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

The mid-infrared spectra (4000-400 cm(-1)) of mixtures of BF3 with cis-2-bu tene, trans-2-butene and 2-methylpropene, dissolved in liquefied argon (88- 127 K), 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 enthal pies Delta H degrees for cis-2-butene.BF3, trans-2- butene.BF3 and 2-methyl propene.BF3 were determined to be - 12.5(2), - 12.4(2) and 12.7(2) kJ mol(- 1), respectively. For all 1:1 complexes studied, the equilibrium geometry a nd the vibrational frequencies were obtained from density functional calcul ations at the B3LYP/6-311++G(d,p) level. Using statistical thermodynamics t o account for the zero-point vibrational and thermal contributions, the com plexation enthalpies of the 1:1 complexes were transformed into complexatio n energies Delta E. The resulting values, -14.5(4), -14.9(5) and -14.8(5) k J mol(-1), respectively, are compared with the values predicted at the B3LY P/6-311 + + G(d,p) and the BSSE corrected MP2/6-311 + + G(3df,2pd)//B3LYP/6 -311 + + G(d,p) levels. (C) 2000 Elsevier Science B.V. All rights reserved.