EXPERIMENTAL AND THEORETICAL DETERMINATION OF THE ELECTRON-AFFINITY OF THE ETHYNYL RADICAL, HC2.

The cross section for electron photodetachment has been measured for the acetylide anion (HC2-) using an ion cyclotron resonance spectrometer in conjunction with a xenon arc lamp. Calculation of the photodetachment behavior near threshold and an estimate of the Franck-Condon factors for the anion→neutral transition allow us to determine EA(HC 2.) = 2.94±0.10 eV. A theoretical determination using eighth-order perturbation theory gives an adiabatic electron affinity of 3.18±0.25 eV, in good agreement with the experimental result. The use of a thermochemical cycle with the experimental electron affinity and gas-phase acidity data gives a C-H bond dissociation energy in acetylene of 132±5 kcal/mol. © 1979 American Institute of Physics.