The complexes of halothane with benzene: the temperature dependent direction of the complexation shift of the aliphatic C-H stretching

The formation of C-H center dot center dot center dot pi bonded complexes of halothane with benzene(-d(6)) has been studied using infrared and Raman spectroscopy of solutions in liquid krypton, in supersonic jet expansions and in room temperature vapour phase. The formation of complexes with 1 : 1 and 2 : 1 stoichiometry was observed. The complexation enthalpy in liquid krypton for the 1 : 1 complex was determined to be -9.8(2) kJ mol(-1) and the enthalpy for the addition of a second halothane molecule to the 1 : 1 complex was determined at -7.0(3) kJ mol -1. The stretching mode of the halothane C-H bond involved in the formation of the complex in the jets was observed to blue shift by 7.7(10) cm(-1). In contrast, for the solutions of liquid krypton and the room temperature measurements a small red shift was observed. Supported by ab initio calculations and Monte Carlo simulations, this shift was explained by the differences in thermal populations of the van der Waals vibrations of the complex in the different experiments.