Global reaction route mapping of water-catalysed gas phase oxidation of glyoxylic acid with hydroxyl radical

Abstract

In order to assess the potential role of a single water molecule as a catalyst, the gas-phase oxidation reaction of hydroxyl radical with glyoxylic acid has been investigated by the means of quantum mechanical computations using CCSD(T), MP2 and DFT methods. The pre-reaction complexes along the oxidation pathways are systematically explored through a global reaction route mapping method. The computations reveal that a single water molecule stabilizes the pre-reaction complexes as well as respective transition states, resulting in the lowering of the energy barrier, however, the transition state theory computed rate constants for the water-catalysed pathways are found to be an order of magnitude lower than that for the water-free pathways. Notably, the abstraction of formyl hydrogen is observed to be most favourable both in the presence and absence of a single water molecule. Besides these, a couple of triple-proton exchange pathways involving simultaneous proton transfer between the glyoxylic acid, OH radical and a single water molecule are also explored. All the pathways are observed to proceed through conventional free radical mechanism when analysed using BHandHLYP exchange–correlation (XC) functional of DFT. However, DFT method using other XC functionals revealed that one of the relevant acidic H-abstraction pathways proceeds through a proton-coupled electron-transfer mechanism and also results in the dissociation of glyoxylic acid. The pathways for trans form of glyoxylic acid has also been compared with those explored for the cis form. The standard Gibbs free-energy profiles for the reactions studied indicate that the hydrogen abstraction, particularly in trans-glyoxylic acid, may be more feasible than the dissociation, particularly at lower temperatures. This study may assist future investigation of similar atmospheric reactions.