NMR solvent effect study of pyrroline-N-oxide spin traps: Microenvironments in sodium dodecyl sulfate micelles

Complete 13C NMR assignments for ten five‐membered ring nitrone (pyrroline‐N‐oxide) spin traps in CHCl3 are presented. A good correlation between the nitronyl 13C NMR chemical shift (δ) and the substituent constant (σ) for the substituted phenyl DMPOs was found. The 13C NMR chemical shift of the nitronyl carbon of these 5,5‐dimethyl‐2‐phenylpyrroline‐N‐oxides (or phenyl‐DMPOs) exhibited a large solvent effect (14 ppm from cyclohexane to water) which could be correlated with the solvent polarity parameter ET(30). Using a paramagnetic relaxation agent, the weighted time‐averaged fast exchange of the nitrone between the interior of a sodium dodecyl sulfate micelle and the bulk aqueous phase could be determined. It was found that the 2‐phenylpyrroline‐N‐oxides reside in microenvironments with polarities close to that of ethanol, methanol and between methanol and water. The fraction of nitrone completely inside the micelle at any given time was found to be around 77 ± 0.05%. This is the first time that the paramagnetic relaxation enhancement method has been used with 13C NMR spin‐lattice relaxation time measurements.