Conformational Information from Vibrational Spectra of Styrene, trans -Stilbene, and cis -Stilbene

Two electron correlation theories, second-order Møller−Plesset perturbation (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 nonplanar conformations of styrene and trans -stilbene are preferred by 0.24 (styrene) and 0.80 kcal/mol ( trans -stilbene), respectively. However, B3LYP, 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. Vibrational spectra of these molecules are calculated at the BLYP/6-31++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 allows the determination of the solution-phase conformation. Both styrene and trans -stilbene are planar in solution, implying that these molecular conformations are mainly determined by the inter molecular forces rather than intra molecular ones.