Vibrational spectra and relative stability of the van der Waals complexes formed between cis-2-butene, trans-2-butene, 2-methyl propene and HCl: a FTIR and density functional study

Abstract

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 evidence was found for the existence of 1:1 and 1:2 van der Waals complexes. Using spectra recorded at different temperatures the complexation enthalpy Δ H° for 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 complexes 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 obtained 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 vibrational and thermal contributions, for all 1:1 complexes approximate values for the dissociation energy D e were calculated from the complexation 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.