Conformational study of α,2-dimethylstyrene: Raman and FT-IR spectra and ab initio calculations

Abstract

Vibrational spectra (Raman and FT-IR) of liquid phase and ab initio SCF MO calculations at the B3LYP/6-31G* level are reported for α,2-dimethylstyrene. The calculations predict two minima with different orientations of the CC double bond relative to the 2-methyl group, but only the minimum with θ (C 2C 1C αC β)=112° (named skew) is deep enough to accommodate torsional levels. The role of intramolecular interactions on the conformational preferences is accessed through the comparison of the ab initio results for α,2-, α,3- and α,4-dimethylstyrene. The Raman spectra of the liquid sample show pairs of bands whose temperature-dependent intensities are ascribed to a conformational equilibrium. From the plot of logarithm of intensity ratio vs. inverse temperature, a Δ H value of 1.1±0.1 kJ mol −1 has been derived. The temperature-dependence of other Raman bands with no measurable conformational splitting can also be related with the conformational equilibrium by assuming different molar Raman intensities. The comparison of the experimental and calculated wavenumber shifts suggests that the gauche form is more stable than the skew form in the liquid, while ab initio results predict a single skew form for the isolated molecule. This discrepancy may arise from intermolecular interactions in the liquid.