C-X•••π halogen and C-H•••π hydrogen bonding: interactions of CF3X (X = Cl, Br, I or H) with ethene and propene

Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF3Cl, CF3Br and CF3I with ethene and propene dissolved in liquid argon has been investigated. For CF3Br and CF3I, evidence was found for the formation of C-X center dot center dot center dot pi halogen bonded 1 : 1 complexes. At a higher ratio of CF3I/propene, weak absorptions due to a 2 : 1 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -5.3(2) kJ mol(-1) for CF3Br center dot ethene, -7.5(2) kJ mol(-1) for CF3I center dot ethene, -5.6(1) kJ mol(-1) for CF3Br center dot propene, -8.8(1) kJ mol(-1) for CF3I center dot propene and -16.5(6) kJ mol(-1) for (CF3I center dot)(2)propene. The complexation enthalpies of the hydrogen bonded counterparts, with CF3H as the Lewis acid, were determined to be -4.6(4) kJ mol(-1) for CF3H center dot ethene and -5.1(2) kJ mol(-1) for CF3H center dot propene. For both hydrogen bonded complexes, a blue shift, by + 4.8 and + 4.0 cm(-1), respectively, was observed for the C-H stretching mode. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.