Dimethylmethoxyborane: vibrational assignment, conformational stability, ab initio calculations and barriers to internal rotation

The infrared (4000-50 cm−1) and Raman spectra (3500-10 cm−1) have been recorded for dimethylmethoxyborane, (CH3)2BOCH3, in both the gaseous and solid states. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values obtained. Only one conformation has been found in all three physical states and, on the basis of the polarized nature of the Raman band assigned as the BC2 antisymmetric stretch, this conformer is identified as being the “planar” form with Cs molecular symmetry. A complete vibrational assignment is proposed based on the depolarization data, infrared gas phase band contours, and group frequencies. Additionally, variable temperature 13C NMR studies have been carried out and the barrier to rotation about the BO bond has been determined as 8.9 kcal mol−1. Structural parameters have been obtained from ab initio Hartree—Fock gradient calculations employing 3-21G and 6-31G* basis sets. The results are compared with the rα values obtained earlier from an electron-diffraction study. Potential-surface calculations have been carried out to determine the conformational stability and barriers to internal rotation. Frequencies and potential-energy distributions for the normal modes have been calculated utilizing the 3-21G basis set. All results are compared to the corresponding data for some other organoboranes.