Influence of surface and bulk water ice on the reactivity of a
  water-forming reaction
  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$_2$ $\rightarrow$ 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.
