648
BRICEG. HOBROCK AXD ROBERT W. KISER
Vol. 67
ELECTRON IMPACT INVESTIGATIONS OF SULFUR COMPOUNDS. 11. 3-METHYL-%THIABUTANE, 4-THIA-l-PENTENE, AND 3,4-DITHIAHEXANE1 BY BRICEG. HOBROCK AND ROBERT Mi. KISER Department of Chemistry, Kansas State University, Manhattan, Kansas Received August 31, 1961 Appearance potentials and relative abundance8 of the principal positive ions from 3-methyl-2-thiabutane, 4-thia-l-pentene, and 3,4-dithiahexane are reported. Probable ionization and dissociation processes are postulated in agreement with the computed energetics. Heatg of formation, consistent with the proposed processes, are given. From appearance potential data, the ionization potential of C2H6SSH is derived to be 9.4 f 0.3 e.v., in agreement with the group orbital calculations. More accurate values for the proton affinities of hydrogen sulfide and methanethiol are found to be - 197 i 7 and 199 i 10 kcal./mole, respectively.
-
Introduction I n the first of a series of papers concerning the electron impact spectroscopy of sulfur compounds, we reported electron impact data for 2-thiabutane, 2thiapentane, and 2,3-dithiabutane.l I n the course of our further investigation of sulfides and disulfides, we have completed studies of 3-methyl-2-thiabutane, 4thia-1-pentene, and 3,4-dithiahexaiie and report the results in this article. Mass spectral cracking patterns as well as ionization and appearance potentials for the principal positive ions have been determined experiment$lly for these compounds. From the measured appearance potentials, probable processes for the formation of the various positive ions are postulated. Some of the interesting species (HSSf, CH,S+) observed in the mass spectra of the compounds in a previous paper2 also were studied from the mass spectra of the compounds reported in this investigation; therefore, slightly revised values for the proton affinities of hydrogen sulfide and methanethiol are reported based on our additional experimental results. As was reported recently,$ the very interesting species, H2S2+,is observed in the mass spectrum of 3,4-dithiahexane. Experimental The experimental data were obtained using a time-of-flight mass spectrometer. A previous description of the instrumentation has been given.4 The mass spectra which we report were obtained at nominal electron energies of 70 e.v. Calibration of the voltage scale in the determination of ionization and appearance potentials was accomplished by intimately mixing xenon with the compound being investigated. Appearance potentials were determined primarily by the extrapolated voltage difference method.6 The technique of Lossing, Tickner, and Bryce6 and the energy compensation technique7 also was used and appeared to give somewhat better values for the ionization potentials of these sulfur compounds than did the extrapolated voltage difference method. I n early experiments we noted some difficulty in obtaining accurate values for the ionization potentials of the organic disulfides. We found that this problem could be resolved by removing any accumulated heavy deposits of electronic "dirt" from the (1) This work was supported by the U. S. Atomic Energy Commission under contract no. AT(11-1)-751 with Kansas State University. This is a portion of a dissertation to be presented by B. G. Hobrock to the Graduate School of Kansas State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. ( 2 ) B. G. Hobrock and R. W . Kiser, J . Phys. Chem., 66, 1648 (1962); note that a minor change has been made in the title of this series of papers t o avoid any confusion over the term electron tmpact spectroscopy. Thib change was made a t the suggestion of S. Meyerson, and we wish t o thank him for his comments. ( 3 ) R. W.Kiser and B. G. Hobrock, d i d . , 66, 1214 (1962). (4) E. J. Gallegos and R. W. Kiser, J . Am. Chem. Sac., 83, 773 (1961). (5) J. W. Warren, Nature, 166, 811 (1950). ( 8 ) P. P. Lossing, A. W. Tickner, and 11'. A. Bryce, J . Chem. Phys., 19, 1254 (1951). (7) R. W. Kiser and E. J. Gallegos, J . Phys. Chem., 66, 947 (1962).
source components. This was accomplished by treating the ion source components with 12% HF in a Sonblaster ultrasonic sound generator* for 2 min. or less. Longer treatment to the 12% HF in the ultrasonic cleaner was found to cause some coloration of the metallic components. The samples of 3,4-dithiahexane, 3-methyl-2-thiabutane, and 4-thia-1-pentene were obtained from commercial sources. The purity of each of the compounds was checked by gas-liquid partition chromatography. The 3,4.-dithiahexane showed no impurities. Both kthia-1-pentene and 3-methyl-2-thiabutane were found to contain small impurities of lower molecular weight, present in quantities less than 3 mole %. The mass spectrum of 3methyl-2-thiabutane shows no significant differences with serial no. 589 and912in theAPI table~~sonointerferencewasexpected. The impurity in 4-thia-1-pentene appeared to be methyl sulfide, since a peak of variable size appeared at m/e 62 in the mass spectrum. Appearance potential measurements on the ion a t m / e 62 indicated that it was a parent molecule-ion and subsequently the impurity was identified as methyl sulfide by comparison with its reported mass s p e ~ t r u m . ~
Results Experimentally determined mass spectra and appearance potentials for the principal ions formed from the three compounds being investigated are given in columns two and three of Tables 1-111. The probable processes for the formation of the various ions are given in column four and the heats of formation consistent with the proposed processes are presented in the last column. The heat of formation of 4-thia-1-pentene was not available and we therefore estimated it to be 10 kcal./ mole, using the method of Frank1in.l" The heats of formation f o r 3-methyl-2-thiabutane and 3,4-dithiahexane are -21.4311 and - 17.42,12 respectively. Other heats of formation employed in our calculations were those tabulated by Ga1leg0s.l~ Discussion Ionization Potentials.-Ionization potentials mere calculated for the complete series of disulfides, CZH6S2 to CeH& (symmetrical and unsymmetrical) using a group orbital treatment14 and the ionization potential of H2S2(10.2 e.v.)8 determined in this study. A C-SS interaction parameter of 2.06 is calculated using the ionization potential of 8.46 e.v. reported for 2,3-dithia( 8 ) "Sonblaster" Ultrasonic Generator, Series 600; The Narda Ultrasonics Corp., Westbury, L. I., New York. (9) "Mass Spectral Data," American Petroleum institute Research Project 44, National Bureau of Standards, Washington, D. C. (IO) J. L. Franklin, Ind. Eng. Chem., 41, 1070 (1949). (11) W. N. Hubbard, W. D. Good, and G. Waddington, J . Phys. Chern., 62, 614 (1958). (12) W. N. Hubbard, D. R. Douslin, J. P. McCullough, D. W. Scott, S. S. Todd, J. F. Messerly, I. A. Hossenlopp, and G. Waddington, J . A m . Chem. Sac., 80, 3547 (1958). (13) E. J. Gallegos, "Mass Spectrometric Investigation of Saturated I-Ietero-cyclics," Doctoral Dissertation, Kansas State University, Manhattan, Kansas, January, 1962. (14) J. L. Franklin, J . Chern. Phw., 22, 1304 (1954).
March, 1963
TABLE I APPEARANCEPOTENTIALS AND HEATSO F FORMATIOS OF THE FROM 3-METHYL-2-THIABUTANE PRINCIPAL IONS
m/e
15 27 39 41 42
Relative abundance 11.5 52.4 35.8 89.5 22.8
43 45
100.0 21.2
46
6.2
47 48 49 59 61
28.1 62.9 34.3 8.6 6.2
75 77 90 92
83.1 3.9 58.9 2.5
Appearance potentials, e.v. Process 19.4 f 0 . 5 CaHioS -+ CHa+ (1) -+ CzHa+ CHa S CH4 (?) 16.5 f . 5 CsHs+ CHs S 2H R z 21.0 f . 5 -+ CaH5+ CHa SH H 15.2 f .2 1 3 . 5 4, .2 -+ CaHa+ CHz HzS -+ &Ha+ C H I + SH -+ CsH7+ CHa S 1 2 . 7 3, . 2 -+ C H S + CaHs H Hz 16.4 i .4 -+ C H S + CzHs CHz Hz -+ CHzS+ CzHa CHa Hi 15.5 f . 4 -+ CHzS+ CaHa 2H 1 3 . 3 3: . 2 -+ CHaS' CzH6 CHz -+ C H & + CzHz CH4 12.0 f . 2 -+ CHsS+ CzHz CHs 12.1 i . 2 -+ CzHaS+ 2CHa H 16.1 f . 3 -+ CZHKS' CZH3 Hz 1 3 . 5 4, .3 -+ CzHsS' CzHa H 11.7 4: .2 -+ CsH7S+ CHI
+ + + + +
-
8.7 i .2
-+
CaHioS
AHr+, koal. f mole
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
292 274 214 226 227 186 300 267 240 227 242 219 172 234 226 225 217
"
179
TABLE I1 APPEARANCEPOTENTIALS AND HEATSO F FORMATIOS OF TH.E FROM 4-THIA-I-PENTENE PRINCIPAL IONS Relative abundm/e ance 15 15.8 27 26.9 35 37 38 39
18.1 6.3 12.4 73.6
40 41 42 45
8.4 82.9 6.9 92.6
Appearance potential, Process
f2.Y.
17.7 f 0 . 5 16.5 z t . 4
285 283 163 308 388 273 275 218
14.8 i 16.6 f 20.3 P 1 6 . 5 =t
.2 .5 .5 .4
12.7
.3
-+
13.8 f .3
-C
zt
44.0 67.3 11.2 5.1 33.8 72.5 6.2 100.0 6.0 5.2
11.4 =k 11.9 P 1 1 . 5 3, 11.7 f 1 2 . 0 3, 11.0 f
.3 .2 .2 .4 .2 .2
8 . 7 0 rt 0 . 2
+ (?) + CHaS + CHz + CHzS + CH3 + CH + CaH4 + CH3 + S 4- 2Hz + CIHaS + H + Hz + CHs + S H + H + CHz + HzS + E[ CaHs+ + C$s + S CHS" + C3Hl + Hz CHS" + CHa + CzHa C H B + + C3Ha + Hz CHaSi + CaHs CHaS+ + C @ P (CHsSa4) + CaH6 CzHaS+ + CzHa CsHsS+ + CHs
AH€", kcal./ mole
CaH&+CHs+ -+ CzHa+ + CzHs+ +Has+ -t CsH' -+ CaHz+ -+ CaHp+ ' -+ CaHa+
-+
46 47 48 49 61 73 87 88 89 90
640
ELECTRON IMPAlCT IiXVESTIGSTIOS O F SULFUR (2OMPOUKDS
-+ 4 -C
-+ -+ -+
-+
+
CaHsS
+
296 284 227 252 229 248 223 232 211
butane.I6 A value of 8.25 e.v. was calculated for 3,4dithiahexane and is in very good agreement with our experimental value of 8.30 e.v., a previously calculated value of 8.36 e.v.,2 and a literature value of 8.27 e.v.16 The ionization potentials of the other disulfide.15 were calculated to fall in the range of 8.2-8.4 e.v. Heats of Formation of Ions.-In general, the AHf+ of the various ions, as summarized in Tables 1-111, are in agreement with literature values. A few specific cases are discussed in the following paragraphs. m/e = 35.-The formation of H3EI+ is noted in the study of two of these molecules. From 4-thia-1-pentene the formation of' H3S+ appears to be accompanied by the neutral fragments CH C3H4. AHf+(H3S)= 163 kcal./mole is calculated, which agrees well with previously determined values.2
+
(15) K. Watanabe, X. Nakayama, and J. Mottl, "Final Report om Xonization Potentials of Molecules by a Photoionization Method," Dtecember, 1959. Department of Army 5BQQ-01-004ORD-TB2-001-00R-1624. Contraot No. DA-04-200 ORD 480 and 737.
APPEARANCEPOTENTIALS
TABLE I11 AND HEATSO F
FORMATION O F THE PRINCIPAL I O N S FROM 3,4-nITHIAHEXAN$
m/e
26 27 29 33
Relative abundance 8.6 65.0 100.0 7.4
Appearance potential, e.v. 19.5 f 0 . 5 1 7 . 2 rt . 4 14.2 f . 2 15.3 f . 2
Process C 4 ~ h ~ z -Cz&+ + (?) -+ &Ha+ CzHsS S Hz -+ CZH5' CZHSS S -+ HasY 2CzHa SH HsS+ CzHz H CzHaS --t CHS CHa CzHsS H -+
45 46 58
1 3 . 5 1 7 . 8 rt . 5 4.5 5.4 18.6 f .5
li9
10.6
+
16.2 i . 3
--t -+
60 61 64 66
68 70 79
10.6 7.8 7.3 66.7 5.8 0.2 5.5
1 2 . 3 =t 12.5 i 14.9 i 12.2 jz 12.2 f
-
.3 .3
-+
.4
.2 .2
-+
-+
14.6 i .2
-+
-+
94 95 96 122 123 124
+ + + + + + + + CzHzS+ + CzHs + S + H + Hz CzHsS+ + CHs + CHsS + H CzHaS+ + CHs + CHzS + HZ ca&S+ + CZH5 + SH CZHKS' + CZHKS sz++ 2CZH6 HzSz" + 2CzHs (HzSS34) + 2CZHa CHsSz+ + CHz + CZH5 CHaSz + CHs + CzHz + Hz CzHsSz' + CzHa + + +* + +
50.2 10.8 f . 3 4.3 4.8 51.9 8.30 f 0.15 5.8 5.4
-+
-+
+
+
C4HiaSz+
AHf', kcal./ mole
287 218 177 154 270
284 234 248 213 232 282 239 239 229 233 219
174
Two processes are believed to contribute to the formation of HIS+ from 2,3-dithiahexane. These processes CzH25 appear to involve the presence of CzHz 3 H and 2C2H3 SH :as neutral fragments. AHf+(H3S) which results from this study and those values obtained in the previous investigation2 is 165 i 6 kcal./mole. A proton affinity of -197 =k 7 kcal./mole is obtained, which is not greatly different from that reported previously.2 m/e = 49.-The interesting species a t m/e = 49 appears in two of the compounds investigated here, 3methyl-2-thiabutane and 4-thia-1-pentene. For 3methyl-2-thiabutane, if the neutral fragments are assumed to be CzHz CH3, AHf+(cH6s)= 172 kcal./ mole. It is believed that the major portion of the ion at m/e = 49 from 4-thia-1-pentene is due to the natural abundance of S34and that the ion is (CH3S34)+. From the appearance potential, 11.7 e.v., which is nearly the same as 11.9 e.v. for m/e = 47, AHf+(CH3ss4)= 248 kcal./mole. An average value for AHf+(CH6S)from previous study2 and the value of 172 kcal./mole obtained in this work is 163 i 10 kcal./mole. From this value and using AHf+(H) = 367 kca,l./mole and AHr(CH3SH) = -5.46 kcal./mole,l6 the proton affinity of methanethiol is calculated to be -199 A 10 kcal./mole. m/e = 61.-The jion a t m/e = 61 is C&b,s+ and appears in significant quantities in each of the investigated compounds. From 3-methyl-2-thiabutane, two processes are thought to be of about equal probability and lead to AHff(C2H6s)= 226 and 225 kcal./mole. (See Table I for the processes involved.) Good internal agreement is noted as AHf+(C2H6S)= 223 kcal./mole in the 4-thia-1-pentene study. The neutral fragment in this case is ethylene. For the appearance potential of the m/e = 61 ion from 3,4-dithiahexane and for the subsequent heat of
+
+
-+
(16) W. D. Good, J. L. Lacina, and J. P. McCullough, J . Phys. Chem., 65, 2229 (1961).
650
BRICE G. HOBROCK AND ROBERT W. KISER
formation of C2H6S+we obtain 12.5 rfr 0.3 e.v. and 232 kcal./mole, respectively. Our appearance potential is 1.3 e.v. higher than that reported by Franklin and Lumpkin.I7 It was feared that any possible dirty conditions in the source of the mass spectrometer (see Experimental) might prevent our obtaining accurate values as in the case of the ionization potentials of the disulfides. This was not the case, however; we found that an appearance potential of 11.2 e.v. could not be obtained under any experimental conditions in our instrument. In addition, the value of AHf+(C2H6S)= 232 kcal./ mole, subject to some uncertainty in our use of AH,(CPHSS)= 39 kcal./mole, agrees well with the values of 223 and 225 or 226 kcal./mole reported above and with values of 225, 213, and 233 kcal./mole reported in our earlier study.2 We conclude that AHf +(CzH5S)= 225 f 5 kcal./mole, and by using our determined appearance potential of 12.5 e.v., that AHf(CzHSS) = 40 f 4 kcal./mole. These values are somewhat different from those reported by Franklin and Lumpkin.17 m/e = 64.-The presence of Sz+is observed in the mass spectrum of 3,4-dithiahexane. From the measured appearance potential, AHf+(Sz)= 282 kcal./mole, which is in excellent agreement with that calculated from the 2,3-dithiabutane case. The derived ionization potential for S2 is unchanged from that obtained previously, 11.0 i 0.2 e.v.s m/e = 66 and 68.-The ion a t m/e = 66 is quite intense in the mass spectrum of 3,4-dithiahexane and is the very interesting specie, HzSz+. The natural abundance of S34and the abundances of ions a t m/e = 68 to 70 indicate that the ion must contain two sulfur atoms. Measurements on both m/e = 66 and 68 give an appearance potential of 12.2 e.v. for HzSz+. The low value of the appearance potential indicates that the ionization and dissociation process leading to HzS2+ should be relatively simple. The formation of two molecules of ethylene as neutral fragments is such a process and leads to AHf+(HzS2)= 239 kcal./mole.16 K i m and Hobrock3have reported that from these data I(HzSz) = 10.2 e.v. and have concluded that D(HSSH) = 59 rfr 3 kcal./mole. m/e = 79.-CH3S2+ appears in the mass spectrum of 3,4-dithiahexane, even as it did in the mass spectrum (17) J. L. Franklin and H. C. Lumpkin, J. Am. Chem. Soc., 74, 1023 (1952).
Vol. 67
of 2,3-dithiabutane. The structure may be CHzSSH+, however, as we note the ease with which hydrogen and sulfur form bonds (H3S+, CH6S+, H2S2+). Either or both of two processes may be responsible for the formation of CH3S2+ and are shown in Table 111. AHf+(CHsS2) = 240 kcal./mole.2 An average value of 234 kcal./mole together with the estimated value of AHf+(CH3S2) = 21 kcal./mole leads to I(CH3S2) = 9.3 rfr 0.3 e.v. m/e = 88.-This is the parent molecule-ion from 4thia-1-pentene. The measured ionization potential of 8.70 e.v. results inA.Hf+(C4HsS)= 211 kcal./mole, which is somewhat higher than AHf+(C4H8S)= 190 kcal./ mole from tetrahydrothiophene.l* Assuming that the tetrahydrothiophene ion retains its cyclic structure, this difference is probably not too surprising. m/e = 90.-This ion appears to a significant extent only in the mass spectrum of 3-methyl-2-thiabutane and is the parent molecule-ion of this compound. AHf+(C4HIoS)= 179 kcal./mole. m/e = 94.-Instead of simple cleavage of a CzH6 group to give an ion a t m/e = 93, a hydrogen atom apparently rearranges to give the ion a t m/e = 94, C2H6Sz+.Assuming that the neutral fragment is CzH4, AHf+(CzH6S2)= 219 kcal./mole. Now, if we assume that the structure is C2H6SSH,and estimating AHf(C2H6SSH) = 2 kcal./mole, we may calculate the ionization potential of CzH6SSH to be 9.4 rfr 0.3 e.v. By use of the group orbital methodT4and parameters by Gallegos and Kiser,19the ionization potential of C2H6SSH is calculated to be 8.95 e.v. Alternatively, using the parameters associated with the treatment of -SSas a group, I(C2H6SSH) is calculated to be 9.05 e.v. The agreement suggests that the structure of CzH&&+ is CzH6SSHf,rather than the ion which might be formed by rearrangement, CH3SSCH5+. m/e = 122.-The value determined for the ionization potential of 3,4-dithiahexane is 8.30 e.v. and agrees well with literature ~a1ues.l~AHff(C4Hlos~)= 174 kcal./ mole, as calculated from the measured ionization potential and AHf(CIHlOS2). Acknowledgments.-We wish to express our indebtedness to Drs. H. C. Moser and A. A. Sandoval for their suggestion to use the 12% HF solution in the ultrasonic cleaner and for their aid in those operations. (18) E. J. Gallegos and R. W . Kiser, J . Phys. Chem., 66, 136 (1962). (19) E. J. Gallegos and R. W. Kiser, {bid., 66, 1177 (1961).
