Abstract: The 1-methylcyclopentyl cation is shown to have the twist conformation through the additivity characteristics of the NMR isotope shifts at C+ induced by &deuteration. The trans- 2, 5-d2 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) ...