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

Abstract

The equilibrium geometries and vibrational frequencies of the H_nBNH_n(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 Møller–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.