The vibrational spectra of the boron halides and their molecular complexes .4. Ab initio predictions of the structures and vibrational spectra of the complexes of boron trifluoride with dimethyl ether and dimethyl sulphide

The optimized structures and vibrational spectra of the 1:1 complexes of boron trifluoride with dimethyl ether and dimethyl sulphide have been determined by means of a series of ab initio calculations using the GAUSSIAN-90 computer program at the Hartree-Fock level with the 6-31G** basis set. The results have been rationalized on the basis of the interactions of the Lewis acid (BF3) with a hard ((CH3)(2)O) and a soft ((CH3)(2)S) base. The magnitudes of the wavenumber shifts, both for the acid and the base fragments, are far larger for the complex with the hard than with the soft base. The computed results have been used as aids in the interpretation of the infrared spectra of BF3 co-condensed with (CH3)(2)O and with (CH3)(2)S in cryogenic matrices, to be reported in a forthcoming publication.