Modelling of interactions between volatile anaesthetics (halothane, enflurane) and aromatic compounds, ab initio study

For many years halothane and enflurane have been used clinically as volatile anaesthetics, however, their mechanism of action is still not fully understood. Recently, it has been suggested that they can act by a direct bonding to neuroreceptors containing the aromatic groups. In this work, the halothane center dot center dot center dot benzene and enflurane center dot center dot center dot benzene complexes were studied by the ab initio MP2 and CCSD(T) methods. All possible structures of the complexes were calculated by means of the counterpoise CP-corrected gradient optimization technique. It has been found that among these species, the C-H center dot center dot center dot pi hydrogen bonded complexes are the most stable. The CCSD(T)/CBS calculated stabilization energies for halothane and enflurane complexes are: -10.56 and -9.72 kcal mol(-1), respectively. The interaction energy is mainly dominated by the dispersion attraction. In the case of enflurane, the C-H bond shows a very small contraction (by -0.0008 angstrom) upon complexation. This change is accompanied by the blue-shift (20 cm(-1)) of the C-H stretching frequency and an increase of the infrared intensity of the corresponding mode by 7 km mol(-1). Similar results were obtained for the halothane complex: a small contraction of the C-H bond; an increase of the C-H stretching frequency by 11 cm(-1) (blue-shift); and an increase of the infrared intensity by 37 km mol(-1). In order to explain the nature of these effects, the halothane and enflurane molecules were studied in the electric field generated by benzene atoms, and Natural Bond Orbital (NBO) analyses were performed. The molecular dipole moments of these molecules were calculated with respect to the C-H bond changes. The positive dipole moment derivative obtained for halothane is in agreement with the literature data, while, in the case of enflurane, an unusual effect is observed, the blue-shift of the C-H stretching frequency is accompanied by the positive dipole moment derivative for one C-H bond and the negative for the other C-H bond. The mechanisms responsible for contraction and strengthening of the CH bonds are discussed. (C) 2010 Elsevier B. V. All rights reserved.