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

The mid-infrared (4000-400 cm(-1)) and the far-infrared (300-50 cm(-1)) spe ctra of cyclopropane/hydrogen chloride mixtures, dissolved in liquefied arg on (LAr) and in liquefied nitrogen (LN2) are discussed. In the spectra of t he LAr solutions, evidence was found for the formation of 1:1 and 1:2 van d er Waals complexes. In the LN2 solutions, only the 1:1 complex could be obs erved. From spectra recorded at different temperatures, the complexation en thalpy 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 L Ar, a value of -14.9(7) kJ mol(-1) was derived. For all investigated specie s, 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 comp lexation energies were calculated at the MP2/6-31 1 + + G(d,p) level. The c alculations predict the existence of two isomers of the 1:2 complex, one wi th 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 vibratio nal and thermal contributions, from the experimental complexation enthalpy the complexation energy for the 1:1 complex was estimated to be - 14.5(6) k J 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 El sevier Science B.V. All rights reserved.