13C NMR spin–lattice relaxation mechanisms of carbocations in superacids

Abstract

AbstractThe contributions to the 13C NMR spin–lattice relaxation rates of both protonated and non‐protonated carbons of a range of carbocations in superacids were determined from the analysis of T1 values measured over a range of temperatures and at different field strengths. From the data for 1‐adamantyl, 2‐methyl‐2‐norbornyl, 3‐methyl‐3‐nortricyclyl, 2‐phenyl‐2‐norbornyl, cumyl (1‐methyl‐1‐phenylethyl) and methylphenylcyclopropylcarbinyl (1‐cyclopropyl‐1‐phenylethyl) cations, dipole–dipole interactions (both intra‐ and inter‐molecular) were found to be entirely responsible for the relaxation of protonated carbons. However, for the cationic carbons, chemical shift anisotropy contributes significantly to their relaxation at low field strengths (2.3 T) and predominantly at high field strengths (5.8 T). The shielding anisotropies (Δσ) of the cationic carbons were determined as 410–540 ppm, which are amongst the largest reported for any carbon atoms.