The Vibrational Spectra of the Boron Halides and Their Molecular Complexes. Part 14. Ab Initio Studies of the Boron Trifluoride-Nitrous Acid Complex

Abstract

A number of electron donor-acceptor complexes formed between boron trifluoride and nitrous acid have been studied, in order to ascertain which of the interacting monomers acts as the acid and which as the base.We have found four complexes in which electron donation occurs in the direction HONO → BF₃. These complexes are bound through the hydroxyl O, the N and the nitrosyl O atoms, in decreasing order of strength of interaction, and in the last case two separate rotational isomers have been identified. The intermolecular structural parameters and the perturbations of the intramolecular bond lengths and angles are consistent with the trends in the interaction energies. The vibrational spectra have also been examined, and the wavenumber shifts and intensity ratios track with the energetic and structural data. The mechanism of complex formation in each case is donation of a lone pair of electrons on the N or O atoms into the &3960;* orbital of BF₃, with back donation in the case of the complex bound through the nitrogen atom froma lone pair on one of the F atoms of BF₃ into the σ*(OH) orbital of HONO. The total amounts of charge transferred vary, in general, with the strengths of interaction, while the charge density topologies and their properties confirm the conclusions derived from the other characteristics discussed.

KEYWORDS Ab initio studies, molecular complexes, boron trifluoride, nitrous acid.

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