Effect of pH on carbon‐13 NMR spectra of N‐dodecyl‐N, N‐dimethylamine oxide solution

Carbon‐13 NMR chemical shifts of the non‐ionic‐cationic surfactant N‐dodecyl‐N, N‐dimethylamine oxide (C12DAO) were measured and identified by an INADEQUATE experiment. On micellization, downfield chemical shifts were found for all carbon atoms except NCH2. The shifts are most pronounced for the carbon atoms in the central part of the chain. Protonation of the micelles resulted in upfield shifts of all the carbon atoms; the magnitudes decreased with increasing distance from the nitrogen atom. A plot of several chemical shifts against pH gave titration curves and pKa values. The pKa values vary with the carbon atom used in the calculation (from pKa = 5.05 to 5.66). An AMI calculation of charge densities suggests that these variations in pKa are caused by the electric field of the OH group. The effect of N‐oxidation on the chemical shifts of the carbon atoms in five solvents was estimated.