Ultraviolet Photoelectron Spectroscopy of Some C₇H₇⁻ Isomers:  Quadricyclanide, Norbornadienide, Cycloheptatrienide, and 1,6-Heptadiynide

The 351 nm photoelectron spectra of several isomers of C 7 H 7 -, obtained by deprotonating quadricyclane, norbornadiene, 1,6-heptadiyne, and cycloheptatriene, are reported. Quadricyclane can be deprotonated from either of two distinct sites by amide anion. The electron affinities (EAs) of the two corresponding C 7 H 7 • isomers are 0.868(6) and 0.962(6) eV, with one well-resolved vibration observed for each state, at 909(20) and 826(20) cm - 1, respectively. Deprotonation of norbornadiene by amide anion leads to formation of one ionic isomer with a binding energy of 1.286(6) eV. Detachment to form an excited state of the neutral is also observed, with a vertical detachment energy (VDE) of 2.8(1) eV. Three vibrational modes are resolved in the ground state region of the spectrum, and one excited state vibration is observed. 1,6-Heptadiyne is deprotonated by hydroxide at two sites, the end carbon (C 1 ) and the third carbon (C 3 ), to produce isomers with binding energies of 3.037(10) and 1.132(6) eV, respectively. One vibration at 500(20) cm - 1 is strongly active in the C 3 -deprotonated portion of the spectrum. Reaction of cycloheptatriene with hydroxide or amide produces a mixture of products at 91 amu which includes the isomer of C 7 H 7 - corresponding to abstraction from the methylene group. The corresponding neutral has an EA ≤ 0.43 eV, in agreement with earlier estimates obtained from thermochemical cycles. The remaining products are assigned to vinyl systems with transitions in the photoelectron spectrum to the ground and first excited states. The measured VDEs of these states are 1.48(1) and 2.97(1) eV, respectively. The spectrum of benzyl anion appears in the spectra of all of the other C 7 H 7 - isomers, probably as a result of toluene contamination in the precursors.