Hydrogen-Transferred Radical Cations of NADH Model Compounds. 1. Spontaneous Tautomerization

The radical cations generated from three NADH model compounds were investigated with particular emphasis on the possible formation of tautomeric enol radical cations. To a small extent, hydrogen transfer occurs upon ionization of all three compounds in argon matrixes. In the bicyclic seven-membered ring derivative enolization continues in a thermal reaction at 12 K (with highly dispersive kinetics), or upon visible irradiation. These processes are not observed in methylcyclohexane glasses at 77 K, which indicates that a cage effect is preventing the attainment of a reactive conformation in this medium. The assignment of electronic spectra of the primary (keto) cations and of their tautomeric (enol) forms is supported by CASSCF/CASPT2 and TD-B3LYP calculations.