Solute-solvent interactions in cryosolutions: a study of halothane-ammonia complexes.

The formation of C-H···N bonded complexes of halothane with ammonia has been studied using infrared and Raman spectroscopy of solutions in the liquid rare gases argon, krypton and xenon, of supersonic jet expansions and of room temperature vapor phase mixtures. For the solutions and for the vapor phase experiments, the formation of complexes with 1:1 and 1:2 stoichiometry was observed. The complexation enthalpy for the 1:1 complex was determined to be -20 (1) kJ mol(-1) in the vapor phase, -17.0 (5) kJ mol(-1) in liquid xenon and -17.3 (6) kJ mol(-1) in liquid krypton. For the 1:2 complex in liquid xenon, the complexation enthalpy was determined to be -31.5 (12) kJ mol(-1). Using the complexation enthalpies for the vapor phase and for the solutions in liquid xenon and krypton, a critical assessment is made of the Monte Carlo Free Energy Perturbation approach to model solvent influences on the thermodynamical properties of the cryosolutions. The influences of temperature and solvent on the complexation shifts of the halothane C-H stretching mode are discussed.

On the weakly C-H···π hydrogen bonded complexes of sevoflurane and benzene.

A vibrational assignment of the anaesthetic sevoflurane, (CF(3))(2)CHOCH(2)F, is proposed and its interaction with the aromatic model compound benzene is studied using vibrational spectroscopy of supersonic jet expansions and of cryosolutions in liquid xenon. Ab initio calculations, at the MP2/cc-pVDZ and MP2/aug-cc-pVDZ levels, predict two isomers for the 1 : 1 complex, one in which the near-cis, gauche conformer of sevoflurane is hydrogen bonded through its isopropyl-hydrogen atom, the other in which the same conformer is bonded through a bifurcated hydrogen bond with the fluoromethyl hydrogen atoms. From the experiments it is shown that the two isomers are formed, however with a strong population dominance of the isopropyl-bonded species, both in the jet and liquid phase spectra. The experimental complexation enthalpy in liquid xenon, ΔH(o)(LXe), of this species equals -10.9(2) kJ mol(-1), as derived from the temperature dependent behaviour of the cryosolution spectra. Theoretical complexation enthalpies in liquid xenon were obtained by combining the complete basis set extrapolated complexation energies at the MP2/aug-cc-pVXZ (X = D,T) level with corrections derived from statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations, resulting in a complexation enthalpy of -11.2(3) kJ mol(-1) for the isopropyl-bonded complex, in very good agreement with the experimental value, and of -11.4(4) kJ mol(-1), for the fluoromethyl-bonded complex. The Monte Carlo calculations show that the solvation entropy of the isopropyl-bonded species is considerably higher than that of the fluoromethyl-bonded complex, which assists in explaining its dominance in the liquid phase spectra.

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...π 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.