Blue-Shifting or Red-Shifting Hydrogen Bonding? Predictions for Haloform Complexes with Dimethyl Ether on the Basis of Perturbation Theory

Frequency shifts of the C−H stretching mode, ν 1, of the haloforms CHF 3, CHClF 2, CHCl 2 F, and CHCl 3 because of complexation with dimethyl ether have been calculated from an ab initio interaction potential between the two molecules, using a perturbative approach. The shift is described as a sum of two terms, a first due to a stiffening of the C−H bond, the second due to the attractive or repulsive nature of the interaction of the C−H bond with the Lewis base. For CHF 3 through CHCl 2 F, both terms are positive, resulting in an overall blue shift of ν 1; for the complex with CHCl 3, the attractive interaction with the Lewis base causes a red shift which is larger than the blue shift because of the stiffening of the C−H bond, resulting in a small overall red shift. The calculated results are compared with experimental literature data on these complexes.