Heteroatom Incorporation Effect in σ- and π-Electron Systems: The sEDA(II) and pEDA(II) Descriptors

The effect of heteroatom or heteroatomic group incorporation into unsaturated five- and six-membered cyclic systems was studied by means of DFT/B3LYP/aug-cc-pVDZ calculations. Two descriptors of the incorporation effect, sEDA­(II) and pEDA­(II), reflecting the influence of the incorporated atom or group on the population of the σ and π valence electrons, were constructed on the basis of natural bond orbital analysis. The sEDA­(II) and pEDA­(II) descriptors were shown to be linearly independent; the former correlated very well with electronegativity scales, whereas the latter correlated with NICS(1)_ZZ and HOMA_CC aromaticity indices. The two descriptors seem to be universal tools for analyzing different chemical and physicochemical effects occurring in unsaturated heterocyclic systems.