tThe tautomeric conversion of pyridazin-3(2H)-one 1 into pyridazin-3-ol 2 has been theoretically studiedusing density functional theory (DFT) methods at the B3LYP/6-311++G** level. Two mechanisms havebeen considered for this process: (i) one in which the hydrogen is directly transferred through TS12;and (ii) another one in which a double hydrogen transfer takes place via TS1122 upon formation of thecorresponding dimer. The former requires a very high activation energy of 42.64 kcal/mol as a conse-quence of the strain associated with the formation of the four-membered TS12, while the latter requiresa much lower activation energy, 14.66 kcal/mol. Implicit, explicit, and a combination of both implicit andexplicit solvation models, using both protic and aprotic polar solvents, have been considered for the firstmechanism. This study allows the establishment of the requirement to use protic polar solvents in orderto reduce the high activation energy associated with TS12

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