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Ionization and Dissociation of Molecules by Electron Impact IONIZATION AND DISSOCIATION OF MOLECULES BY ELECTRON IMPACT by MINA.THERESA TUNG-FAI YU B.Sc., University of British 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 this thesis in partial fulfilment of the requirements for an advanced degree at the University of • British Columbia,, I agree that the Library shall make it freely available for reference and study, I further agree that per• mission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that;copying or publi• cation.of this thesis for financial gain shall not be allowed without my written permission* Department of (3HEMISTRY The University of British 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 all 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 all the C-S and S-S bond strengths were made. • Ill 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 all the non-cyclic sulphides and disulphides. I would like to thank also all 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 PAGE CHAPTER 1. SULPHUR BONDING IN ALKYL SULPHIDES, CHAPTER 2. IONIZATION AND DISSOCIATION OF MOLECULES BY ELECTRON IMPACT. (1) Methods of determining dissociation energies. 3 (2) Ionization by electron impact. 4 (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 its interpretation. 9 (6) Heats of formation and dissociation energies. 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 vi 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. Circuit 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 earliest classical thermochemical studies of simple alkane thiols and sulphides date back to 1886 when J. Thomsen first did his experiments (1). Since then many people have entered the field as organic sulphur compounds have become more important in petroleum technology and in 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 in 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 in 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 later Gowenlock, Kay and Majer (5) supported this finding with their electron impact data.'' Other values for these bond strengths have also been reported and the highest value obtained is 77.3 kcal/mole for both (6)'. In this work, a series of six related compounds was studied. The substitution of a more electronegative group for the methyl group shows a definite tendency in the respective bond dissociation, energies. - 2 - That the important factor in determining the bond energy of a slightly 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 orbital is virtually 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 will be concentrated outside the binding regions. The ionization energy of the s lone pair electron will 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, it is changed only slightly 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 affinity of the group attached to sulphur, the more nearly will the bond angle approach that predicted for p£ geometry. This is presumably because electrons will be on the average farther from sulphur and therefore less likely to utilize the lower energy s orbital. Hence the non-bonding pair will have more s character and a higher ionization energy. The bond will 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 dissociation 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 limits, extrapolations to band convergence limits, long-wavelength limits of absorption continua and photodissociations (14). An absorption continuum in the visible or near ultra violet can, according to Gaydon (15), nearly always be assigned to a dissociation process. Absorption due to ionization is rarely seen because the corresponding continuum lies well down in the ultra-violet. 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 identification of the electronic states of the products. Determination of the heat of reaction of the gas phase process A + B = AB, gives directly the dissociation energy in thermal units.. However this method involves heat and temperature measurements, - k - which at times can be quite difficult to make. The error introduced is generally large. In actual fact the only method other than kinetic (14) for obtaining dissociation energies in polyatomic molecules, requires appearance potential determinations. The electron impact technique is employed in the present work and will be discussed in more detail below. (2) Ionization by electron impact When an electron collides with an atom or molecule, it can lose its energy in one of two ways. In an elastic collision the electron loses part of its energy to the targe.t particle in such a way that the translational energies of the two body system is conserved. The second kind of collision is inelastic. Internal energy changes take place within the molecule or atom. If the bombarding electron has high enough energy, an inelastic collision may lead to ionization and/or dissociation of the molecule.
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