The Threshold Photoelectron Spectrum of the Vinylcyclopentadienyl Radical, a C ₇ H ₇ Resonance-Stabilized Radical

High Resolution Image Download MS PowerPoint Slide We generated the aromatic resonance-stabilized vinylcyclopentadienyl radical, C 5 H 4 –CH=CH 2, in the gas phase via flash pyrolysis of meta -vinylanisole, CH 3 O–C 6 H 4 –CH=CH 2 . Double-imaging photoelectron photoion coincidence spectroscopy was used to measure the photoion mass-selected threshold photoelectron spectrum of the vinylcyclopentadienyl radical. Spectral assignments were enabled by independent, high-level ab initio coupled cluster calculations of adiabatic ionization energies originating from the ground electronic state of the doublet neutral, X̃ 2 A″, to both the ground state of the cation, X̃ + 1 A′, and the lowest triplet state, ã + 3 A′. The experimentally determined band origin energies are found to be (7.81 ± 0.02) eV (X̃ + 1 A′ ← X̃ 2 A″) and (8.08 ± 0.02) eV (ã + 3 A′ ← X̃ 2 A″), in agreement with the respective calculated values of (7.83 ± 0.01) eV and (8.09 ± 0.01) eV. The combined experimental and theoretical characterization presented here provides a foundation to identify important resonance-stabilized radicals in complex gas-phase environments.