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 (D1) lifetimes than their benzyl radical prototypes. The 3-picolyl radical (3-(ĊH2)C5H4N) has an excited-state D1 lifetime of ∼9 ns, and the 2,5-lutidyl radical (2-(CH3),5-(ĊH2)C5H3N) has an excited-state D1 lifetime of ∼245 ns. The near-degeneracy of the D1 and D2 states of the benzyl radical is not found for these pyridine-based radicals, and the strong D1-D2 vibronic interaction is missing as well in these species. A number of ab initio calculations, at the (5 × 5) CASMP2 and (7 × 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 sevenmembered 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-state vibrational modes ν6a, ν6b, and ν1 are assigned for the picolyl and lutidyl radicals.