σ‐ and π‐electron contributions to the substituent effect: natural population analysis

Abstract Two independent σ ‐ and π ‐ substituent effect descriptors, s EDA and p EDA, were constructed using the Natural Population Analysis (NPA) approach. The descriptors are based on parameters of 32 monosubstituted benzene molecules calculated at the B3LYP/6‐31G** level which is easily applicable to large molecular systems. The s EDA and p EDA descriptors have a clear physical meaning—they show to what extent the σ and π electrons are donated or withdrawn by the substituent from the substituted system. The descriptors were successfully tested to be independent of either of the applicable theoretical methods (DFT or MP2), basis set (6‐31G** or cc‐pVTZ), and solvent presence. We also demonstrated that the s EDA descriptor described equally well methane derivatives, whereas the p EDA descriptor explained behavior of ethene derivatives. Moreover, the two descriptors work out well for as different molecular systems as triazoles. A comparison of the s EDA and p EDA descriptors with the selected well‐known substituent effect scales shows that the s EDA descriptor correlates well ( R = 0.961) with the Boyd & Boyd/Boyd– Edgecombe χ descriptor and that so does also the p EDA descriptor ( R = 0.943) with the Taft–Topsom σ R ‐ resonance constant. Copyright © 2009 John Wiley & Sons, Ltd.