Influence of surface and bulk water ice on the reactivity of a water-forming reaction

Abstract

On the surface of icy dust grains in the dense regions of the interstellar medium a rich chemistry can take place. Due to the low temperature, reactions that proceed via a barrier can only take place through tunneling. The reaction H + H${}_2$O${}_2$ $\to$ H${}_2$O + OH is such a case with a gas-phase barrier of $\sim$26.5 kJ/mol. Still the reaction is known to be involved in water formation on interstellar grains. Here, we investigate the influence of a water ice surface and of bulk ice on the reaction rate constant. Rate constants are calculated using instanton theory down to 74 K. The ice is taken into account via multiscale modeling, describing the reactants and the direct surrounding at the quantum mechanical level with density functional theory (DFT), while the rest of the ice is modeled on the molecular mechanical level with a force field. We find that H${}_2$O${}_2$ binding energies cannot be captured by a single value, but rather depend on the number of hydrogen bonds with surface molecules. In highly amorphous surroundings the binding site can block the routes of attack and impede the reaction. Furthermore, the activation energies do not correlate with the binding energies of the same sites. The unimolecular rate constants related to the Langmuir-Hinshelwood mechanism increase as the activation energy decreases. Thus, we provide a lower limit for the rate constant and argue that rate constants can have values up to two order of magnitude larger than this limit.

Accepted for publication in ApJ

References in corpus (9)

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• Dual-Level Approach to Instanton Theory
• Experimental Characterization of the Energetics of Low-temperature Surface Reactions
• Adsorption of H${}_2$ on Amorphous Solid Water Studied with Molecular Dynamics Simulations
• Methanol Mapping in Cold Cores: Testing Model Predictions
• Extension of the HCOOH and CO2 solid-state reaction network during the CO freeze-out stage: inclusion of H2CO
• Low-Temperature Kinetic Isotope Effects in CH3OH+H -> CH2OH+H2 Shed Light on the Deuteration of Methanol in Space
• Divide-and-Conquer Method for Instanton Rate Theory
• Processing of hydroxylamine, NH2OH, an important prebiotic precursor, on interstellar ices
• Hydrogenation of acetaldehyde on interstellar ice analogs reveals limited destruction
• Investigating the impact of reactions of C and CH with molecular hydrogen on a glycine gas-grain network
• Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains
• Simulation of proton radiolysis of H2O and O2 ices with the Nautilus code