Investigation of the structure and properties of the potentially tautomeric 5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazines in the gas and aqueous phases using the AM1 and PM3/COSMO solvation method

Abstract

The potentially tautomeric 5.7-dimethyl-6H-pyrrolo[3,4-d]pyidazines 2H and 6H, and their fixed tautomeric forms were studied in order to predict the most stable form by the restricted Hartree-Foch approach using semiempirical PM3 and AM1 quantum chemical calculations at the SCF level in the gas and aqueous phases. Both methods predicted that the 6H form, 5, is more stable than the other forms, 1-4, in both gas and aqueous phases, The results obtained were found to be in agreement with the experimental data. Monoprotonated forms of 5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazines were also examined. Proton affinity calculations predicted that the first protonations take place on the N6 atom in the 2H form and on the N2 atom in the 6H form, resulting in a common cation 13