Ab initio study of the vibrational frequencies of H n BNH n (n= 1, 2, 3) and related compounds

Abstract

The equilibrium geometries and vibrational frequencies of the HnBNHn(n= 1, 2, 3) molecules have been calculated at the SCF level using a 4-31 G** basis set. Comparative calculations were carried out for the isoelectronic hydrocarbons. The borane–ammonia adduct was treated at the correlated second-order Moller–Plesset level, with analytic calculation of second derivatives. The B—N stretching vibration in H3 BNH3 is poorly described at the SCF level and shifts from 604 to 679 cm–1 on inclusion of electron correlation. Over the whole set of molecules, calculated SCF harmonic frequencies are uniformly higher than the experimental fundamentals by 10%, with a few exceptions that lead to proposed reassignments of some spectroscopic bands. A correlation between BH stretching frequency and nominal hybridisation is determined and its application to some related compounds discussed.