Observation of the Ã−X̃ Electronic Transitions of Cyclopentyl and Cyclohexyl Peroxy Radicals via Cavity Ringdown Spectroscopy

The Ã−X̃ electronic absorption spectra of cyclopentyl, cyclohexyl, and cyclohexyl-d 11 peroxy radicals have been recorded at room temperature by cavity ringdown spectroscopy. By comparing the experimental spectra with predictions from ab initio and density functional calculations, we have assigned the band origins and vibrational structure of each of these species. The spectrum of cyclopentyl peroxy is interpreted primarily in terms of two overlapping gauche conformers, while that of cyclohexyl peroxy appears to be a superposition of axially and equatorially substituted gauche conformers, both based on the chair conformation of cyclohexane. Expectations from calculated Boltzmann factors indicate comparable populations for cis-conformers; however, no bands uniquely assignable to cis-conformers of either peroxy can be identified. Plausible assignments for cis-conformers are considered, and possible explanations for their absence are offered, including specifically lower oscillator strengths than for the gauche conformers. Mode mixing appears to be responsible for the appearance of multiple vibrations with COO bending character for both peroxies, particularly for cyclohexyl peroxy.