Vibrational ab initio calculations and spectra of C–H bonds of trimethylboron

Abstract

Visible laser photoacoustic and near-IR spectra of the overtones of the C–H stretches of (CH3)3B in the gas phase are reported. Two bands are assigned to nonequivalent methyl C–H bonds. The interaction of an empty 2p orbital of the boron atom with the C–H bonds of a methyl group changes the strength of the C–H bonds during the internal rotation. The most intense, higher energy absorption band in each overtone region is assigned to the CH bonds in the molecular plane (C–H∥) and the least intense, lower energy absorption band to the CH bonds out of the molecular plane (C–H⊥). To interpret the experimental results, overtone transitions are described in terms of the local mode model. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to particular transitions. Ab initio molecular orbital calculations were also performed. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartree–Fock level using the 3–21G and 6–31G* split valence basis set. Several geometries were calculated. The minimum energy corresponds to a geometry in which the CH3 groups are aligned with a single CH bond in the molecular plane (C3h symmetry). Another geometry in which two CH3 groups are aligned with a C–H bond in the plane of the molecule and one CH3 group is aligned with a C–H bond perpendicular to the molecular plane is found to be the saddle point. A third geometry in which each CH3 group has a single CH bond aligned perpendicular to the molecular plane (C3v symmetry) is higher in energy than the first two and does not correspond to a minimum. Calculations were performed on the deuterated molecule (CHD2)B(CD3)2 using the most stable C3h conformation and the conformation of the saddle point. In this way, the isolated C–H∥ and C–H⊥ bond lengths, the corresponding C–H stretching force constants, and the vibrational frequencies were obtained.