CH-Bindungsmomente und IR-Intensitäten von Benzol-Derivaten—I. Einführung

It is shown that to a good approximation the CH-stretching vibrations in benzene derivatives are localized in the CH bonds and that the total intensity of a CH-stretching vibration of such a compound is given by the sum of the squares of the derivatives ∂μ|∂R_j: I_CH = $\text{j}\text{́}$($\text{∂μ}\text{∂}\text{R}{}_{\mathrm{j}}$) ∂μ|∂R_j is the change of the molecular dipole moment with respect to a change of the stretching coordinate R_j of the jth CH bond. Experimental data demonstrate the validity of this approximation.

In continuation of our former investigations on the dependence of the CH-stretching vibrations on different substituents we have examined a considerable number of deuterated benzene monoderivatives, para-disubstituted benzene derivatives and 2-, 3- and 4-substituted pyridine derivatives. We have found a relation of general validity between I_CH and the Taft substitution constant σ_I: I_CH = F(σ_I). The interpretation of this result leads to conclusions concerning the effect of substituents on the CH dipole of benzene derivatives.