The unimolecular dissociation of protonated glyoxylic acid: Structure and dynamics of a step-by-step process

The unimolecular chemistry of protonated glyoxylic acid [HCOCOOH]H+, has been investigated by analyzing the fragmentation of metastable ions (MI) during their flight through a sector-field mass spectrometer. The only significant ionic product in the MI experiments is H3O+, indicating the loss of two carbon monoxide molecules. High-level ab initio calculations have been used to model the relevant parts of the potential energy surface (PES). Starting from the most stable isomer, H(CO)C(OH)2+ (1), fragmentation takes place as the result of three successive intramolecular proton transfer reactions. The first CO molecule is liberated in-between the second and the third proton transfer and moves slowly away. This mechanism is fully confirmed by ab initio direct dynamics calculations. In addition, the dynamics calculations reveal that the majority of the potential energy, which is liberated ends up as OH vibrations of the H3O+ moiety. After passage through the critical transition state, dissociation of the H2OH+⋯CO (6) complex appears to be a limiting step. In order to acquire sufficient relative translational energy to depart, the hydroxonium ion and carbon monoxide have to reside together for 1-2 ps.