Ab initio study of guanine tautomers in the S0 and D0 states

The electronic and stabilization energies of the guanine six most stable tautomers were calculated in the neutral ground electronic, S0, and the cationic ground, D0, states. Calculations using ab initio MP2 method shows that guanine 9‐H keto is the most stable tautomer in the S0 state. It was found that upon ionization to the D0 state, guanine 9‐H‐enol‐trans is the most stable form. The ionization energy of the six tautomers of guanine was calculated and found to agree well with the values reported from experiments and with theoretically predicted ionization energies. The molecular structures in the S0 and D0 states were predicted and the results from geometry optimization show that all tautomers are nonplanar in the S0 state. It was predicted that the ionization of all guanine forms to the D0 state may originate from the removal of one of the electron pairs on the terminal NH2 group of the pyrimidine ring, and the other electron enhances the interaction between nitrogen atoms of the NH2 and the immediate carbon atom neighbors via hyperconjugation. These interactions force the molecule to become planar in the D0 state. The planar geometry is justified by the electron delocalization in and outside both rings. The calculations of atomic charges and dipole moments support the findings of electrons delocalization around the ring causing the geometrical changes in the S0 and D0 states of all tautomers. The calculations of the rotational constants show that the geometries are different for all tautomers, in both the S0 and D0 states, verifying the findings from atomic charge and dipole moment analysis that these tautomers suffer large geometrical changes upon ionization.