J . Phys. Chem. 1988, 92, 1794-1803
1794
Photoelectron Spectroscopy of Sulfur Ions Sean Moran and G. Barney Ellison* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5 (Received: September 4, 1987)
Photoelectron spectra of a number of sulfur-containingnegative ions have been measured, and the following electron affinities are reported: EA(S2) = 1.670 h 0.015 eV, EA(HS2) = 1.907 0.023 eV, EA(DS2) = 1.912 f 0.015 eV, EA(CH3S2)= 1.757 h 0.022 eV, EA(CD3S2)= 1.748 f 0.022 eV, EA(CH3CH,S) = 1.947 0.013 eV, EA(CH3SCH2)= 0.868 f 0.051 eV, EA(CH3S) = 1.871 A 0.012 eV, and EA(CH2S) = 0.465 f 0.023 eV. For the first five anions, the photoelectron spectra consist of excitation of the S-S stretching mode of the radical. Wherever possible, the spectroscopic information obtained is used to elucidate geometries and thermochemistry of these reactive intermediates. Results from these sulfur-containing ions and radicals are compared with what is known about their oxygen analogues.
*
Introduction The technique of negativeion photoelectron spectroscopy (PES) is useful for studying the properties (electron affinities, vibrational frequencies, and geometries) of transient species.' One set of such reactive intermediates that has been investigated with PES is the group of oxygen-containing compounds known to be involved in combustion p r o c e s ~ e s . ~Early ~ ~ workes focused on the photoelectron spectra of 02-, OH-, HOC, CH30-, and CH3CH20-. From a theoretical standpoint, it would be interesting to substitute sulfur for oxygen and see how this affects the properties of these intermediates. U p to this point much attention has been lavished on the oxidized forms of sulfur (such as SO, SO2,and SO3)since these play an important role in air pollution and acid rain. Less well studied are the divalent, reduced forms of sulfur. In this paper we report the photoelectron spectra of a number of reduced sulfur-containing anions. Some of these (S2-, HS2-, CH3S;, CH3S-, and CH3CH2S-) are the sulfur analogues of the oxygen-containing species important in combustion. Another, CH3SCH2-, is an example of a carbanion with an electron-donating substituent; its radical, CH3SCH2,is postulated to be an important intermediate in the reaction of OH radicals with dimethyl ~ u l f i d e . ~CH2S-, the radical anion of thioformaldehyde,10is the sulfur analogue of a proposed intermediate in the reduction of carbonyl compounds.' 1 ~ 1 2 Experimental Section Photoelectron spectra of negative ions are taken by forming beams of the ions of interest (M-), crossing them with laser radiation of a fixed frequency (hao), and measuring the kinetic energy distribution of the detached electrons: M-
+ hao
-
M
+ e- (KE)
(1)
Subtracting the electron's kinetic energy (KE) from the energy (1) Mead, R. D.; Stevens, A. E.; Lineberger, W. C. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic: New York, 1984; Vol. 3. (2) Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. Rea. 1985, 85, 235. (3) Benson, S. W.; Nangia, P. S. Acc. Chem. Res. 1979, 12, 223. (4) Celotta, R. J.; Bennett, R. A.; Hall, J. L.; Siegel, M. W.; Levine, J. Phys. Rev. A 1972, 6, 631. (5) Celotta. R. J.; Bennett, R. A.; Hall, J. L. J . Chem. Phys. 1974. 60,
1740. (6) Oakes, J. M.; Harding, L. B.; Ellison, G. B. J . Chem. Phys. 1985, 83, 5400. (7) Engelking, P. C.; Ellison, G. B.; Lineberger, W. C. J. Chem. Phys. 1978,69, 1826. ( 8 ) Ellison, G . B.; Engelking, P. C.; Lineberger, W. C.J. Phys. Chem. 1982, 86, 4873. (9) Atkinson, R. Chem. Reo. 1985, 85, 69. (10) Moran, S.;Ellison, G. B. In?.J . Mass Spectrom. Ion Processes 1987, 80. 83. ( I 1) Pradhan, S. K. Tetrahedron 1986, 4 2 , 6351. (12) House, H . 0.Modern Synthetic Reactions, 2nd ed.; Benjamin: Menlo Park, CA, 1972; Chapter 3.
0022-3654/88/2092-1794$01 S O / O
*
of the photon furnishes the electron affinity of the neutral, EA(M). The instrument and procedure used to obtain the photoelectron spectra in this paper have been described earlier.l39l4 Briefly, some precursor gas with a pressure of a few tenths of a Torr is introduced into an electric discharge ion source. Negative ions are extracted from the ion source, formed into an ion beam by a modified Einzel lens, and mass selected by a Wien filter. The mass-selected ions then enter a high-vacuum region. Here they are intersected at right angles by the intracavity radiation of a Spectra Physics Model 171 argon ion laser operating a t approximately 75 W of C W power on the 488-nm line. Detached electrons enter a double hemispherical electrostatic analyzer and are dispersed according to their energy. This analyzer has a resolution of 20-25 meV (160-200 cm-l) fwhm at a constant transmission energy of roughly 3.8 eV. Since the electron's kinetic energy is measured in the laboratory frame of reference, a transformation to the center-of-mass (CM) frame of reference must be carried out: KE = KEca1 + y(vca1- v) + m w l : ( l / M ) - (l/McaJI (2) Here, KE is the C M kinetic energy of an electron detached from a negative ion of mass M, V is the voltage through which the electron must be accelerated to reach the transmission energy. To fix the kinetic energy scale, a calibration ion of mass Md whose electron affinity is well-known (KE, = h w o - E&J is detached along with the anion of interest; V,, is the acceleration for the electrons detached from the calibration ion. W is the ion-beam kinetic energy (600 eV), m is the mass of the electron, and y is a dimensionless energy scale compression factor measured from the photoelectron spectrum of an ion15*16 (such as NH- or Cr-) whose neutral has two electronic states with a separation of 1 eV or more; typically y is found to be 1.00 f 0.03. The ions studied in this paper were generated from a variety of precursors. These compounds were obtained commercially and used without further purification. With a tungsten filament and a 15-20-mA emission current, 1-3-nA beams of Sy and HS; were produced from CH3SH (Matheson), CH3SCH3 (Eastman Organic), or CH3SSCH3(Aldrich). A tantalum filament operating at 5-10-mA emission current with CH3SCH3or CH3SSCH3as a source gas generated 0.5-1 .O-nA beams of CH,SF. CD3SCD3 (Aldrich) was used to produce the deuteriated anions (DS2- and CD,S;). CH3CH2S-and CH3SCH2-were produced by starting with 1-2-nA beams of NH2- from ammonia and then adding a small amount of either CH3CH2SH (tungsten filament run at 7-10-mA emission current) or CH3SCH3(tantalum filament with 5-mA emission current) to the ion source to generate 200-300-pA beams of the M - 1 anion. Varying amounts of CH3S- were produced by all precursor compounds used. The best yields of (13) Ellis, H. B., Jr.; Ellison, G. B. J . Chem. Phys. 1983, 78, 6541.
(14) Ellis, H. B., Jr. Ph.D. Thesis, University of Colorado, 1983. (15) Engelking, P. C.; Lineberger, W. C. J. Chem. Phys. 1976,65,4322. (16) Feigerle, C. S.; Corderman, R. R.; Bobashev, S. V.; Lineberger, W. C. J. Chem. Phys. 1981, 7 4 , 1580.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1795
Photoelectron Spectroscopy of Sulfur Ions
( and HS,- 1 m / Z = 6 4 amu Laser = 2.540 eV
S;
00
20
60
40
MASS
BO
( A M U )
Figure 1. Mass spectrum of anions produced in electric discharge ion source with CH3SSCH3 as a source gas. Mass resolution ( M I A M ) is approximately 40.
0 5
10
CM PHOTOELECTRON KINETIC ENERGY ( eV ) Figure 3. Photoelectron spectrum of S ; with small amount of HSC taken at 5 mevlpoint. Peak A is origin of S2ground state; range in which excited IAs state of S2 is proposed to lie is marked by bracket. Ions are produced in a discharge of CH3SCH3and H,O.
TABLE I: Peak Positions for SIm/Z = 6 4 amu Laser= 2 . 5 4 0 eV
peak
C M KE, eV
a A' A
0.926 f 0.014 0.905 f 0.015 0.856 f 0.012 0.817 f 0.013 0.766 f 0.01 1 0.727 f 0.013 0.676 f 0.01 1 0.586 f 0.01 1
B' B C' C
D
spliting from origin, cm-' 570 f 100 400 f 100 310 f 90 720 f 70 1040 f 90 1450 f 80 2170 f 80
assignt" (O,O*) (0,O)
(1,0*) (190) (2,0*) (290) (3x9
"Asterisk indicates a transition out of the excited spin-orbit state
(2111,2)of the anion; see Figure 5. 00
os
10
15
20
25
CM PHOTOELECTRON KINETIC ENERGY ( eV ) Figure 2. Survey (10 meV/point) photoelectron spectrum of S2-taken over full energy range of 488-nm laser line; photon energy is marked by hwo. The source gas is CD3SCD3.
CH2S- (up to 500 PA) were obtained from CH3SSCH3with a tungsten filament run at 10-mA emission currents. Results and Discussion By use of various precursors, copious amounts of sulfur-containing negative ions, both in terms of variety and beam current, are produced. Figure 1 shows the mass spectrum obtained from dimethyl disulfide; similar mass scans result from any precursor containing sulfur, carbon, and hydrogen. For this mass scan, conditions in the ion source were optimized to give the best mass resolution between mass peaks 77 and 79 amu; by varying ion source conditions, more intense ion beams (1-2 nA) of the major anions can be produced, but at the expense of mass resolution. The mass peak labeled 32, 33 consists of a mixture of S- ( m / z 32) and HS- ( m / z 33); both and threshold photodet a ~ h m e n t ' studies ~ . ~ ~ of these anions have been carried out previously. Since the electron affinities of both these radicals have been quite accurately determined, their ions are used to calibrate nearly all the spectra discussed in this paper. The thiomethoxide ion (CH,S-), m / z 47, has been scrutinized earlier.' On account of the slightly higher resolution of our apparatus, we are able to observe a number of features that were not seen in the previous spectrum; these features and their interpretation will be discussed or H - C s below. Mass 57 is likely to be either H-S-CkCC-S-. When a deuteriated precursor is used (see Figure 6), the mass of this anion increases by 1 amu, indicating it contains one hydrogen atom. Using the 488-nm (2.540-eV) line of the argon (17) Breyer, F.; Frey, P.; Hotop, H. Z.Phys. A 1978, 286, 133. (18) Breyer, F.; Frey, P.; Hotop, H. 2.Phys. A 1981, 300, 7. (19) Hotop, H.; Lineberger, W. C. J . Phys. Chem. Re$ Data 1985, 14, 731. (20) Janousek, B. K.; Brauman, J. I. Phys. Reu. A 1981, 23, 1673.
ion laser, we see no detachment of this anion. I . S2-. The mass peak labeled 64, 65 is a mixture of S2-and HS;. Figure 2 shows a photoelectron spectrum of S2-extracted from a discharge of CD3SCD3 (see Figure 6) so as to minimize contamination from HS2-. The spectrum covers the entire range of electron kinetic energies accessible at hwo = 2.540 eV with the data points separated by 10 meV. By inspection, the EA can be estimated by subtracting the kinetic energy of the fastest electrons (roughly 0.9 eV) from the photon energy (2.540 eV). This yields an approximate electron affinity for S2of 1.64 eV, which agrees with a earlier determination5 of 1.663 0.040 eV. Just like oxygen, S2 has a 3Z; ground state and a low-lying lAg excited state; the interval between these states is not wellknown. The ground state of SF is 211g;matrix studies suggest that there is appreciable spin-orbit splitting in this anion. Figure 3 shows an expanded spectrum of S2-produced from a mixture of CH3SCH3and H 2 0 in the ion source. Peak A corresponds to the origin of the spectrum. This assignment is based on the agreement of the peak's binding energy with the previously measured EA(S2). Also, the splitting between peaks A and B is 730 f 70 cm-l, which matches the ground-state vibrational frequency of S2 (we = 725.65 cm-1),21while the separation between peak A and the adjacent peak with higher kinetic energy is less than 600 cm-'. This suggests that features with kinetic energies higher than peak A result from detachment of vibrationally excited S2- ions. Peaks A through D are fairly narrow, but below 0.5-eV kinetic energy the peaks are less well-defined. Two plausible explanations come to mind. The first one is that detachment to the low-lying lAg electronic state of S2 is beginning to take place a r o u n d 0.5 eV. There have been various theoretical prediction^^^-^^ and
*
(21) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Norstrand Reinhold:
New York, 1979. (22) Theodorakopoulos, G.; Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. Lett. 1981, 81, 413. (23) Staemmler, V.;Jaquet, R. Theor. Chim. Acta 1981, 59, 501. (24) Swope, W. C.; Lee, Y.-P.; Schaefer, H. F., I11 J . Chem. Phys. 1979, 70, 947.
Moran and Ellison
1796 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
HS2- and S,A
1:
I
cn
-
m / Z = 6 5 , 6 4 amu Laser = 2 5 4 0 eV
X
I
C
s,
3z;
‘E 0
n
0
t-
v
>-
a
K
w z
W 0 0
CM
05
PHOTOELECTRON
IO
KINETIC ENERGY ( e V )
Figure 4. High signal-to-noise (2.8 meV/point) photoelectron spectrum of mixture of S , and HS,. Peaks A through D, A’ through C’, and a are due to detachment of S2-; their kinetic energies are listed in Table I. The energies of peaks resulting from HS2- detachment (peaks Z through T) are collected in Table 11. CH3SH is used as a source gas.
indirect experimental estimate^^^^' of the energy splitting between this excited state and the 32[ ground state; all of these place the IAg excited state between 4000 and 6000 cm-I above the ground state. Blending of peaks due to detachment into the two different electronic states would create a congested spectrum below 0.5 eV. The second possibility is that around 0.5 eV we are starting to see detachment of another ion which has a mass nearly equal to that of S2-,such as HS2-. If there are really two different ions in the spectrum shown in Figure 3, it should be possible to change conditions in the ion source so as to vary their ratio. Figure 4 shows a spectrum of a mixture of S2-and HS2-;only CH3SH was used as a source gas. Since changing ion source conditions should not affect the intensity of detachment into two different electronic states of the same neutral, it is now clear that there are two different ions being detached. Table I lists the peak positions for S2-obtained from this spectrum. By subtracting the kinetic energy of peak A from the laser energy, we obtain a raw electron affinity (uncorrected for rotational broadening and sequence bands) of 1.684 f 0.012 eV. Unfortunately, the enhanced signal from HS2- obscures some of the transitions into higher vibrational states of S2, but the higher signal-to-noise (relative to Figure 3) yields some information about the anion. Peak a is a transition originating in the first excited vibrational state of S;. From its splitting from the origin of the spectrum (peak A), we obtain a vibrational frequency for the anion of 570 f 100 cm-’, which agrees with an estimate of 550 cm-’ from the Raman spectroscopy of S2- in alkali-metal halide crystals.28 The primed peaks A’, B’, and C’ are electronic hot bands. Since the term symbol for the ground state of S2-is zI18,there are two for R, the component of the total different values, 3 / 2 and angular momentum along the internuclear axis. The spin-orbit interaction shifts the energy of these two levels by AAZ, where A is the spin-orbit coupling constant and A ( = I ) and L: are the components of the orbital and spin angular momentum along the internuclear axis. EPRZ9and zero-phonon absorption30 studies in alkali-metal halide crystals have determined that the levels are inverted so that the R = 3 / 2 level is lower in energy and have predicted a spin-orbit coupling constant of -420 cm-I. Figure 5 shows harmonic potential curves representing the 2r11/2 and 2r1312spin-orbit levels of S2- along with that for S2(3Z,-). Transitions from a particular vibrational level of the 2r11,2spinorbit state of Sc to a given vibrational level of the neutral will require less energy than the transition from the same vibrational level of the 2113,2spin-orbit state of the negative ion. In Figure (25) Bondybey, V. E.; English, H. J. J . Chem. Phys. 1980, 72, 3113. (26) Barnes, I.; Becker. K. H.; Fink, E. H. Chem. Phys. Letr. 1979. 67, 314. (27) Carleer, M.; Colin, R. J . Phys. E 1970, 3, 1715. (28) Holzer, W.; Murphy, W. F.; Bernstein, H. J. J . Mol. Spectrosc. 1969, 32, 113. (29) Vannotti, L. E.; Morton, J. R. Phys. Reu. 1967, 161, 282 (30) Vella, G. J.: Rolfe, J. J . Chem. Phys. 1974, 61, 41.
J
5
IZ
3
1 s2-
w
I- 2 O
a
I -
0.-
I
1.8
I .9
2.1
2.0
BOND LENGTH
(8)
Figure 5. Harmonic potentials for the two spin-orbit states of S , and the ground state of S2. Primed peaks in Figure 4 correspond to transi-
tions from the excited spin-orbit state of anion (dashed vibrational levels) to the neutral ground state. For the anion, Re = 2.005 f 0.015 8, and we = 570 100 cm-I; for the neutral, R, = 1.889 8, and a, = 725 cm-’. The change in bond length upon detachment is approximately -0.12 A.
*
4 the splitting between the unprimed peaks, which arise in the R = 3/2 spin-orbit state of the anion, and the primed peaks, which state, is 410 f 90 cm-’. Assuming originate in the excited R = that the Franck-Condon factors for transitions out of the two different spin-orbit states are the same (the two spin-orbit states of the anion have the same bond length), and that the population of the excited states of the anion can be described by a Boltzmann distribution, the intensity pattern of the R = ‘I2transitions should be the same as that for the R = 3/2 transitions with the R = ‘ I 2 transitions scaled down by a Boltzmann factor:
(3) In expression 3, O* is the ground vibrational state of the excited spin-orbit level of the anion, hE (=2AAB) is the splitting between the spin-orbit levels of the negative ion (420 cm-’ in this case), k B is Boltzmann’s constant, and T is the ion temperature. By measuring the intensity ratio of the primed peaks to the unprimed peaks in Figure 4, we find an ion temperature of 625 f 200 I(. In Figure 3, peaks A through F correspond to detachment into the six lowest vibrational levels of the neutral. To model our spectra, we first need to determine the vibrational frequencies. The energy levels of an anharmonic o ~ c i l l a t o rcan ~ ~ be written (4) The splittings of peaks B through F from the origin (peak A) can be fit to an expression of the form G ~ ( u=) G(u) - G(0) = W,U
-
w,x,(u’
+ U)
(5)
The spectroscopic constants resulting from (5) are W , = 720 f 50 cm-’ and W,X, = -3 f 14 cm-I; for the most part the values of W,X, in this paper are so small compared with our spectral resolution (160 cm-’) that they can be ignored. The S2- photoelectron spectrum can be modeled as transitions from a harmonic S< negative ion with a vibrational frequency of 570 cm-’ into a harmonic S2neutral with a bond length2’ of 1.889 8, and a frequency of 725 cm-’ (see Figure 5). Vibrational wave functions (31) Herzberg, G. Molecular Spectra and Molecular Structure I . Spectra of Diatomic Molecules; Van Norstrand Reinhold: New York, 1950.
Photoelectron Spectroscopy of Sulfur Ions
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1797
TABLE II: Peak Positions for HS2-
DS2-
HS2-
peak
CM KE, eV 0.625 f 0.022 0.550 f 0.015 0.475 0.010 0.392 f 0.012 0.313 f 0.010 0.231 f 0.012 0.156 f 0.018
Z Y X
610 f 190 1210 f 170 1880 f 180 2510 f 170 3180 f 180 3780 f 210
*
W V U
T awj =
splitting from origin, cm-l
splitting from origin, cm-l
CM KE, eV 0.620 f 0.01 0.548 f 0.018 0.474 f 0.018 0.398 f 0.021 0.323 f 0.020 0.250 f 0.024
(030)
580 f 1180 f 1790 f 2400 f 2990 f
140 140 160 160 190
3; 30
:! 30 3;
S-S stretch.
I
I
I
I
I
I
I
I
7
0 0
CM
M A S S ( A M U ) Figure 6. Mass scan of ions formed in electric plasma discharge using CD3SCD3as a source gas. Mass resolution ( M I A M ) is roughly 40.
are calculated by numerically solving the vibrational Schriidinger equations for the anion and the neutral in a basis set of cubic spline^.^^^^^ The intensity of each transition is proportional to the Franck-Condon factor. We vary the bond length of the ion until the calculated Franck-Condon factors match the experimentally measured intensities. We obtain the best agreement for an S< bond length of 2.005 f 0.015 A, which agrees quite well with the value of 2.004 A predicted by c a l c ~ l a t i o n sof~ ~the photodetachment cross section of S2-. 2. HS2-. The HSC ion has a closed-shell ‘A’ ground state; detachment of this species can access either the ,A” ground state of HS2 or the low-lying 2A’ excited state. The peaks labeled Z through T in Figure 4 belong to the photoelectron spectrum of HS,-; the energies of these peaks are listed in Table 11. Two observations support this identification. First, the mass scan in Figure 6 shows that when a deuteriated precursor is used, the mass peak labeled 64, 65 in Figure 1 splits into two resolvable peaks with masses 64 and 66. (The peak labeled 62 is believed to be CH2=CHS-, the sulfur analogue of the acetaldehyde enolate anion; since only small amounts, 1100 PA, of this anion were generated, the possibility of detaching it was not investigated.) The second piece of evidence that aids in assigning this ion is the infrared emission spectrum of the HS, radical observed in a flow tube.35 A frequency of 596.3 cm-’ is reported for the S-S stretching motion of the XZA” ground state of HS,, while for DS, this frequency is 593.1 cm-’. Fitting peaks Z through T to a vibrational progression yields a frequency of 610 f 80 cm-’. The assignment of the origin of the spectrum to peak Z (in fact, the identification of a peak between peaks C and D at all) may seem somewhat dubious, until the deuteriated spectrum shown in Figure 7 is considered. Here, the amount of S2- being simultaneously detached is greatly reduced, and peak Z is much more pronounced. (32) (33) (34) (35)
assignta
DeBoor, C. J . Approx. Theory 1962, 6, 50. Shore, B. W. J . Chem. Phys. 1973, 59, 6450. Stehman, R. M.; Woo, S. B. Phys. Reo. A 1981, 23, 2866. Holstein, K. J.; Fink, E. H.; Wildt, J.; Zabel, F. Chem. Phys. Lett.
1985, 113, I .
1
0 5
DS; and S,m / Z = 66,64 amu 0 eV
I O
PHOTOELECTRON
KINETIC ENERGY ( eV ) Figure 7. High signal-to-noisespectrum of DSF (peaks Z through U) with small amount of S c detached simultaneously (peaks A through D); the splitting between data points is roughly 2.8 meV/point. Peak positions for DS,- are recorded in Table 11.
Deuteriation has a very small effect on the frequency of the S-S stretch, which in Figure 7 is fit to be 580 f 90 cm-I. The raw electron affinity for HS,- is 1.915 f 0.022 eV, while that of DS2is 1.920_f 0.014 ev. The detachment process for HS, is similar to the X’A“ X’A’ transition observed earlier6 for H02-; however, the A’A’ excited state of the HS, radical is reported to be 7255 cm-’ above the ground state,35 so we cannot see detachment into this state with 488 nm photons. In the IR emission work, the bending mode in the ground state of the radical was also observed, and frequencies of z 9 0 0 cm-’ for HS2and r700 cm-’ for DS, were reported. Starting with peak X in Figure 4, there appear to be low-energy shoulders on each of the peaks in the S-S stretching progression. These features are split from the major peaks by roughly 300 cm-l, so the separation from the preceding peaks in the S-S stretching progression is about 900 cm-’. It is possible that these shoulders are combination bands made up of one quantum of the bending mode built on each of the peaks in the S-S stretching progression. Since deuteriation causes the frequency of the bending mode to drop, these shoulders may not be observable in the deuteriated spectrum. Attempts to model this spectrum are complicated by the fact that somewhere around or below 0.5-eV kinetic energy, our electrostatic analyzer begins discriminating against low-energy electrons. The low-energy cutoff is the reason that the intensity profiles for the S-S stretching progressions in HS2 are DS2 are so different (peak X is the most intense for HS, and the progression extends through seven vibrational levels, while for DS2 peak Y is the most intense and the progression only extends through six vibrational levels). Nevertheless, it is still possible to estimate a lower limit to the change in the S-S bond length in going from the negative ion to the neutral. We assume that the intensity profile reaches a maximum somewhere between peaks X and V and that the S-S stretching frequency of HS2- is shifted down by the same percentage as CH3SF (see below). A number of theoretical calculations on the geometry of the HS2radical have been performed;3639the concensus seems to be that the S-S bond +-
(36) De, B. R.; Sannigrahi, A. B. J . Comput. Chem. 1980, I , 334 (37) Hinchliffe, A. J . Mol. Struct. 1980, 66, 235.
1798 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
Moran and Ellison
TABLE III: Peak Positions for Methyl Thioperoxide
__
CD&_ _
CHS-
peak
CM KE, eV
b
0.909 0.844 0.779 0.702 0.626 0.550 0.473
a A
B C D E Ow7
splitting from origin, cm-'
f 0.024 f 0.025 f 0.021 f 0.018 f 0.021 f 0.024 f 0.031
1050 f 210 530 f 220 610 f 1230 f 1840 f 2470 f
170 200 220 270
splitting from
CM KE, eV 0.918 0.851 0.787 0.711 0.637 0.564 0.485
f 0.030 f 0.030 f 0.020 f 0.021 f 0.020 f 0.022 f 0.032
origin, cm-'
assignt"
1050 f 230 SO0 f 230
7; 7: (0,O)
620 f 1220 f 1800 h 2440 f
7:
160 150
7;
170
730
250
740
= S-S stretch.
I
C HsSzm / Z = 7 9 amu
0.0
05
1.0
15
20
'T
25
CM PHOTOELECTRON KINETIC ENERGY ( e V ) Figure 8. Survey spectrum of CH3SYtaken at 10 meV/point resolution with 488-nm line of an argon ion laser.
length is 2.04 A. The additional electron in HS1 would most likely be localized on the terminal sulfur atom, but its presence there would increase the electrostatic repulsion between the sulfur atoms and thereby increase the bond length and lower the S-S vibrational frequency in the ion. We obtain a minimum geometry change of -0.15 f 0.05 A, where the negative sign indicates R(HS-S-) > R(HS-S). It is useful to note that the change in the 0-0bond length6 upon detachment of H02- is roughly -0.17 A, so the estimated change in the S-S bond length for HS2- is the right order of magnitude. 3. CH3S2-. In Figure 1 there are two high-mass ions unaccounted for. The first is assigned a mass of 77 amu and is postulated to be the sulfur equivalent of the formate ion, HCS2-. Again, this assumed structure is based on the mass shift observed when a deuteriated precursor is used (78 amu; see Figure 6) and the inability of 488-nm photons to detach this ion [EA(HCO,) = 3.6 eV].40 The second ion has a mass of 79 amu, which shifts to 82 amu upon deuteriation; this indicates the ion contains three hydrogens and suggests that it is CH3S2-. (The assignment of the masses of both these negative ions is further strengthened by the observation that under certain ion source conditions a small amount of CS2- ( m / z 76) is produced and simultaneously detached4' with CH3S2-when mass resolution is poor.) The CH3S2ion has been studied in solution-phase NMR42and detected in the mass spectrum of the products of electron impact on CH3SSCH3!3 The radical resulting from detachment of this ion has been observed by ESR spectroscopy following photolysis of CH3SSCH3'- in an aqueous 6 M HCl glassMand is proposed to be a primary product of the reaction of 3P, mercury with CH3(38) Sannigrahi, A. B. J . Mol. Struct. 1978, 44, 223. (39) Sannigrahi, A. B.; Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. Lett. 1977,46, 415. (40) Yamdagni, R.;Kebarle, P. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 181. (41) Oakes, J. M.; Ellison, G . B. Tetrahedron 1986, 42, 6263. (42) Nilvebrant, N.-0.; Wwddannstrom, S.; Tormund, D. J . Wood Chem. Technol. 1985, 5, 247. (43) Jiiger, K.; Henglein, A. 2.Naturforsch. A : Astrophys., Phys., Phys. Chem. 1966, ZIA, 1251. (44) Isaeva, 0. V.; Razskazovsky, Y. V.; Mel'nikov, M. Y. J . Radioanal. Nucl. Chem. 1986, 106, 281.
CH,S2-
10
0 5
CM
PHOTOELECTRON
KINETIC
ENERGY ( eV )
Figure 9. High signal-to-noise photoelectron spectrum of CH&; data points are split by 22.8 meV. Peak positions are listed in Table 111. 20
B
1
m
0
c
15-
;}.
CD,S,m / Z = 82 amu Laser = 2 5 4 0 eV
ji 2 > i i ,
v
I
L. 10
0 5
CM PHOTOELECTRON KINETIC ENERGY ( e V ) Figure 10. High signal-to-noise spectrum of CD& with 2.8 meV separation between data points. Kinetic energies of peaks are listed in Table 111.
SSCH3.45A survey photoelectron spectrum of this ion is shown in Figure 8; as wilh HS2-, detachment to only one electronic state of the neutral (X2A" X'A') is observed. The vibrational structure discernible even at this low resolution indicates a large geometry change upon detachment of the anion. Subtracting the fastest electron kinetic energy ( ~ 0 . 8eV) from the laser energy gives an approximate EA(CH3S2)of 1.75 eV. A slower scan yields the spectrum shown in Figure 9; the peak positions obtained from this spectrum are compiled in Table 111. Peak A is assigned as the origin of the spectrum based on the difference in the peak splittings at kinetic energies above and below peak A; peaks with higher kinetic energies are split by roughly 550 cm-', while peaks with lower kinetic energies are split by r 6 0 0 cm-'. The raw EA obtained from peak A is 1.761 f 0.021 eV. Fitting peaks A through E to a vibrational progression as before yields a frequency of 610 k 160 cm-' for CH,S,; the splitting between peaks A and a fixes the CH3S2-frequency at 530 f 220 cm-I. Figure 10 shows the spectrum obtained when a deuteriated precursor is used. For the most part the spectrum remains un-
-
(45) Jones, A.; Yamashita, S.; Losing, F. P.Can. J . Chem. 1968, 46, 833.
Photoelectron Spectroscopy of Sulfur Ions
The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988
-- c
CH,CH,S-
A
m / Z = 61 amu
6
I
L a s e r = 2 . 5 4 0 eV
CH ,SC H 2m / Z = 61 amu L a s e r = 2.540 eV
A
N
2
Y
1799
4-
v)
"
I-
z
3 20 '
d
0 0
0 0 0.0
os
10 1.0
I1.5 S
2 .O 2 0
25
C M PHOTOELECTRON KINETIC ENERGY ( eV 1 Figure 12. Survey (10 meV/point) photoelectron spectrum of CH3SCH,; photon energy marked by hwo.
1
I
I
0.3
0.5
0.7
C M P H O T O E L E C T R O N KINETIC ENERGY ( e V ) Figure 11. Photoelectron spectrum of CH,CH,S-; the splitting between the data points is roughly 2.8 meV. Peak positions and assignments are collected in Table IV.
TABLE I V Peak Positions for CH?CH,Speak a
A B C
D
CM KE, eV 0.674 f 0.023 0.593 f 0.013 0.536f 0.020 0.500 f 0.018 0.470 f 0.018
splitting from origin. cm-I 660 f 180
450 f 160 750 f 140 990 f 140
assignto 10; (030) 11; 10; 9;
10
I 5
20
CM PHOTOELECTRON KINETIC ENERGY ( e V ) Figure 13. Spectrum of CH3SCH2- taken at 2.8 meV/point resolution. Estimated eIectron affinity of CH3SCH, is marked by vertical arrow, while the horizontal arrow indicates the large uncertainty in the EA.
11i(?) #cog = C-C stretch; wIo = C-S stretch; w l l = C-C-S bend.
changed; the raw EA decreases to 1.753 f 0.020 eV,' the radical's frequency stays at 610 f 140 cm-I, and the anion's frequency drops to 500 f 230 cm-'. This would seem to indicate that the vibrational mode being excited in the radical is the S-S stretch. It is also possible that the C-S-S bending mode would show little frequency shift upon deuteriation, but this mode would be expected to have a much lower frequency (the C-S-S bend46in CH3SSH is 314 cm-I; the C-S-Cl bend4' in CH3SCl is 245 cm-I). To first order, one would not expect the substituting of a methyl group for a hydrogen (comparing CH3S2-to HS2-) to the change the photoelectron spectrum much since the effects of detachment should be localized in the disulfur region of the molecule. As was the case with HS2-,the low energy electron cutoff makes modeling the spectrum difficult, but we can still make an estimate of the lower bound to the geometry change. Proceeding as before, we obtain ARs+ = -0.11 f 0.02 A. 4. CH3CH2S-. When DS2- was studied, there was originally some question as to the identity of the negative ion with m / z 66 amu; CD3CD2S-also has the same mass-to-charge ratio. For this reason we measured the photoelectron spectrum of CH3CH2Swhich is shown in Figure 11; the peak positions obtained from this spectrum are listed in Table IV. Since the spectrum in Figure 11 looks nothing like the spectrum in Figure 7, we concluded that m / z 66 was indeed DS2-. In Figure 11 peak A is assigned as the origin and yields a raw EA of 1.947 f 0.013 eV; this value is in close agreement with a previous determination of the electron affinity of CH3CH2S (1.953 f 0.006 eV) from threshold detachment studies in an (46) Grassi, G.; Tyblewski, M.; Bauder, A. Helu. Chim. Acta 1985, 68, 1876. (47) Winther, F.; Guarnieri, A,; Nielsen, 0. F. Spectrochim. Acta, Part A 1975, ~ I A689. ,
ICR!8 Both the thioethoxide anion and the radical that results from its detachment possess a plane of symmetry; hence they belong to the C, point group. For detachment originating from the ground vibrational state of the negative ion, only those vibrations of the neutral that are likewise totally symmetric are likely to be active modes. We have assigned the resolvable vibrational structure as single quanta of excitation in u l l ,the C-C-S bend, wl0, the C-S stretch, and wg, the C-C stretch. These assignments differ somewhat from those reported in the threshold detachment work; higher resolution and a deuteriated spectrum would allow for a more definitive assignment of the vibrational frequencies of the CH3CH2Sradical. Since there may be as many as three different vibrational modes of the radical excited upon detachment, and since the low energy electron cutoff may be causing inaccurate peak intensities, attempts to model this spectrum did not yield any reliable conclusions. 5. CH3SCH2-. A number of substituted carbanions, CH2X-, have been s t ~ d i e dthese ; ~ ~anions ~ ~ possess electron-withdrawing or resonance-stabilizing substituents. We have made ion beams of CH3SCH2-,a carbanion with an electron-donating substituent, using a technique previously shown to generate this ion in an ICR,53Le., proton abstraction of dimethyl sulfide with NH2-. Figure 12 is a survey scan photoelectron spectrum of this anion. Only detachment to the electronic ground state of the CH3SCH2 radical is observed; the EA of CH3SCH2is roughly 1.0 eV. At 10 meV/point scanning speed there is no discernible vibrational (48) Janousek, B. K.; Reed,K. J.; Brauman, J. I. J. Am. Chem. Soc. 1980, 102, 3125. (49) Oakes, J. M.; Ellison, G. B. J. A m . Chem. SOC.1983, 105, 2969. (50) Oakes, J. M.; Ellison, G. B. J . A m . Chem. SOC.1984, f06,7734. (51) Moran, S.;Ellis, H. B., Jr.; DeFrees, D. J.; McLean, A. D.; Ellison, G. B. J . Am. Chem. SOC.1987, 109, 5996. (52) Moran, S.; Ellis, H. B., Jr.; DeFrees, D. J.; McLean, A. D.; Paulson, S . E.; Ellison, G. B. J . A m . Chem. SOC.1987, 109, 6004. (53) Ingemann, S.; Nibbering, N. M. M. Can. J . Chem. 1984,62,2273.
Moran and Ellison
1800 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
I
CH,S-
m/i
2 -
I
1
B
47 amu Laser = 2 540 eV =
A
. . . . ..
>.
CH,SCALCULATED SPECTRUM
I
(6,O) (0.0'
-
Iv,
z W I-
* I
.-
..".
.
.,
.,-A ,..:..;
o
, y . . . ' * ' .
vi,.
d-..-:-,
0 0 50
0 50
TABLE V: Peak Positions for CH& C M KE, eV
b a
0.763 f 0.024 0.727 f 0.017 0.667 f 0.01 1 0.636 f 0.01 1 0.575 f 0.013 0.540 f 0.015 0.499 f 0.015 0.446 f 0.014
A
B C D E F
splitting from origin, cm-' 770 f 190 480 f 130
80 100 120 120 110
0 75
CM PHOTOELECTRON KINETIC ENERGY ( e V ) Figure 15. Calculated spectrum (solid line) for CH,S- overlaid on experimental spectrum (data points); geometries used to obtain calculated spectrum are listed in Table VI. Vertical lines are stick spectrum of calculated transitions; they are positioned along the horizontal axis according to their energy, and their height is proportional to the intensity of the transition they represent.
assignt" 3: 3Y (0,O)
260 f 750 f 1030 f 1360 f 1790 f
I
-.-._ .*.--
0 75
C M PHOTOELECTRON KINETIC ENERGY ( e V ) Figure 14. High signal-to-noise (2.8 meV/point) photoelectron spectrum of CH,S-; peak positions and assignments are listed in Table V.
peak
I .I
b
'* I,
/S'
I
(0,O) 3: 3; 2; 3;
An overhead bar indicates a transition into the excited spin-orbit state (2E,i2)of the CH,S. w 2 = CH, umbrella; w , = C-S stretch.
structure. Even at higher signal-to-noise (Figure 13) there is no sign of individual vibrational peaks. There might be a number of reasons for this lack of resolvable vibrational structure. If CH3SCH2- has a pyramidal geometry around the carbanion center, detachment should lead to excitation of the umbrella motion. These modes tend to have low frequencies in the range of 300-400 cm-I. The gas-phase acidity of dimethyl sulfide has been measured in an ICR to be 393.2 f 1.9 k ~ a l / m o l since ; ~ ~ the the proton gas-phase acidity of ammonia is 403.6 f 1.0 kcal/m01,~~ abstraction of dimethyl sulfide by amide is approximately 10 kcal/mol exothermic. If a large portion of this 3500 cm-' of excess energy goes into the CH3SCH2-product, it could lead to vibrational excitation of the ion and thus lead to intense hot bands and sequence bands. Both the ion and the radical should possess other low-frequency torsional modes whose excitation would cause further spectral congestion. Increasing the resolution or finding a means to vibrationally relax the anions may lead to a more analyzable spectrum. The electron affinity of CH,SCH2 is roughly fixed at 0.868 f 0.051 eV. 6. CH,S-. The photoelectron spectrum of CH,S- has been previously measured7 with a spectrometer which had a resolution of 50 meV (400 cm-I) fwhm, so it was unable to resolve the spin-orbit splitting of the *E ground state of the radical. Subsequently, the threshold detachment spectrum of CH3S- in an ICRS5was also recorded; from an observed doubling of the detachment thresholds to various vibrational levels of the radical, a spin-orbit coupling constant of -280 i 50 cm-' was deduced (the coupling constant is negative because, as in the case of S,discussed above, the R = 3 / 2 spin-orbit state is lower in energy). Laser-induced fluorescence (LIF) experiments on the CH,S radical56 have been reported; here also a spin-orbit coupling constant of -280 f 20 cm-I for the 2E ground state was observed. Figure 14 shows a high signal-to-noise (2.5 meV/point) photoelectron spectrum of CH3S-; the peak energies from this spectrum are listed in Table V. The binding energy of peak A gives a raw (54) Bartmess, J . E.; McIver, R. T.,Jr. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic: New York, 1979; Vol. 2. ( 5 5 ) Janousek, B. K.; Brauman, J. I. J . Chem. Phys. 1980, 72, 694. ( 5 6 ) Suzuki, M.; Inoue, G.; Akimoto, H. J . Chem. Phys. 1984,81, 5405.
TABLE VI: Geometries for CH3S and CH3SCH,S microwave derived calcd geom,b ab initio geom,O fi = ' I 2 geomC param fi = 3 / 2 1.791 1.78 f 0.01 1.839 R(C-S), A 1.072 1.090 1.09 R(C-H), 110 112f 1 112.5 L(H-C-S), deg
CH,S' derived geomb 1.845 f 0.015 1.09 113.5 f 0.5
"Reference 58. *This work. 'Reference 59
EA of 1.873 f 0.01 1 eV, in good agreement with the previous EA determinations from PES (1.882 f 0.024 eV) and threshold detachment (1.861 f 0.004 eV). The splitting between peak A and peak B is 260 f 80 cm-I, which agrees with the two previous measurements of the spin-orbit splitting. The same splitting should be observed between all peaks due to transitions originating in some vibrational level of the ion, ending up in a given vibrational level of the radical, and differing only in which of the two spinorbit states of the radical the transition terminates. The weak features a and b are hot bands resulting from transitions from a negative ion with one quantum of excitation in the C-S stretching mode into the vibrational ground states of the two spin-orbit levels of the radical; they are split by 290 f 210 cm-I. Peaks C and D are due to excitation of one quantum of the C-S stretch in the radical; their splitting is 280 f 130 cm-I. It is interesting to note that peaks C and D have different intensities, while peaks A and B have roughly the same intensity. At first we thought there might be another transition that coincided with peak D and enhanced its intensity, but the threshold detachment work and LIF studies have already identified all the symmetric low-frequency modes. In addition, calculations accompanying the threshold detachment work indicate that any Jahn-Teller distortion of the CH3S X2E state is small, so that the excitation of asymmetric modes as in the case5' of CH30- is unlikely. In the LIF studies different rotational band structures were observed in emission depending on which of the spin-orbit levels of the ground state the radical ended up in; this suggested that the two spin-orbit states of CH3S have different geometries. The difference in the peak intensities in the photoelectron spectrum is hence due to different Franck-Condon factors for detachment into the two different spin-orbit states. The high-resolution microwave spectrum of CH3S has been taken$* by assuming the C-H bond length to be 1.090 %, and the H-C-S angle to be l l O o , a C-S bond length of 1.791 A for the ground state is obtained from the B rotational constant. As a first approximation to the anion's geometry, we used the bond lengths (57). Brassard, S. D.; Carrick, P. G.; Chappell, E. L.; Hulegaard, S. C., Engellung, P. C. J . Chem. Phys. 1986, 8 4 , 2459. ( 5 8 ) Endo, Y . ;Saito, S : Hirota, E. J . Chem. Phys. 1986, 85. 1770
Photoelectron Spectroscopy of Sulfur Ions
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
1801
TABLE VI: PES Results for CH,Sspecies CH2S B(jA2) CH,S - R('A,) . .. CHIS- R(2B1)
Tn,eV 1.813 f 0.055 (1.799) 0.0 -0.461 f 0.023 (-0.29)
w,,
cm-I
(859) 1110 f 110 (1059) 860 f 220
RP-G A
wd. cm-I
x,,, cm-'
ref
(312)
(1.683)
71
(990) 450 f 120
(1.611) 1.72 f 0.02 (1.777)
71
10 f 40 72
TABLE VIII: Final Electron Affinities (eV) species
s2 HS2 DS2 CH3S2 CD3SI
CH3CHzS CH3SCH2 CH3S CHzS
raw EA 1.684 f 0.012 1.915 f 0.022 1.920 f 0.014 1.761 f 0.021 1.753 f 0.020 1.947 f 0.013 0.868 f 0.051 1.873 f 0.011 0.468 f 0.023
rotational cor 0.007 f 0.002 0.008 f 0.005 0.008 f 0.006 0.003 f 0.003 0.003 f 0.003
sequence-band cor -0.007 f 0.009
0.002 f 0.004 0.005 f 0.003
0.000 f 0.001 0.002 f 0.002
and the angle predicted in a set of ab initio calculation^.^^ The geometry of the anion was varied until the experimental intensities of the transitions into the 'E312 state of the radical were reproduced; this fixes the geometry of the anion and allows the geometry of the excited spin-orbit state of the radical to be varied until the intensities calculated fbr transitions into the radical's 2El12state matched the experimental intensities. The resulting fit to the experimental spectrum is shown in Figure 15; Table VI lists the geometries derived in the above manner along with the microwave and a b initio geometries. 7. C H 2 S . In a separate paperlo we reported the photoelectron spectrum of the radical anion of thioformaldehyde. The results of these studies are listed in Table VII. Briefly, detachment of CH2S- gives a progression in the C-S stretching mode of the neutral; a hot band fixes the C-S stretching frequency of the anion a t 860 f 220 cm-'. From sequence bands in the out-of-plane umbrella mode, the inversion frequency of the radical anion is determined to be approximately 440 cm-l; any barrier to inversion is less than 100 cm-I high and 10' wide. The electron affinity of thioformaldehyde is 0.465 f 0.023 eV. These results are included in Tables VI11 and IX for the sake of comparison. Table VI11 contains the raw electron affinities of all the sulfur species studied in this paper. These raw EA'S need to be corrected for rotational broadeningms6' and sequence-band effects.60 The rotational and sequence-band corrections for each anion are collected in Table VI11 along with the final electron affinities.
Conclusions In this paper, photoelectron spectra and the molecular parameters that can be gleaned from them have been reported for a large number of sulfur-containing anions. In order to discern trends in the properties of the species studied, the results for the various anions have been grouped in Table IX. For the anions containing two sulfur atoms, adding a substituent to one of the sulfur atoms causes the electron affinity of S2 to increase. This might be because the substituent destabilizes the radical more than the anion by tying up an electron which in S2is involved in back-bonding. The smaller increase in EA for CH3S2could be due to the inductive electron-donating tendency of the alkyl group which would destabilize the anion somewhat. A substituent also lowers the S-S stretching frequency in the neutral and, to a lesser extent, the anion. For HS2the change in the vibrational frequency due to the increase in the reduced m a d 2 relative to S2is less than 1%, so the lowering of the S-S (59) Magnusson, R. J. Am. Chem. Soc. 1986, 108, 11. (60) Nimlos, M. R.; Ellison, G. B. J . Am. Chem. Soc. 1986, 108, 6522. (61) Engelking, P. C. J. Phys. Chem. 1986, 90, 4544. (62) The reduced mass used for all the stretching modes discussed here is approximated by treating the molecule as a diatomic so that, for example, MHS-S
= mSHMS/(mSH + m S ) .
final EA 1.670 f 0.015 1.907 f 0.023 1.912 f 0.015 1.757 f 0.022 1.748 f 0.022 1.947 f 0.013 0.868 f 0.051 1.871 f 0.012 0.465 f 0.023
-0.001 f 0.005 -0.002 f 0.008
TABLE I X PES Results Obtained for Sulfur Ions Disulfur Anions species
s2 HS2 DS2 CH3S2 CD&
EA, eV 1.670 f 0.015 1.907 f 0.023 1.912 f 0.015 1.757 f 0.022 1.748 f 0.022
w ' ~ - ~cm-I ,
w " ~ - ~cm-I ,
725 f 610 f 580 f 610 f 610 f
570 f 100
90
80 90 160 140
530 f 220 500 f 230
Monosulfur Anions species
EA, eV
w ' * - ~ , cm-'
w"~+., cm-'
HS CHjS CH3CH2S CHIS
2.077 120 f 0.000001 2.317 f 0.002 1.871 f 0.012 1.947 f 0.013 0.465 f 0.023
2702 750 f 100 750 f 140 1110 f 110
2648 770 660 860
S
f 110 f 190
f 180 f 220
stretching frequency is caused by a weakening of the S-S bond. With CH3S2half of the decrease in frequency relative to S2 is due to the larger reduced mass of CH3S2. Again, the presence of the substituent lowers the bond order between the sulfur atoms by binding to one of the unpaired electrons which would otherwise participate in a-orbital back-bonding; the bond weakening is less for CH3S2because the methyl group inductively strengthens the S-S bond through the u-bond framework. If another substituent is added to the radical, the frequency of the S-S stretch drops ,~~ another 100 cm-' (wss(HSSH) = 509 ~ m - ' ws+(CH3SSCH3) = 517 U ~ - ~ ( C H , S S H=) 512 ~ m - l ~ ~These ). S-S stretching frequencies (w(S2) = 725 cm-I, w(XS2) 600 cm-', w(X2S2) 500 cm-') correlate well with the bond strengths determined from thermochemistry: DHO (S-S) = 102.5 kcal/ mol,64DH'(HS-S) = 79 f 1 kcal/m01,6~DHo(CH3S-S) = 83 f 1 kcal/m01,~~DHo(HS-SH) = 66 f 2 kcal/m01,~~ and DHo(CH3S-SCH3) = 67.8 f 2 kcal/m01.~~It should be pointed out that for FSSF both the frequency62(us+ = 615 cm-') and the bond strength66 (DH'(FS-SF) = 87 f 10 kcal/mol) are higher than those of the alkyl disulfides, an indication that at least with this dihalodisulfide the bonding is different. In the second part of Table IX, the effects of putting alkyl substituents on a single sulfur atom are charted. Adding a hydrogen atom to sulfur makes it more electronegative, but attaching alkyl groups decreases its ability to bind an electron. As mentioned
=
=
(63) Shimanouchi, J. J. Phys. Chem. Ref. Data 1977, 6, 993. (64) Benson, S. W. Chem. Rev. 1978, 78, 23. (65) Shum, L. G. S.; Benson, S.W. Int. J . Chem. Kinet. 1983, I S , 433. (66) From AHf"(FS) = 3.1 f 1.5 kcal/mol and AHIo(FSSF) = -80.4 f 10.0 kcal/mol (Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A,; Syverud, A. N. J . Phys. Chem. ReJ Data 1985, 14, Supplement 1).
1802 The Journal of Physical Chemistry, Vol. 92, No. 7, I988 TABLE X Thermochemistry of Sulfur Compounds (Energies in kcal/mol) species HS-H CH3S-H CHSCHZS-H HSCH2-H HSS-H CH3SS-H CH3SCH2-H
DHO ref (H-A) 18 93.2 f 2 55 8 8 . 3 f 2 48 88.9 f 2 96 f 1 339.6 f 1.6 43.98 f 0.53 a 70 f 1.5 343.1 f 1.6 40.53 f 0.51 a 70 f 1.5 393.2 f 1.9 53 20.0 f 1.2 u 99.6 i 2.2 AHro(HA) 353.4 f 2 359.0 f 2 357.4 f 2
ref EA(A) 54 53.43 f 0.05 54 42.92 f 0.09 54 45.04 f 0.1
ref
65 64 64
AHfo(A) = DHo(H-A) - AHfo(H) + AHfo(HA)
0-
17
OH 1.827 670 f 0.000 021 3737.76, 0.96966 -139.21 - 0 . 2 7 , ~
S484.3 f 1.6 0.0
ref 19 14 74
17
75
SH 2.317 f 0.002
18
21 21 21
2711., 1.3409 -376.9,
21 21 21
OH3684 f 74 0.9738
18 18
SH2648f 110 1.36
18 18
(6)
CH30 1.570 f 0.022 1045 f 3 1.376 -98 f 1 1
7 57 76 57
CH,S 1.861 f 0.004 740 f 4 1.791 -280 f 20
55 56 58 56
(7)
CH301.361
59
CH3S770 f 190 this work 1.845 0.015 this work
CHpCHzO 1.726 f 0.033 1067
8 77
CHjCH2S 1.953 f 0.006 48 750 f 140 this work
above, this could be caused by the electron-donating nature of the alkyl substituent which would tend to destabilize the anion. The high X-S frequency for HS is due to the low mass of the substituent and the tighter 1s-3p bond a hydrogen atom can make. For alkyl substituents, the heavier substituents and the weaker 2p-3p bonds make for lower X-S stretching frequencies. The results for CHzS included in this table are of course anomalous because of the closed-shell nature of the neutral and the C-S double bond in thioformaldehyde. Often the electron affinity of a radical can be used together with the gas-phase acidity of a compound to determine the homolytic bond strength and the heat of formation of the radical:54
+ EA(A)
TABLE XI: Comparison of Oxygen and Sulfur Species oxygen sulfur species ref species 0 S 1.461 122 f 73 2 . 0 7 7 1 2 0 f 0.000003 0.00000 1 226.5 74 573.6 158.5 74 396.8 0.0 0.0 177 f 2 0.0
QThiswork.
DHo(H-A) = A.Ho,,id(H-A) - IP(H)
Moran and Ellison
Table X lists the S-H and C-H bond strengths for some sulfur-containing species. The gas-phase acidities of the thiol hydrogen for H,S, CH3SH, and CH3CHzSHhave all been bracketed in ICR experiment^;^^ using the electron affinities of HS, CH3S, and CH3CHzSwith (6) yields the first three listed RS-H bond strengths listed in Table XI. The acidities of the alkyl hydrogens for CH3SH and CH3CH2SHhave not been measured; estimates place the C-H bond strength of CH3SH at 96 f 1 k ~ a l / m o l . ~ ~ Also unknown are the gas-phase acidities of HSS-H and CH3SS-H. RSz-H bond strengths have been estimated to be 70 f 1.5 kcal/moI;@ inverting (6) gives approximate gas-phase acidities for these two compounds of 339.6 f 1.6 and 343.1 f 1.6 kcal/mol, which would make them very strong acids. The gas-phase acidity of CH3SCHz-H has been measured in an ICR;53combining this value with our electron affinity for CH3SCH2provides the homolytic C-H bond strength. This value is slightly higher than the value obtained from iodine-catalyzed pyrolysis6' of dimethyl sulfide (96.6 f 1.0 kcal/mol). Table XI compares the molecular constants of the various sulfur-containing species studied in this paper with those of their oxygen analogues. Looking first at the electron affinities, it appears that when the substituent is small, replacing an oxygen atom with a sulfur atom increases the electron affinity of the neutral by about 0.5 eV; for species containing two group VIA atoms, such as HS2, the effect appears to be cumulative; i.e., the electron affinity increases by rl .O eV for two oxygen-for-sulfur replacements. It could be argued that this effect is caused by a stabilization of the sulfur anions relative to their oxygen-containing counterparts; in the sulfur anions the extra electron is located in the 3p valence shell of the sulfur atom where it has more room to minimize its unfavorable interaction with the neutral's compliment of electrons. Certain electronic states possess spin-orbit splittings that are naturally larger for the sulfur species due to sulfur's higher atomic number. Turning next to a comparison of frequencies, we see that vibrations of both the R-X type and the X-X type (R = H, CH3, CH3CH2;X = 0, S) are lower for the sulfur species. For R-X stretching vibrations the frequencies decrease about 30% in going from X = 0 to X = S. There is an insignificant change in the reduced mass of the vibration going from OH to SH, so the frequency reduction is due almost entirely to the shorter bond (67) Shum, L. G. S.; Benson, S.W . Inr. J . Chem. Kiner. 1985, 17, 277.
0 2
0.440 f 0.008 1580.193 1.20752 7918.1
*
s2
4 21 21 21
1.670 f 0.015 this work 725.65 21 1.8892 21
1090 1.35 -160
4 4 21
570 f 100 this work 2.005 i 0.015 this work -410 f 90 this work
H02 1.078 f 0.017 1097.6258 7029.48
6 6 6
HS2 1.915 f 0.022 this work 596.3 35 7255 35
02-
s2-
CH82 1.757 f 0.022 this work 610 f 160 this work CH3SCH2 0.868 f 0.051 this work
length and greater bond strength of the H-O bond yhich involves tighter bonding 2p orbitals on the oxygen; for the alkyl substituents the larger change in reduced masses accounts for about half the frequency reduction. X-X stretching frequencies drop roughly 50% when sulfur replaces oxygen; more than half of this decrease can be attributed to the larger reduced mass for the sulfur species. R-X bond lengths increase 0.3-0.4 A when sulfur is substituted for oxygen, while the increase for X-X bond lengths is about twice as large (0.6-0.7 A). The properties of two mixed species have been studied. The electron affinity of SO measured by PES68is 1.126 i 0.01 3 eV, consistent with one sulfur-for-oxygen substitution; the neutral ground-state vibrational frequencyz2is 1149.2, cm-', intermediate between that of Ozand S2with half the frequency decrease relative to 0, due to the heavier reduced mass of SO. The visible-near-IR emission of HSO has been observed,69and the frequency of the (68) Bennett, R. A. Ph.D. Thesis, University of Colorado, 1972
J. Phys. Chem. 1988, 92, 1803-1807
SO stretching vibration in the ground state is 1013 f 5 cm-I. This is somewhat anomalous since the actual frequency reduction relative to H 0 2 is less than that predicted by the increase in the reduced mass. The X-X bond lengths of the mixed species (RW = 1.481087 8, for SO, 1.54 8, for HSO) are also intermediate between those of the 0-0 and S-S compounds. The splitting between the triplet ground state and the lowest excited singlet state of O2is about 1.0 eV; this splitting is less well known for S2,but is somewhere around 5000 cm-’. For the mixed compound SO the alA X 3 B splitting is 5890 f 20 cm-’. The excited 2A’ electron states have been observed for H 0 2 , HSO, and HS,; it is interesting that this splitting is 14 367 cm-I for HSO, more than 0.5 eV higher than H 0 2 and HS,. We were unable to observe the 2A’ excited state of CH3S2,presumably because its excitation energy puts it beyond the reach of our laser photons. However, C H 3 0 2has a low-lying 2A’ excited state which is less than 1 eV above the ground state;’O if the electron affinity of
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(69) Schurath, U.; Weber, M.; Becker, K. H. J. Chem. Phys. 1977, 67, 110. (70) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1976, 64, 3488.
1803
CH302 is roughly equal to that of H02 (as is the case for CH3S2 and HS2), then it might be possible to detach CH302- into the 2Af excited state of the radical.
Acknowledgment. We thank the Department of Energy for support under Contract DE-AC02-80ER10722. Conversations with Pave1 Rosmus were helpful in identifying CHIS-. RM~tryNO.Sz-, 12185-15-8; HST, 26693-74-3; DSZ-, 113109-54-9; CH&, 97141-28-1; CD,ST, 113109-55-0; CHSCHZS-, 20733-13-5; CH3SCH
