Matrix isolation radiation chemistry and photochemistry: electronic absorption spectra of o-xylylene and benzocyclobutene radical cations; localization of koopmans and non-koopmans bands in the photoelectron spectra of o-xylylene, styrene, toluene, o-xylene and benzocyclobutene

Abstract

Ar matrix-isolated benzocyclobutene, o-xylylene and styrene have been X-irradiated and the products of the radiation reactions elucidated. From a chemical point of view the easy reaction of benzocyclobutene to neutral o-xylylene is quite remarkable. Besides o-xylylene, the radiolysis of benzocyclobutene yields the radical cations of o-xylylene and of styrene. These cations as well as the benzocyclobutene radical cation are independently produced by resonant two-photon ionization of matrix-isolated benzocyclobutene ( o-xylylene +· and benzocyclobutene +·) and by radiolysis of matrix-isolated styrene (styrene +·). Benzocyclobutene +· is easily phototransformed into o-xylylene +·. The UV-visible absorption spectra of the radiolysis products as well as of the photolyzed radiolysis products have been recorded. High-resolution photoelectron spectra of benzene, toluene, o-xylene, benzocyclobutene and styrene have been obtained. The electronic excitation spectra of the radical cations as well as the ionization spectra of the various molecules have been calculated using the semiempirical methods for spectroscopy, LNDO/S PERTCI and MNDO/S PERTCI, and the interrelations between both sorts of spectra analyzed. The agreement between theory and experiment is on the whole very satisfactory. It is shown that the investigation of the UV-visible absorption spectra of the radical cations of the substituted benzenes leads to an une-quivocal localization of a higher π band amid several σ bands in the corresponding photoelectron spectra and that the study of the UV-visible absorption spectrum of o-xylylene radical cation unequivocally confirms a low-energy shake-up band in the photoelectron spectrum of o-xylylene.