Experimental characterization of C-X···Y-C (X = Br, I; Y = F, Cl) halogen-halogen bonds.

Using FTIR and Raman spectroscopy, we investigated the formation of halogen bonded complexes of the trifluorohalomethanes CF3Cl, CF3Br, and CF3I with the halomethanes CH3F and CH3Cl and the haloethanes C2H5F and C2H5Cl dissolved in liquid krypton. For CF3Br and CF3I, evidence was found for the formation of C-X···F and C-X···Cl halogen bonded 1:1 complexes. Using spectra recorded at different temperatures, we determined the complexation enthalpies for the complexes to be -7.0(3) kJ mol(-1) for CF3Br·CH3F, -7.6(1) kJ mol(-1) for CF3I·CH3F, -5.9(2) kJ mol(-1) for CF3Br·CH3Cl, -8.3(3) kJ mol(-1) for CF3I·CH3Cl, -7.1(1) kJ mol(-1) for CF3Br·C2H5F, -8.7(2) kJ mol(-1) for CF3I·C2H5F, -6.5(2) kJ mol(-1) for CF3Br·C2H5Cl, and -8.8(3) kJ mol(-1) for CF3I·C2H5Cl. For all halogen bonded complexes with a fluorine-electron donor, a blue shift ranging from +0.6 to +1.5 cm(-1) was observed for the C-X stretching mode. The results from the cyrospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.