Deuterium isotope effects on ¹³C nuclear shielding as a measure of tautomeric equilibria

Large deuterium isotope effects of both signs have been observed on the 13C nuclear shielding in proton‐chelating tautomeric forms of β‐thioxoketones, and small isotope effects of both signs have been found in related forms of Schiff's bases of salicylaldehydes. These effects are interpreted in terms of shifts in the tautomeric equilibria on deuterium substitution of the enolic proton. The observation of deuterium isotope effects is suggested as a useful method for the detection of fast tautomeric equilibria in systems of the following type. (Formula Presented.) .