Deuterium Isotope Effects on the Carbon-13 Chemical Shifts in 2-Substituted-2-Norbornyl Cations

Sa？etak Deuterium isotope effects on carbon-13 chemical shifts have been determined for 2-methyl-2-norbornyl-2-dj cation, 2-methyl -2-norbornyl-exo -G-<21 cation, 2-methyl-2-norbornyl-encfo-G-di cation, 2-methyl-2-n orbo rnyl-exo,ero- 5,6-d2 cation, 2-methyl-2-norbornyl-exo, anli-6,7-？2 cation, 2-aryl-2-norbor- nyl-1 -dj cation, 2-aryl-2-norbornyl-exo-G-cf1 cation, 2-aryl-2-norbornyI-en？o-6- di cation, 2-aiyl-2-norbornyl-exo,exo-5,6-cf2 cation, and 2-aryl-2-norbomyl- exo,anti-G,l-d2 cation. The deuterium isotope effects exhibit different magnitudes for the different isotopomers. For the 2-methyl-2-norbornyl cation, the isotope effect at C2 is 1.4 ppm for exo-G-dlt 0.9 ppm for endo-G-d^, and 0.1 ppm for 1-dj. Large positive 3-bond isotope effects are highly unusual but are consistent with the bridged structure for this ion. The isotope effects, AC(D), at the cation center have been plotted vs. oc+ constants for each of the deuterated 2-aryl-2-norbornyl cation series and compared to plots of the isotope effects, AC(D), vs. I3C chemical shifts at the cation center. The nonlinear behavior of these plots is consistent with the onset of bridging. The onset of delocalization in the 2-aryl-2-norbornyl cation as a result of electron withdrawing groups on the phenyl ring gives enhanced positive values for the isotope shifts at C2 for deuterium substitution at Cg and reduced positive values for deuterium substitution at Cj. Isotopic substitution appears to readily perturb the vibrational averaging occurring in the relatively flat an- harmonic potential well for these systems as the electron demand increases