The cross section for electron photodetachment has been measured for the acetylide anion (HC2minus;) 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 Franckndash;Condon factors for the anionrarr;neutral transition allow us to determine EA (HC2sdot;) = 2.94plusmn;0.10 eV. A theoretical determination using eighthhyphen;order perturbation theory gives an adiabatic electron affinity of 3.18plusmn;0.25 eV, in good agreement with the experimental result. The use of a thermochemical cycle with the experimental electron affinity and gashyphen;phase acidity data gives a Cndash;H bond dissociation energy in acetylene of 132plusmn;5 kcal/mol.
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