Room-Temperature Cavity Ring-Down Spectroscopy of Methylallyl Peroxy Radicals.

We report room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of 1-, 2-, and 3-methylallyl peroxy (MAOO) radicals produced by 193 nm photolysis of methyl-substituted allyl chlorides in the presence of O. Vibronic structure of experimentally observed spectra was simulated using calculated relative populations of MAOO conformers, their electronic transition frequencies and oscillator strengths, as well as their vibrational frequencies and Franck-Condon factors of the à ← X̃ electronic transition. The reaction intermediate for the production of 1- and 3-MAOO radicals, CHCHCHCH, is a resonance-stabilized free radical. CRD spectra of 1- and 3-MAOO radicals obtained using different precursors suggest that allylic rearrangement between the two resonance structures (CHCH=CHCH and CHCHCH=CH) is significantly faster than oxygen addition. Branching ratio between terminal and nonterminal oxygen addition was predicted to be 52:48 on the basis of calculated spin densities, which agrees qualitatively with the experimental CRD spectra of 1- and 3-MAOO radicals.