STRUCTURES AND SPECTROSCOPIC CHARACTERISTICS OF 5,6-DIHYDRO-6-THYMYL AND 5,6-DIHYDRO-5-THYMYL RADICALS BY AN INTEGRATED QUANTUM-MECHANICAL APPROACH INCLUDING ELECTRONIC, VIBRATIONAL, AND SOLVENT EFFECTS

A recently developed quantum mechanical approach devoted to the study of unstable species in solution was applied to isomeric radicals resul ting from the addition of hydrogen atoms to thymine. The computational protocol includes either post-Hartree-Fock or density functional elec tronic computations, together with simulation of the solvent by a pola rizable continuum, and averaging of spectroscopic properties over thr most important vibrational motions. Concerning electronic computations , hybrid Hartree-Fock/density functional models (here B3LYP) provide r eliable results both for structural and spectroscopic parameters. In c ontrast. pure Hartree-Fock or low-order perturbative many-body approac hes (here MP2) stand against considerable difficulties in the treatmen t of open-shell systems. Starting from B3LYP computations, vibrational averaging by the out of plane motions and, to a lower extent, conside ration of solvent effects lend to remarkable agreement between the com puted hyperfine coupling constants and experimental data.