THERMODYNAMIC CHARACTERIZATION OF TAUTOMERIC EQUILIBRIA BY MULTINUCLEAR MAGNETIC-RESONANCE - APPLICATION TO 3-HYDROXYPYRIDINE

A marked temperature dependence of the C-13 and N-15 chemical shifts o f 3-hydroxypyridine in D2O (pD = 7.0) was observed. This dependence is explained in terms of thermally induced equilibrium shifts between th e neutral form and the zwitterionic form of 3-hydroxypyridine. A metho d was developed to characterize the thermodynamics of the equilibria t hat are rapid on the NMR time scale, using the observed average C-13 a nd N-15 chemical shifts at different temperatures. This method was app lied to the thermodynamics of the tautomerization equilibria of 3-hydr oxypyridine. The equilibrium constants and the standard enthalpy chang e of the tautomerization equilibrium obtained by C-13 and N-15 methods agree very well with the results obtained by other spectroscopic meth ods.