Distortion of electronic structure in solvated molecules: Tautomeric equilibrium of 2-pyridone and 2-hydroxypridine in water studied by the RISM-SCF method

As well recognized, the electronic structure of a molecule in the solution phase is distorted from that in the gas phase. We present a new procedure that enables us to partition the electronic distortion energy caused by solvation into the energy contribution of each atom (or each moiety), with which one can evaluate the atomic-level change of electronic structure. In the present study, we combine the procedure with the RISM-SCF method that can treat solvent molecules explicitly. This method is successfully applied to the tautomerization of 2-pyridone in aqueous solution, whose equilibrium is known to show a marked solvation effect, and provides us a new detailed feature of this equilibrium. (1) As expected, electronic structures of oxygen and nitrogen atoms, which are strongly solvated, are distorted significantly. (2) However, the electronic distortion energies of the oxygen and nitrogen atoms are considerably compensated by the microscopic solvation. (3) One of the determining factors of the equilibrium is the carbon atom, with which the oxygen atom is bound, because the electronic distortion cannot be compensated by the microsolvation due to its rather inside position, and (4) the other factor is the distortion energy of the proton that directly relates to the equilibrium probably because the electronic structure changes too much to be compensated by microsolvation. These new findings lead to deep and correct understanding of this equilibrium.