Raza, Z; (2012) Proton ordering and reactivity of ice. Doctoral thesis , UCL (University College London).

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Abstract

Cubic ice Ic is a rarely-observed ambient pressure phase of water implicated in the catalysis of atmospheric reactions. It forms between 160K and 243K, in droplets smaller than 5µm 3µm in diameter. It is metastable with respect to hexagonal ice Ih and is poorly characterised both experimentally and theoretically. The proton ordered ground state for cubic ice has I4­1md symmetry and is named ice XIc. We find that the ground state proton ordered configurations of hexagonal and cubic ice, XI and XIc are isoenergetic. The surface energy of ice is strongly dependent on proton ordering. The “striped” configuration has the lowest surface energy, and clustering of dangling OH bonds increases the surface energy. Cubic ice has a surface energy approximately 10% higher than hexagonal, and is more reactive. Elementary steps on the ice surface reconstruct to lower the step formation energy; undercoordinated molecules on the step edge relax to form an additional hydrogen bond with the lower terrace. We examine five different steps: the low energy A- and B1 -steps and the high energy B1∗-, B2- and B2∗-steps. Different growth rates for these steps combined with a proton disorder are in part responsible for the isotropic bilayer growth of ice observed by Sazaki et al. Glycolaldehyde, the simplest sugar, has been observed recently in the interstellar medium and in a solar-type protostar. We evaluate two potential mechanisms for its formation on icy dust grains at 10 K, finding that activation barriers are greatly reduced by the ice surface, and that the most likely route is a reaction between H2COH and HCO radicals, which are formed by the sequential hydrogenation of carbon monoxide.

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