VIBRATIONAL-SPECTRA AND RELATIVE STABILITY OF THE VAN-DER-WAALS COMPLEXES FORMED BETWEEN CIS-2-BUTENE, TRANS-2-BUTENE, 2-METHYL PROPENE ANDHCL - A FTIR AND DENSITY-FUNCTIONAL STUDY

The mid-infrared spectra (4000-400 cm(-1)) of cis-2-butene/HCl, trans- 2-butene/HCl and 2-methylpropene/HCl mixtures, dissolved in liquefied argon (88-127 K), were investigated. In all spectra, experimental evid ence was found for the existence of 1:1 and 1:2 van der Waals complexe s. Using spectra recorded at different temperatures the complexation e nthalpy Delta H degrees far cis-2-butene HCl, trans-2-butene . HCl and 2-methylpropene . HCl were determined to be -11.9 (5), -11.6 (4) and -11.2 (3) kJ mol(-1), respectively, while for the corresponding 1:2 co mplexes the complexation enthalpies were found to be - 19.2 (20), - 18 .0 (20) and - 17.0 (22) kJ mol(-1). For all 1:1 bonded species studied , the equilibrium geometry and the vibrational frequencies were obtain ed from density functional calculations at the B3LYP/6-311++G** level. Using a SCRF/SCIPCM scheme to correct for the solvent influences, and using statistical thermodynamics to account for the zero-point vibrat ional and thermal contributions, for all 1:1 complexes approximate val ues for the dissociation energy D-e were calculated from the complexat ion enthalpies. The resulting values, -16.8 (5), -16.5 (4) and -15.9 ( 3) kJ mol(-1), respectively, are compared with the ab initio values. ( C) 1998 Elsevier Science B.V.