REVERSAL OF THE USUAL NU(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 o f chloroform, deuterochloroform and bromoform have been investigated i n mixed systems containing such proton accepters as carboxy, nitro and sulpho compounds. Intermolecular hydrogen bond formation is accompani ed by a shift to higher wavenumbers of the absorption band of the stre tching 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 eff ect is the opposite to the usual occurrence of C-H/D bond weakening du e to hydrogen bonding and could be considered as a strengthening of th e C-H/D bond due to an increase of its s character caused by molecular deformation as a result of intermolecular forces, Quantum chemical mo delling of the vibrational spectrum of the chloroform molecule demonst rates an increase in the nu(C-H) wavenumber with increasing Cl-C-H bon d angle. However, calculation of the vibrational spectrum of the chlor oform-nitromethane complex predicts an overall reduction of the nu(C-H ) wavenumber. (C) 1997 Elsevier Science B.V.