RESUMO
The formation of weak molecular complexes of cyclopentene with HCl and BF3, dissolved in liquid argon is investigated using infrared spectroscopy. Evidence is found for the formation of 1:1 complexes in which the Lewis acid under study binds to the CC double bond. At higher concentrations of HCl, weak absorption bands due to 1:2 species are also observed. From spectra recorded at different temperatures between 92 and 127 K, the complexation enthalpies for CP.HCl and CP.BF3 are determined to be -9.5(3) and -16.1(9) kJ mol-1, while for CP.(HCl)2 a value of -17.0(6) kJ mol-1 is obtained. For the 1:1 and 1:2 complexes, structural and spectral information is obtained from ab initio calculations at the MP2/6-31+G(d) level. Using free energy perturbation Monte Carlo simulations to calculate the solvation enthalpies and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the complexation energies for CP.HCl and CP.(HCl)2 are estimated from the experimental complexation enthalpies to be -17.4(14) and -34.0(20) kJ mol-1, while the value for CP.BF3 was derived to be -23.4(22) kJ mol-1. The experimental complexation energies are compared with the theoretical values derived from the MP2/6-31+G(d) potential energy surfaces and with single point energies calculated at the MP2/6-311++G(3df,2pd) level.