . Mol. Spec. 1 , 124 ( 1 9 5 7 ) 15 Hisatsune and Eggers, J . Chem. Phys. 22, 4 8 7 ( 1 9 5 5 ) 16 Benedict and P l y l e r , N. B. S. J . Research, 46, 246 ( 1 9 5 1 ) 17 E. M. Andrew, Nuclear Magnetic Resonance. Cambridge Univ-e r s i t y Press, Cambridge, 1 9 5 5 18 Dalby and Nielsen, J . Chem. Phys. 22, 9 3 ^ ( 1 9 5 6 ) 19 Gutowsky and Pake, J . Chem. Phys. 18, 163 ( 1 9 5 0 ) 2 0 Decius, J . Chem. Phys. 2 2 , 1 9 4 1 , 1946 ( 1 9 5 ^ ) 2 1 Bass and Broida, Formation and Trapping; of Free Radicals. Academic Press, New York, i 9 6 0 " " 2 2 Walker, Formaldehyde. Relnhold Publishing Corp., New York, 1953 23 Herzberg, Infrared and Raman Spectra of Polyatomic Molecules. Van. Nostrand Co., Princeton, i 9 6 0 (ninth printing) 41 24 Wilson, Decius and Cross, Molecular Vibrations. McGraw-H i l l Book Company, New York, 1955 25 B. A. Morrow and K. B. Harvey (private communication) 26 Hornlg, Discussions of the Faraday Society, No. 9, 115, 1950 2? Hexter, J . Chem. Phys. 22, 1833 (i960) 28 Sirkar, Indian J . Physics, 10, 189 (1936) 29 H. P. Shurvell and K. B. Harvey (private communication) 30 Robinson and McCarty, J . Chem. Phys. 22, 999 (1959) 31 K. B. Harvey (private communication) 

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