Intramolecular proton transfer in tautomeric 2-imidazolone and 2-thioimidazolone

The enol reversible arrow keto and thiol reversible arrow thione prototropic tautomerisms in 2-imidazolone and 2-thioimidazolone were studied by ab initio methods in the frame of MO theory. The energetics of these reactions were derived using various basis sets and levels including both polarization and diffuse functions. To account for electron correlation, second-, third- and fourth-order Moller-Plesset perturbation theory was applied. The thermodynamics for both reactions indicate that the keto and thione are the predominant species in the gas phase, whereas in solution the enol and. keto forms are present in comparable concentrations. For both tautomerization reactions the transition states for the intramolecular mechanism were found and fully characterized. The calculated energetic barrier for the enol-keto system is 41.6 kcal mol(-1), whereas that for the thiol-thione system is 32.5 kcal mol(-1) (I kcal = 4.184 U). The study of the solvent effect, using the PCM model, on E-a reveals that solvents lower the barrier by ca I I and 7 kcal mol(-1) for the enol-keto and thiol-thione systems, respectively. Copyright (C) 2002 John Wiley Sons, Ltd.