Intrinsic and Equilibrium NMR Isotope Shift Evidence for Negative Hyperconjugation

Article abstract of DOI:10.1021/ja00269a006

Intrinsic NMR isotope shifts are found at 19F due to CD3 substitution for CH3 in N-methyl- and N,N-dimethyl-4-fluoroaniline and 4-fluoroanisole.These intrinsic shifts demonstrate isotopic perturbation of negative hyperconjugation, in which C-H(D) bonds act as electron acceptors.Comparison with carbocations and a carbanion through the use of a perturbational analysis based on MO calculations supports the hyperconjugative origin of intrinsic long-range isotope shifts.Equilibrium isotope effects on the protonation of anilines, measured through observation of equilibrium NMR isotope shifts, are also consistent with negative hyperconjugation.