CH-Bindungsmomente und IR-Intensitäten von Benzol-Derivaten—V. Theorie

It is shown that a complete parametrisation of the intensities of the CH-stretching vibrations of benzene and its derivatives is possible. The intensity I_CH for such a compound can be expressed by a relation I_CH=

(frcase|∂μ/∂R_j)²=F(a_j, σ_I) which can be explicitly stated. σ_I is the Taft substituent constant and the a_j can be empirically determined. From the above relation there was obtained a function for the derivative of the dipole moment of the j-th CH bond with respect to the j-th stretching coordinate: $\text{∂μ}{}_{\mathrm{j}}\text{∂}\text{R}{}_{\mathrm{j}}$ = F(a_j, σ_I) Under reasonable assumptions this function can be integrated, thus leading to a functional relation between the CH dipole moment μ_j and the substituent σ_I. The results show that for benzene and most benzene derivatives the polarity of the CH bond is C⁺H⁻. In fact, all substituents having σ_I— values greater than zero induce a C⁻H⁺ polarity. There are, however, some para disubstituted benzene derivatives where the dipole moments induced by the substituents are so great as to lead to a C⁻H⁺ polarity of the CH-bond.