The electronic absorption spectrum of cyclopentadienyl peroxy radical, c-C5H5OO center dot, has been recorded over the range 7000-8700 cm (1) by cavity ringdown spectroscopy. Using electronic structure calculations, the observed features are assigned to the G conformer, with a band origin at 7479 cm (1). A second form, the C conformer, is predicted to absorb weakly in a region obscured by signals arising from HO2 radical and cannot be definitively identified. Simulations of the sequence band structure based on computed OOCH torsional potentials suggests that the barrier separating the enantiomeric G/G' minima is ca. 600 cm (1), significantly larger than values predicted by B3LYP/cc-pVDZ (138 cm (1)) and G2 (314 cm (1)) calculations. (C) 2011 Elsevier B. V. All rights reserved.