A cryospectroscopic study of the van der Waals complexes between vinyl fluoride and boron trifluoride: Evidence for the existence of sigma and pi complexes

The complexes formed between vinyl fluoride and BF3 have been investigated by studying the mid-infrared (4000-400 cm(-1)) spectra of solutions of the compounds. in liquefied argon and in solid argon matrixes. In both types of spectra, evidence was found for 1:1 van der Waals complexes. Ab initio mol ecular orbital calculations at the MP2/6-31+G(d) level indicate that the co mplexation can occur via the fluorine atom, giving rise to a sigma complex, or via the C=C bond, resulting in a pi complex. From a comparison of the e xperimental and theoretical frequencies, it was concluded that all complex bands observed in the spectra of the cryogenic solutions are due to the sig ma species. In the spectra of the solid matrixes not only bands of the sigm a complex, but also weak bands due to the pi complex are observed. Using sp ectra recorded at several temperatures between 97 and 123 K, the complexati on enthalpy Delta H degrees for the sigma complex in LAr was determined to be -11.2(3) kJ mol(-1). Using a SCRF/SCIPCM model to correct for the solven t influences and using statistical thermodynamics to account for zero-point vibrational and thermal contributions, the gas-phase complexation energy f or this complex was derived to be -15.2(3) kJ mol(-1).