Direct Observation of a Fundamental Arylium Species Using Photoelectron Spectroscopy: Singlet-Triplet Gap in Phenylium.

Reactive intermediates such as aryl cations play crucial roles in organic and astrochemical processes, yet their characterization remains a significant experimental challenge. The present study offers the first partially vibrationally resolved photoelectron spectrum of the phenyl radical (C6H5), focusing on ionization thresholds to the lowest singlet and triplet states of its cation. Using synchrotron-based photoelectron spectroscopy, we directly determine the adiabatic ionization energy of the first triplet state of phenylium (C6H5+, IEad(ã+ ← X̃) = 9.300(10) eV), and use this as an anchor to derive the singlet-triplet energy gap (ΔE(S-T) = 1.043(32) eV) with assistance from advanced quantum chemical computations. These findings provide new constraints to thermochemical properties essential in arylium ground state determination (and thus essential for a better understanding of arylium chemistry), and establish a framework for future joint spectroscopic-computational studies of more complex arylium compounds.