Exploring the C-X…π Halogen Bonding Motif: An Infrared and Raman Study of the Complexes of CF(3)X (X = Cl, Br and I) with the Aromatic Model Compounds Benzene and Toluene

The formation of halogen bonded complexes formed between the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I and the Lewis bases benzene and toluene at temperatures below 150K was investigated using FTIR and Raman spectroscopy. Experiments using liquid krypton as solvent show that for both CF(3)Br and CF(3)I substantial fractions of the monomers can be involved in 1:1 complexes. In addition, weak absorptions illustrating the formation of 2:1 complexes between CF(3)I and benzene are observed. Using spectra recorded at temperatures between 120 and 140 K, observed information on the relative stability was obtained for all complexes by determining the complexation enthalpies in solution. The resulting values for CF(3)Br·benzene, CF(3)I·benzene and (CF(3)I)(2)·benzene are −6.5(3), −7.6(2) and −14.5(9) kJ mol(−1). The values for CF(3)Br·toluene and CF(3)I·toluene are −6.2(5) and −7.4(5) kJ mol(−1). The experimental complexation enthalpies are compared with theoretical data obtained by combining results from MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) ab initio calculations, from statistical thermodynamical calculations and from Monte Carlo Free Energy Perturbation simulations. The data are also compared with results derived for other C-X···π halogen bonded complexes involving unsaturated Lewis bases such as ethene and ethyne.