EFFECTS OF BASIS-SET AND ELECTRON CORRELATION ON THE STRUCTURE AND VIBRATIONAL-SPECTRA OF DIBORANE

The effects of basis sets and polarization functions on the ground sta te equilibrium geometry, harmonic force: constants and vibrational fre quencies at the self consistent held and second order Moller-Plesset p erturbation levels have been investigated for diborane. Basis sets ran ging from double-zeta to triple-zeta quality including higher order po larization functions have been utilized. Inclusion of polarization fun ctions in the basis set is important for calculating the equilibrium g eometry. The ring region of diborane has been found to respond differe ntly from the terminal regions towards various methods of ab initio ca lculations. The relatively larger errors in the predicted ring stretch ing frequencies: nu(7) and nu(17), of diborane have been explained. Ha rmonic force fields for diborane have been obtained from each of the a b initio calculations by directly mapping the experimental harmonic fr equencies of diborane on to the ab initio predicted vibrational eigenv ector space. This force constant set, termed RECOVES, reproduces, by d efinition, 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 dib orane. This procedure has yielded a good quality force constant set. T he calculated vibrational intensities were found to match qualitativel y the experimental features.