Effects of basis set and electron correlation on the structure and vibrational spectra of diborane

Abstract

The effects of basis sets and polarization functions on the ground state equilibrium geometry, harmonic force constants and vibrational frequencies at the self consistent field and second order Møller-Plesset perturbation levels have been investigated for diborane. Basis sets ranging from double-zeta to triple-zeta quality including higher order polarization functions have been utilized. Inclusion of polarization functions in the basis set is important for calculating the equilibrium geometry. The ring region of diborane has been found to respond differently from the terminal regions towards various methods of ab initio calculations. The relatively larger errors in the predicted ring stretching frequencies, ν 7 and ν 17, of diborane have been explained. Harmonic force fields for diborane have been obtained from each of the ab initio calculations by directly mapping the experimental harmonic frequencies of diborane on to the ab initio predicted vibrational eigenvector space. This force constant set, termed RECOVES, reproduces, by definition, the vibrational frequencies of the parent molecule exactly and its reliability is assessed by the ability of the force constants to predict the vibrational frequencies of different isotopomers of diborane. This procedure has yielded a good quality force constant set. The calculated vibrational intensities were found to match qualitatively the experimental features.