Ab initio and Semiempirical Studies on H3C-B=O and H-B=O. Their Structure and Vibrational Spectra

Abstract

The electronic structure of H3C -BO and H - BO is elucidated employing one determinant ab initio calculations with STO-3G and 4-31G basis sets and the semiempirical MNDO theory. The vibrational spectra of both molecules including various isotopic shifts have been calculated at the 4-31G level. The influence of methyl substitution on the vibrational frequencies, BO stretching force constant, Mulliken charges, and vertical ionization potential is examined and compared with that in the pairs H-CN/H3C-CN and H-NC/H3C-NC , isoelectronic with H-BO/H3C-BO. As judged from a comparison of the force constants for stretching the XY bonds in the H3C -XY and H-XY molecules, the XY bond is “softened” upon methyl substitution. This effect is found to decrease in the order H3C-BO>H3C-CN>H3C-NC. For XY=BO the difference between the first ionization potentials of H-XY and H3C-XY is found to be somewhat smaller than for XY = CN but slightly higher than for the isonitril compounds (XY = NC).