Competition between Abstraction and Addition Channels for the Reaction between the OH Radical and Vinyl Alcohol in the Interstellar Medium

Vinyl alcohol (Vy) and hydroxyl radical (OH) are involved in several processes, which take place in environments characterized by very different physical-chemical conditions, ranging from the low pressures and temperatures typical of the interstellar medium (ISM) up to the high temperatures of interest for combustion processes. A gas-phase reaction mechanism involving Vy and OH has been proposed as a possible path for the formation of (Z)-1,2-ethenediol (Et), a molecule recently identified in the ISM. Et, the enolic form of glycolaldehyde, is considered a key precursor for the formation of sugars in both interstellar and prebiotic chemistry. We have therefore undertaken a detailed quantum chemical study of possible reaction channels starting from the interaction between the OH radical and both conformers of Vy (syn and anti). The formation of a prereactive complex always represents the first step of the reaction, which can then proceed through the attack to the C═C double bond (leading in turn to the formation of different dissociation products) or through hydrogen abstraction, which eventually produces a radical species and water. Then, a master equation approach based on ab initio transition state theory has been employed to calculate the reaction rate constants of different products for temperatures up to 500 K. A comparison of the kinetic results for the different reaction channels shows that hydrogen abstraction is strongly favored for both Vy conformers and leads to the formation of water and CH2CHO radical. As a matter of fact, formation of Et is strongly disfavored under the harsh conditions of the ISM from both kinetic and thermodynamic points of view because of the high activation energy and strong endothermicity of the corresponding reaction path.