Electronic Spectrum\nof α‑Hydrofulvenyl\nRadical (C<sub>6</sub>H<sub>7</sub>), and a Simple and Accurate Recipe\nfor Predicting Adiabatic Ionization Energies of Resonance-Stabilized\nHydrocarbon Radicals

Using a combination of resonant two-photon two-color\nionization\n(R2C2PI) and laser-induced fluorescence/dispersed fluorescence spectroscopy,\nwe have examined the à <sup>2</sup><i>A</i>″\n← X̃ 2<i>A</i>″ transition\nof the resonance-stabilized α-hydrofulvenyl radical, produced\nfrom methylcyclopentadiene dimer in a jet-cooled discharge. Like the\nrelated 1,4-pentadienyl and cyclohexadienyl radicals, the α-hydrofulvenyl <i>Ã</i>-state lifetime is orders of magnitude shorter than\nthe predicted <i>f</i>-value implies, indicative of rapid\nnonradiative decay. The transition is fully allowed by symmetry but\nconsiderably weakened by transition moment interference. Intensity\nborrowing among <i>a</i>′ modes brings about static\n(i.e., Condon) and vibronic (i.e., Herzberg–Teller) moments\nof similar size, the result being a spectrum substantially less origin-dominated\nthan is usually observed for extensively delocalized radicals. Twenty Ã-state modes and twelve X̃-state modes are identified with high confidence\nand assignments for several others are suggested. In addition, from\na series of two-color appearance potential scans with the Ã-state zero-point level serving as an intermediate,\nwe obtain a field-free adiabatic ionization energy (AIE) of 7.012(1)\neV. For a set of 21 resonance-stabilized radicals bearing 5 to 11\ncarbon atoms, it emerges that the field-free AIE obtained by R2C2PI\nmethods under jet-cooled conditions lies very close to the average\nof B3LYP/6-311G++(d,p) (with harmonic zero-point energy) and CBS-QB3\n0 K calculations, with a mean absolute deviation of only 0.010(7)\neV (approximately 1 kJ/mol). On average, this represents a nearly\n10-fold improvement in accuracy over CBS-QB3 predictions for the same\nset of radicals.