6986
J. Phys. Chem. 1993,97, 6986-6989
Ab Initio Vibrational Frequencies of the Triflic Acid Molecule Shridhar P. Gejji, Kersti Hermansson, and Jan Lindgren' Institute of Chemistry, University of Uppsala, Box 531, S-75121 Uppsala, Sweden Received: February 18, 1993; In Final Form: April 14, I993
The optimized geometry, harmonic vibrational frequencies, and infrared intensities of the trifluoromethanesulfonic (triflic) acid, CF3S020H, have been determined by ab initio self-consistent Hartree-Fock calculations and second-order Maller-Plesset perturbation theory with 6-3 1G** and lower basis sets. The optimized geometry of the triflic acid molecule is in good agreement with that reported from electron diffraction experiments. The overall symmetry for the molecule, however, is C1,and not Csas suggested from the experiments. The symmetric CF3 and SO2 stretching vibrational modes of the triflic acid are seen to be reversed compared to the assignment from the infrared spectra, reported earlier in the literature. The CF3 stretchings and the O=S=O bending normal modes include strong couplings of different internal coordinates. The vibrational frequencies and the infrared intensities are sensitive to the basis set choice as well as to electron correlation effects. n
Introduction Trifluoromethanesulfonic (triflic) acid, which is a powerful proton donor,' has found many useful applicationsin the synthesis of coordination compounds and in organometallicchemistry2 ever since it was first synthesized by Haszeldine and Kidd.3 A review of its uncommon properties and its reaction chemistry has been given by Howells and M ~ C o w n .Haszeldine ~ and Kidds assigned the frequencies for S=O and C-F stretching vibrations in the acid on the basis of a comparison with CH3SO3 and other related compounds. Balicheva et aL6 have presented infrared spectra of the acid and its aqueous solution. The assignment of the vibrational frequencies presented in their work was based on a comparison with spectra of the deuterated compound and the potassium salt of the triflic acid. The Raman spectrum of the liquid substance has also been presented.' The infrared and Raman spectra of the acid in the solid, liquid, and gaseous phases under different experimental conditionshave been measured and assigned by Varetti.8 The geometry of the triflic acid molecule has been investigated by electron diffraction.9 The position of the hydrogen atom in the acid, however, could not be determined, and it was conjectured that the molecule as a whole exhibits Cssymmetry. Recently, Bencivenni et a1.lOstudied the geometry of the triflic acid molecule by ab initio methods considering both the staggered and eclipsed conformationswith two orientations for the 0-H bond, viz., the 0-H bond pointing out of the O=S-0 plane and the 0-H bond constrained within the O=S-0 plane. The calculations showed that the former orientation (with overall C1 point group symmetry) is more stable by -9 kJ mol-'. In this paper we report the fully optimized geometry and the harmonic vibrational frequenciesof the triflic acid molecule from ab initio calculations with the second-order Mdler-Plesset (MP2) theory using the 6-31G** basis.
Computational Method Ab initio Hartree-Fock (HF) self-consistent-field molecular orbital calculations have been performed using the GAUSSIAN 90 program" with three different basis sets, viz., (a) 3-21G* (polarization functions only on the sulfur atom), (b) 6-31G* (polarization functions on all atoms except hydrogen), and (c) 6-31G** (polarization functions on all the atoms). The equilibrium geometries in the H F calculations were obtained by the gradient relaxation method of Pulay12 using Berny's algorithm,13 which evaluates the gradients analytically. The harmonic vibrational frequencies were obtained from the diagonalization of the force constant matrices. A complete geometry optimization 0022-3654/93/2097-6986$04.00/0
Figure 1. OptimizedMP2/6-3 1G** geometryof the triflicacid molecule.
of the triflic acid molecule was also carried out at the MP2 level with the basis sets as in (b) and (c). For the evaluation of force constants and frequencies from the correlated MP2 theory analytical first-derivative techniques were used, and the second derivatives of the energies were computed numerically. For the HF- and MP2-derived force constant matrices additional normal-mode analyses were performed, where potential energy distributions (PED'S) expressed in internal vibrational coordinates were obtained. These calculations were performed using a local version of a normal-coordinate analysis program originally written by Gwinn.14
Results and Discussion The resulting optimized geometries, normal modes, force constants, vibrational frequencies, and infrared intensities from the ab initio calculations are discussed in the following. OptimizedGeometries. The triflic acid molecule in its staggered configuration, with C1 point group symmetry, is shown in Figure 1. Optimized geometry parameters for bond lengths and bond angles of the triflic acid molecule from the ab initio HF/3-21G*, HF/6-31G*, HF/6-31G**, MP2/6-31G*, and MP2/6-31G** calculations are presented in Table I. The extension of the basis set from 3-21G* to 6-31G* at the H F level leads to an increase in the C S and S-01 bond lengths by 0.048 and 0.014 A, respectively, and a decrease of 0.03 A for the different C-F bond lengths. The S-02 and S-03 bonds are remarkably insensitive to the basis set quality. The addition of 2p polarization functions on the hydrogen atom (6-31G** basis) results in a decrease of 0 1993 American Chemical Society
Vibrational Frequencies of Triflic Acid
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6987
TABLE I: Optimized Geometry Parameters (Bond Lengths in A and Bond Angles in deg) for the Triflic Acid Molecule (see Figure 1) HFA HFA H V MPY MP2/ ex t ~~
3-21
*
6-31
6-31
1.816 1.571 1.410 1.418 1.305 1.304 1.310 0.956 3.193 2.417 99.4 107.4 107.2 108.8 110.1 109.2 110.9 112.8 134.3 177.7 120.4 120.1 98.8 13.1
1.816 1.568 1.410 1.418 1.305 1.304 1.310 0.951 3.188 2.418 99.4 107.3 107.2 108.8 110.1 109.2 111.2 112.9 134.2 177.7 120.4 120.1 99.7 12.2
**
6-31
*
~~~
6-31G**
ref9 1.832(3) 1.557(2) 1.417(1) 1.417( 1) 1.330(1) 1.330(1) 1.330( 1) ~~
RG-9 R(S-01) R(S-02) R(S-03) R(C-Fl) R(C-F2) R(C-F3) R(O 1-H) R(O2-H) R(03-H) L01s-C L02s-c L03-S-C LF1-C-S LF2-C-S LF3-C-S LS-Ol-H LOls-c-02 L02s-C-03 LO 1S-C-F 1 LF 1-C-S-F2 LF2-C-S-F3 LH-01s-C LH-01S-03
1.768 1.557 1.410 1.418 1.339 1.336 1.342 0.973 3.221 2.551 97.8 107.4 107.4 108.9 110.8 109.2 117.7 113.4 134.1 177.6 120.5 120.3 109.9 1.6
1.833 1.622 1.445 1.453 1.334 1.331 1.338 0.981 3.257 2.452 98.5 107.3 107.0 108.6 110.2 109.2 108.1 111.6 135.6 178.2 120.3 120.4 97.6 13.7
1.833 1.621 1.445 1.452 1.334 1.331 1.338 0.972 3.247 2.447 98.5 107.3 107.0 108.6 110.2 109.2 108.0 111.6 135.6 178.2 120.3 120.4 97.6 13.7
bond could not be determined by experiment. The H-01-S-C dihedral angle is predicted to be nearly 97O from the MP2/631G** calculations. The zero-point vibrational energies at the HF/3-21G*, HF/ 6-31G*, HF/6-31G**, MP2/6-31G*, and MP2/6-31G** levels are 26.75,27.32, 27.39,24.85, and 24.98 kJ mol-', respectively. The total electronicenergy for the optimized triflic acid geometry at thevarious levels are-953.895 06, -958.769 246, -958.776 190, -960.123 243, and -960.134 607 au. Thedipolemoment for the triflicacidmoleculeat theoptimized MP2/6-31GS* geometry is 2.725 D. The Mulliken net atomic
chargesforS,C,01,02,03,F1,F2,F3,andHare1.610,0.873, 102.3(16) 105.3(8) 105.3(8) 110.3(2) 110.3(2) 110.3(2)
169.5( 13)
0.005'A in the 01-H bond length and a decrease of 0.003 A in the S-01 distance. The S-01-H bond angle decreases by 7 O when the basis set is improved from 3-21G* to 6-31G* but is virtually unaffected by the further extension to the 6-3 1G** basis. The remaining bond angles are nearly unchanged by the basis set improvements. Electron correlation gives rise to lengthenings of the C S and S-01 bonds by 0.0160.017 and 0.048-0.051 A, respectively. TheS-02 and S-03 and the different C-F bond lengths increase by -0.03 A and the 01-H bond by -0.02 A. The bond angles are nearly insensitive to the electron correlation effect, except for the S-01-H bond angle which decreases by 3'. In Table I we also report the molecular structure of triflic acid obtained from electron diffraction experiments6 It should be noted that in the model used in the determination of the experimental geometry, a pyramidal symmetry for the CFj group was assumed and the 01-H bond length and S-01-H bond angle werefixed at 0.96Aand 115O. Theoverallexperimentalgeometry parameters, however, are in good agreement with the MP2/631G** results. The position of 01-H bond relative to the S-01
-0.670, -0.600, -0.636, -0.326, -0.321, -0.339, and 0.411 au, respectively. Normal-Mode Analysis. In order to analyze the computed normal coordinates with respect to a molecule-fixed coordinate system, we defined a nonredundant set of the internal coordinates consisting of eight stretching coordinates, uiz., C S , S-01, S-02, S-03, C-F1, C-F2, C-F3 and 01-H stretchings, 11 bending coordinates, uiz., 01-S-C, 02-S-C, 0 3 4 4 2 , F1-CS, F2C S , F 3 - C S , 0 2 S - 0 1 , 0 3 S - 0 2 , F2-C-F1, F3-C-F2, and S-01-H bendings, and the two internal torsions defined as the dihedral angles F2-CS-02 and H-01-S-C. The CF3 rocking coordinates are thus expressed in terms of bending coordinates in the present work. The PED matrix components greater than 7% from the MP2/6-31G** calculations are presented in Table I1 for all the normal modes. For most of the normal modes, the PED components do not sum up to loo%, indicating that the secondary (or interaction) force constants are important. Force Constants. The primary internal force constants from the different H F and MP2 calculations are presented in Table I11 for the stretching as well as the bending modes along with some interaction constants involving the different C-F and S-0 1 stretchings. The S - 0 1 force constant is much smaller than the corresponding one for S-0 as is reflected in the different SO bond lengths presented in Table 1. The force constant for the C-F3 stretching is slightly different from that for C-F1 or C-F2, a consequence of the loss of pyramidal symmetry at the CF3 end of the molecule. The different SO/SO interaction constants are observed to be very small. The different CF/CF interaction constants, on the other hand, are nearly 10% of their primary force constants. The full force constant matrix is available from the authors on request. Vibrational Frequencies and Infrared Intensities. The uibrational frequencies of the triflic acid molecule from the HF/3-
TABLE II: The PED Matrix (from MP2/631G**) for the Triflic Acid Molecule normalmode frw,cm-l
1 60
2 185
3 191
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 282 308 336 350 446 483 558 561 608 774 845 1155 1184 1278 1293 1300 1470 3808
C S stretch 41.4 7.9 S-01 stretch S - 0 2 stretch S-03 stretch C-F1 stretch C-F2 stretch C-F3 stretch 01-H stretch 01-S-C bend 28.5 68.1 12.9 14.0 21.9 0 2 S - C bend 123.4 9.3 31.1 31.5 9.0 03-S-Cbend 8.2 33.5 104.0 39.9 11.2 F 1-CS bend 45.2 64.4 33.7 38.2 9.9 F2-C-S bend 14.0 100.4 31.2 55.0 12.5 F 3 - C S bend 102.0 15.9 47.3 16.0 7.1 50.5 31.3 0 2 4 - 0 1 bend 7.8 62.7 0 3 4 - 0 2 bend 8.6 37.1 86.3 9.5 F2-C-F1 bend 9.1 9.6 19.1 7.2 35.9 35.1 F3-C-F2 bend 24.1 28.9 63.7 S-01-H bend 8.3 C S torsion 112.5 S O 1 torsion 8.0 16.1 51.7 17.2 18.1
15.6 13.4 79.1 10.3 9.4 9.7
12.6 10.1 28.9 9.0 28.2 9.4
9.3 52.6 8.0 10.8 40.8 35.8 42.8 17.6 68.3 62.9 18.2 100.1
11.3 23.0 22.1 9.8 7.2 7.4 43.4 7.6
7.6
11.6 7.6
9.3 15.8
7.4 8.2
25.4 15.8 114.8 38.7
13.3
Gejji et al.
6988 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
TABLE In: Internal Force Constants (in mdyn Triflic Acid Molecule force constant f(CS) AC-Fl) f(C-F2) f(C-F3) AS-01) AS-02) f(s-03) f(O1-H) f(F1 -C-F2) f(F2-C-F3) f ( F 1- C S ) AF2-CS) f(F3-C-S) f(OlS-02) f(02S-03) f(0lS-c) f(o2-s-C) f(03S-C) f(S-01-H) f(CFl/CF2) f(CF1 /CF3) f(S02/S03) f(SOl/S02) A S 0 1 /S03)
of the
HF/ 3-21G*
HF/ 6-31G*
HF/ 6-31GS*
MP2/ 6-31G*
MP2/ 6-31G**
4.6 8.1 8.2
4.2 8.5 8.5 8.2 6.3 13.0 12.5 9.0
4.2 8.5 8.5 8.2 6.4 13.0 12.5 9.4 2.0 2.0 1.2 1.4 1.3 1.4 1.9 0.9 1.5 1.3 0.7
3.3 7.0 7.0 6.8 4.7 11.3 10.9 7.6 1.6 1.7 1.1 1.2 1.1 1.1 1.5 0.7 1.1 1.o 0.6
3.3 7.0 7.0 6.8 4.8 11.3 10.9 8.1
0.8 0.8 0.0 0.2 0.2
0.7 0.7 -0.1 0.1 0.1
0.7 0.7 -0.1 0.1 0.1
8.O 7.3 13.3 12.9 8.2 1.7 1.8 1.2 1.4 1.2 1.5 2.0 1.o 1.5 1.3 0.5
2.0 2.0 1.2 1.4 1.3 1.4 1.9 0.9 1.5 1.3 0.7 0.8 0.8 0.0 0.2 0.2
0.5 0.5 -0.1 0.2 0.2
experiment. The MP2/6-3 1G** vibrational frequencies for the S02, S-01, CF3, and 01-H stretchings are -90, 125, 130, and 300 cm-I lower than the HF values, respectively. As shown in Table IV the infrared intensities are sensitive to basis set quality and electron correlation effects. Extension of the basis set from 3-21G* to 6-31G** leads to an enhancement of the infrared intensities of the SO2 stretchings and the CF3 stretching (at 1300cm-I) modes. The intensities for the S-01 and 01-H stretchings are lowered. The infrared intensities for the low-lying bending modes, except for those at 213, 344, and 391 cm-I in the HF/6-3 1G** calculations, decrease as the basis set is extended. Comparison with Experimental Vibrational Frequencies. In this subsection, we compare the vibrational frequencies from the ab initio MP2/6-31G** calculations with those observed in the infrared spectrum of the monomeric gaseous acid8 in the region 450-3590 cm-l. No such comparisons were possible for the lowwavenumber vibrations since the gas-phase measurements for the triflic acid in this region were not available. SOz, CFj, and C-S Stretching. The bands at 1277 and 1456 cm-1 observed in the infrared spectra of the gaseous triflic acid both at 100 OC and a t room temperature were assigned in ref 8 to the symmetric and antisymmetric SO2 stretching modes, respectively. We have assigned the 1470-cm-I mode (no. 20 in Table IV) in the MP2/6-3 1G** calculations as an 502 stretching mode. Two modes at 1300 and 1293 cm-l (no. 18 and no. 19) in the calculations have been correlated with the observed 1277-cm-I band. Our assignment of these two modes to CF3 stretchings, however, differs from the experimental work where, as said above, the 1277-cm-1 band was assigned as SO2 antisymmetric stretching. The observed bands at 1220 and 1157 cm-I were assigned to antisymmetric and symmetric CF3 stretchings, respectively, by VarettL8 The calculated vibrational frequencies at 1278 (no. 17) and 1184cm-l (no. 18) have been correlated with these bands. It may be noted that our assignment of the 1184-cm-I band to an SO2 stretching mode thus differs from the assignment based on the observed spectra. Furthermore, the 1278-cm-1 mode in the calculations is from Table I1 seen to be comprised of C S stretching in addition to the CF3 stretchings.
-
1.6 1.7 1.1 1.2 1.1 1.1 1.5 0.7 1.1 1.o 0.6
'The bending constants were normalized with the factor (rlr2)-', r l and r2 being the lengths (in A) of the bonds forming the angle.
-
21G*, HF/6-31G**, and MP2/6-31G** calculations are presented in Table IV. All vibrational frequencies below 550 cm-I change only by 5-10 cm-I with an expansion of the basis from 3-21G* to 6-31G*, while those in the range 550-1 600 cm-I change by 20-30 cm-1. The frequencies for the S-01-H bending and the 0-H stretching modes show a strong basis set dependence. The vibrational frequencies with the 6-31G* basis are not presented in this table since the only modes which are affected by the presence of 2p polarization functions on the hydrogen atom are the S-01-H bending (decreased by 21 cm-I) and the 01-H stretching (increased by 74 cm-I). The effect of electron correlation is important and invariably brings the calculated frequencies into closer agreement with
TABLE I V Vibrational Frequencies (cm-') and Infrared Intensities (km mol-') (in Parentheses) of the Triflic Acid Molecule obs ref 8
ref 8
assignmentd Dresent work CSt CF3 b
201'
CF3r
312'
CSs
340'
wag S-C-F3 wags-0-H
569'
CF3ab
621' 771b 857' 1122' 1157' 1220c 1277'
S02b CF3sb S-01 s Sal-Hb CF3ss CF3 as SO~SS
1456' 3585'
S02as 01-HS
HF/6-31G** int
V
MP2/6-31G** V int
normal mode
+ SO2 b
73 206
(3.4) (32)
66 207
(1.8) (13)
60 185
(1.4) (14)
1 2
CF3 b + SO2 b SO2 b CS s + CF3 b SO2 b CF3 b
216 282 350 371
(4.3) (132) (2.2) (4.8)
213 328 344 375
(4.8) (66) (3.9) (2.1)
191 282 308 336
(3.1) (61) ( 1.O) (1.0)
3 4 5 6
382 493
(22) (34)
391 497
(44) (29)
350 446
(32) (30)
7 8
538 585 600 669 825 998 1165 1299 1401 1420 1429 1584 3825
(55) (3.7) (9.9) (241) (17) (310) (112) (241) (51) (226) (192) (330) (263)
544 613 62 1 684 859 969 1245 1277 1407 1426 1440 1556 4096
(44) (1.2) (3.4) (221) (4.4) (254) (80) (341) (81) (222) (280) (391) (239)
483 558 561 608 774 845 1155 1184 1278 1293 1300 1470 3808
(23) (0.9) (0.8) (143) (18) (211) (76) (255) (85) (193) (221) (245) (149)
9 10 11 12 13 14 15 16 17 18 19 20 21
+
SO2 b SO2 b 496*
HF/3-21GS int
V
+ CF3 b
SO2 b CF3 b CF3 b SO2 b CS s + CF3 s s-01 s Sal-H b so2 s CFas CS s CF3 s CF3 s so2 s 01-H s
+
a From ref 7. IR spectrum of saturated gas at ca. 100 OC. IR spectrum of a gas at room temperature. The following notations are used: s = stretch, ss = symmetric stretch, as = antisymmetric stretch, b = bend, sb = symmetric bend, ab = antisymmetric bend, t = torsion, r = rock.
Vibrational Frequencies of Triflic Acid The vibrational frequencies from the MP2/6-31G1* calculations are on the average 30 cm-' higher than those observed experimentally in the region 1470-1 150 cm-I. Our assignment of the C S stretching for the mode at 308 cm-1 (no. 5) agrees with the one reported by Varett? for the liquid and solid acid. It should be noted, however, that observed vibrational band at 771 cm-1 was assigned to C S stretching by Balicheva et ale6and Edwards.15 Our normal-mode analysis shows that the C S stretching coordinate in fact contributes to several different normal modes at higher wavenumbers, including mode no. 13 at 774 cm-1. The S-01-H Vibrations. The calculated 01-H stretching mode (no. 21) occurs at 3808 cm-I, Le., 223 cm-I above the experimental frequency of the gas a t room temperature. This discrepancy is largely due to the neglect of the anharmonicity correction, which is 150-200cm-I for 0-H stretchingvibrations of unbound 0-H bonds.'' The in-plane S-01-H deformation in the triflic acid was observed at -1122 cm-l in both the liquid- and gas-phase experimental spectra. The calculated S a l - H bending frequency is found at 1155 cm-I (mode no. 15). A C-S-01-H torsional or out-of-plane S-01-H bending mode was observed at -700 cm-I for the liquids and at -660 cm-I for the solid.8 No such assignment was presented for the gaseous samplein ref 8. It may be noticed from Table I1 that the torsional coordinate contributes to several normal modes. The normal modes with vibrational frequencies at 184 and 282 cm-I (no. 2 and no. 4) have substantial contributions from this internal coordinate. No single normal mode could therefore be assigned as H-014-C torsion for the free triflic acid molecule. The rather high wavenumbers observed for the condensed phases suggest that this mode is very sensitive to bonding effects, e.g., from hydrogen bonding. CF3 and +S=O Bendings. The symmetric and antisymmetric CF3 bendings have been assigned* to bands at 771 and 569 cm-1, respectively, in the infrared spectra of gaseous triflic acid a t ca 100 OC. The calculated frequencies are 774 (no. 13) and 558 cm-1 (no. 10). A strong coupling of the SO2 and the CF3 bending coordinates is also seen for the latter normal mode. The normal mode a t 774 cm-1, on the other hand, does not include O-S-0 bending but involves very strong couplings of the S-01, C S , and CF3 stretching coordinates in addition to the CF3 bendings. The O=S=O bending in the infrared spectrum is found at 621 cm-1, which is 13 cm-1 higher than the calculated frequency (mode no. 12). Comparison between the ab Initio Results for the Triflic Acid and the Triflate Anion. Force Constants. Calculated force constants for the triflate anion were reported in ref 16. The C S , S-0, and C-F primary force constants from the MP2/6-31G** theory were 3.2, 9.5, and 6.2 mdyn A-', respectively. Thus, the addition of a proton to the triflate anion is accompanied by (a) an increase in the S-02 and S-03 force constants by 19 and 15%, (b) an increase in the different C-F force constants by about lo%, and (c) a nearly 50% decrease for the S-01 stretching force constant. The C-S force constant remains unchanged. Vibrational Frequencies. The symmetric and antisymmetric CF3stretchings in the triflate anion occur at 1284 and 1214cm-l, respectively. For the triflic acid the CF3 stretchings were found at 1278, 1293, and 1300 cm-1. The addition of a proton to the anion results in a wavenumber upshift of 160 cm-I for one of the SOz stretching normal modes (no.20) as compared to the case of the anion. The normal mode at 774 cm-1, which is comprised of C S and CF3 stretchings in the acid, is shifted to 757 cm-I for
-
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6989 the free anion. The relative insensitivity of this complicated mode to the bonding environment is intriguing. The SO2 bending mode at 608 cm-1 in the acid (no. 12) shows a great resemblance to the mode at 641 cm-1 in the anion, not only regarding the coordinates involved but also regarding the infrared intensities which are rather similar (143 km mol-' in the acid and 172 km mol-' in the anion) and large compared to their respective neighboring modes. The degenerate CF3 bending mode at 571 cm-I in the anion is split into two modes at 558 and 561 cm-I (no. 10 and 11) in the acid. The C-S stretching mode at 308 cm-I (no. 5) in the acid is found at 3 13 cm-I in the anion. A comparison of the PED'S in the two cases shows that all internal coordinates involved in this mode are the same, except for a contribution of the 01-S-C bending in the case of the acid molecule. No useful comparison is possible for the low-lying modes since here one member of the degenerate mode in the anion is coupled to the H-0143-C torsion in the acid.
Conclusions Optimized geometry, force constants, and the vibrational frequencies of the triflic acid have been obtained using different ab initio molecular orbital methods. The hydrogen is not equidistant from the oxygens atoms in the O=S=O group of the acid. The molecule belongs to the C1 symmetry point group. The harmonic vibrational frequencies from the MP2/6-3 1G** calculations agree well with those observed in the infrared spectra. With a few exceptions, the discrepancies between observed and calculated frequencies are less than 30 cm-I in the whole range from60 to l5OOcm-l. Asexpected, the highlyanharmonic01-H stretching differs as much as 220 cm-1. The present assignment of the vibrational modes shows a reversal of the CF3 and SOz stretching normal modes when compared with those reported previously.8
Acknowledgment. This work was supported by the Swedish Natural Science Research Council and the Swedish Board for Technical Development, which are gratefully acknowledged. The authors thank Dr. Anders Eriksson for useful discussions. References and Notes (1) Gramstad, T. Tiddskr. Kjemi Bergues. Metall. 1959, 19, 62. (2) Johansson, M.; Persson, I. Inorg. Chim. Acta 1987, 127, 15. (3) Haszeldine, R. N.; Kidd, J. M. J . Chem. SOC.1954, 4228. (4) Howells, R. D.; McCown, J. D. Chem. Reo. 1977, 77, 69. (5) Haszeldine, R. N.; Kidd, J. M. J. Chem. SOC.1955, 2901. (6) Balicheva, T. G.; Ligus, V. I.; Fialkov, Yu. Ya. Russ. J . Inorg. Chem. 1973, 18, 12. (7) Katsuhara, T. G.;Hammaker, R. M.; Desmarteau, D. D. Inorg. Chem. 1980, 19, 607. (8) Varetti, E. L. Spectrochim. Acta 1988, 44A, 733. (9) Schultz, G.;Hargittai, I.; Seip, R. Z . Naturforsch. 1981,36A, 917. (10) Bencivenni, L.; Caminiti, R.; Feltrin, A,; Ramondo, F.; Sadun, C. J. Mol. Struct. 1992, 257, 369. (11) Gaussian 9 0 Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.;Raghavachari, K.; Robb, M. A,; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.;Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1990. (12) Pulay, P. Mol. Phys. 1969, 17, 197. (13) Schlegel, H. B. J . Comp. Chem. 1982, 3, 214. (14) Gwinn, W. D. J. Chem. Phys. 1971, 55,477. (15) Edwards, H. G.M. Spectrochim. Acta 1989, 45A, 715. (16) Gejji, S. P.; Hermansson, K.; Lindgren,J. J . Phys. Chem. 1993.97, 3712. (17) Sandorfy, C. In The Hydrogen Bond Recent developments in theory andexperiment; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland Publishing: Amsterdam, 1976; Vol. 2, p 613.
