Key points are not available for this paper at this time.
The photoionization spectrum of vinyl radical is reported, from its observed threshold to 1160 Å. Two methods of preparation have been employed; (a) the abstraction reaction of F atoms with C2H4, and (b) the pyrolysis of divinyl mercury at 1200 K. In both experiments, relatively sharp autoionization structure is observed, and interpreted as a Rydberg series converging to the excited 3A″ state of vinyl cation. The analysis leads to an adiabatic ionization energy of ∼10.7 eV for this state, with a structure similar to that of vinyl radical but with an increased C–C distance. The observed ionization threshold for the ground state of vinyl cation is 8.59±0.03 eV with the F atom reaction, and 8.43±0.03 eV with the pyrolysis method. The lower value in the latter experiment is interpreted as a hot band. The relatively low value of the photoionization cross section near threshold implies a large geometry change between vinyl radical and ground state vinyl cation. A progression in the in-plane C–H bending vibration is indicated in the photoionization spectrum; it is quite possible that the vibrational 0–0 transition lies one quantum lower than our detected limit. With this bracketed adiabatic ionization potential and the appearance potential of C2H+3 (C2H4), a C–H bond energy in ethylene of 107–110 kcal/mol (0 K) is deduced.
Berkowitz et al. (Wed,) studied this question.