Ab Initio and Cryospectroscopic Investigation of the Van der Waals Complexes of Methylcyclopropane with Hydrogen Chloride and Boron Trifluoride

Abstract

The mid infrared spectra of methylcyclopropane-HCl and methylcyclopropane-BF 3 mixtures, dissolved in liquid argon (LAr) or liquid nitrogen (LN 2 ), have been examined. In LAr solutions, evidence was found for the formation of two different isomers of the 1:1 van der Waals complexessan asymmetric complex, 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 found for the formation of a chain-type 1:2 complex. In LN 2 solutions, only the two 1:1 isomers of the HCl complex have been observed. Using spectra recorded at different temperatures, the complexation enthalpy ∆H°in LAr was determined to be -9.5(4) and -8.3(3) kJ mol -1 for the symmetric and asymmetric MCP‚ HCl complexes, and -9.1(2) and -6.7(5) kJ mol -1 for the symmetric and asymmetric MCP‚BF 3 complexes, respectively. In addition, for all investigated species, full MP2/6-31+G(d,p) geometry optimizations and B3LYP/6-311++G(d,p) frequency calculations were performed, and the complexation energies 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 strength, while the asymmetric MCP‚BF 3 complex is significantly less stable than its symmetric counterpart. Using Free Energy Perturbation Monte Carlo simulations to calculate the solvent influences, and using statistical thermodynamics to account for zero-point vibrational and thermal contributions, from the experimental complexation enthalpies, the complexation energies are estimated to be -15.7(8) and -16.0(9) kJ mol -1 for the symmetric and asymmetric 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‚BF 3 species.

Infrared and Raman Measurements of Halogen Bonding in Cryogenic Solutions

“…Figure 43 shows the correlation of the experimental complexation enthalpies and theoretical values obtained by correcting the ab initio energies for zero-point vibrational, thermal and solvent influences. The coefficients of the linear regression line Δ exp H ¼ a + bΔ calc H a ¼ À0.87(85) kJ mol À1 and b ¼ 0.796 (73). These results illustrate that although most theoretical values overestimate the strength of the halogen bond, a good correlation is obtained.…”