Ab Initio Studies of the Glyoxal Unimolecular Dissociation Pathways

Abstract

Glyoxal (C2H2O2) unimolecular dissociation has been the subject of many experimental and theoretical studies, and some questions remain regarding the predominant pathway for dissociation. Molecular beam photodissociation studies identify the major pathway as that which leads to the formation of formaldehyde (CH2O) and carbon monoxide (CO). Thermal decomposition experiments and theoretical studies, on the other hand, predict the triple whammy pathway that leads directly to H2 + 2CO to be predominant. The objective of this study is to elucidate this discrepancy by performing a thorough ab initio exploration of the free energy surface for glyoxal dissociation. Gaussian-3 (G3) theory was shown to be an appropriate method for studying glyoxal unimolecular dissociation since the calculated heats of reaction agree well with reliable experimental values. On the other hand, other standard model chemistries, including the popular hybrid-density functional theory methods, predict poor heats of reaction for glyoxal dissociation. Various pathways for ground-state glyoxal unimolecular dissociation were then explored with G3 theory. We identified nonplanar transition state structures that lie lower in energy than the previously reported planar stationary points for both the triple whammy and formaldehyde channels. Three pathways, i.e., the formaldehyde, triple whammy and hydroxymethylene channels, have activation free energies that fall below typical experimental glyoxal photoexcitation energies and are thus possible under the experimental conditions. The formaldehyde channel was shown to be predominant at low temperatures, in agreement with molecular beam experiments and in contrast to previous theoretical predictions. As temperature increases, however, the ordering of the activation barriers is reversed and the predominant channel becomes the triple whammy channel, as observed in high-temperature thermal experiments. The temperature dependence of the activation barriers is attributed to the vibrational structure of the transition states relative to that of glyoxal, which significantly affects the entropic contributions to the activation barriers. The present study ultimately reconciles various sets of experimental findings and theory.