Using Implicit and Explicit Solvation Models on Theoretical Study Tautomerization of 3,4-dihydropyrimidin-2(1H)-imine

A theoretical study at the B3LYP/6-311++G (d, p) level on tautomerization of 3,4-dihydropyrimidin-2(1H)-imine(3,4DHP) into 1,6-dihydropyrimidin-2-amine(1,6DHP) and 1,4-dihydropyrimidin-2-amine(1,4DHP) was performed at 298.15K. Two mechanisms have been considered for these processes: (i) one in which the hydrogen is directly transferred, mechanisms A, through TS13 and TS24 in two different pathways, called P(a) and P(b), respectively and (ii) another one in which a double hydrogen transfer takes place via TS1133, TS1234 and TS2244 by formation of the corresponding dimer, mechanisms B. The results associated with the gas phase reveal that overcame to TS13 and TS24, need high activation free energies of 38.31, 40.29 kcal/mol, respectively as a consequence of the strain associated with the formation of the four-membered transition state, while overcame to TS1133, TS1234 and TS2244, require much lower activation free energies, 7.01, 8.19 and 8.98 kcal/mol, respectively. To investigate the effect of solvent on the kinetics and thermodynamics of the tautomeric process in p(a) and p(b) paths, Implicit, explicit, and a combination of both implicit and explicit solvation models in various media (protic and aprotic polar solvents) have been considered. The results show that the use of a protic polar solvent reduces the energy barrier of the corresponding transition states in a very significant amount.