Vibrational spectra and relative stabilities of the van der Waals complexes of boron trifluoride with cis-2-butene, trans-2-butene and 2-methyl propene

Abstract

The mid-infrared spectra (4000–400cm−1) of mixtures of BF3 with cis-2-butene, trans-2-butene and 2-methylpropene, dissolved in liquefied argon (88–127K), have been investigated. In all spectra, evidence was found for the formation of a 1:1 van der Waals complex, while at higher concentrations of BF3 weak bands indicating the presence of a 1:2 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies ΔH° for cis-2-butene·BF3, trans-2-butene·BF3 and 2-methylpropene·BF3 were determined to be −12.5(2), −12.4(2) and 12.7(2)kJmol−1, respectively. For all 1:1 complexes studied, the equilibrium geometry and the vibrational frequencies were obtained from density functional calculations at the B3LYP/6-311++G(d,p) level. Using statistical thermodynamics to account for the zero-point vibrational and thermal contributions, the complexation enthalpies of the 1:1 complexes were transformed into complexation energies ΔE. The resulting values, −14.5(4), −14.9(5) and −14.8(5)kJmol−1, respectively, are compared with the values predicted at the B3LYP/6-311++G(d,p) and the BSSE corrected MP2/6-311++G(3df,2pd)//B3LYP/6-311++G(d,p) levels.