Van der Waals complexes between unsaturated hydrocarbons and boron trifluoride:: an infrared and ab initio study of ethyne • BF3, propyne • BF3 and propyne • (BF3)2

The mid-infrared spectra of C2H2/BF3 and C3H4/BF3 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 BF3 bands of a 1:2 species C3H4 . (BF3)(2) were also observed. Using spectra recorded at different temperatures, the complexation enthalpy for C2H2 . BF3 was determined to be - 9.3(4) kJ mol(-1), while for C3H4 . BF3S and C3H4 . (BF3)(2) the Delta H degrees 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 C2H2 . BF3, C3H4 . BF3 and C3H4 . (BF3)(2) are - 13.3(8) kJ mol(-1), - 17.8(8) kJ mol(-1) and - 28.2(24) kJ mol(-1), respectively. (C) 1999 Elsevier Science B,V. All rights reserved.