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

Abstract

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⋯π 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·ethene, −7.5(2) kJ mol⁻¹ for CF₃I·ethene, −5.6(1) kJ mol⁻¹ for CF₃Br·propene, −8.8(1) kJ mol⁻¹ for CF₃I·propene and −16.5(6) kJ mol⁻¹ for (CF₃I·)₂propene. 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·ethene and −5.1(2) kJ mol⁻¹ for CF₃H·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.