Conformational Dependence of Isotope Effects for Hyperconjugating Methyl Groups. Nonadditivity of NMR Isotope Shifts in Benzylic Ions

Article abstract of DOI:10.1021/ja00365a044

Deuterium substitution in the methyl groups induces long-range downfield shifts in 13C NMR signals of the ortho and para positions of the phenyldimethylcarbenium ion.Similar downfield isotope shifts occur in 19F signals of (p-fluorophenyl)carbenium ions upon deuteration of α-methyl groups.These NMR isotope shifts are analogous to secondary β-deuterium isotope effects on rates and equilibria and arise from hyperconjugative interactions.The effects of substituting entire CD3 groups for CH3 groups are additive, but the effects of deuterium substitution within a methyl group are not additive.The nonadditive behavior is attributed to unequal populations of the possible methyl conformation for partially deuterated methyl groups, so that each C-H(D) bond is not equally involved in hyperconjugation.This interpretation is supported by the observation of an isotope effect on the vicinal 1H-19F coupling constant in the phenylmethylfluorocarbenium ion, PhCFCH3⁺.