Intensities of vibration bands. I. Bending vibrations of benzene derivatives and the dipole of the C-H link

Abstract

A simplified potential energy function has been applied to calculate the frequencies and relative amplitudes of out-of-plane vibrations of benzene and its mono-, di- and trisubstituted derivatives. The frequencies and extinction coefficients of the infra-red absorption bands corresponding to the most intense mode of this class have been measured for twenty compounds in which the substituents were (CH 3 ) or a halogen. The observed and calculated frequencies show satisfactory agreement. The relative intensities calculated for the methyl derivatives are in accord with those predicted from the calculated amplitudes. By applying the same treatment to compounds containing one or more halogen substituents, an estimate has been made of the magnitude and direction of the dipole moment of the C-H link. It is shown unequivocally that the positive end of the dipole is the hydrogen atom, and the mean values is close to 0.42D.