Quantum-Chemical Studies on the Favored and Rare Tautomers of Neutral and Redox Adenine

All possible twenty-three prototropic tautomers of neutral and redox adenine (nine amine and fourteen imine forms, including geometric isomerism of the exo NH group) were examined in vacuo {DFT­(B3LYP)/6-311+G­(d,p)}. The NH → NH conversions as well as those usually omitted, NH → CH and CH → CH, were considered. An interesting change of the tautomeric preference occurs when proceeding from neutral to reduced adenine. One-electron reduction favors the nonaromatic amine C8H–N10H tautomer. This tautomeric preference is similar to that (C2H) for reduced imidazole. Water molecules (PCM model) seem to not change this trend. They influence solely the relative energies. The DFT vertical detachment energy in the gas phase is positive for each tautomer, e.g., 0.03 eV for N9H–N10H and 1.84 eV for C8H–N10H. The DFT adiabatic electron affinity for the favored process, neutral N9H–N10H → reduced C8H–N10H (ground states), is equal to 0.18 eV at 0 K (ZPE included). One-electron oxidation does not change the tautomeric preference in the gas phase. The aromatic amine N9H–N10H tautomer is favored for the oxidized molecule similarly as for the neutral one. The DFT adiabatic ionization potential for the favored process, neutral N9H–N10H → oxidized N9H–N10H (ground states), is equal to 8.12 eV at 0 K (ZPE included). Water molecules (PCM model) seem to influence solely the composition of the tautomeric mixture and the relative energies. They change the energies of the oxidation and reduction processes by ca. 2 eV