Modelling of tautomeric equilibria of 5-hydroxyisoxazole in aqueous solution

Abstract

The structure and energetics of the three tautomers of 5-hydroxyisoxazole have been studied theoretically in the gas phase and in aqueous solution. The structures of the species have been optimised in the gas phase and in solution using the self-consistent reaction field model, with a multipole expansion up to l= 7, at the 6–31G**//6–31G** level. Electron correlation up to the MP4SDTQ level has been included in the gas phase calculations. The predicted solvation energies are consistent with those from the polarisable continuum model and from molecular dynamics simulations, and predict the CH tautomer to be dominant in water, in agreement with experiment. The NH tautomer is predicted to be the next most stable tautomer, although its energy separation from the CH form is larger than that suggested by experiment.