Water, being the predominant molecule coating dust grains in the early stages of star formation, impacts greatly on the chemistry that can occur in the ice. In this contribution, a computational chemistry point of view on various aspects of the influence of water on the reactivity in ices will be presented. First of all, the intermediates formed during water formation can react with other species as well. The most obvious reactive intermediate is the hydroxyl radical which can react with, e.g., CO forming CO2 whilst the water layer is still forming around it. Other reactions where OH plays a role in the formation of more complex molecules will be discussed. Furthermore, the structure of ices that consist mainly of water is governed by hydrogen bonding, even when those ices are amorphous. As such, molecules that can participate in a hydrogen bonding interaction may take part in a chain of hydrogen bonds, whereas apolar molecules will in fact disrupt these chains. The influence of the relatively strong hydrogen bonds on inhibiting reactions will be showcased and a brief comment will be made on the binding energies of species on water ice. Thirdly, water can be an active participant in a reaction by accepting and donating a hydrogen atom. In this way reactants no longer need to be direct neighbors. One obvious example is the exchange reaction OH + H2O → H2O + OH, but other, more interesting, cases are known as well or can be postulated and these will be touched upon. Finally, the activation energy of a reaction can be altered by its direct surrounding. The extent to which this occurs has been investigated for a variety of reactions and will be summarized. Note that chemistry on grains is aided by the constant presence of a third body that allows energy to dissipate via, e.g., surface and bulk phonons that are specific to the molecular composition of the ice (see contribution by Jörg Meyer).

Influence of water on reactivity in and on icy grain surfaces

2019 Fall National Meeting

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