The C-Cl•••N halogen bond, the weaker relative of the C-I and C-Br•••N halogen bonds, finally characterized in solution

The formation of halogen bonded complexes between trimethylamine (TMA) and the trifluoro halo-methanes CF3I, CF3Br and CF3Cl dissolved in liquid xenon, krypton and argon, respectively, has been investigated using FTIR spectroscopy. For all Lewis acids, evidence was found for the formation of C-X center dot center dot center dot N halogen bonded 1:1 complexes. This study provides the first experimental characterization of a complex formed under thermodynamic equilibrium conditions in solution via a C-Cl center dot center dot center dot N halogen bond. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -28.7(1) kJ mol(1) (CF3I center dot TMA), -18.3(1) kJ mol(1) (CF3 center dot Br TMA) and -8.9(2) kJ mol(1) (CF3Cl center dot TMA). The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/6-311++G(d,p) + LanL2DZ* level. (C) 2010 Elsevier B.V. All rights reserved.