Deuterium isotope effects on the carbon-13 chemical shifts in 2-substituted 2-nonbornyl cations

Deuterium isotope effects on 13C chemical shifts have been examined as a function of location of deuterium in the C3-exo or C3-endo positions and as a function of increasing electron demand in the series 2-norbornanone, 2-aryl-2-nonbornyl cations, and 2-methyl-2-norbornyl cation. Comparison with isotope shifts in 2-aryl-2-propyl cations demonstrates a change in the type of response to isotopic perturbation in 2-aryl-2-norbornyl cations as electron demand increases. The results are consistent with the onset of σ-bridging as electron demand increases. The observed isotope shift at a cation center is suggested to be a sum of contributions from a small upfield shift due to perturbation of the sigma-framework (inductive-type perturbation), a downfield shift due to perturbation of hyperconjugation, and a potentially large upfield shift from perturbation of three-center, two-electron bonding. NMR isotope shifts may be a particularly sensitive probe of sigma-bridging because of the vibrational origin of the isotope effect