Abstract

Ab initio and semiempirical calculations have been carried out for the five tautomeric forms of 8-azapurine (Azapu) both in the gas phase and in aqueous solution. In the gas phase the stability order is Azapu(9) > Azapu(8) > Azapu(7) > Azapu(3) > Azapu(1), whereas in solution, the above order is Azapu(8) > Azapu(9) > Azapu(7) > Azapu(1) > Azapu(3) according to the SM2 solvation model. When applying ab initio into SCRF, Azapu(7) shows up as the most stable species. The ?G? soln values for the equilibria involving the three more important tautomers indicate that Azapur(7) is formed from Azapu(9) and Azapu(8).