Dissociative photoionization and threshold photoelectron spectra of polycyclic aromatic hydrocarbon fragments: an imaging photoelectron photoion coincidence (iPEPICO) study of four substituted benzene radical cations.

Abstract

Four molecules were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy: 1-propynylbenzene, indene, ethynylbenzene, and benzocyclobutene. Their threshold photoelectron spectrum was obtained and electronic transitions were assigned by OVGF (outer valence Green's function) calculations. Vibrational progressions observed in the electronic ground and excited states were simulated by calculating Franck-Condon factors based on the neutral as well as the cation ground and excited state geometries. iPEPICO was used to obtain ion dissociation data in threshold photoionization as a function of photon energy, which were modeled with RRKM theory to extract kinetic parameters for the reactions C9H8(+•) (1-propynylbezene) → C9H7(+) + H (R1); C9H8(+•) (indene) → C9H7(+) + H (R2); C8H8(+•) (benzocyclobutene) → C8H7(+) + H (R3); C8H8(+•) (benzocyclobutene) → C6H6(+) + C2H2 (R4); C8H6(+•) (1-ethynylbenzene, aka phenylacetylene) → C6H4(+) + C2H2 (R5). These results were compared to G3 level calculations. In addition, the enthalpy of formation of the indenyl radical was estimated to be ΔfH°0K = 249 ± 50 kJ mol(-1) based on a previously measured IE and a cation ΔfH°0K = 976 kJ mol(-1), determined herein.