Reversal of the usual ν(C-H/D) spectral shift of haloforms in some hydrogen-bonded complexes

Infrared spectra in the region of the nu(C-H/D) stretching frequency of chloroform, deuterochloroform and bromoform have been investigated in mixed systems containing such proton accepters as carboxy, nitro and sulpho compounds. Intermolecular hydrogen bond formation is accompanied by a shift to higher wavenumbers of the absorption band of the stretching vibration nu(C-H/D) of the haloform molecules, to the extent of 3-8 cm(-1) in comparison with its position in CCl4 solution. This effect is the opposite to the usual occurrence of C-H/D bond weakening due to hydrogen bonding and could be considered as a strengthening of the C-H/D bond due to an increase of its s character caused by molecular deformation as a result of intermolecular forces, Quantum chemical modelling of the vibrational spectrum of the chloroform molecule demonstrates an increase in the nu(C-H) wavenumber with increasing Cl-C-H bond angle. However, calculation of the vibrational spectrum of the chloroform-nitromethane complex predicts an overall reduction of the nu(C-H) wavenumber. (C) 1997 Elsevier Science B.V.