Effect of Conjugation on Electron Distributions. Separation of σ and π Terms.

The ground states of unsaturated molecules with C(s) symmetry have separate σ and π molecular orbitals. This makes it possible to separately calculate the σ and π Hirshfeld populations and to analyze the effect of structural changes in more detail. A number of types of compounds have been studied: vinyl derivatives, 1-substituted hexatrienes, phenyl derivatives, and six-membered ring heterocycles. The results are shown as plots of changes in population vs atoms. The σ and π components vary widely, depending on π conjugation, σ inductive effects, and σ/π polarization. When an electron withdrawing group is attached to an alkene carbon, significant σ density is transferred to it, and to compensate π density is transferred to the substituent. Such interactions are similar to those found when a carbonyl or other ligands are attached to a metal.