Threshold photoionization spectra of benzyl radical: Cation vibrational states and {ital ab} {ital initio} calculations
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
We measured threshold photoionization spectra of benzyl{sup +}-{ital h}{sub 7}, benzyl{sup +}-{alpha}{ital d}{sub 2}, and benzyl{sup +}-{ital d}{sub 7} in the ground electronic state ({tilde {ital X}}{sup +1}{ital A}{sub 1}) using resonant two-photon excitation and detection of electrons by pulsed field ionization. The adiabatic ionization potentials of benzyl-{ital h}{sub 7}, benzyl-{alpha}{ital d}{sub 2}, and benzyl-{ital d}{sub 7} are 58468{+-}5 cm{sup -1}, 58418{+-}5 cm{sup -1}, and 58386{+-}5 cm{sup -1}. Excitation through a variety of vibronically mixed {tilde {ital A}}{sup 2}{ital A}{sub 2}-{tilde {ital B}}{sup 2}{ital B}{sub 2} neutral excited states allows observation of cation vibrations of both {ital a}{sub 1} and {ital b}{sub 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 {ital ab initio} calculations, we assign 35 of 48 observed levels in the -{ital h}{sub 7} isotopomer, 15 of 22 levels in -{alpha}{ital d}{sub 2}, and 25 of 30 levels in -{ital d}{sub 7}. {ital 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 similar geometries with relatively short exocyclic CC bond (1.371 A and 1.372 A, respectively) and with the aromatic ring compressed along the {ital C}{sub 2} symmetry axis. The neutral free radical has a longer exocyclic CC bond (1.413 A) 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 more important in the anion and cation than in the neutral. {copyright} {ital 1996 American Institute of Physics.}
- DOE Contract Number:
- FG02-92ER14306
- OSTI ID:
- 286169
- Journal Information:
- Journal of Chemical Physics, Vol. 104, Issue 22; Other Information: PBD: Jun 1996
- Country of Publication:
- United States
- Language:
- English
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