In this research study, relative stability of all the tautomers of phthalazinone ring in the gas phase and the solvent effect on the tautomeric equilibrium were evaluated using the density functional theory-polarizable continuum model at the B3LYP/6-311++G(d,p) basis set. In addition, variation of the dipole moments in the gas phase and solution, the specific solvent effect on the transition state of proton transfer assisted by a water molecule and the NBO calculated charges on the atoms were investigated. The water-assisted tautomerization with one molecule revealed that, the free energy activation barrier was reduced compared to those for the uncatalyzed systems. In the all the tautomers of phthalazinone rings, when going from gas phase to more polar solvents, the net charges on the O atoms slightly increased.
