662 J. Am. Chem. Soc., Vol. 122, No. 4, 2000
Bahou et al.
Figure 1. Partial IR absorption spectrum of a SCS/Ar (1/2000) matrix sample before irradiation (A), difference spectrum after irradiation at 193
nm for 2 h (B), difference spectrum after further irradiation at 532 nm for 1 h (C), and difference spectrum of a SCS/N2 (1/2000) matrix sample
after irradiation at 193 nm for 2 h (D). The absorbance scale was expanded 10-fold in the region 1285-510 cm-1 and 5-fold (except spectrum A)
in the region 2185-2155 cm-1
.
Typically 10 mmol of a sample mixture was deposited over a period
of 2-3 h. The molar fraction of SCS in argon or nitrogen was typically
1/2000-1/5000.
optimization, and vibrational frequencies were calculated analytically
at each stationary point.
IR absorption spectra were recorded with a Fourier transform infrared
(FTIR) spectrometer (Bomem DA8) equipped with a KBr beam splitter
and a Hg/Cd/Te detector (77 K) to cover the spectral range 430-4000
cm-1. Typically 600 scans with resolution of 0.2 cm-1 were recorded
at each stage of the experiment. The optical path between the FTIR,
the cryogenic system, and the detector was purged with nitrogen to
avoid atmospheric absorption of H2O and CO2.
An ArF excimer laser (193 nm), operated at 10 Hz with energies
2.0-4.0 mJ pulse-1, was employed to photodissociate the matrix
sample. Light sources for secondary photolysis include a KrF excimer
laser (248 nm), a XeCl laser (308 nm), a frequency-doubled Nd:YAG
laser (532 nm), and a tunable dye laser pumped with a Nd:YAG laser
to yield emission in a region 560-620 nm (Rhodamine 590 and DCM
dyes).
Results and Discussion
A. Experimental Observations and Assignments. 1. Ex-
periment with SCS in Natural Abundance. The IR spectrum
of SCS isolated in solid Ar is well characterized; it exhibits
site splitting. Observed lines at 1528.2 (and 1526.4) cm-1
(Figure 1, trace A) are consistent with previous report;21 they
are assigned to the asymmetric CS-stretching (ν3) mode of SCS.
Wavenumbers listed in parentheses are associated with a minor
matrix site with integrated intensity 0.1% of that of the major
site. The symmetric CS-stretching (ν1, 657.7 cm-1 in solid Ar)
is IR inactive, and the weak bending (ν2) mode is reported to
lie at 395.1 cm-1 21
, outside the spectral range of our experiment.
Lines at 2177.9 (and 2177.4) cm-1 are assigned as the ν1 + ν3
combination band. Lines observed at 1524.5 (1522.9) and 2165.2
cm-1 correspond to the ν3 and ν1 + ν3 modes of 32SC34S,
respectively. The ratio of integrated intensities of 32SC34S/
32SC32S is ∼9.4%, consistent with the natural abundance of the
isotopomer 32SC34S. Very weak lines at 2323.8 (2323.3) and
2825.4 cm-1 are assigned to the 2ν2 + ν3 and 2ν1 + ν3 modes,
respectively. Observed wavenumbers of various isotopomers of
SCS are listed in Table 1. Integrated intensities of ν1 + ν3, 2ν2
+ ν3, and 2ν1 + ν3 are ∼0.026, 0.0022, and 0.0002 of that of
ν3, respectively; absorption of ν3 might be saturated.
34SC34S (Cambridge Isotope Laboratories, listed isotopic purity of
90%) was used without purification. Scrambled 32S- and 34S-isotopic
species was produced in a Pyrex vacuum manifold by mixing 32SC32S
and 34SC34S in equal proportions followed by electric discharge with a
tesla coil for a few minutes.
Computational Method
The equilibrium structure, vibrational frequencies, IR intensities, and
single-point energies were calculated with the Gaussian 94 program.15
We used three methods: second-order Moller-Plesset theory16 employ-
ing all active orbitals (MP2-full) and two types of density functional
theory (DFT) calculations, BLYP and B3LYP. The BLYP method uses
Becke’s exchange functional,17 which includes Slater exchange func-
tional with corrections involving a gradient of the density, and a
correlation functional of Lee, Yang, and Parr,18,19 with both local and
nonlocal terms. The B3LYP method uses Becke’s three-parameter
hybrid exchange functional.20 The standard 6-31+G* basis set and
Dunning’s correlation-consistent polarized valence triple-ú basis set,
augmented with s, p, d, and f functions (aug-cc-pVTZ) were used in
all methods. Analytic first derivatives were utilized in geometry
Irradiation of the SCS/Ar (1/2000) matrix with an ArF laser
at 193 nm for 2 h produced new lines at 1270.7, 1270.0, 881.1,
876.5, 522.7, and 517.7 cm-1 and decreased absorption of lines
attributed to SCS, as shown in the difference spectrum in Figure
1, trace B. A positive feature indicates production after
irradiation, whereas a negative feature indicates destruction.
Approximately 15 ( 5% of SCS was photodissociated. Further
irradiation of the matrix sample with laser light at 532 nm from
a frequency-doubled Nd:YAG laser for 1h diminished lines at
876.5, 881.1, 517.7, and 522.7 cm-1 nearly completely, whereas
lines at 1270.7 and 1270.0 cm-1 remained unchanged, as
illustrated in the difference spectrum in Figure 1, trace C. The
photodissociated SCS precursor was recovered to nearly 65 (
10% of original content after this process.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Mongomery, J. A.; Raghavachari, K.; Al-laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen. W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision D3; Gaussian
Inc.: Pittsburgh, PA, 1995.
The line at 1270.0 (1270.7) cm-1 is readily assigned to the
CS photofragment; the wavenumber is consistent with previous
report (1275.4 and 1270.2 cm-1).22,23 C2S2, which has an intense
(16) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988,
153, 503-506.
(17) Becke, A. D.; Phys. ReV. A. 1988, 38, 3098-3100.
(18) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B. 1988, 37, 785-789.
(19) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200-206.
(21) Givan, A.; Loewenschuss, A.; Bier, K. D.; Jodl, H. J.; Chem. Phys.
1986, 106, 151-159.
(22) Bohn, R. B.; Hannachi, Y.; Andrews, L. J. Am. Chem. Soc. 1992,
114, 6452-6459.
(20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.