O^- + acetaldehyde reaction products: Search for singlet formylmethylene, a Wolff rearrangement intermediate

The mass-resolved anionic products of the reaction of O^•? with acetaldehyde, H₃CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H₂CCHO^?, formylmethylene anion, HCCHO^•?, and ketenylidene anion, CCO ^•?. From photoelectron spectra of HCCHO^•?, the electron affinity of triplet (ground state) formylmethylene (1.87 ± 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO^•?, whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H₄⁺ abstraction. A possible mechanism is proposed, involving an [H ₂CCO^•?]* intermediate. From the measured electron affinities of HCCHO (above), H₂CCHO (1.82 ± 0.01 eV), and CCO (2.31 ± 0.01 eV), several new thermochemical properties are determined, including the C?H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O^•? with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.