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Vibrational analysis of stilbene and its isotopomers on the ground state potential energy surface

Optimized geometries and vibrational frequencies of trans- and cis-stilbene in the S-0 state are calculated by the semiempirical RHF/AM1 method and by means of ab initio RHF method using the 6-31G(d) basis set. Fully optimized geometry and vibrational frequencies of the perpendicular stilbene are calculated by means of the UHF function of the two MO methods. With only one imaginary frequency, the perpendicular configuration is a transition state on the ground-state potential energy surface. The three stilbene isomers possess C-2 symmetry. The reaction coordinate leading either to trans or cis isomer is not only the rotation around the central bond but also includes significant changes in bond lengths and angles of the ethylenic moiety. A high value of the secondary kinetic isotope effect for thermal isomerization of cis-stilbene and alpha, alpha'-D-2-cis-stilbene calculated using the ab initio frequencies agrees very well with the recent experimental result. Infrared and Raman spectra for the ground state cis-stilbene and eleven of its deuterated isotopomers (D0-, 4-D1-, 4, 4'-D2-, 2, 3, 4, 5, 6-D5- (D-5-cS), 2, 3, 4, 5, 6, 4'-D6- and 2, 3, 4, 5, 6, 2', 3', 4', 5', 6'-D-10-cS (D-10-cS) in group I, alpha-D1-, 4, alpha'-D2-, 2, 3, 4, 5, 6, alpha-D6-, and D-11-cS in group II, and alpha, alpha'-D2- and D-12-cS in group III) have been recorded. The vibrational assignments for both stilbene isomers were deduced to the largest possible extent on the basis of band intensities in solution and solid phases, isotopic frequency shifts and phenyl characteristic frequencies. The experimental frequencies of three isotopomers (D-0, alpha, alpha'-D-2 and D-10) were used simultaneously to obtain the scaled quantum chemical force fields for the trans and cis isomer separately, i.e. no scaling factor was given an arbitary value. The semiempirical force field after scaling reproduces the experimental frequencies with an average error of 13 cm(-1) which is twice as large as the error of the scaled ab initio force field. The possibility of using the same set of scaling factors for different isomers is considered. The eight modes (5A + 3B) involved in the low-frequency region are thoroughly analyzed and reasons are given why the phenyl torsions of trans-stilbene can only be properly explained by a non-planar ground-state structure. The AM1 barrier hindering the thermal transition between the chiral ground-state cis-stilbene of C-2 symmetry to its enantiomer is rather low (1.3 kcal mol(-1)) and comparable to that of trans-stilbene (1.7 kcal mol(-1)). It accounts for the large bandwidths of some composite low-frequency Raman bands (below 400 cm(-1)).

stilbene ; isotopic frequency shifts ; normal coordinate analysis ; scaled force field

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