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Ionization and dissociation of molecules by electron impact Yu, Mina Theresa Tung-Fai 1966

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IONIZATION AND DISSOCIATION OF MOLECULES BY ELECTRON IMPACT by MINA.THERESA TUNG-FAI YU B.Sc., University of Br it ish Columbia, 1964, A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Chemistry• We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA APRIL,1966. In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the U n i v e r s i t y of • B r i t i s h Columbia,, I agree that 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 reference and study, I further agree that per-mission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representatives. I t i s understood that;copying or p u b l i -c a tion.of t h i s t h e s i s for f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission* Department of (3HEMISTRY The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada Date APRIL,1966. ABSTRACT A series of related alkyl and perfluoromethyl sulphides and di'sulphides has been studied by electron impact. The trend in the ionization potentials shows that when the bonding electrons are drawn further away from the sulphur atom(s), as in the case of the perfluoromethyl compounds, more energy is required to ionize the molecule. From the appearance potentials and subsidiary thermo-chemical data, the upper limits of a l l the heats of formation of the non-cyclic compounds were derived. The upper limits of the heats of formation of the principal fragment ions and radicals were also calculated. From the derived heats of formation, estimates of a l l the C-S and S-S bond strengths were made. • I l l ACKNOWLEDGEMENTS This' work was done under the supervision of Dr. D. C. Frost to whom I express my sincere appreciation. ! I would also like to thank Professor C. A. McDowell and Dr. C. E. Brion for their interest in the work. Special thanks are extended to Dr. W. R. Cullen for providing samples of a l l the non-cyclic sulphides and disulphides. I would like to thank also a l l my Colleagues in the mass spectrometry group and others who have given me their valuable support and help in the preparation of this Thesis. | IV TABLE OF CONTENTS CHAPTER 1. SULPHUR BONDING IN ALKYL SULPHIDES, PAGE CHAPTER 2. IONIZATION AND DISSOCIATION OF MOLECULES BY ELECTRON IMPACT. (1) Methods of determining dissociation energies. (2) Ionization by electron impact. (3) The Franck-Cpjidon Principle and the dissociation of molecules. (4) Secondary (and other) processes in a mass spectrometer. (5) The ionization efficiency curve and i t s interpretation. (6) Heats of formation and dissociation energies. 3 4 9 11 CHAPTER 3. EXPERIMENTAL METHOD. 13 (1) Theory of mass spectrometry. 13 (2) The instrument. 14 (3) Experimental 18 CHAPTER 4. RESULTS AND DISCUSSION. 22 (1) Ionization potentials of thesulphides. 22 1 (2) Mass spectra and fragmentation of molecules. 25 (3) Ionization and appearance potential measurements. 32 (4) Heats of formation and bond energies. 43 (5) Discussions on individual molecules 51 a) Dimethyl sulphide 51 b) Dimethyl disulphide 53 c) Methyl-perfluoromethyl sulphide 56 d) Methyl-perfluoromethyl disulphide 56 e) Bis-perfluoromethyl sulphide 58 f) Bis-perfluoromethyl disulphide 58 g) Thiophene 59 (6) Conclusions 61 V LIST OF TABLES PAGE I. The Ionization Potentials of Some Sulphur Compounds 23 II. The Ionization Potentials of Some Oxygen Compounds 24 III. Mass Spectrum of Dimethyl Sulphide 27 IV. Mass Spectrum of Dimethyl Disulphide 28 V. Mass Spectrum of Methyl-Perfluoromethyl Sulphide 29 VI. Mass Spectra of Methyl-Perfluoromethyl Disulphide and Bis-Perfluoromethyl Disulphide 30 VII. Mass Spectrum of Thiophene 31 VIII. Ionization Potential Differences of Rare Gases 33 IX. Appearance Potentials of the Principal Ions of Dimethyl Sulphide 34 I X. Appearance Potentials of the Principal Ions of Dimethyl Disulphide 35 XI. Appearance Potentials of the Principal Ions of Methyl-perfluoromethyl Sulphide 36 XII. Appearance Potentials of the Principal Ions of Methyl-perfluoromethyl Disulphide 37 XIII. Appearance Potentials of the Principal Ions of Bis-perfluoromethyl Sulphide 38 XIV. Appearance Potentials of the Principal Ion of Bis- | perfluoromethyl Disulphide 39 XV. Appearance Potentials of the Principal Ion of Thiophene 40 • • • • ' • - ] i XVT. "Heats of, Formation of Some Useful Species I 43 XVIT. Electron Impact Induced Reactions of the Sulphides 46 XVIIT. Heats of Formation of Sulphides and Their Radicals 47 XIX. C-S and S-S Bond Energies SO XX. Heats of Formation of Positive Ions 63 v i LIST OF FIGURES PAGE 1. The Franck Condon Principle 7 2. A Typical Ionization Efficiency Curve 10 3. Warren Extrapolated Differences 10 4. Schemetic Diagram of an Electron Bombardment Ion Source 15 5. "Ihe MS 9 Mass Spectrometer 16 6. C ircuit for Mass Measurement 19 7. The Gas Inlet System 20 8. Ionization Efficiency Curves of the Rare Gases 32a - 1-CHAPTER 1 SULPHUR BONDING IN ALKYL SULPHIDES The e a r l i e s t c l a s s i c a l thermochemical studies of simple alkane t h i o l s and sulphides date back to 1886 when J . Thomsen f i r s t did his experiments (1). Since then many people have entered the f i e l d as organic sulphur compounds have become more important i n petroleum technology and i n rubber and protein chemistry. To supplement thermo-chemical data on these compounds, other workers began to study them using electron impact techniques. In 1952, Franklin and Lumpkin (2) determined the C-S and S-S bond energies i n dimethyl sulphide and dimethyl disulphide respectively, using a Westinghouse type LV mass spectrometer. They found the two bonds to have equal strength (73.2 kcal/mole). Later i n a study of the heats of combustion and vaporization of the same two compounds, Mackle and Mayrich (3) discovered that the S-S bond was 5 kcal/mole weaker than the C-S bond. They suggested that the electron impact processes be re-studied. Palmer and Lossing (4) and l a t e r Gowenlock, Kay and Majer (5) supported this f i n d i n g with t h e i r electron impact data.'' Other values for these bond strengths have also been reported and the highest value obtained i s 77.3 kcal/mole f o r both (6)'. In t h i s work, a series of s i x related compounds was studied. The substitution of a more electronegative group for the methyl group shows a d e f i n i t e tendency i n the respective bond dissociation, energies. - 2 -That the important factor in determining the bond energy of a s l ight ly polar bond is the ionization energy is shown by Peters (7) in his theoretical paper on Localized Molecular Orbitals. He showed that the ionization energy of a lone pair s atomic orbita l is v i r tua l ly independent of the molecular environment of the atom. However, this is not true of the p atomic orbital lone pair electrons. With the inclusion of a few percent of p atomic orbital in the s lone pair, the electrons w i l l be concentrated outside the binding regions. The ionization energy of the s lone pair electron w i l l therefore be decreased. He also concluded that while the ionization energy of the lone pairs is modified by the changes which occur in the coupling of the spin and orbital angular momentum, i t is changed only s l ight ly by the polarity of adjacent bonds. Bent (8) in a study of the use of sulphur s-atomic orbitals in bonding found that the greater the electron a f f in i ty of the group attached to sulphur, the more nearly w i l l the bond angle approach that predicted for p£ geometry. This is presumably because electrons w i l l be on the average farther from sulphur and therefore less l ike ly to u t i l i z e the lower energy s orb i ta l . Hence the non-bonding pair wi l l have more s character and a higher ionization energy. The bond w i l l become weaker because of the positive and negative lobes of the p orbitals tend to cancel out part of the overlap. - 3 -CHAPTER 2 THE IONIZATION AND DISSOCIATION OF MOLECULES BY ELECTRON IMPACT (1) Methods of determining di s s o c i a t i o n energies . Two most generally used methods involve spectroscopic (9) and thermal measurements. Others are concerned with appearance potential (10, 11), chemical kinetics . (12), and shock and detonation (13) experiments. Spectroscopic methods are by far the most accurate. Measurements are taken on band convergences, predissociation l i m i t s , extrapolations to band convergence l i m i t s , long-wavelength l i m i t s of absorption continua and photodissociations (14). An absorption continuum i n the v i s i b l e or near u l t r a v i o l e t can, according to Gaydon (15), nearly always be assigned to a diss o c i a t i o n process. Absorption due to ionization i s rarely seen because the corresponding continuum l i e s well down i n the u l t r a - v i o l e t . Unfortunately, this method has limited application because of the complexity of the.spectra of most polyatomic . molecules. Besides, ambiguity can also arise as to the i d e n t i f i c a t i o n of the electronic states of the products. Determination of the heat of reaction of the gas phase process A + B = AB, gives d i r e c t l y the diss o c i a t i o n energy i n thermal units.. However this method involves heat and temperature measurements, - k -which at times can be quite d i f f i c u l t to make. The error introduced i s generally large. In actual fact the only method other than k i n e t i c (14) for obtaining d i s s o c i a t i o n energies i n polyatomic molecules, requires appearance potential determinations. The electron impact technique i s employed i n the present work and w i l l be discussed i n more d e t a i l below. (2) Ionization by electron impact When an electron c o l l i d e s with an atom or molecule, i t can lose i t s energy i n one of two ways. In an e l a s t i c c o l l i s i o n the electron loses part of i t s energy to the targe.t p a r t i c l e i n such a way that the tran s l a t i o n a l energies of the two body system i s conserved. The second kind of c o l l i s i o n i s i n e l a s t i c . Internal energy changes take place within the molecule or atom. I f the bombarding electron has high enough energy, an i n e l a s t i c c o l l i s i o n may lead to io n i z a t i o n and/or diss o c i a t i o n of the molecule. The i o n i z a t i o n potential of an atom or molecule i s the o r e t i c a l l y defined as the energy required to completely remove an electron from the neutral species i n i t s ground state. XY + e" --.-» XY + + 2e~ 2.V In practice, however, the io n i z a t i o n potentials measured do not necessarily correspond to this d e f i n i t i o n because of the p o s s i b i l i t y that the product(s) of the ioni z a t i o n process may be excited. We 5" -therefore define the ionization potential in electron impact studies, as the minimum energy of the bombarding electrons at which the for-mation of 'parent' ions can be detected. No attempt will be made to.discuss the theory of electron-molecule collisions since i t covers a whole field of its own (16). It suffices to say, here, that when the energy of the electron is less than the ionization potential of the system studied, ionization transitions cannot take place and the ionization cross section is zero. As the energy of the electron is increased above the critical voltage, the cross section for the transition between two given levels increases. Theoretical treatment of the ionization in the vicinity of the threshold is difficult and in most cases i t remains a problem to be solved. (3) The Franck-Condon Principle (17,18) and the dissociation of molecules Qualitatively, this principle may be stated as follows: no changes in the positions and velocities of the nuclei of a molecule undergoing an electronic transition occur during the course of the transition. The system has the same nuclear configuration immediately after the transition as i t had immediately beforehand. More simply, the point on the potential energy surface of the molecule representing the configuration before the transition lies directly below the point on the potential energy surface of the molecule-ion representing the configuration after the transition. The energy accompanying such a transition is called the vertical ionization potential. - 6 -After the ioni z a t i o n has occurred a new electronic state i s formed and the forces acting on the nuclei are dif f e r e n t . Consequently, the nuclear configuration and nuclear motions w i l l change. The equilibrium internuclear distance may. not be the same as before (figure 1). This accounts for the fact that the ion may not necessarily be formed i n the ground state. In the case of transitions to a repulsive upper state or to an att r a c t i v e state above i t s dissociation asymptote, dissociation occurs Hence for the diatomic molecule undergoing such a process, as depicted i n figure l c : XY + e" * X + + Y + 2e" 1.1 the appearance potential of the ion X + i s given by the equation AP(X +) = D(X— Y) + I(X) + K.E. + E.E. 2. 3 where I(X) = ionization potential of the atom X K.E. = t o t a l k i n e t i c energy with which X + and Y part E.E. = any excitation energy with which the ion or the neutral atom may be endowed In the case of polyatomic molecules, potential curves are replaced by multidimensional potential energy surfaces. Dissociation occurs f i r s t at the weakest bond. Equations analogous to[2.2) and(23) can be written for such a process: ABCD + e >A+ + BCD + 2e~ 2 .4 AP(A +) = D (A - BCD ) + 1(a) + K.E. + E.E. 2..5* The appearance potentials and dissociation energies so measured are only upper l i m i t s . FIGURE I THE FRANCK CONDON.PRINCIPLE - 8 -(4) Secondary processes in a mass spectrometer A variety of reactions other than those mentioned also take place when molecules are bombarded by a stream of energetic electrons. J . J . Thomson (19) f i r s t noted the various effects of secondary processes - the formation of new ionic species corresponding to no known molecular species that appeared as sharp l ines, diffuse l ines, bands, and general background on the screen of his original parabola mass spectroscope. However, these secondary processes usually only occur at high sample pressures. Molecules may be electronical ly and vibrational ly excited without ionization or dissociation occurring. On the other hand, at high' energy multiple ionization is possible. Negative ions can also be formed from electron capture or ion pair processes. In detecting the excited product, one isolates i t from the rest of the gas according to i t s mass to.charge rat io. Many complications can then be s implif ied. Dissociative rearrangement of a molecule may give rise to peaks in the mass spectra which cannot be accounted for by direct fragmentation. The- extent of any rearrangement depends on the kinds of free neutral radicals produced in a scission of bonds and the react iv i t ies of them. A Skeletal rearrangement reactions in sulphides and disulphides are described by J . 0. Madsen, C. Nolde, S. 0. Lawesson and G. Schroll (20); • - 9 -(5) The ionization eff iciency curve and i ts interpretation An ionization efficiency curve is a graphical re-presentation of the variation of the ion current intensity as a function of the bombarding electron energy. A typical curve based on the graphical method of Hagstrum and Tate (21) for calculating the probability of a transition can be found in the l i terature (22). The curve can be.effectively divided into two parts (figure 2), a straight portion (be) and a curved part (ab) near the threshold due mainly to thermal spread of the energy of the bombarding electrons (23). Ideally, that point on the energy scale at which ions are f i r s t formed represents the ionization or appearance potential of the ion. Owing to contact, potentials (24) however, the recorded potential does not necessarily correspond to the.energy of the electrons. A means of calibrating the energy scale is essential. This can be accomplished by introducing an inert gas, whose ionization potential has been determined spectroscopically, into the system. A comparison of the onset potentials of the ion under investigation and the inert gas ion givesthe ionization potential of the.former. Different workers have different views on how this Comparison should be made. Nicholson (25) gives an account of eight methods •available. A l l of these are subjective and there is no real ly re l iab le universal method. The appearance and ionization potentials throughout this work were calculated on the basis of extrapolated voltage differences (26). - 10 -Electron Energy FIGURE 2. IONIZATION EFFICIENCY CURVE Unknown Electron Energy IGURE 3. WARREN'S EXTRAPOLATED DIFFERENCES -11.-The ion i z a t i o n efficiency curves for the ion studied and an ion to calibrate the electron voltage scale are determined with both gases present i n the mass spectrometer. The ordinate scales are chosen1 so as to make the straight portions of the curves p a r a l l e l (figure 3) The differences of the ion voltage (AV) corresponding to various values of the ion current I are measured, and a graph of AV against I i s drawn and extrapolated to zero ion current. The value of AV at zero ion current i s taken as the difference between the appearance potentials. This method l i k e any other i s a r b i t r a r y , and' Warren (26) points out that the extrapolation of any curve other than a straight l i n e i s open to objection. Moreover, the method i s not suitable for measuring the appearance potentials of ions formed with a very.low i n t e n s i t y . Since most fragment ions have long " t a i l s on t h e i r ionization e f f i c i e n c y curves, the accuracy of the appearance potentials measured for such i s , of course, debatable. For parent ions the method i s generally recognized to give I.P.'s accurate to about 0.1 eV. (6) Heats of formation and dissociation energies Having determined the appearance potentials of the. main fragments of a compound, one can calculate various bond dissociation energies (2). I f we assume that the energy of the two electrons remaining after the following process i s zero AB + e~ > A + + B + 2e" 2. 6 we can calculate the heat of formation of B, AHf(B) - 12 -AHf° (AB) = AHf (A +) + AHf (B) - AP (A+/AB) 2-7 Si m i l a r l y , we can fi n d the heats of formation of the neutral species i n the following reaction, knowing AH£ (AC) AC + e > A + + C + 2e 2. 8 AHf (AC) = AH f (A +) .+ AHf(C) - AP(A+/AC) 2.^ Now, i f we have a compound CD, we may write CD ->C + D 2. 10 AH£ (CD) = AHf (C) + AH £ (D) - D (C-D> 2.11 where D(C-D) = dissociation energy or bond strength. - 13- -CHAPTER 3 EXPERIMENTAL METHOD (1) Theory of mass spectrometry Excellent reviews (27, 28, 29) have been written on this subject. I t i s the author's intention to present only the "mass spectrometry equation" here. Positive ions produced i n the io n i z a t i o n chamber are forced by a p o s i t i v e l y charged r e p e l l e r electrode (figure 4) into the ion gun. Here they are accelerated through a def i n i t e potential difference X. The f i n a l s l i t of the ion gun selects a narrow beam of ions which then pass through the magnetic analyser where, they are de-fle c t e d . The force exerted on the ion by the magnetic f i e l d i s equal to the centripetal force due to i t s motion, that i s Hev =• mv2/R 3- 1 where H = magnetic f i e l d strength v •= v e l o c i t y of the ion m = mass of the ion e = charge of the ion •' • R = radius of the ion trajectory. Assuming that the ions are produced with zero i n i t i a l k i n e t i c energy, the k i n e t i c energy of the ions after acceleration i s given by 1/2 m v 2 = X e 3-1 - Ik -Combining (3,1). and(3,2), we get and R = H"1' (2 m X/ e ) 1 / 2 3-3 m / e = H 2 R 2 / 2 X 3. 4 I t i s evident from equation (3) that the ion trajectory i s determined by i t s mass to charge r a t i o f o r a given magnetic and e l e c t r i c :fields. I f either H or X i s varied, ions of different mass to charge r a t i o may be collected and.identified as such. (2) The instrument The instrument used f o r this work i s a AEI model MS9 mass spectrometer. One advantage the MS9 has over other.spectrometers l i e s i n the fact that i t has both an e l e c t r o s t a t i c and magnetic analyser. This double focussing device provides a high resolving power with narrow s l i t widths. Only a few special features of the MS9 w i l l be mentioned here, as a detailed description of the machine can always be obtained from the instrument manual. Figure (5) shows the main parts of the instrument (30). As we follow the path of the pos i t i v e ions out of the ion source we encounter a source i s o l a t i n g valve. This consists of a b a l l valve which can be used to close off the source from the analyser at a point immediately beneath the object s l i t . This enables the source 10 ^ 6 7 8 1. Filament 2. Draw out p l a t e 3. G r i d 4. R e p e l l e r electrode 5. G r i d 6. 7. 8. 9. 10. Trap g r i d s with a p p l i e d a c c e l e r a t i n g p o t e n t i a l I o n i z a t i o n chamber region FIGURE 4. SCHEMATIC DIAGRAM OF AN ELECTRON BOMBARDMENT ION SOURCE - 16 -1. Source Isolating Valve 9. Pump 2. Solid Sample Probe 10. Cold Trap 3. Adjustable S l i t 11. To Mult ip l ier and Recorder 4. Ion Source 12. Auxil iary Collector 5. Electrostatic Analyser 13. . Adjustable S l i t 6. Monitor Collector 14. Magnetic Analyser 7. Gas Inlet .15. Electron Mult ip l ier 8. Cold Trap FIGURE 5. MS9 MASS SPECTROMETER - 17 -to be removed without admitting a i r to the analyser regions. Much time is saved in this way whenever the source has to be removed for cleaning or replacement of the filament. Otherwise, the whole system would have to be evacuated and baked out after each operation. It takes f ive to six hours to bake out the analysers and about the same time again for them to cool down to room temperature. A monitor col lector situated betwe-en the analysers inter-cepts a fraction of the total beam passing through the system. The current is amplified and displayed on a panel meter. This provides a more accurate indication of the total amount of sample admitted into the source than the ionization gauge (which is connected just above the diffusion pump). Thus the monitor serves a twofold.purpose as i t also helps in the setting up of the instrument for maximum resolving power. ' Since by this point in the ion trajectory no mass'separation has taken place, the monitor current provides a measure of the total ion beam intensity and indicates the relat ive amount of sample actually present. At the exit end of the magnetic analyser are two col lectors. The principal col lector has incorporated in i t a high sensit iv i ty electron mult ipl ier, and is used for high resolution work with very narrow s l i t s or fast scanning with a wide collector s l i t . The.electron 4 mult ipl ier steps up the signal by a factor of 10 . The auxil iary col lector, intended for routine work at low resolution has a similar design to that in a standard system. One can svitch from the principal - 18. -to the a u x i l i a r y c o l l e c t o r or vice versa just by turning the HR and LR knob on the panel. And when the HR and LR knob i s at LR, the ;spectrum can be scanned at fast and slow speed and displayed on the oscilloscope incorporated on the panel. This provides the f a c i l i t y for measurement. of the mass to charge r a t i o of an "unknown ion". The relationship between the unknown mass to charge r a t i o and that of a known reference i s determined accurately by comparison of the ion accelerating voltage necessary to bring the ion beams on to the co l l e c t o r s l i t at a constant magnetic f i e l d (figure 6). Provided that the masses of the two ions do not differ.more than 10%., the s t a b i l i t y of the c i r c u i t s and the resolving power of the instrument i s s u f f i c i e n t for the measure-6 ment of mass r a t i o s with a precision of a few parts'in 10 . Solids are inserted d i r e c t l y into the io n i z a t i o n chamber through the s o l i d sample probe (figure 5). There are two other i n l e t systems (figure 7). The instrument i s as yet not equipped with a device f o r producing monoenergetic electrons. (3) Experimental Before any sample i s introduced, the whole system i s switched on and the electronics allowed to warm up. With the accelerating voltage at 8 keV, and the trap current regulating at 100 uA, - 19 -Ion v o l t power u n i t Analyser voltage Saw-tooth time base A m p l i f i e r O s c i l l o s c o p e FIGURE 6. CIRCUIT FOR MASS MEASUREMENT •- 20 -1. Source ' 4. Metal valves 2 . Ion chamber 5 . Sample bulbs 3 . M e t r o s i l leaks 6 . Doser Manometer Cold System 6 ' °" Hot valve system ( 3 5 0 ° C ) FIGURE 7. THE GAS INLET SYSTEM - 21 -the instrument is ..focussed on nitrogen at m/e = 28. The electron energy is set at 70 eV. The instrument is set up for maximum resolving power at the desired s l i t widths. This is usually at a resolving power about 3000. A background spectrum is always taken before the sample was introduced. The compounds used had been prepared by Dr. Cullen in his laboratory in this department. They were introduced into the mass spectrometer without further pur i f icat ion. The dimethyl compounds were liquids with high vapour pressures so that enough sample can be introduced through the cold in let system. The other compounds were introduced as gases except bis-perfluoromethyl sulphide. The working pressure ranged from 2.0 x 10 ^ to--4.0 x 10 ^ Torr. This is the total pressure of the sample and the reference gas krypton. The monitor indicated a range of 1.5 to 5.0 on Range 100. Several spectra of each".'.sample were taken to give an accurate picture of i ts cracking pattern. The amount of reference gas introduced was adjusted to give a peak of similar intensity to the main fragment peaks of the sample. When the mass to charge ratio of a fragment peak was not clear, this was measured against the krypton 84 or nitrogen 28 peaks. Appearance potentials of individual fragments were measured at a trap current of 20 yA. As fluorine compounds attack,the tungsten filament, a rhenium filament with lower work function was used for the bis-fluoromethyl compounds. - 22 -CHAPTER 4 , RESULTS. AND DISCUSSION (1) Ionization potentials of the sulphides The f i r s t ionization potential of a sulphide compound probably corresponds to the removal of an electron from one of the lone pairs on sulphur. Theoretically i f there is no hybridisation of atomic orbitals when, bonds are formed, these sulphide compounds should have the same ionization energy as sulphur atom i t s e l f . This is true.in the case of hydrogen sulphide whose ionization potential is only 0.11 eV higher than that of S (Table 1). Replacement of hydrogen.atoms by methyl groups lowers the ionization potential . and substitution by perfluoromethyl groups increases i t . The f i r s t table shows the effect of electron donating and electron withdrawing groups on the ionization potentials of the sulphides and disulphides as found in this work. The two substituting groups CH^ and CF^ have opposite effects on the ionization potential of the sulphur atom. The f i r s t is an electron donating group and the latter an electron with-drawing group. In the case of the dimethyl compound, there is charge flow towards the sulphur atom and hence the lone pair electrons are subjected to more electronic repulsion than in a normal isolated atom. This is reflected in a decrease of ionization potential. In - 23 -TABLE 1 THE IONIZATION POTENTIALS OF SOME SULPHUR COMPOUNDS COMPOUND I. P.(eV) REFERENCE S 10.357 31 H2S 10.47 31 CH3SCH3 8.71 This work CH3SCF3 9.75 This work CF3SCF3 11.28 This work S2 10.8 31 CH3SSCH3 8.85 This work CH3SSCF3 9.58 This work CF3SSCF3 10.68 This work Thiophene 9.06 This work bis-perfluoromethyl compounds the opposite effect is true. The SCH3 group like the OCH3 is slightly electron withdrawing compared to the CH3 group. The ionization potential of.dimethyl disulphide is found to be slightly higher than the corresponding monosulphide. This depends on the relative intensities of the effects of lone-pair—lone-pair repulsions on adjacent sulphur atoms and of the electron affinity of the SCH3 group. The normal concept that in a smaller molecule the electrons are more tightly bound does not represent the whole picture. The ionization potential of thiophene is higher than that of dimethyl sulphide because the CH r a d i c a l s , , to which the sulphur atom is - 2k -bonded, are less electron donating compared to methyl group. The deviation of the ionization potentials of the sulphide compounds from that of the sulphur atom is not as great as in the oxygen series. The ionization potentials of this latter series are shown below: TABLE 11 THE IONIZATION POTENTIALS OF SOME OXYGEN COMPOUNDS COMPOUND I. P. • REFERENCE 0 .13.61 31 H20 •'. 12.61 31 CH3OCH3 10.5 31 0 2 12.2 31 H 2 0 2 12.1 31 Sulphur, being an element in the third row of the periodic table has vacant d orbitals available for back donation of electrons. Hence, the lone pair electrons in the sulphide will.experience a smaller repulsion than is the case with an oxide. The bond distance between fluorine and carbon is less than that of the C - C distance (32).. - 25. -BOND BOND DISTANCE C H 1.09 A C F 1.36 A C Cl 1.76 A C C 1.54 A The replacement of fluorine for hydrogen in hydrocarbon structures does not distort or strain normal carbon-to-carbon bonds. This is also true in the case of thio-alkanes. Electron di f fract ion studies (33) showed that the S - S bond distance in the perfluoroalkyl and alkyl disulphide compounds are v i r tua l ly ident ica l . A comparison of the bond strengths in the two groups of compounds, therefore, shows the true bond nature of the sulphur atom in the presence of electron donating and electron withdrawing groups. (2) Mass spectra and fragmentation A mass spectrum is extremely useful for the elucidation . of structures and the fragmentation behaviourof the compounds studied here. Isotope label l ing is a general method for determining fragmentation processes. This involves introducing a heavier isotope of atoms into the molecule studied, e.g. CF^SCD^ (5). Identification of the fragment ions w i l l indicate the bond(s) broken and the - 26 -rearrangement processes present. Another substantial recognition of the mechanisms results from the i d e n t i f i c a t i o n - of metastable ions. These are formed by spontaneous decomposition of the ions i n a region between the io n i z a t i o n chamber and the analysers. Metastable ions do not necessarily appear at int e g r a l mass uni t s . Derivations•for the parent mass and the apparent mass of the metastable can easily be found i n l i t e r a t u r e (51). m2 m = 4 1 o * where m = apparent mass of metastable ion m0. = mass of parent ion formed i n the io n i z a t i o n chamber m = mass of the fragment ion after decomposition of the parent.. Other unsubstantiated mechanistic paths can be arrived at by measuring the appearance potentials of the fragment peaks and deriving processes which are energetically plausible. Djerassi, Budzikiewicz and Williams ( 3 4 ) believed that "a plausible path i s preferable to the proverbial wiggly l i n e which lacks any rat i o n a l e . " In his recent paper, C. E. Brion ( 3 5 ) indicated some of the possible thermal hazards associated with the use of electron impact for the study of fragmentation. To ensure that conditions were the same on different days, the source was allowed to at t a i n a certain equilibrium temperature before use. I t was found that the measured appearance potentials of fragments were higher than the corresponding values obtained i n free r a d i c a l studies. This showed - 27 -that in most cases there was a bond rupture during the impact by electrons. Tables III to VII show the spectra of the sulphides, disulphides and thiophene obtained in this work. The ion accelerating potential was 8-keV and the electron trap current 100 uA using 70 volt electrons. The results are the average of four spectra taken for each compound. Peaks ar is ing from different isotopes of sulphur and carbon are labelled ( i ) . TABLE III MASS SPECTRUM OF DIMETHYL SULPHIDE % ABUNDANCE m/e ION THIS WORK . LITE: 15 CH3. 5.8 6.5 27 , C 2 H 3 17.0 24.3 34 ,SH2 28.6 35 SH 3 39.4 45 CHS 25.3 50.4 46 ' C H 2 S 32.2 • 43.1 47 CH3S 75.5 108.3 48 CH3SH 5.4 49 CH 3SH 2 5.5 61 CH3SCH2 33.5 36.3 62 CH 3SCH 3 100.0 100.0 63 6.6 64 (i) 7.5 TABLE IV MASS SPECTRUM OF DIMETHYL DISULPHIDE ION THIS CH 3 4.6 CS CHS 34.6 CH2S 24.7 CH3S 20 v 4 CH3SH . 11.7 CHjSH-, 4.5 CH SCR, 12.6 CH3SCH3 4.7 S 2 8.6 c s 2 7.7 CH 2S 2 3.2 CH3SS 49.2 CH3SSH (3.2) CH3SSH2 (5.3) CH3SSCH2 CH3SSCH3 100.0 (i) . 5.8 ( i ) 10.1 % ABUNDANCE LITERATURE (36) . 1 4 .3.5 58.6 39.5 • 27.4 14.8 . 5.3 16.7 .9.6 3.1 54.3 2.6 4.9 1.9 100 " 5 . 6 9.3 - 29 -TABLE V MASS SPECTRUM OF METHYL-PERFLUOROMETHYL SULPHIDE m/e ION % ABUNDANCE 15 CH 3 12.3 . 44 CS 4.1 45 ' CHS 29.6 46 CH2S 16.1 47 CH3S 64.5 48 CH3SH 2.3 49 CH3SH2 3.4 63 CSF 11.8 66 CH3SF . 2.4 69 CF 3 51.6 76 ' CS 2 . 4.6 82 SCF4 ' 5.1 97 CH3SCF2 7.5 115 C H 2 S C F 3 7 - 9 116 C H 3 S C F 3 1 0 0 - ° 117 [i}. 4.5 118 " C i j 4.9 - 30 -TABLE VI MASS SPECTRA OF METHYL-PERFLUOROMETHYL DISULPHIDE AND BIS-PERFLUOROMETHYL DISULPHIDE m/e ION % ABUNDANCE . METHYL-PERFLUORO BIS-PERFL 15 C H 3 4.3 31 CF 2.5 32 S • 3.4 44 CS 3.3 45 CHS 21.1 46 CH2S 12.1 47 CH3S 10.5 63 CFS 3.5 4.8 64 S 2 10.2 14.8 69 C F 3 22.6 100.0 76 c s 2 21.0 78 CH2SS 8.0 79 CH SS 63.7 80 CH3SSH 2.8 81 CH3SSH2 6.5 82 CF 2S 8.0. (12.1) 94 CH3SSCH3 5.3 .95 CFSS 4.8 101 CF3S 4.8 114 CF2SS 20.6 129 ' CH3SSCF2 3.4 133 CF3SS 5.3 ::22.5 .148 CHjSSCFj 100.0 149 ( i ) 5.1 150 (i) 10.1 183 CF 2SSCF 3 10.7 202 CF 3SSCF 3 84.4 203 (i) 4.0 204' (i) 9.6 TABLE VII MASS SPECTRUM OF THIOPHENE m/e ION ' % ABUNDANCE 39 C 3 H 3 14.2 45 CHS+ 25.5 58 C 2 H 2 S + 41.6 84 C 4 H 4 S + 100.0 Since bis-perfluoromethyl sulphide was obtained by Dr. Cullen from the irradiat ion of the disulphide by ultra violet l ight, i t is not surprising to f ind a relat ive high percentage of the disulphide in the sample. An accurate mass spectrum due to fragmen-tation of the monosulphide i t s e l f cannot be determined. But from the spectra obtained, we found that the principal peak corresponds to the trifluoromethyl ion. The percentage abundances were calculated with the largest peak in the spectra as reference. In the straight chain compounds, peaks with percentage abundances less than 3 are not recorded. In the case of thiophene, only the 4 major peaks of abundance greater than 10% are stated. The parent molecule ion gave the largest peak in a l l the - 32 -spectra except those of the bis-perfluoromethyl compounds. The particular stability of the trifluoromethyl group accounts for its i prominence in the'crack/ing pattern of compounds containing such a group. A different effect than that discussed in the last paragraph in the first chapter is encountered when CF^ is attached to sulphur. There is a reduction of the conjugation between the non-bonding electrons on the sulphur atoms (37). The relatively high percentage abundance of the S 2 peak in the spectrum of bis-perfluoromethyl disulphide as compared to dimethyl disulphide can be attributed to the stability of the CF^ fragment. Energetically, i t is also found that the appearance potential of S* is lower in the bis-perfluoromethyl compound. (3) Ionization and appearance potential measurements Contact potentials and the nature of the gas in the ionization chamberwere found to modify the nominal electron energy (38). To verify the linearity of the energy scale the ionization potentials of each species in a mixture of rare gases is measured.. The ionization potentials are spectroscopically known. Figure 8 shows the ionization efficiency curves of xenon, krypton, argon and helium. The differences between any two ionization potentials agree within experimental error with the differences of accepted values. This provides a check for the energy scale between 12eV and 25eV. The procedure was carried out from time to time during the course of this research. FIGURE 8 IONIZATION EFFICIENCY CURVES OF THE RARE GASES 10 11 12 13 . 14 15 23 E l e c t r o n Energy (eV) - 33 -TABLE VIII IONIZATION POTENTIAL DIFFERENCES (in eV) OF RARE GASES PAIR OF GASES THIS WORK FROM LITERATURE (39) Kr-Xe 1.89 1.88 . Ar-Kr 1.78 , 1.75 He-Ar 8.66 8.82 He-Xe 12.40 12.45 Krytpon was used as the calibrating gas except when ambiguity arose around fragment peaks with mass to charge rat io equalto 84 or 42. This occurred in the case of thiophene, and so xenon was used as the cal ibrating gas. Tables IX to XIV give the ionization and appearance potentials of. major fragments together with probable processes. In the f i r s t column is recorded the mass to charge rat io. Since we are only concerned with singly charged ions, this represents the mass number of the ion. In column two the process responsible for the ion is predicted. The number of determinations is also included to give an indication of the r e l i a b i l i t y and reproduc ib i l i t y of the values quoted. In column four, the cal ibrating gas is quoted for completeness. Column f ive gives the experimental values with standard deviations obtained in this work together with previous results (when available) from the l i terature. TABLE IX APPEARANCE POTENTIALS OF THE PRINCIPAL IONS. OF DIMETHYL SULPHIDE m/e PREDICTED PROCESS . No. OF DETERMINATIONS CAL. GAS A. P. THIS WORK REF C5) 62 CH3SCH3 } CH 3SCH 3 + 10 Kr 8.76± 0.18 9.0 61 •--> CH3SCH2 10 11.95* 0.16 47 —> CH S + CH 3 10 11.09* 0.15 11.3 46 -} CH2S + CH3'.+ H 10 10.97* 0.13 •F-45 CHS++ CH 3 + H 2 14.16± 0.08 35 -> SH3++ (C 2H 3) 13.85 34 •> SH2T+ (C 2H 4) 14.29* 0.04 27 •» C 2 H 3 + (SH3) 15.02* 0.10 14.7 15 CH 3 + SCH' 10. 46(rough) 13.0 -vl4 (first break) TABLE X APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF DIMETHYL DISULPHIDE m/e PREDICTED PROCESS No. OF DET. CAL. GAS A. P. THIS WORK REF. C56 3 p + (84+) CH SSCH 69 64 62 61 49 48 47 46 45 15 > CH SSCH -> CHjSS + CH "> S 2 + 2 C H 3 CH3SCH + S -> CH5S + CS + H — *j CH.S + CS + H„ 4 2 CH 3S + + CH3S CH2S + CH 4 + S -> CHS + CH.S + H •> CH + SSCH 10 10 10 5 10 5 5 10 Kr Kr Kr 10 10 8.85*0.20 11.73*0.24 15.01*0.13 8.96*0.06 10.66±0.05 11.44±0.15 9.72*0.09 11.23*0.11 10.61*0.11 13.43*0.09 10.02(rough) 9.1*0.2 12.1*0.2 15.4*0.3 10. 9*0.-2. V^J 11.9*0.2 11.5*0.2 13.0*0.4 12.2*0.2 15,5*0.3 15.7*0.3 12.9*0.4 (first break) TABLE XI APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF METHYL-PERFLUOROMETHYL SULPHIDE PREDICTED PROCESS No. OF DET. CAL. GAS A. P. THIS WORK CH 3SCF 3 • CH 3SCF 2 + F CF 3 + SCH3 CSF + CH + F 2 Kr 9.75*0.11 15.7 ±0.24 13.07*0.09 15.32*0.13 CH3S •+ CF 3 11.78*0.03 CH2S + CF 3 + H ^14 CHS + CF 3 + H 2 14.88*0.06 CH 3 + . SCF 3 14.24*0.13 TABLE XII APPEARANCE POTENTIALS OF THE PRINCIPAL IONS  OF METHYL PERFLUOROMETHYL DISULPHIDE No. OF m/e PREDICTED PROCESS PET. 148 CH3SSCF3 — CH 3SSCF 3 + 6 . . ' 133 — C F 3 S S + + CH 3 6 129 CF 3SSCF 2 +.+ F . 5 94 CH3SSCH3 CH 3SSCH 3 + 3 79 CH3SSCF3 -) CH3SS+ + CF 3 6 69 -> C F 3 + + CH SS S 47 CH 3S + + S• + CF 3 • . 6 46 -> CH 2S + + SH + CF 3 6 45 CHS++ CSF + H 2 6 15 > CH 3 + + CF 3 + SS 2 CAL. GAS K r A. P. THIS WORK 9 . 5 8 * 0 . 1 4 14.77*0.12 ~15 9 . 2 7 * 0 . 0 8 11 .29*0.20 1 2 . 5 0 * 0 . 0 8 12.84*0.11 13 .43*0.24 1 4 . 8 3 * 0 . 0 8 10 'v 14 ( f i r s t break) TABLE XIII APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF BIS-PERFLUOROMETHYL SULPHIDE m/e PREDICTED PROCESS No. OF DET. CAL. GAS A. P. THIS WORK 170 CF 3SCF 3 > C F 3 S C F 3 Xe 11.28*0.04 151 -> C F 3 S C F 2 + F Xe M5 CO 69 -> CF 3 + SCF 3 Xe 12.54*0.06 m/e TABLE XIV APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF BIS-PERFLUOROMETHYL DISULPHIDE PREDICTED PROCESS No. of DET. CAL. GAS A. P. THIS WORK 202 CF 3SSCF 3 183 133 114 101 95 82 69 64 550 32 • 31 — ± CF 3SSCF 3 .—^ CF 3SSCF 2 + F - — > CF3SS +.CF CF2SS + CF 4 CF 3S + SCF 3 CFSS + CF 3 + F 2 > CF 2S + CF 3S + F CF 3 + SSCF3 — ~> S2 + + C F 3 C F 2 + + SSCF'3 + F > S + + SCF 3 + CF 3 CF + '+ SSCF3 > F 2 8 5 6 5 6 5 5 10 6 4 1 1 Kr 10.68*0.19 14.64*0.07 13.31*0.05 11.28*0.06 : 14.43*0.08 15.60*0.10 10.93*0.17 12.07*0.13 13.22*0,17 13.63*0.05 ^13 ^16 TABLE XV APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF THIOPHENE No. OF , CAL. A. P. PREDICTED PROCESS DET. GAS THIS WORK } C 4H 4S + 6 Xe 9.06*0.16 > C-H0S+ + C-H ' 5 " 12.36*0.06 '2 2 2 2 > CHS+ + C 3H 3 5 » 13.51*0.08 > C 3 H 3 + + CHS 3 " 13.31*0.10 - kl -In the determination of the above appearance potentials, the kinetic energy and excitation energy terms are neglected since suitable relevant data is seldom available. The appearance potentials therefore give an upper limit for the energy required for ion formation. Other factors such as those introduced in the interpretation of the efficiency curves for the ion under study and the calibrating gas, and the energy distribution of the electron beam affect the accuracy of the measured appearance potentials. However electron impact results for fragment ions are usually reliable to about 0.1 or 0.2 eV (31). The appearance potentials of CH^S* ions from dimethyl sulphide and disulphide were f i r s t determined by Franklin and Lumpkin They found that in both cases the OH^ S* ion appeared at an electron energy of 11.38 eV and so they arrived at the conclusion that the strength of the C-S bond varied but l i t t l e with the second group attached to sulphur. Later workers found that the strength of the S-S bond in the disulphide was less than the C-S bond in the mono-sulphide compound by approximately 4 kcal/mole (3, 4, 5). In the present work, i t is found that though the appearance potentials of the same ions differed only sli g h t l y (Tables IX and X) and agreed with those cited in the literature, about 11 eV, they showed a different relationship. The appearance potential of CH^S* ion is greater for the disulphide than the monosulphide. From a study of the CH_S radical, Lossing (4) found that the ionization potential -14.2 -of the free radical is 8.06±0.1 -eV. This shows that the S-S bond of the disulphide i s stronger than the S-C bond in the sulphide by 0.14 eV. The difference i s increased to 1.45 eV in the case of the bis-perfluoromethyl compounds (Table XIX). It is interesting to note that in Majer's paper (5), the appearance potential of CH^S* from the mono-sulphide i s greater than that of CH^S* from the disulphide by 0.2 eV, but the appearance potential of an equivalent species C2H^S+ from diethyl sulphide was less than that,from the disulphide by 0.1.eV. It i s unfortunate that an analgous comparison cannot be made for the bis-perfluoromethyl compounds, the reason being the low probability for the formation of SCF^* (Tables V and VI) and the consequent i n a b i l i t y to find i t s appearance potential accurately. + In the determination of the appearance potentials of CH^ ions from dimethyl sulphide, dimethyl disulphide and the methyl perfluoromethyl compounds, i t was found that the f i r s t onset potential was around 10 eV. This corresponds to the ionization potential of the CH^ radical, 10 eV (31). The C-S bond must be therefore broken thermally prior to ionization. A sudden increase of ion intensity around 14 eV indicates that not a l l of the molecules have decomposed, and that CH^ may be produced st this energy as a result of C-S bond scission by electron impact. The break in the ionization efficiency curves was 12.9*0.4 eV in the case of CH^* from dimethyl disulphide and 14.24 eV + for CH_ from methyl-perfluoromethyl sulphide. - l | 3 -(4) Heats of formation arid bond energies Whilst the ionization potential of any molecule indicates how firmly the outermost electron i s bound, the appearance potentials of i t s fragment ions are by themselves of limited value. However, when we combine such information with thermochemical data we can derive the energies of the broken bonds as well as the heats of formation of the ions. These quantities are important in elucidating the chemical properties of the whole molecule. Table XVI gives the heats of formation of some species that are useful in later calculations. TABLE XVI HEATS OF FORMATION OF SOME USEFUL SPECIES (in eV) NEUTRAL (GASEOUS) SPECIES AH REFERENCE 1.41 40 -5.09 41 -0.39 42 CH3SSCH3 -0.25 42 (To be cont'd) TABLE XVI (Cont'd) IONIC SPECIES REFERENCE + 11.36 40 + 5.01 41 A conversion factor of 1 eV = 23.06 kcal/mole i s used to interchange thermal-chemical values and electron volts. The heat of temperature, Lossing (4) had chosen their heats of formation in the gaseous state to be those values derived from the heats of vaporisation (42). From them and appropriate ionic appearance potentials the heats of formation of the other compounds and their primcipal ions can be calculated. The accuracy of these w i l l , of course, depend on the accuracy of the thermal information in Table XVI. formation of CF^+ was derived from the heat of formation of the free radical and i t s ionization potential (41). Since dimethyl sulphide and disulphide are liquids at room From Tables IX and XVI, we obtain AH f(CH 3S +) < AHf (CH3SCH3)- AH f (CH3) + AP (CH3S+/CH3SCH3) < -0.39 - 1.41 + 11.09 < 9.29 eV With this derived heat of formation, we can obtain the heat of formation of SGH from 3 CH3SSCH3 > CH 3S + + SCH3 . 4 . 4 where AH £ (SCHj) ='AHf (CHgSSCH )- AH f (CH 3S +) + AP (CH3S+/CH3SSCH3) 4. 5" <_ -0.25 - 9.29 + 11.23 <1.69 eV Similar calculations were made for the heats of formation of the other compounds. A summary of these calculations is given in Table XVII. The most probable reactions involved are given, and the heat of formation of each reactant and product appear above the particular species involved. These are obtained from Table XVI or derived. The appearance potential of the ion produced in the reaction appears below the ionic symbol. The heat of formation of the ion, radical or molecule derived from the reaction i s given within brackets. A l l numbers are quoted in eV. - Ii-6 -TABLE XVII ELECTRON IMPACT INDUCED REACTIONS OF THE SULPHIDES. (AHf in eV) : o .39 [9.29) . 1.41 CH3SCH3 — > CH 3S + + CH3 11.09 "0.25 9.29 (l.69> CH.SSCH, . - —--> CH,S+ + SCH, 3 3 113 2 3 3 ("7.58) 9.29 "5.09 CH3SCF3 > CH 3S + +.CF - 11.78 "7.58 11.36 (-4.70) ' CH3SCF3 -> CH3 + SCF 3 14.21+ (-12.23) , 5.01 "4.70 CF 3SCF 3 > CF 3 + + SCF 3 12.54 (-8.25) 9 . 2 9 - 4 . 7 0 CH3SSCF3 —.—.-> CH 3S + + SCF 3 12.84 -8 .25 (5 .1 l ) 1.41 CH SSCF >\ CF SS + + CH ("13.29) 5.11 "5.09 CF,SSCF, --•—-> CF„SS + + CF. 3 3 13331 3 "13.29 5.01 (-6.23) CF 3SSCF 3 — --—> CF 3 + + SSCF3 12.07 -0.25 11.36 Cl .3l) CH3SSCH3 > CH 3 + + SSCH3 12.9 - 1 + 7 -With the derived heats of formation of the neutral molecules and radicals, We can compare the different S-C and S-S bond strengths. The values required "for such calculations are summarised in Table XVIII. TABLE XVIII HEATS OF FORMATION OF SULPHIDES AND THEIR RADICALS (in-eV) A H f RADICALS THIS WORK PREVIOUS WORK CH3 1.41 C40J 1.43 C2] C H 3 S : J - 6 9 . , {1.38 C4] CH3SS 1.31 CF 3 -5.09 Of) CF3S -4.70 CFjSS -6.23 (To0be cont'd) — 1+8 — TABLE XVIII (Cont'd) COMPOUNDS CH3SCH3 CH3SSCH3 CH3SCF3 -7.58 CH3SSCH3 -8.25 CF 3SCF 3 -12.23 CF 3SSCF 3 -13.29 From the figures in Table XVIII, i t i s evident that the differences in AH£ between the disulphide and i t s corresponding sulphide increases with the presence of CF 3 groups in the compound. A typical calculation (for the upper limit of the bond dissociation energy of the CF 3 - SSCF3 bond in bis-perfluoromethyl disulphide) i s given below. CF 3SSCF 3 ——-> CF 3 + SSCF3 4.6 AH f (CF3SSCF3) = AH £ (CF3) + AHf (SSCFj) -D (CF 3 - SSCF3) 4 . 7 THIS WORK PREVIOUS WORK -0.39 [42] -0.25 C4 2] - 14-9 -therefore, D (CF 3 - SSCF3) = AH f (CFj) + AH £ '(SSCFj) - A H £ (CF3SSCF3) 4. £ <_-.-5.09•+ (-6.23) - (-13.29) 1 1-97 eV The bond strengths calculated in this manner are presented in Table XIX. The C-S bond strength for a symmetrical disulphide i s the energy required to break the f i r s t C-S bond. This is different from the second C-S bond energy as well as the average C-S bond strength (32) in the molecule. T n e heats of formation of the radicals and of the molecule have been inserted. The CH 3S—CH 3 bond energy (3.49 eV) is about 0.3 eV higher than previously accepted values. Field and Franklin obtained 3.17 eV (2) compared to 3.16 eV by Maynick (3), 3.18 eV by Lossing (4) and 3.21 eV by Majer (5). The CH3S-CH3 bond was found to be slightly weaker than CHjS—SCH 3. This is contrary to what other workers have found; the S-S bond energy reported in the literature ranges from 2.91 eV (3) to 3.17 eV (.2) In both the sulphide and bisulphide series, the C-S and S-S bonds are strongest when the molecule is asymmetrical; that is when both CH3 and CF 3 groups are present. This must be due to an increase £+ 6_ 6-t 6-in conjugation along; the chains H„C — S — C F , and H,C— S^S—CF,, s i n c e o n e e n d i s a n e l e c t r o n w i t h d r a w i n g g r o u p a n d t h e o t h e r e l e c t r o n d o n a t i n g . TABLE XIX C-S AND S-S BOND ENERGIES (UPPER LIMITS)  BOND ENERGIES (IN eV) CH.-S o CF, S—S 1.41 1.69 CH 3 - SCH3 "0.39 3.49 1.41 -4.70 CH3 — SCF 3 4.29 "7.58 1-69 5.09 CH3S — CF 3 4 . 1 8 . "7.58 "4.70 . "5.09 CF 3S — CF 3 2.44 "12.23 . 1.41 1.31 CH — SSCH3- 2.97 "0.25 1.69 1.69 CH -S—SCH -0 .25 3.63 1.41 :-6.25 CH 3 — SSCF3 3.41 "8.25 1.31 -5.09 CH3SS CF 3 4.47 -•8.25 1.69 -4.70 'CH S — SCF 3 "8.25 5.24 "6.23 "5.09 CF3SS — CF 3 1.97 • "13.29 4.70, -4.70 C.F S — SCF3 " 13.29 3.89 - 51 -The low C-S bond energy in the bis-perfluoromethyl compounds correlates with the fact that SCF 3 + are formed with very low intensity. (5) Discussions on individual molecules In the following section, heats of formation are calculated for a number of principal fragment ions of particular molecules. a) Dimethyl sulphide The ionization potential of dimethyl sulphide was found in this work to be 8.76 ± 0.18 eV. This is lower than 9.0 eV reported by Majer (5). With the heat of formation of the molecule equal to -0.39 eV (42), we can calculate the heat of formation of the molecule ion. Thus, AHf (CH3SCH3+) = 8.37 ± 0.18 eV. The percentage abundances of the fragment peaks reported in this work differ slightly from those of Majer's (5). This could be due to different experimental conditions, in particular source temperature (35) and also of course the general mass spectrometer design 47+ The heats of formation of this ion and the corresponding free radical are given in Table XVII. The ionization potential of CHjS may then be determined: I (CH3S) i.= AHf (CH3S+) - AHf )CH3S) 4-3 . <_ 9.29 - 1.69 < 7.60 eV - 52 -This i s about 0.4 eV lower than the 8.06±0.1 eV found by Lossing (4). The lower IP of the CH^S r a d i c a l as compared to the sulphur atom results from a 'backward'flow of charge from the CH^ group. 34 + AP (SH_+) = 14.29±0.04 eV. This value i s comparable to the l .... . . . | • • appearance potential of SH^+ reported by Majer at 13.85 eV (5). The r e l a t i v e abundance' of the ion i n the two spectra indicate that they may i n fact be the same species. Levy (43) reported a spectrum with m/e from 32 to 35, the most abundance peak within t h i s group being 35. However, a mass r a t i o of t h i s peak compared to nitrogen m/e =28 peak gives m / e W = 1.213468 m/3 (28) Hence m/e (34) = 33.9846 A calculation of the mass of SH 2 from atomic masses shows that m(SH2) = 33.9877 The difference of about 30 parts in 100,000 l i e s within the accuracy of the apparatus at the time of measurement. This peak arises from rearrangement after the breaking of two C-S bonds. 27* AP = 15.02*0.10 eV. The high value for the appearance potential indicates that t h i s i s another rearrangement peak. Mass measurements .give: -m / e ( 2 8 ) = 1.036355 ; m/e (27) - 53 -This gives m/e (27) = 27.0237. as compared to m(C2H ) = 27.0347. The appearance potential of this rearrangement'peak agrees with Majer's value of 14.7 eV (5). 15 A long t a i l appearing in the ion eff ic iency curve of mass 15 renders i t very d i f f i c u l t to obtain an accurate value for the AP. An estimate of 10*0.5 eV shows that the ion may be formed by ionization of the free radical resulting from thermal decomposition in the ion chamber. There is a sudden change in slope in the ionization eff ic iency curve around 14 eV indicating the presence of another process. This is probably CH3SCH3+e" > CH 3 + + SCH3 + 2e~ 4 . 1 0 b) Dimethyl disulphide 84^ . IP (CH3SSCH3) = 8.85*0.2 eV. This is higher than the photo-ionization result of 8-46 eV (44), as most electron impact data are. But i t is about 0.2 eV lower than the 9.1 eV reported by Kiser (36). The heat of formation of this molecule is" -0.25 eV (42). Hence, AH f (CH3SSCH3+) = AH £ (CH SSCH ) + IPCCHjSSCHj) 4. 1 1 V 4 8.60 eV 79+ AP (CH3SS+) = 11.73 * 0.24 eV The probable process is CH3SSCH3 >CH3SS+ + CH 3 4. 11 - 51}- -This gives AH f (CH^SS ) = AHf (CH3SSH3) - AH£(CH3) + AP (CH3SS+/CH3SSCH3) 4-13 = 10.07 eV Therefore, IP(CH3SS) = AHf (CH3SS+) - AH£(CH3SS) 4 . . ; l 4 = 9.76 eV 64* AP (S 2 +) = 15.09 ±0.13 eV. The probable process is CH3SSCH3 > S 2 + + 2 C H 3 This gives AH£ (S 2 +) = AHf(CH3SSCH3) - 2AH£(CH3) + AP (S2+/CH3SSCH3) 4 4 ? ~ 12.74 eV ;# ' From Table I, AH £(S 2) = AH£ (S 2 +) - I(S 2) = 12.74 - 10.8 =' 1.94 eV This is higher than 1.34 eV reported by Evans (45). Most probably the S 2 + ion is formed in an excited vibrational state, so that the AH £ (S 2) calculated does not necessarily correspond to that of S 2 in the ground state. 62"*" AP (CH3SCH3) = 8.97 eV. This is probably an impurity or the monosulphide resulting from the decomposition of the disulphide; since i t is only 0.21 eV above the measured IP of CH3SCH3. . 48 + AP (CH 4S +) = 9.72 AO.09 eV The probable process is CH3SSCH3 —--> CH 4S + + CS + H2- 4. l& The heat of formation of CS is.2.34 eV (31) - 55 -This g i v e s , AH £ (CH 4 S + ) = AH £ (CH 3SSCH 3) - AH f(CS) - AH £ (H2) + AP (CH 4 S + /CH 3 SSCH 3 ) '4.17 ' " 6 7-13 eV From reference (46), AH £ (CH 3SH +) = - 0 . 2 4 eV, hence the 48 + peak corresponds to CH 4 S + and not CH 3 SH + . 4 5 + AP (CHS+) = 13.43 ±0.09 eV. Th is i s f a r lower than 15.5 eV obtained by K i s e r (36). The probable process i s CH 3 SSCH 3 > CHS+ + CH 3S + H 2 ; AH1 L £ (CHS ) = AH £ (CH 3SSCH 3) - AH £ (CH3S) AH £ (H 2) + AP..(CH 3 S + /CH 3 SSCH 3 ) 4-19 ^ 11.49 eV. This i s the same as the value obtained by K i s e r i n a study o f 2-thiapentane (36), the reac t ion being C 4 H 1 Q S — - > CHS + + C 3 H 6 + H + H 2 . 4 . i 0 But i n the study o f 2, 4 - d i th iabutane , K i s e r obtained AH £ (CHS ) = 12.96 eV from the r e a c t i o n , CH 3 SSCH 3 - — - -> CHS+ + CH4S + H . ' 4 . 2 1 This r e a c t i o n invo lves rearrangement to give CH.S. 15"*" As in the case of the s u l p h i d e , the long t a i l on the ion e f f i c i e n c y curve f o r t h i s fragment prevents an accurate measurement of the onset p o t e n t i a l . However, a break i n the e f f i c i e n c y curve i s observed at 12.9 eV. . Th is corresponds to the i o n i z a t i o n / d i s s o c i a t i o n - 56 -process: CH3SSCH3 + e" > CH 3 + + SSCH3 + 2e~. '4.2 2 c) Methyl-perfluoromethyl sulphide  116* I P (CH3SCF3) = 9 .75 ± o . l l eV From Table XVIII, AHf (CH 3SCF 3 +) = AH f (CH3SCF3) + IP (CH3SCF3) 4.2 5 - 2.17 eV 45 + A P (CHS+) = 14.88 eV. From the reaction: CH3SCF3 > CHS+ + CF 3 + H 2 4.24 AHf (CHS+) = AH f (CH3SCF3) - AH£ (CF^ - AHf(H2) + A P (CHS+/CH3SCF3) 2 S - 12.39 eV This is higher than ithe 11.49 eV calculated for AHf (CHS+) from dimethyl sulphide, but s t i l l agrees with the values quoted by Kiser (36) at 12.44 eV, 12.96 eV and 11.49 eV. 15+ A P (CH 3 +) = 14.24 eV. This i s the appearance potential of the ion coming from the process CH3SCF3 + e" > CH 3 + + SCF 3 + 2e' 4. 2 £ d) Methyl-perfluoromethyl disulphide 148+ IP (CH3SSCF3) = 9.58 ± 0.14 eV This corresponds to AHf (CF 3SSCH 3 +) < + 1.33 eV. - 57 -94* AP (CHjSSCH*-' = 9.27 eV This ion must be.the result of some rearrangement in the ion source. The AP is 0.52 eV higher than the IP of CH SSCH . 79 + AP (CH SS +) = 11.29 ± 0.20 eV 3 The probable process is CH3SSCF3 > CH3SS+ + CF 3 4 2.7 This gives AH f (CH3SS+) = AHf (CH SSCFj) - AH f (CFj) + AP (CH3SS+/CH3SSCF3) 4.2B 1 8-15 eV-and IP '(CHjSS) = AH (CH3SS+) - AH f (CH^S) 4.l9 <_ 6.81 eV Both of these are much lower than the corresponding energies obtained from dimethyl disulphide, AHf (CH3SS+) being 10.07 eV and the IP (CH3SS) being 9.76 eV. Since from Table XIX, the bond strength of 2.97 eV for CH 3- SSCH3 is less than 4.47 eV for CF 3 — SSCH3, the appearance potential may correspond to the energy required to separate CH3SS+ from CH3SSCH3, which i s formed from rearrangement. 64 + AP (S 2 +) = 12.46 eV. This low value for the appearance potential of S 2 + as compared to the same measurement from dimethyl disulphide indicates that one or both S-C bonds might be thermally broken. If we assume the following process CH3SSCF3 > S 2 + + CH3 + CF 3 4,30 AH f (S 2 +) = AH£ (CH3SSCF3) - AHf (CHj) - AH f (CF3) + AP (S 2 +/CH 3SSCF 3) 4.31 ^ 7.89 eV. - 58 -Energetically, this is not possible, since i t w i l l give, AH£ (S 2) = AHf (S 2 +) - IP (S 2) 4 f5 2-<_ 7.89 - 10.8 1 -2.91 eV as compared to 1.34 eV reported by Evans (45) 45* AP (CHS+) = 14.83 ± 0.08 eV If we take the most probable process as CH3SSCF3 > CHS+ +SCF3 + H 2 4 .3 3 we get AHf (CHS+) 6 11.67 eV This agrees reasonably well with 11.49 eV obtained for AH£ (CHS+) from dimethyl disulphide. e) Bis-perfluoromethyl sulphide 117+ IP (CF 3SCF 3) = 11.28 ± 0.04 eV From Table XVIII, AH f (CF 3SCF 3 +) = AHf (CF 3SCF 3) + IP (CF 3SCF 3) .4.34 <_ -0.95 eV f) Bis-perfluoromethyl disulphide . 202* IP (CF3SSCF3) = 10.68 ± 0.19 eV From Table XVIII, AHf (CF 3SSCF 3 +) = AHf (CF3SSCF3) + IP (CFjSSCFj) 4.3S < -2.61 eV - 59 -64+' AP (S 2 +) = 13.22 ± 0.17 eV As in the case of methyl-perfluoromethyl disulphide this low appearance potential indicates that there is thermal decomposition. If we take the most probable reactiorvas CF 3SSCF 3 > S 2 + + 2CF 3 AHf (S 2 +) = AHf (CF3SSCF3) - 2AH '(CFj) + AP (S 2 +/CF 3SSGF 3) 4.3 6 <_ 10..2 9 eV which is lower than I P ( S 2 ) of 10.8 eV. This w i l l give AH f (S 2) S <_ -0.51 eV. If this S 2 + ion is produced from the following reactions ' CF3SSCF > CF3.+ SSCF3 4.37 CF3SS — - > S 2 + + CF 3 4.3J AH£ (S 2 +) = AHf (CF SS.) - AH £ (CFj) • + AP (S 2 +/CF 3SS) 4.39 <_ 12.08 eV. This gives AH £(S 2) =12.08-10.8 <_ 1.28 eV. This is in f a i r agreement with 1.34 eV obtained by Evans (45) and supports the fact that there is thermal decomposition or that S 2 + does not come from the parent molecule. g) Thiophene 84+ IP (C.H.S) = 9.06 ± 0.16 eV ^ 4 4 / This agrees quite well with the spectroscopic value 8.95 eV (47). - 60 -The heat of formation of thiophene was found by Franklin and Lumpkin to be 1.19 eV (2). AHf (C 4H 4S +) = AH£ (C4H4S) + IP ( C ^ S ) 4.40 <_ 10.25 eV. This agrees f a i r l y well with 10.14 eV quoted by Franklin (31). 58 + AP (C 2H 2S +) =12.36 ± 0.06 eV. This is lower than 12.6 ± 0.2 eV given by Hissel (48). 45* AP (CHS+) = 13.51 ± 0.08 eV. This does not d i f f e r very much from Hissel's 13.6 ± 0.2 eV (48). The probable process is C 4 H4S > CHS+ + C 3H 3. If we take AH^ (CHS+) to be 11.85 eV — the average of the three values obtained in this work A H £ (C 3H 3) = AH£ (C4H4S) - AH£ (CHS+) . + AP (CHS+/C4H4S) ^2.85 eV. The ionization potential of C 3H 3 was previously determined to be 8.25 ± 0.08 eV (49). This gives AH f(C 3H 3 +) = AH £ (C 3H 3) + IP ( C ^ ) .4.42. <_ 2. 85 + 8.25 1 11-10 eV' 39 + AP (C 3H 3) = 13.3 ± 0.1 eV The probable process, i s : - 61 -4-. 4 3 AH£ (CHS) = AHf (C4H4S) « AH£ (C 3H 3 +) + AP (C3H3+/C4H4S) 4.44 < 3.39 eV. This gives IP (CHS) = AH- (CHS+) -AH- (CHS+) - AH,. (CHS) 4-.4S" < 8.46 eV. 6) Conclusions The heats of formation of various species are fairly consfetent except in the presence of thermal decomposition. This proves and Mayrichrs finding that the C-S and S-S bonds in dimethyl sulphide and dimethyl disulphide should be different (3). But contrary to previous findings (3, 4, 5), the S-S bond is found to be 0.14 eV or 3.2 kcal/mole stronger than the C-S bond. From Table XIX, we note that the calculated strengths of these bonds depend very much on the accuracy of the heats of formation of SCH3, CH3 and the two molecules CH3SCH3 and CH3SSCH3. The monosulphide (AH£ = -0.39 eV) is more stable than the disulphide (AH£ = -0.25 eV) by 0.14 eV. The relative strengths of the C-S and S-S bonds then depend on whether AH£ (CH3) is greater or less than AH£ (SCH^  . If AH£ (SCH3) - AH£ (CH3) = 0.14 eV,' then the two bond strengths are the same.' From the literature, the the validity of the proposed processes. A':;comparison of the various bond energies supports Mackle - 62 -lowest and highest values for AHf (SCH3) are 1.32 eV (3) and.1.57 eV (2). These values are within ± 0.16 eV of 1.41 eV quoted for AH (CH ) by Stevensons (40). Besides, this value lies well within the error range in thermo-chemical data (about 5 kcal/mole) and electron impact results (about 0.1 to 0.2 eV) to be of significant value. In this work i t is found that AHf(SCH3) - AHf(CH3) <_0.28 eV. The particularly low C-S bond energy (1.97 eV) found in bis-perfluoromethyl disulphide can be attributed to the particular stability of the two products with respect to the parent reactant. A certain amount of conjugation is present to strengthen the bonds in asymmetrical sulphides. It was found (37) that from electro-negativity considerations alone, a perfluoroalkyl group, and particularly trifluoromethyl, is. to be considered as a pseudo-halogen rather than as a fluorine substituted alkyl group. This means that there is a considerable reduction in the availability of the electron lone-pairs of the atom to which the perfluoroalkyl group is attached. In the asymmetrical sulphides, the presence of the electron-donating methyl group compensates for this reduction, so that the bond orders can well be higher than 1; CH3 — S — CF3 and CH3 —" S — S — CF3. . D(CF3S — SCH3) was found to be 5.24 eV., — higher than 4.4 ± 0.1 eV quoted by Gaydon (14) for S^-In both the dimethyl and bis-perfluoromethyl series, the C-S bond is stronger in the monosulphide. This is due to the greater - 6 3 -s t a b i l i t i e s ' o f the-dissociated products of the disulphides; AH £ (SSCH3) = 1,37 eV compared to AH£ (SCH3) = 1.69 eV and [ AH £ (SSCF3) = -6.23 eV compared to AH£ (SCF3) = -4.70 eV, the other products being the same. A summary of the heats of formation of the various ions found in this work is given in Table XX. TABLE XX 1 HEATS OF FORMATION OF POSITIVE IONS (UPPER LIMITS) IONS PARENT THIS wnprAHf (An- eV)- P R E VI0US WORK CH3 1 11.36 (40) CHS+ CH,SCF,. 12.39 11.49 3 3 CH SSCH 11.49 CH^SSCF^ 11.67 12.44 12.96 (36) CH S CH SCH 9.29 9.63 ( 2) 9.43 . ( 4) S * CH SSCH 12.74 12.09 (31) CH3SSCF3 12.08 CF 3S + CF 3SSCF 3 - 5.84 CH3SCH3+ C H 3 S C H 3 ' / ' 8 - 3 7 CH3SSCH3+ CH3SSCH3 8.60 8.84 (36) CH 3SCF 3 + C H 3 S C F 3 , 2 . 1 7 CH 3SSCF 3 + CH3SSCF3 1.33 CF 3SCF 3 + C F 3 S C F 3 -0.95 CF 3SSCF 3 + CF 3SSCF 3 - 2.61 - 6).|. -REFERENCES 1. J . Thomsen, Thermodchemsche, J . Am. Chem. S o c , Untersuchungen Vol. IV Leipzig (1886). 2. J . L . Franklin and H.E. Lumpkin, J . Am. Chem. Soc. 74, 1023 (1952). 3. H. Mackle and R.G. Mayrich, Trans. Faraday S o c , 58_, 33 (1962). 4. T.F. Palmer and F.P. Lossing J . Am. Chem. S o c , 84, 4661 (1962). 5. B.G. Gowenlock, J . Kay, J.R. Majer, Trans. Faraday Soc. 59, 2463 (1963) 6. T.L. Al len, J . Chem. Phys. 31_, 1039 (1959). 7. D. Peters, J . Chem. 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