Electron Delocalization in Electron-Deficient Alkenes and Push-Pull Alkenes
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We have studied pi 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 pi 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 pi electron stabilizing energy of a push-pull system and as much as 45% in a strongly electron-deficient tetracyanoethene. The Acc -> C= C bond pi 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 NO2 the smallest. Increase in the number of Acc groups increases percentage contribution of Acc -> C= C bond p electron delocalization to a system stabilization. A... difference in pi*(C=C) orbital occupancy between isomers can be related with their chemical reactivity.
Keywords:density functional theory / electron-deficient alkene / electron delocalization / electronic structure / push-pull alkene
Source:Chemistryselect, 2017, 2, 1, 42-50
- Wiley-V C H Verlag Gmbh, Weinheim
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