Raman, infrared and NMR spectra and structure of dimethylvinylborane

Abstract The Raman spectra of gaseous, liquid and solid dimethylvinylborane have been recorded from 30 to 4000 cm −1 . The infrared spectra of gaseous and solid dimethylvinylborane have been recorded over the frequency range of 80–3500 cm −1 . A complete vibrational assignment is proposed. From the observation of depolarized Raman lines, it is concluded that the molecule has a plane of symmetry. Since both the in-phase and out-of-phase symmetric CH 3 stretches are polarized, it appears that the molecule exists as the conformer in which the molecular plane contains all of the heavy atoms rather than as the conformer with the vinyl group perpendicular to the CBC plane. The vinyl internal torsional mode was observed at 100 cm −1 in the Raman spectrum of the gas and a two-fold periodic barrier of 4.2 kcal mol −1 was calculated. Both 1 H and 13 C low temperature NMR studies were carried out and the results were consistent with the relatively low barrier to internal rotation of the vinyl group. The 13 C chemical shift of C β of the vinyl group is more shielded than that of any other vinylborane studied thus far, which implies that the delocalization is small. The structural conclusions are discussed relative to some recent theoretical predictions for this molecule.