NMR isotope shift and perturbational MO study of conformation and hyperconjugation in cyclopentyl cations

Abstract

The 1-methylcyclopentyl cation is shown to have the twist conformation through the additivity characteristics of the NMR isotope shifts at C/sup +/ induced by ..beta..-deuteration. The trans-2,5-d/sub 2/ isotopomer has an isotope shift that is smaller than expected on the basis of additivity. This small isotope shift is due to an isotope effect on the conformational equilibrium that is only possible in the twist conformation. The 1-(4-fluorophenyl)cyclopentyl cation also exists in the twist conformation. The ratio of ..beta..-CD/sub 2/ to ..beta..-CD/sub 3/ effects in these and other cations indicates that hyperconjugation is stronger for endocyclic methylene bonds than for exocyclic methyl bonds. A theoretical model, based on MINDO/3 calculations and a perturbational approach, provides a possible explanation for the relative magnitude of these isotope shifts.