Ab initio theoretical study of interactions in borazane molecule

Some structural and energetical properties of the borazane molecule have been calculated by the Moller-Plesset perturbation theory accurate to the second, third, and fourth orders (MP2, MP3, MP4), the coupled cluster [CCSD(T)] approach, and the symmetry-adapted perturbation theory (SAPT). The geometry has been fully optimized at the MP2 level. The theoretical values for bond distances, bond angles, rotational barrier, dipole moment, vibrational frequencies, and the nuclear quadrupole coupling constants agree well with the experimental data. The dissociation energy, the BN bond distance, and the vibrational frequencies obtained indicate that borazane is to a considerable extent a floppy system, which has to be classified as rs strong van der Waals complex rather than a molecule similar (isoelectronic) to ethane. The best estimate for the complex formation enthalpy corrected for the basis set superposition error is equal to 25.7+/-2 kcal/mol. As revealed by the SAPT analysis the main binding contributions are the induction and electrostatic effects. The dipole moment of the complex increases very strongly [from 1.53 to 5.30 D at the CCSD(T) level] upon the interaction due mainly to the umbrella structural polarization of the BH3 molecule and to the polarization of the electron cloud. (C) 1999 American Institute of Physics. [S0021-9606(99)31401-X].