Van der Waals complexes between unsaturated hydrocarbons and boron trifluoride: an infrared and ab initio study of ethyne [rad]BF 3, propyne [rad]BF 3 and propyne [rad](BF 3) 2

Abstract

The mid-infrared spectra of C 2H 2/BF 3 and C 3H 4/BF 3 mixtures dissolved in liquefied argon are discussed. For both Lewis bases, the formation of a 1:1 van der Waals complex was observed, while at higher concentrations of BF 3 bands of a 1:2 species C 3H 4 (BF 3) 2 were also observed. Using spectra recorded at different temperatures, the complexation enthalpy for C 2H 2 BF 3 was determined to be −9.3(4) kJ mol −1, while for C 3H 4 BF 3 and C 3H 4 (BF 3) 2 the ΔH o are −13.2(4) kJ mol −1 and −19.8(12) kJ mol −1, respectively. For the 1:1 complexes the equilibrium geometry and the vibrational frequencies were calculated using ab initio calculations at the MP2/6–31+G(d) level. Using a SCRF/SCIPCM scheme to correct for solvent influences, and statistical thermodynamics to account for zero-point vibrational and thermal contributions, approximate values for the gas phase complexation were calculated. The resulting complexation energies for C 2H 2 BF 3, C 3H 4 BF 3 and C 3H 4 (BF 3) 2 are −13.3(8) kJ mol −1, −17.8(8) kJ mol −1 and −28.2(24) kJ mol −1, respectively.