Valence-Bond Theory of Resonance: Aniline, Phenol, and Fluorobenzene

A perturbation-type valence-bond theory is developed for the computation of pi-orbital charge distributions in substituted benzenes. Application is made to aniline, phenol, and fluorobenzene, employing for comparison approximate charge distributions derived from proton chemical shift measurements. An important finding is that the previously employed approximation of including only single-electron transfers between adjacent atoms is not valid. The inclusion of second-order transfer interactions permits one to compute charge distributions which agree well with the experimental data. However, one must employ empirical integral parameters which are slightly different from the estimated values. Detracting from the theory is the fact that the computations apparently place too much charge on the carbon atom adjacent to the substituent.