Deuterium Isotope Effects on Carbon-13 NMR Shifts and the Tautomeric Equilibrium in N-Substituted Pyridyl Derivatives of Piroxicam

Article abstract of DOI:10.1021/ja00199a015

N-Alkylated pyridyl derivatives of piroxicam are formally zwitterionic but can, at the expense of aromaticity in the pyridyl ring, revert to neutral structures by means of fast internal proton transfers.The large change in dipole moment accompanying these tautomeric shifts makes the equilibrium unusually sensitive to changes in chemical substitution, solvent, and temperature.This property is used to formulate, test, and verify a new type of deuterium isotope effect on carbon-13 NMR shifts recently proposed by Hansen and, in the process, establish close correspondencewith the general theory of isotope effects on proton transfer equilibria.