VAN-DER-WAALS COMPLEXES BETWEEN UNSATURATED-HYDROCARBONS AND BORON-TRIFLUORIDE - AN INFRARED AND AB-INITIO STUDY OF ETHENE-CENTER-DOT-BF3 AND PROPENE-CENTER-DOT-BF3

The mid-infrared (4000-400 cm(-1)) spectra of ethene/BF3 and propene/B F3 mixtures, dissolved in liquid argon (93-125K) and in liquid nitroge n (80-118K) are discussed. In all spectra, the formation of a 1:1 van der Waals complex, in which the electron deficient BS molecule binds t o the C=C double bond, was observed. Using spectra recorded at differe nt temperatures, the complexation enthalpy Delta H degrees for ethene BS was determined to be -10.0 +/- 0.2 kJ mol(-1) in liquid argon and - 5.4 +/- 0.3 kJ mol(-1) in liquid nitrogen, while for propene . B-3 the Delta H degrees are -11.8 +/- 0.2 and -6.9 +/- 0.3 kJ mol(-1), respec tively. For both complexes a converged structure and the harmonic vibr ational frequencies were calculated using ab initio calculations at th e MP2/6-31+G level.