Predictions of tautomeric equilibria in 2-hydroxypyridine and pyridine-2-thiol: correlation effects and possible discrepancies with experiment

Abstract

Calculations of the free energy differences between the tautomer pairs, 2-hydroxypyridine/2-pyridone, and pyridine-2-thiol/pyridine-2-thione, are reported using large (TZV2P) basis sets and electron correlation at the QCISD(T) level. For the hydroxy/oxo system the predicted energy difference (2.9 kJ mol^–1) is the same as the experimental value (2.9 ± 0.5 kJ mol^–1). For the thiol/thione system, the predicted energy difference (15.2 kJ mol^–1) is somewhat larger than the experimental value (9.9 ± 0.5 kJ mol^–1). The importance of correlation energy contributions beyond second order is highlighted.