The End of a 30-Year-Old Controversy? A Computational Study of the B−N Stretching Frequency of BH 3 −NH 3 in the Solid State

The molecular geometry and vibrational frequencies of borane ammoniate, BH 3 −NH 3, are studied by ab initio calculations in the solid state. The B−N bond distance shortens by ∼0.08 Å, and the corresponding stretching frequency increases by 200 cm -1, compared to the same characteristics of the molecule in the gas phase. A reassignment of the experimental argon-matrix vibrational spectrum in an earlier work [Smith, J.; Seshadri, K. S.; White, D. J. Mol. Spectr. 1973, 45, 327−337], on the basis of these calculations, unifies all the experimental vibrational data available for this molecule. The shortening of the B−N bond is analyzed in terms of the atoms-in-molecules theory.