PHOTO-DISSOCIATION SPECTROSCOPY OF ALKANE RADICAL CATIONS

Photodissociation spectra are reported for gas-phase radical cations of straight-chain and cyclic alkanes C5 to C8 (along with N-butane). Large photodissociation cross sections, in the neighborhood of 10-17 cm2, are observed for these ions. The spectrum of cyclopentane ion suggests ring opening to give a pentene ion structure, but the other cycloalkane ions appear to retain a saturated hydrocarbon structure. The n-alkane ion spectra are compared with photoelectron spectra of the neutrals and do not correspond very closely. Calculations of the predicted optical spectra of the ions are made by using a conformation-ally averaged MINDO/3 approach with the inclusion of transition moment weighting, and the correspondence of theory and experiment is encouraging. The long-wavelength portions of the spectra of hexane and octane ions are unexpectedly weak, and some influence by threshold truncation effects is suggested. © 1979, American Chemical Society. All rights reserved.