CONFORMATIONAL INFORMATION FROM VIBRATIONAL-SPECTRA OF STYRENE, TRANS-STILBENE, AND CIS-STILBENE

Two electron correlation theories, second-order Moller-Plesset perturb ation (MP2), and density functional (DFT) methods have been adopted to obtain fully optimized structures of styrene, trans-stilbene, and cis -stilbene; Full geometry optimizations with MP2 shows that the nonplan ar conformations of styrene and trans-stilbene are preferred by 0.24 ( styrene) and 0.80 kcal/mol (trans-stilbene), respectively. However, B3 LYP, BLYP, and BVWN prefer a planar conformation contradicting the MP2 results. Due to the disorder of the crystal, X-ray experimental data of C=C double bond length of trans-stilbene seem to be too short. Vibr ational spectra of these molecules are calculated at the BLYP/6-3++G* level without any empirical scaling. The agreement with experiment is excellent, some normal modes are reassigned. The dependence of the IR spectrum as a function of conformation in the 700-800 cm(-1) region a llows the determination of the solution-phase conformation. Both styre ne and trans-stilbene are planar in solution, implying that these mole cular conformations are mainly determined by the intermolecular forces rather than intramolecular ones.