The structure of the boron trifluoride-ammonia complex: a Fourier transform matrix isolation infrared spectroscopic and ab initio molecular orbital study

Abstract

The structures, interaction energies and Mulliken atomic charges of the 1:1 electron donor-acceptor complexes formed between boron trifluoride and ammonia have been determined by means of ab initio molecular orbital calculations. The calculations have been extended to include a prediction of the infrared spectra of the complexes, leading to the identification of the staggered conformer as the minimum energy structure and the eclipsed species as a transition state. The computed wavenumber shifts of the complex bands, relative to their positions in the monomers, and the complex/monomer infrared band intensity ratios have been determined and rationalized in the light of the nature and strength of the interaction. The infrared spectra of boron trifluoride and ammonia co-deposited in nitrogen and argon matrices have been recorded at cryogenic temperatures. The spectra have been interpreted in terms of the Lewis acid-base complex predicted theoretically to be the preferred structure, and band assignments proposed for most of the complex bands. The spectroscopic properties derived experimentally and theoretically have been compared with one another.