Quantum-chemical studies of the consequences of one-electron oxidation and one-electron reduction for imidazole in the gas phase and water

Consequences of one-electron oxidation and one-electron reduction were studied for imidazole (I) in the gas phase and in water (apolar and polar environment, respectively) using quantum-chemical methods {DFT(B3LYP)/6-311+G**, G2, G2(MP2), G3B3, and PCM//B3LYP/6-311+G(d,p)}. For calculations, all possible prototropic tautomers (NH and CH) were considered for the neutral and redox forms of I. Independently on environment (gas phase or water), an interesting change of the composition of the tautomeric mixture takes place when going from the neutral to reduced form of imidazole. One-electron reduction (I + e → I−) increases stability of the CH tautomers (non-aromatic forms). On the other hand, one-electron oxidation (I − e → I+) has no important effect on the tautomeric preferences. The NH tautomers predominate for both neutral (I) and oxidized imidazole (I+). For the favored forms in the gas phase, the oxidation Gibbs energy is close to 200 kcal mol−1, and the reduction Gibbs energy is more than ten times lower. In water, these energies are lower by more than 50 kcal mol−1. Independently on the state of oxidation, π-electrons are more delocalized for the NH than CH tautomers.