Large Intrinsic Nuclear Magnetic Resonance Isotope Shifts Associated with Bending Motion along the Bridging Coordinate in Carbocations

Article abstract of DOI:10.1021/ja00258a004

Deuterium isotope effects on the 13C NMR chemical shifts have been determined for the 2-methyl-2-bicyclo<2.2.2>octyl and 2-methyl-2-bicyclo<2.2.1>heptyl cations.These tertiary carbocations have isotope shifts that are larger than 1 ppm per deuterium, which is an order of magnitude larger than ordinary intrinsic shifts found in nonionic model compounds and in other carbocations.For deuteriation at C₃ methylene or methyl group, the same pattern occurs in both cations: isotope shifts that are large and upfield at C₂, downfield at C₁, and upfield at the remaining carbon directly bonded to C₂.The similarity in the pattern of the isotope shifts suggests that the force field and shielding influences in both ions are similar.The specific results are interpreted as indicating the presence of a shallow potential surface for the bending motion along the direction associated with ?-bridging.Since the existence of the shallow potential does not depend on the actual extent of bridging, the magnitude of the isotope effect is not proportional to the extent of bridging.