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.
The ionization potential of an atom or molecule is
theoretically defined as the energy required to completely remove an electron from the neutral species in its ground state.
XY + e" --.-» XY+ + 2e~ 2.V
In practice, however, the ionization potentials measured do not necessarily correspond to this definition because of the possibility
that the product(s) of the ionization 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 it 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 it 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 it 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 ionization has occurred a new electronic state
is formed and the forces acting on the nuclei are different.
Consequently, the nuclear configuration and nuclear motions will 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 in the ground state.
In the case of transitions to a repulsive upper state or to an attractive state above its dissociation asymptote, dissociation occurs Hence for the diatomic molecule undergoing such a process, as depicted in figure lc:
XY + e" * X+ + Y + 2e" 1.1 the appearance potential of the ion X+ is 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. = total kinetic 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 first 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 limits. 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) first noted the various effects of secondary processes - the formation of new ionic species corresponding to no known molecular species that appeared as sharp lines, diffuse lines, 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 electronically and vibrationally 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 it from the rest of the gas according
to its mass to.charge ratio. Many complications can then be simplified.
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 reactivities 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 efficiency curve and its 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 literature (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 first 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. All of these are subjective and there is no really reliable 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 ionization efficiency curves for the ion studied and an ion to calibrate the electron voltage scale are determined with both gases present in the mass spectrometer. The ordinate scales are chosen1 so as to make the straight portions of the curves parallel (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 is drawn and extrapolated to zero ion current. The value of AV at zero ion current is taken as the difference between the appearance potentials. This method like any other is arbitrary, and'
Warren (26) points out that the extrapolation of any curve other than a straight line is open to objection. Moreover, the method is not suitable for measuring the appearance potentials of ions formed with a very.low intensity. Since most fragment ions have long "tails on their ionization efficiency curves, the accuracy of the appearance potentials measured for such is, of course, debatable. For parent ions the method is 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). If we assume that the energy of the two electrons remaining after the following process is 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
Similarly, we can find the heats of formation of the neutral species
in the following reaction, knowing AH£ (AC)
AC + e > A+ + C + 2e 2. 8
+ + AHf (AC) = AHf (A ) .+ AHf(C) - AP(A /AC) 2.^
Now, if 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. It is the author's intention to present only the "mass spectrometry equation" here.
Positive ions produced in the ionization chamber are forced by a positively charged repeller electrode (figure 4) into the ion gun. Here they are accelerated through a definite potential difference X. The final slit of the ion gun selects a narrow beam of ions which then pass through the magnetic analyser where, they are de• flected. The force exerted on the ion by the magnetic field is equal to the centripetal force due to its motion, that is
Hev =• mv2/R 3- 1 where H = magnetic field strength
v •= velocity 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 initial kinetic energy, the kinetic energy of the ions after acceleration is given by
1/2 m v2 = X e 3-1 - Ik -
Combining (3,1). and(3,2), we get
R = H"1' (2 m X/ e)1/2 3-3 and
m / e = H2 R2 / 2 X 3. 4
It is evident from equation (3) that the ion trajectory is determined by its mass to charge ratio for a given magnetic and electric :fields.
If either H or X is varied, ions of different mass to charge ratio may be collected and.identified as such.
(2) The instrument
The instrument used for this work is a AEI model MS9 mass spectrometer. One advantage the MS9 has over other.spectrometers lies in the fact that it has both an electrostatic and magnetic analyser. This double focussing device provides a high resolving power with narrow slit widths.
Only a few special features of the MS9 will 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 positive ions out of the ion source we encounter a source isolating valve. This consists of a ball valve which can be used to close off the source from the analyser at a point immediately beneath the object slit. This enables the source 10 ^6
7
8
1. Filament 6. Trap
2. Draw out plate 7. grids with applied 3. Grid 8. accelerating
4. Repeller electrode 9. potential
5. Grid 10. Ionization 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 Slit 11. To Multiplier and Recorder
4. Ion Source 12. Auxiliary Collector
5. Electrostatic Analyser 13. . Adjustable Slit
6. Monitor Collector 14. Magnetic Analyser
7. Gas Inlet .15. Electron Multiplier
8. Cold Trap
FIGURE 5. MS9 MASS SPECTROMETER - 17 -
to be removed without admitting air 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 five to six hours to bake out the analysers and about the same
time again for them to cool down to room temperature.
A monitor collector 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
it 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 relative amount of sample
actually present.
At the exit end of the magnetic analyser are two collectors.
The principal collector has incorporated in it a high sensitivity
electron multiplier, and is used for high resolution work with very
narrow slits or fast scanning with a wide collector slit. The.electron 4
multiplier steps up the signal by a factor of 10 . The auxiliary
collector, 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 auxiliary collector or vice versa just by turning the HR and
LR knob on the panel.
And when the HR and LR knob is at LR, the;spectrum can be scanned at fast and slow speed and displayed on the oscilloscope incorporated on the panel. This provides the facility for measurement. of the mass to charge ratio of an "unknown ion". The relationship between the unknown mass to charge ratio and that of a known reference is determined accurately by comparison of the ion accelerating voltage necessary to bring the ion beams on to the collector slit at a constant magnetic field (figure 6). Provided that the masses of the two ions do not differ.more than 10%., the stability of the circuits and the resolving power of the instrument is sufficient for the measure- 6 ment of mass ratios with a precision of a few parts'in 10 .
Solids are inserted directly into the ionization chamber through the solid sample probe (figure 5). There are two other inlet systems (figure 7).
The instrument is as yet not equipped with a device for producing monoenergetic electrons.
(3) Experimental
Before any sample is introduced, the whole system is 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 volt power unit
Analyser voltage
Saw-tooth time base
Amplifier
Oscilloscope
FIGURE 6. CIRCUIT FOR MASS MEASUREMENT •- 20 -
1. Source ' 4. Metal valves
2. Ion chamber 5. Sample bulbs
3. Metrosil leaks 6. Doser Manometer
Cold System 6 ' °" Hot valve system (350°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 slit 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 purification. The dimethyl compounds
were liquids with high vapour pressures so that enough sample can be
introduced through the cold inlet 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 its 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 first ionization potential of a sulphide compound probably corresponds to the removal of an electron from one of the lone pairs on sulphur. Theoretically if there is no hybridisation of atomic orbitals when, bonds are formed, these sulphide compounds should have the same ionization energy as sulphur atom itself. 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 it. The first 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 first 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
S 10.8 31 2
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 radicals,, 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
02 12.2 31
H202 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 diffraction studies (33) showed that the S - S bond distance in the perfluoroalkyl and alkyl disulphide compounds are virtually identical. 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 labelling 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 will indicate the bond(s) broken and the - 26 -
rearrangement processes present. Another substantial recognition of the mechanisms results from the identification- of metastable ions. These are formed by spontaneous decomposition of the ions in a region between the ionization chamber and the analysers.
Metastable ions do not necessarily appear at integral mass units.
Derivations•for the parent mass and the apparent mass of the metastable can easily be found in literature (51). m2
m = 4 1 o * where m = apparent mass of metastable ion
m0. = mass of parent ion formed in the ionization 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 (34) believed that "a plausible path is preferable to the proverbial wiggly line which lacks any rationale."
In his recent paper, C. E. Brion (35) 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 attain a certain equilibrium temperature before use. It was found that the measured appearance potentials of fragments were higher than the corresponding values obtained in free radical 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 arising 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 17.0 24.3 , C2H3
34 ,SH2 28.6
35 SH3 39.4
45 CHS 25.3 50.4
CH S 32.2 • 46 ' 2 43.1
47 CH3S 75.5 108.3
48 CH3SH 5.4
49 CH3SH2 5.5
33.5 36.3 61 CH3SCH2
62 CH3SCH3 100.0 100.0
63 6.6
64 (i) 7.5 TABLE IV
MASS SPECTRUM OF DIMETHYL DISULPHIDE
% ABUNDANCE ION THIS LITERATURE (36)
CH3 4.6 .14
CS .3.5
CHS 34.6 58.6
CH2S 24.7 39.5
CH3S 20 v 4 • 27.4
CH3SH . 11.7 14.8 .
CHjSH-, 4.5 5.3
CH SCR, 12.6 16.7
CH3SCH3 4.7
8.6 .9.6 S2
cs2 7.7
CH2S2 3.2 3.1
CH3SS 49.2 54.3
CH3SSH (3.2) 2.6
CH3SSH2 (5.3) 4.9
CH3SSCH2 1.9
CH3SSCH3 100.0 100
(i) . 5.8 "5.6
(i) 10.1 9.3 - 29 -
TABLE V
MASS SPECTRUM OF METHYL-PERFLUOROMETHYL SULPHIDE
m/e ION % ABUNDANCE
15 CH3 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 CF3 51.6
76 ' CS2 . 4.6
82 SCF4 ' 5.1
97 CH3SCF2 7.5
115 CH2SCF3 7-9
116 CH3SCF3 100-°
117 [i}. 4.5
118 " Cij 4.9 - 30 - TABLE VI
MASS SPECTRA OF METHYL-PERFLUOROMETHYL DISULPHIDE AND BIS-PERFLUOROMETHYL DISULPHIDE
% ABUNDANCE m/e ION . METHYL-PERFLUORO BIS-PERFL
15 CH 4.3 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 10.2 14.8 S2
69 CF 22.6 100.0 3
76 cs2 21.0
78 CH2SS 8.0 79 CH SS 63.7
80 CH3SSH 2.8
81 CH3SSH2 6.5
82 CF2S 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 CF2SSCF3 10.7
202 CF3SSCF3 84.4 203 (i) 4.0 204' (i) 9.6 TABLE VII
MASS SPECTRUM OF THIOPHENE
m/e ION ' % ABUNDANCE
39 C H 14.2 3 3 45 CHS+ 25.5
58 C H S+ 41.6 2 2
+ 84 C4H4S 100.0
Since bis-perfluoromethyl sulphide was obtained by Dr.
Cullen from the irradiation of the disulphide by ultra violet light, it is not surprising to find a relative high percentage of the disulphide in the sample. An accurate mass spectrum due to fragmen• tation of the monosulphide itself 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 all 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 S2 peak in the spectrum of bis-perfluoromethyl disulphide as compared to dimethyl disulphide can be attributed to the stability of the CF^ fragment. Energetically, it 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
Electron 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 ratio equalto 84 or 42. This occurred in the case of thiophene, and so xenon was used as the calibrating gas.
Tables IX to XIV give the ionization and appearance potentials of. major fragments together with probable processes. In the first column is recorded the mass to charge ratio. 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 reliability and reproducibility of the values quoted. In column four, the calibrating gas is quoted for completeness.
Column five gives the experimental values with standard deviations obtained in this work together with previous results (when available) from the literature. TABLE IX
APPEARANCE POTENTIALS OF THE PRINCIPAL IONS. OF DIMETHYL SULPHIDE
. No. OF CAL. A. P. m/e PREDICTED PROCESS DETERMINATIONS GAS THIS WORK REF C5)
+ 62 CH3SCH3 } CH3SCH3 10 Kr 8.76± 0.18 9.0
61 •--> CH3SCH2 10 11.95* 0.16
47 —> CH S + CH3 10 11.09* 0.15 11.3
46 -} CH2S + CH3'.+ H 10 10.97* 0.13 •F-
+ 45 CHS + CH3 + H2 14.16± 0.08
+ 35 -> SH3 + (C2H3) 13.85
T 34 •> SH2 + (C2H4) 14.29* 0.04
27 •» C2H3 + (SH3) 15.02* 0.10 14.7
15 CH3 + SCH' 10. 46(rough) 13.0
-vl4 (first break) TABLE X
APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF DIMETHYL DISULPHIDE
No. OF CAL. A. P. PREDICTED PROCESS GAS THIS WORK m/e DET. REF. C56 3 p+ (84+) CH SSCH > CH SSCH 10 Kr 8.85*0.20 9.1*0.2
69 -> CHjSS + CH 10 Kr 11.73*0.24 12.1*0.2
64 S + 2CH 10 Kr 15.01*0.13 15.4*0.3 "> 2 3
62 CH3SCH + S 5 8.96*0.06
61 10 10.66±0.05 10. 9*0.-2. V^J
49 -> CH5S + CS + H 5 11.44±0.15 11.9*0.2
9.72*0.09 11.5*0.2 48 — *j CH.S + CS + H„ 5 4 2
+ 47 CH3S + CH3S 10 11.23*0.11 13.0*0.4
46 CH2S + CH4 + S 10.61*0.11 12.2*0.2
45 -> CHS + CH.S + H 10 13.43*0.09 15,5*0.3
15 •> CH + SSCH 10 10.02(rough) 15.7*0.3
12.9*0.4 (first break) TABLE XI
APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF METHYL-PERFLUOROMETHYL SULPHIDE
No. OF CAL. A. P. PREDICTED PROCESS DET. GAS THIS WORK
CH3SCF3 Kr 9.75*0.11
• CH3SCF2 + F 15.7 ±0.24
CF3 + SCH3 13.07*0.09
CSF + CH + F2 15.32*0.13
CH3S •+ CF3 11.78*0.03
CH2S + CF3 + H ^14
CHS + CF3 + H2 14.88*0.06
CH3 +.SCF3 14.24*0.13 TABLE XII APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF METHYL PERFLUOROMETHYL DISULPHIDE No. OF CAL. A. P. m/e PREDICTED PROCESS PET. GAS THIS WORK
+ 148 CH3SSCF3 — CH3SSCF3 6 . . ' Kr 9.58*0.14
+ 133 —CF3SS + CH3 6 14.77*0.12
+ 129 CF3SSCF2 .+ F .5 ~15
+ 94 CH3SSCH3 CH3SSCH3 3 9.27*0.08
+ 79 CH3SSCF3 -) CH3SS + CF3 6 11.29*0.20
+ 69 -> CF3 + CH SS S 12.50*0.08
+ 47 CH3S + S• + CF3 • . 6 12.84*0.11
+ 46 -> CH2S + SH + CF3 6 13.43*0.24
+ 45 CHS + CSF + H2 6 14.83*0.08
+ 10 15 > CH3 + CF3 + SS 2 'v 14 (first break) TABLE XIII
APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF
BIS-PERFLUOROMETHYL SULPHIDE
No. OF CAL. A. P. m/e PREDICTED PROCESS DET. GAS THIS WORK
CF 170 CF3SCF3 > 3SCF3 Xe 11.28*0.04
151 CF SCF + F Xe M5 -> 3 2 CO
69 -> CF3 + SCF3 Xe 12.54*0.06 TABLE XIV APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF BIS-PERFLUOROMETHYL DISULPHIDE
No. of CAL. A. P. m/e PREDICTED PROCESS DET. GAS THIS WORK
8 202 CF3SSCF3 — ± CF3SSCF3 Kr 10.68*0.19
183 .—^ CF3SSCF2 + F 5 14.64*0.07
133 - —> CF3SS +.CF 6 13.31*0.05
114 CF2SS + CF4 5 11.28*0.06
101 CF3S + SCF3 6 : 14.43*0.08
95 CFSS + CF3 + F2 5 15.60*0.10
82 > CF2S + CF3S + F 5 10.93*0.17
69 CF3 + SSCF3 10 12.07*0.13
64 S + + CF 6 13.22*0,17 — ~> 2 3
550 + CF2 + SSCF'3 + F 4 13.63*0.05
+ 32 > S + SCF3 + CF3 1 ^13
+ • 31 CF '+ SSCF3 > F2 1 ^16 TABLE XV
APPEARANCE POTENTIALS OF THE PRINCIPAL IONS OF THIOPHENE
No. OF , CAL. A. P. PREDICTED PROCESS DET. GAS THIS WORK
+ } C4H4S 6 Xe 9.06*0.16
+ > C-H0S + C-H ' 5 " 12.36*0.06 '2 2 2 2
+ > CHS + C3H3 5 » 13.51*0.08
> C3H3+ + 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 first 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 little 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, it is found that though the appearance potentials of the same ions differed only slightly (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 is stronger than the S-C bond in the sulphide by 0.14 eV.
The difference is 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 is 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 is 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
inability to find its appearance potential accurately.
+
In the determination of the appearance potentials of CH^
ions from dimethyl sulphide, dimethyl disulphide and the methyl perfluoromethyl compounds, it was found that the first 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 all 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 is bound, the appearance potentials of its 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 is used to
interchange thermal-chemical values and electron volts. The heat of
formation of CF^+ was derived from the heat of formation of the free
radical and its ionization potential (41).
Since dimethyl sulphide and disulphide are liquids at room
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 will, of
course, depend on the accuracy of the thermal information in Table XVI.
From Tables IX and XVI, we obtain
+ AHf(CH3S ) < AHf (CH3SCH3)- AHf (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 > CH3S + SCH3 . 4.4
+ where AH£ (SCHj) ='AHf (CHgSSCH )- AHf (CH3S )
+ + 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 is given within brackets.
All 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 — > CH3S + 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 > CH3S +.CF - 11.78
"7.58 11.36 (-4.70)
' CH3SCF3 -> CH3 + SCF3 14.21+
(-12.23) , 5.01 "4.70
+ CF3SCF3 > CF3 + SCF3 12.54 (-8.25) 9.29-4.70
+ CH3SSCF3 —.—.-> CH3S + SCF3 12.84
-8.25 (5.1l) 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)
+ CF3SSCF3 — --—> CF3 + SSCF3 12.07
-0.25 11.36 Cl.3l)
+ CH3SSCH3 > CH3 + 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 Hf RADICALS THIS WORK PREVIOUS WORK
CH3 1.41 C40J
1.43 C2]
CH3S : J-69 . , {1.38 C4]
CH3SS 1.31
CF3 -5.09 Of)
CF3S -4.70
CFjSS -6.23
(To0be cont'd) — 1+8 —
TABLE XVIII (Cont'd)
COMPOUNDS THIS WORK PREVIOUS WORK
CH3SCH3 -0.39 [42]
CH3SSCH3 -0.25 C4 2]
CH3SCF3 -7.58
CH3SSCH3 -8.25
CF3SCF3 -12.23
CF3SSCF3 -13.29
From the figures in Table XVIII, it is evident that the differences
in AH£ between the disulphide and its corresponding sulphide increases
with the presence of CF3 groups in the compound.
A typical calculation (for the upper limit of the bond
dissociation energy of the CF3 - SSCF3 bond in bis-perfluoromethyl
disulphide) is given below.
CF3SSCF3 ——-> CF3 + SSCF3 4.6
AHf (CF3SSCF3) = AH£ (CF3) + AHf (SSCFj)
-D (CF3 - SSCF3) 4.7 - 14-9 - therefore,
D (CF3 - SSCF3) = AHf (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 is the energy required to break the first 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. Tne heats of formation of the radicals and of the molecule have been inserted.
The CH3S—CH3 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—SCH3. 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 CF3 groups are present. This must be due to an increase £+ 6_ 6-t 6- in conjugation along; the chains H„C —S—CF, and H,C— S^S—CF,, since one end is an electron withdrawing group and the other electron donating.
TABLE XIX C-S AND S-S BOND ENERGIES (UPPER LIMITS) BOND ENERGIES (IN eV)
CH.-S CF, S—S o
1.41 1.69
CH3 - SCH3 3.49
"0.39
1.41 -4.70 1-69 5.09
CH3 — SCF3 4.29 CH3S — CF3 4.18 "7.58 . "7.58
"4.70 . "5.09
CF3S — CF3 2.44
"12.23 .
1.41 1.31 1.69 1.69
CH — SSCH3- 2.97 CH -S—SCH 3.63
"0.25 -0 .25
1.41 :-6.25 1.31 -5.09 1.69 -4.70
CH3 — SSCF3 3.41 CH3SS CF — 3 4.47 'CH S SCF3 5.24 "8.25 -•8.25 "8.25
"6.23 "5.09 4.70, -4.70
CF3SS — CF3 1.97 C.F S — SCF3 3.89 • "13.29 " 13.29 - 51 -
The low C-S bond energy in the bis-perfluoromethyl compounds
+ correlates with the fact that SCF3 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 is about 0.4 eV lower than the 8.06±0.1 eV found by Lossing (4).
The lower IP of the CH^S radical 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 is comparable to the l ...... | • • appearance potential of SH^+ reported by Majer at 13.85 eV (5).
The relative abundance' of the ion in the two spectra indicate that they may in fact be the same species. Levy (43) reported a spectrum with m/e from 32 to 35, the most abundance peak within this group being 35. However, a mass ratio of this 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 SH2 from atomic masses shows that
m(SH2) = 33.9877
The difference of about 30 parts in 100,000 lies 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 this is another rearrangement peak.
Mass measurements .give: -
m/e (28) = 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 tail appearing in the ion efficiency curve of mass 15 renders it very difficult 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 efficiency curve around 14 eV indicating the presence of another process. This is probably
+ CH3SCH3+e" > CH3 + SCH3 + 2e~ 4.10 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 it 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, AHf (CH3SSCH3 ) = AH£ (CH SSCH ) + IPCCHjSSCHj) 4. 1 1
V4 8.60 eV
+ + 79 AP (CH3SS ) = 11.73 * 0.24 eV
+ The probable process is CH3SSCH3 >CH3SS + CH3 4. 11 - 51}- -
This gives AHf (CH^SS ) = AHf (CH3SSH3) - AH£(CH3)
+ + AP (CH3SS /CH3SSCH3) 4-13
= 10.07 eV
+ Therefore, IP(CH3SS) = AHf (CH3SS ) - AH£(CH3SS) 4..;l4
= 9.76 eV
+ 64* AP (S2 ) = 15.09 ±0.13 eV.
> S + + 2CH The probable process is CH3SSCH3 2 3
+ This gives AH£ (S2 ) = AHf(CH3SSCH3) - 2AH£(CH3)
+ + AP (S2 /CH3SSCH3) 4 4?
~ 12.74 eV ;# '
+ From Table I, AH£(S2) = AH£ (S2 ) - I(S2)
= 12.74 - 10.8
=' 1.94 eV
This is higher than 1.34 eV reported by Evans (45). Most probably
+ the S2 ion is formed in an excited vibrational state, so that the
AH£ (S2) calculated does not necessarily correspond to that of S2 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
it is only 0.21 eV above the measured IP of CH3SCH3. .
+ + 48 AP (CH4S ) = 9.72 AO.09 eV
+ The probable process is CH3SSCH3 —--> CH4S + CS + H2- 4. l&
The heat of formation of CS is.2.34 eV (31) - 55 -
+ This gives, AH£ (CH4S ) = AH£ (CH3SSCH3) - AHf(CS)
+ - AH£ (H2) + AP (CH4S /CH3SSCH3) '4.17
' " 6 7-13 eV
+ + From reference (46), AH£ (CH3SH ) = -0.24 eV, hence the 48 peak
+ + corresponds to CH4S and not CH3SH .
45+ AP (CHS+) = 13.43 ±0.09 eV. This is far lower than 15.5 eV
obtained by Kiser (36).
The probable process is
+ CH3SSCH3 > CHS + CH3S + H2 ;
1 AHL£ (CHS ) = AH£ (CH3SSCH3) - AH£ (CH3S)
+ AH£ (H2) + AP..(CH3S /CH3SSCH3) 4-19
^ 11.49 eV.
This is the same as the value obtained by Kiser in a study of 2-
thiapentane (36), the reaction being
+ C4H1QS —-> CHS + C3H6 + H + H2. 4 . i0
But in the study of 2, 4- dithiabutane, Kiser obtained
AH £ (CHS ) = 12.96 eV from the reaction,
+ CH3SSCH3 - — --> CHS + CH4S + H . '4.21
This reaction involves rearrangement to give CH.S.
15"*" As in the case of the sulphide, the long tail on the ion efficiency curve for this fragment prevents an accurate measurement of the onset potential. However, a break in the efficiency curve is
observed at 12.9 eV. . This corresponds to the ionization/dissociation - 56 -
process:
+ CH3SSCH3 + e" > CH3 + SSCH3 + 2e~. '4.2 2
c) Methyl-perfluoromethyl sulphide
116* IP (CH3SCF3) = 9.75 ± o.ll eV
+ From Table XVIII, AHf (CH3SCF3 ) = AHf (CH3SCF3) + IP (CH3SCF3) 4.2 5
- 2.17 eV
45+ AP (CHS+) = 14.88 eV.
+ From the reaction: CH3SCF3 > CHS + CF3 + H2 4.24
+ AHf (CHS ) = AHf (CH3SCF3) - AH£ (CF^ - AHf(H2)
+ + AP (CHS /CH3SCF3) 2 S
- 12.39 eV
+ This is higher than ithe 11.49 eV calculated for AHf (CHS ) from dimethyl sulphide, but still agrees with the values quoted by
Kiser (36) at 12.44 eV, 12.96 eV and 11.49 eV.
+ + 15 AP (CH3 ) = 14.24 eV.
This is the appearance potential of the ion coming from the process
+ CH3SCF3 + e" > CH3 + SCF3 + 2e' 4. 2 £
d) Methyl-perfluoromethyl disulphide
+ 148 IP (CH3SSCF3) = 9.58 ± 0.14 eV
+ This corresponds to AHf (CF3SSCH3 ) < + 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 + CF3 4 2.7
+ This gives AHf (CH3SS ) = AHf (CH SSCFj) - AHf (CFj)
+ + AP (CH3SS /CH3SSCF3) 4.2B 18 -15 eV-
+ and IP '(CHjSS) = AH (CH3SS ) - AHf (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
CH3- SSCH3 is less than 4.47 eV for CF3 — SSCH3, the appearance
+ potential may correspond to the energy required to separate CH3SS
from CH3SSCH3, which is formed from rearrangement.
+ + 64 AP (S2 ) = 12.46 eV.
+ This low value for the appearance potential of S2 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 > S2 + CH3 + CF3 4,30
+ AHf (S2 ) = AH£ (CH3SSCF3) - AHf (CHj)
+ - AHf (CF3) + AP (S2 /CH3SSCF3) 4.31
^ 7.89 eV. - 58 -
Energetically, this is not possible, since it will give,
+ AH£ (S2) = AHf (S2 ) - IP (S2) 4f5 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 + H2 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 (CF3SCF3) = 11.28 ± 0.04 eV
+ From Table XVIII, AHf (CF3SCF3 ) = AHf (CF3SCF3) + IP (CF3SCF3) .4.34
<_ -0.95 eV
f) Bis-perfluoromethyl disulphide .
202* IP (CF3SSCF3) = 10.68 ± 0.19 eV
+ From Table XVIII, AHf (CF3SSCF3 ) = AHf (CF3SSCF3) + IP (CFjSSCFj) 4.3S
< -2.61 eV - 59 -
+ + 64 ' AP (S2 ) = 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
+ CF3SSCF3 > S2 + 2CF3
+ AHf (S2 ) = AHf (CF3SSCF3) - 2AH '(CFj)
+ + AP (S2 /CF3SSGF3) 4.3 6
<_ 10..2 9 eV
which is lower than IP(S2) of 10.8 eV. This will give
AHf (S2) S <_ -0.51 eV.
+ If this S2 ion is produced from the following reactions
' CF3SSCF > CF3.+ SSCF3 4.37
+ CF3SS —-> S2 + CF3 4.3J
+ AH£ (S2 ) = AHf (CF SS.) - AH£ (CFj)
+ • + AP (S2 /CF3SS) 4.39
<_ 12.08 eV.
This gives AH£(S2) =12.08-10.8
<_ 1.28 eV.
This is in fair agreement with 1.34 eV obtained by Evans (45) and
+ supports the fact that there is thermal decomposition or that S2 does not come from the parent molecule.
g) Thiophene
84+ IP (C.H.S) = 9.06 ± 0.16 eV ^44/ 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 (C4H4S ) = AH£ (C4H4S) + IP (C^S) 4.40
<_ 10.25 eV.
This agrees fairly well with 10.14 eV quoted by Franklin (31).
+ + 58 AP (C2H2S ) =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 differ very much from Hissel's 13.6 ± 0.2 eV (48).
The probable process is
+ C4 H4S > CHS + C3H3.
If we take AH^ (CHS+) to be 11.85 eV — the average of the three values obtained in this work
+ A H£ (C3H3) = AH£ (C4H4S) - AH£ (CHS ) .
+ + AP (CHS /C4H4S) ^2.85 eV.
The ionization potential of C3H3 was previously determined to be
8.25 ± 0.08 eV (49). This gives
+ AHf(C3H3 ) = AH£ (C3H3) + IP (C^) .4.42.
<_ 2. 85 + 8.25
1 11-10 eV'
+ 39 AP (C3H3) = 13.3 ± 0.1 eV
The probable process, is: - 61 -
4-. 4 3
+ AH£ (CHS) = AHf (C4H4S) « AH£ (C3H3 )
+ + 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 the validity of the proposed processes.
A':;comparison of the various bond energies supports Mackle 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 - 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 it 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 - 63 -
stabilities'of 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)- PREVI0US WORK
1 CH3 11.36 (40)
CHS+ CH,SCF,. 12.39 11.49 3 3 12.44 12.96 (36) CH SSCH 11.49 CH^SSCF^ 11.67
CH S CH SCH 9.29 9.63 ( 2) 9.43 . ( 4)
S * CH SSCH 12.74 12.09 (31)
CH3SSCF3 12.08
+ CF3S CF3SSCF3 - 5.84
+ CH SCH 8 37 CH3SCH3 3 3 ' / ' -
+ CH3SSCH3 CH3SSCH3 8.60 8.84 (36)
+ CH SCF CH3SCF3 3 3 ,2.17
+ CH3SSCF3 CH3SSCF3 1.33
+ CF SCF CF3SCF3 3 3 -0.95
+ CF3SSCF3 CF3SSCF3 - 2.61 - 6).|. -
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