Van der Waals Complexes between Unsaturated Hydrocarbons and Boron Trifluoride: An Infrared and ab Initio Study of Ethene·BF3 and Propene·BF3

Abstract

The mid-infrared (4000−400 cm-1) spectra of ethene/BF3 and propene/BF3 mixtures, dissolved in liquid argon (93−125K) and in liquid nitrogen (80−118K) are discussed. In all spectra, the formation of a 1:1 van der Waals complex, in which the electron deficient BF3 molecule binds to the CC double bond, was observed. Using spectra recorded at different temperatures, the complexation enthalpy ΔHo for ethene·BF3 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·BF3 the ΔHo are −11.8 ± 0.2 and −6.9 ± 0.3 kJ mol-1, respectively. For both complexes a converged structure and the harmonic vibrational frequencies were calculated using ab initio calculations at the MP2/6-31+G* level.