Kinetics of direct and water-mediated tautomerization reactions of five-membered cyclic amides or lactams

As part of a series of studies of hydrogen-atom transfer or tautomerization reactions of imidic acid–amide species, $H─O═C─N─\rightleftharpoons O═C─N─H$, we report the rate constants for a set of 24 five-membered cyclic compounds at low, 50–300 K, and high, 500–1500 K, temperatures. These rate constants are for both the high temperature direct reaction and for that mediated by an additional water molecule which facilitates the hydrogen transfer reaction at low temperatures. In the latter case we show that the rate of reaction from a pre-reaction complex can be rapid at temperatures down to 50 K on a 1 ms timescale and is dominated by quantum mechanical effects as evaluated by small-curvature and quantised-reaction-states tunnelling. In addition, we present thermochemical data such as enthalpies of formation, entropies, isobaric heat capacities and enthalpy functions for these largely unknown species which span a range of compounds from pyrolidinone to oxo-tetrazoles.