Ab initio and cryospectroscopic investigation of the Van der Waals complexes of methyleyclopropane with hydrogen chloride and boron trifluoride

The mid infrared spectra of methylcyclopropane-HCl and methylcyclopropane-B F3 mixtures, dissolved in liquid argon (LAr) or liquid nitrogen (LN2), have been examined. In LAr solutions, evidence was found for the formation of t wo different isomers of the 1:1 van der Waals complexes-an asymmetric compl ex, in which the electron acceptor interacts with the C-C bond adjacent to the methyl. group, and a symmetric complex in which the interaction occurs with the opposite C-C bond. At higher concentrations, indications were foun d for the formation of a chain-type 1:2 complex. In LN2 solutions, only the two 1:1 isomers, of the HCl complex have been observed. Using spectra reco rded at different temperatures, the complexation. enthalpy AW in LAr was de termined to be -9.5(4) and -8.3(3) kJ mol(-1) for the symmetric and asymmet ric MCP-HCl complexes, and -9.1(2) and -6.7(5) kJ mol(-1) for the symmetric and asymmetric MCP.BF3 complexes, respectively. In addition, for all inves tigated species, fall MP2/6-31+G(d,p) geometry optimizations and B3LYP/6-31 1++G(d,p) frequency calculations were performed, and the complexation. ener gies were calculated at the MP2/6-311++G(3df,2pd) level. These calculations indicate that the isomers of the MCP-HCl complex are of comparable strengt h, while the asymmetric MCP.BF3 complex is significantly less stable than i ts symmetric counterpart. Using Free Energy Perturbation Monte Carlo simula tions to calculate the. solvent influences, and using statistical thermodyn amics to account for zero-point vibrational and thermal contributions, from the experimental complexation. enthalpies, the complexation energies are e stimated to be - 15.7(8) and -16.0(9) kJ mol(-1) for the symmetric and asym metric MCP.HCl complexes, which is in agreement with the. ab initio results , and -14.0(8) and -15.3(5) kJ mol(-1) for the corresponding MCP.BF3 specie s.