Hydroxyl radicals in ice: insights into local structure and dynamics.

The hydroxyl radical and its reactivity within ice environments are crucial to many important atmospheric reactions. The associated molecular mechanisms are largely unknown due to challenges posed by direct experimental measurements and computational studies of this transient species. Here we report insights into the local structure and behaviour of the hydroxyl radical in bulk ice through an extensive study utilizing Car-Parrinello molecular dynamics simulations. Interstitial and in-lattice hydroxyl radicals in hexagonal ice were investigated at primarily 190 K. Our findings, utilizing both HCTH/120 and BLYP functionals, show that OH* can exhibit greater mobility than other ice defects (the trapping energy estimated to be only 0.09 eV). We observe the formation of a two-center three-electron hemibond structure between the hydroxyl radical and an in-lattice water molecule; while controversial, such a structure in ice may be amenable to experimental detection due to its relative stability. Our results show that interstitial water molecules can strongly influence the mobility of the hydroxyl radical in bulk ice through the displacement of the radical to an interstitial location. We also demonstrate that the H-transfer reaction from an interstitial water to the radical is a rare event in ice. Together, these results predict that the radical can be a reactive species in bulk ice, as both interstitial and in-lattice OH* can be available for reactions with other species. These microscopic insights should contribute to our understanding of the reactivity of OH* in ice and its implications to atmospheric reactions.