Theoretical study of the properties of BH3NH3

Borane monoammoniate (BH3NH3) has been studied using several ab initio electronic structure methods and Gaussian basis sets. Equilibrium geometries have been computed at the Hartree--Fock level and, using the electron-correlated Moller--Plesset perturbation method, carried out to third order (MP3) with double-zeta polarized quality basis sets. The computed MP3 geometry is in close agreement with recent microwave data; electron correlation is found to be necessary for a proper description of the B--N distance. Hartree--Fock dipole moments and harmonic vibrational frequencies are presented and discussed. Moller--Plesset perturbation theory carried out to fourth order with triple-zeta plus polarization basis sets is used to compute a B--N dissociation energy of 34.7 kcal mol-1 and a (Hartree--Fock zero-point corrected) rotational barrier of 2.065 kcal mol-1, which is in excellent agreement with the experimental value. Analysis of the dissociation energy as a function of perturbation order indicates that terms involving triple and quadruple substitutions are required in the dissociation energy