An ab initio and cryospectroscopic study of the hydrogen chloride and boron trifluoride complexes of cyclopropene

Mid-infrared spectra of cyclopropene (c-C3H4) mixtures with HCl or BF3, dissolved in liquefied argon and in liquefied nitrogen have been examined. Evidence was found for the formation of two different isomers of the 1:1 complexes: a π-type complex in which the interaction occurs with the π-bond of cyclopropene, and a σ-type complex in which the electron acceptors interact with one of the CC single bonds. At lower temperatures, indications for the formation of a 1:2 chain-type complex were found. Using spectra recorded between 90 and 124 K in liquid argon, the complexation enthalpies for the π- and σ-complexes with HCl were determined to be −8.8(3) and −7.9(3) kJ mol−1, respectively. For the π-complex with BF3 a value of −7.4(3) kJ mol−1 was found. Structural and spectral information on the 1:1 complexes was obtained from B3LYP/6-311++G(d,p) and MP2(FC)/6-31+G(d) calculations. Using Free Energy Perturbation Monte Carlo simulations to calculate the solvent influences, and statistical thermodynamics to account for zero-point vibrational and thermal contributions, the solution enthalpies were transformed into complexation energies with values of −15.5(6) and −13.8(8) kJ mol−1 for the π- and σ-isomers of c-C3H4·HCl, respectively, and of −12.7(8) kJ mol−1 for the π-type BF3 complex.