3-picolyl and 2,5-lutidyl radicals: Generation, optical spectroscopy, and ab initio calculations

Electronic spectra are reported for two ring-substituted benzyl-like radicals, 3-picolyl and 2, 5-lutidyl. These pyridine-based radicals have much shorter excited state (D-1) lifetimes than their benzyl radical prototypes. The 3-picolyl radical (3-(CH2)C5H4N) has an excited-state DI lifetime of similar to 9 ns, and the 2,5-lutidyl radical (2-(CH3),5-(CH2)C5H3N) has an excited-state D-1 lifetime of similar to 245 ns. The near-degeneracy of the D-1 and D-2 states of the benzyl radical is not found for these pyridine-based radicals, and the strong D-1-D-2 vibronic interaction is missing as well in these species. A number of ab initio calculations, at the (5 x 5) CASMP2 and (7 x 7) CAS levels with DZP and d95** basis sets, are employed to rationalize these results and assign the spectra. One possibility for the absence of other isomeric radical spectra is a rearrangement to a seven-membered radical ring form. The dynamics of radiationless transitions, ring expansion and rearrangement, and excited-state lifetimes are all factors for the spectroscopic detection of these radicals. Based on the calculations, the excited-stare vibrational modes upsilon(6a), upsilon(6b), and upsilon(1) are assigned for the picolyl and lutidyl radicals.