Electron Delocalization in Electron‐Deficient Alkenes and Push‐Pull Alkenes

We have studied π electron delocalization in electron‐deficient alkenes and push‐pull alkenes by means of natural bond orbital analysis at the B3LYP/6‐311+G(d,p) level. The study revealed that the rarely mentioned π electron donation from an electron‐accepting group (Acc) toward the C=C double bond in push‐pull alkenes and electron‐deficient alkenes can provide up to 10 % of total π electron stabilizing energy of a push‐pull system and as much as 45 % in a strongly electron‐deficient tetracyanoethene. The Acc → C=C bond π electron donation is more intense in s‐trans than in s‐cis conformational arrangement, but is less dependent on Z/E isomerism in push‐pull alkenes, being slightly more pronounced in Z isomers. Among different Acc substituents, CN and COO ‐ groups contribute the largest percent of stabilizing energy and NO 2 the smallest. Increase in the number of Acc groups increases percentage contribution of Acc → C=C bond π electron delocalization to a system stabilization. A difference in π * C=C orbital occupancy between isomers can be related with their chemical reactivity.