A large structural polarization by intermolecular forces: Ab initio theoretical investigation of the H3N center dot center dot center dot BH2CN and H3N center dot center dot center dot B(CN)(3) interactions

The structure, binding energies, harmonic vibrational frequencies, and nucl ear quadrupole coupling constants of the cyanodihydroborate and boron tricy anide (known compounds, but no structural experimental data available) comp lexes with ammonia were calculated using the Moller-Plesset perturbation th eory through the fourth order and by the coupled-cluster method including s ingle, double, and approximate triple excitations. The origins of the bondi ng in these complexes were investigated by symmetry-adapted perturbation th eory. The computed complex dissociation energies corrected for the basis-se t superposition error are large: 27 and 36 kcal/mol, respectively. A peculi ar change of the character of the HOMO orbital is taking place when the mol ecules approach each other. In both complexes, a large (umbrellalike) struc tural change of the boron moiety occurs due to the interaction with the vir tually unchanged ammonia molecule and the accompanying large change of the dipole moment is taking place. (C) 1999 John Wiley & Sons, Inc.