Ab initio theoretical study of interactions in borazane molecule

Some structural and energetical properties of the borazane molecule have be en calculated by the Moller-Plesset perturbation theory accurate to the sec ond, third, and fourth orders (MP2, MP3, MP4), the coupled cluster [CCSD(T) ] approach, and the symmetry-adapted perturbation theory (SAPT). The geomet ry has been fully optimized at the MP2 level. The theoretical values for bo nd distances, bond angles, rotational barrier, dipole moment, vibrational f requencies, and the nuclear quadrupole coupling constants agree well with t he experimental data. The dissociation energy, the BN bond distance, and th e vibrational frequencies obtained indicate that borazane is to a considera ble extent a floppy system, which has to be classified as rs strong van der Waals complex rather than a molecule similar (isoelectronic) to ethane. Th e 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 S APT analysis the main binding contributions are the induction and electrost atic effects. The dipole moment of the complex increases very strongly [fro m 1.53 to 5.30 D at the CCSD(T) level] upon the interaction due mainly to t he umbrella structural polarization of the BH3 molecule and to the polariza tion of the electron cloud. (C) 1999 American Institute of Physics. [S0021- 9606(99)31401-X].