Mechanisms of photodissociative ionization of HCOOH: The heat of formation of COOH+

The dissociative ionization of HCOOH to form COOH+ +H+e has been reinvestigated. Experiments with DCOOH and HCOOD demonstrate that rearrangement competes with direct bond cleavage in this dissociation, even near threshold. Extrapolation of the photoion yield curve of COOH+ to the background level is ambiguous. A photoelectron–photoion coincidence experiment was thereupon performed, to obtain a breakdown diagram. From the 0 K cross over energy of 12.30±0.02 eV, we infer ΔHf00 (COOH+)=143.2±0.5 kcal/mol, and proton affinity (PA) (CO2)=129.2±0.5 kcal/mol. Some possible mechanisms for the rearrangement process are explored, but the calculated barriers are too high to explain the observation.