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

Two independent sigma- and pi- substituent effect descriptors, sEDA and pEDA, 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 sEDA and pEDA descriptors have a clear physical meaning-they show to what extent the sigma and pi electrons are donated or withdrawn by the substituent from the substituted system.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 (6-31 G** or cc-pVTZ), and solvent presence. We also demonstrated that the sEDA descriptor described equally well methane derivatives, whereas the pEDA descriptor explained behavior of ethene derivatives. Moreover, the two descriptors work out well for as different molecular systems as triazoles. A comparison of the sEDA and pEDA descriptors with the selected well-known substituent effect scales shows that the sEDA descriptor correlates well (R = 0.961) with the Boyd & Boyd/Boyd-Edgecombe chi descriptor and that so does also the pEDA descriptor (R = 0.943) with the Taft-Topsom sigma(R)- resonance constant. Copyright (c) 2009 John Wiley & Sons, Ltd.