A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol

It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH₂CHO) to 1,2-ethenediol (HOCHCHOH) and the subsequent ^•OH-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto–enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol^–1 and involves a double-hydrogen shift mechanism to give the lower-energy <i>Z</i> isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by ^•OH is initiated by addition of the radical to the π system to give the ^•CH­(OH)­CH­(OH)₂ radical, which is subsequently trapped by O₂ to form the peroxyl radical ^•O₂CH­(OH)­CH­(OH)₂. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH­(OH)₂CHO] and HO₂^• through an α-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of ^•OH through a β-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric ^•OH concentrations, the lifetime of 1,2-ethenediol toward reaction with ^•OH is calculated to be 68 h