Threshold photoionization spectra of benzyl radical: Cation vibrational states and ab initio calculations

We have measured threshold photpionization spectra of benzyl⁺h₇, benzyl⁺-αd₂, and benzyl⁺-d₇ in the ground electronic state (X̃^{+ 1}A¹) using resonant two-photon excitation and detection of electrons by pulsed field ionization. The adiabatic ionization potentials of benzyl-h₇, benzyl-αd_2, and benzyl-d₇ are 58468±5 cm^-1, 58 418±5 cm^-1, and 58 386±5 cm^-1. Excitation through a variety of vibronically mixed à ²A₂-B̃ ²B₂ neutral excited states allows observation of cation vibrations of both a ₁ and b ₁ symmetries. We directly measure in-plane fundamentals and infer the frequencies of certain out-of-plane fundamentals from their involvement in combinations or overtones. By comparison with harmonic frequencies from ab initio calculations, we assign 35 of 48 observed levels in the -h₇ isotopomer, 15 of 22 levels in αd₂, and 25 of 30 levels in -d₇. Ab initio calculations permit a detailed comparison of the geometry, chemical bonding, and vibrational frequencies in the benzyl anion, neutral, and cation. The anion and cation, both closed-shell species, have remarkably similar geometries with relatively short exocyclic CC bond (1.371 Å and 1.372 Å, respectively) and with the aromatic ring compressed along the C₂ symmetry axis. The neutral free radical has a longer exocyclic CC bond (1.413 Å) and a more nearly sixfold symmetric ring. The natural resonance theory provides bond orders and resonance-structure weights in all three species. While no single resonance structure dominates in any of the three species, the structure with an exocyclic CC double bond is significantly more important in the anion and cation than in the neutral.