Tautomeric equilibria of maleic hydrazide in the gas phase and in aqueous solution: an experimental and theoretical study

Abstract

Theoretical predictions of the tautomeric equilibria in maleic hydrazide in the gas phase and in aqueous solution are presented. The use of pK_a data to obtain tautomer populations in aqueous solution is discussed and the values obtained compared with our theoretical predictions. Ab initio calculations of the gas phase species including a high level of electron correlation are combined with both molecular dynamics simulations and continuum methods, to model solvation effects. Both in the gas phase and in aqueous solution the monohydroxy-monoketo tautomer is predicted to be dominant. The effect of solvation is predicted to stabilize preferentially the diketo form so that both forms may be observed in solution, whilst the dihydroxy form is essentially absent. These conclusions are in agreement with available experimental data.