Conformers of 1-propyl and 2-propyl peroxy radicals in both their (X) over tilde and (A) over tilde electronic states were investigated by quantum chemical calculations including density functional theory, coupled cluster, second-order Moller-Plesset perturbation theory, equation of motion coupled cluster, and G2 methods. The calculations yielded geometries, relative energies, harmonic vibrational frequencies in both states, (A) over tilde-(x) over tilde excitation energies of the different conformers, and relevant barrier heights between the conformers in their ground state. The computational results were combined with recent experimental observations to analyze and assign the origin transition and the O-O stretching vibration of (A) over tilde-(X) over tilde spectra of particular conformers of the propyl peroxy radicals. (c) 2005 Elsevier B.V. All rights reserved.