Cavity ringdown spectroscopy of the NIR A∼-X∼ electronic transition of allyl peroxy radical (H2CCH–CH2OO·)

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 O₂. 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, CH₃CHCHCH₂, 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 (CH₃CH=CHCH₂^• and CH₃CH^•CH=CH₂) 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.

Probing alkenoxy radical electronic structure using anion PEI spectroscopy

Photoelectron imaging spectra of three alkenoxide radical anions (3-buten-1-oxide, 3-buten-2-oxide, and 2-propenoxide) are presented and analyzed with supporting results of density functional theory calculations. In all spectra, intense detachment features are observed at approximately 2 eV electron binding energy, which is similar to the electron affinities of saturated neutral alkoxy radicals [Ramond et al., J. Chem. Phys. 112, 1158 (2000)]. Photoelectron angular distributions suggest the presence of several overlapping transitions which are assigned to the X̃ and à states of multiple energetically competitive conformers. The term energy of the à state of the 2-propenoxy radical, 0.17 eV, is higher than that of 3-buten-2-oxy (0.13 eV) and 3-buten-1-oxy (0.05 eV) radicals. Comparing the butenoxy radicals, we infer that stronger interactions between the non-bonding O 2p orbitals and the π bond increase the splitting between the ground and the first excited state in the 3-buten-2-oxy radical relative to the 3-buten-1-oxy radical.

tert-Butyl peroxy radical: ground and first excited state energetics and fundamental frequencies

The lowest adiabatic electronic transition origin and fundamental vibrational frequencies are computed, with high accuracy, for the tert-butyl peroxy radical.