Investigation of prototropic equilibria by carbon‐13 relaxation time analysis

Quaternary 13 C dipole–dipole spin–lattice relaxation times are used to analyse prototropic equilibria of 2‐acetylbenzimidazole (1) and 4‐azabenzimidazole in DMSO‐ d 6 solutions. For the first compound, fast exchange between the two chelated structures prevails for 13 C relaxation times, whereas proton and 13 C chemical shifts characterize a slow exchange. For 4‐azabenzimidazole, 13 C and 1 H spectra do not exhibit absorptions from structures in slow exchange. Analysis of the T 1 values of carbons 8 and 9 before and after isotopic exchange of the labile amino hydrogen clearly show that the major component of the equilibrium is the 3 H structure.