On the nature of unusual intensity changes in the infrared spectra of the enflurane⋯acetone complexes

Abstract

Recently, the blue-shifting hydrogen bonded complexes between enflurane and acetone have been investigated [W. Zierkiewicz, B. Czarnik-Matusewicz and D. Michalska, J. Phys. Chem. A, 2011, 115, 11362]. It has been shown that enflurane is a particularly interesting system due to its ability to form the blue-shifting C–H⋯O hydrogen bonded complexes accompanied by both a decrease and an increase of IR intensity of the corresponding ν(C–H) bands. In the current paper the origin of different intensity changes of the two C–H stretching vibrations in these complexes is elucidated based on solid quantitative grounds. The derivatives of the components of the dipole moment along the C–H stretching normal modes in the enflurane monomer and their changes upon complexation are discussed. Moreover, ten different exchange–correlation functionals were tested with regard to their performance in the prediction of IR intensity changes of the ν(C–H) stretching vibrations, in the title complexes.