A cryospectroscopic and ab initio study of the cyclopropane•(HCl)x van der Waals complexes

The mid-infrared (4000-400 cm(-1)) and the far-infrared (300-50 cm(-1)) spectra of cyclopropane/hydrogen chloride mixtures, dissolved in liquefied argon (LAr) and in liquefied nitrogen (LN2) are discussed. In the spectra of the LAr solutions, evidence was found for the formation of 1:1 and 1:2 van der Waals complexes. In the LN2 solutions, only the 1:1 complex could be observed. From spectra recorded at different temperatures, the complexation enthalpy Delta H degrees for the 1:1 complex was determined to he -7.8(2) kJ mol(-1) in LAr and -5.6(3)kJ mol(-1) in LN2, while for the 1:2 complex in LAr, a value of -14.9(7) kJ mol(-1) was derived. For all investigated species, ab initio geometry optimizations and frequency calculations were made at the MP2/6-31 + G(d,p) and at the B3LYP/6-311+ + G(d,p) level, and the complexation energies were calculated at the MP2/6-31 1 + + G(d,p) level. The calculations predict the existence of two isomers of the 1:2 complex, one with the two HCl molecules bonded to different carbon-carbon bonds, while in the other the second HCl molecule is bonded to the first. Using Free Energy Perturbation Monte Carlo simulations to calculate the solvent influences, and using statistical thermodynamics to account for the zero-point vibrational and thermal contributions, from the experimental complexation enthalpy the complexation energy for the 1:1 complex was estimated to be - 14.5(6) kJ mol(-1). Comparison of the calculated with the experimental data suggests that the observed 1:2 complex is a mixture of the two isomers. (C) 2000 Elsevier Science B.V. All rights reserved.