Coupled QM/MM molecular dynamics simulations of HCl interacting with ice surfaces and water clusters - Evidence of rapid ionization

We present the results of coupled quantum mechanics and molecular mechanics (QM/MM) classical molecular dynamics simulations for HCl sticking to the (0001) basal plane of ice Ih. Interatomic forces and energies of hydrogen chloride and up to 24 water molecules in the top ice bilayer were obtained from semiempirical molecular orbital calculations based on the PM3 method. A few PM3 parameters were adjusted so that structural and energetic properties of small neutral and ionic systems match available ab initio and experimental data. This QM region was coupled to the remainder of the ice surface (the MM region), which was treated using the analytic TIP4P force field. The surface temperature was between 0 and 180 K, and the dynamics was followed for 100 ps. On surface impact, HCl binds to a dangling (free) H2O oxygen via a ClH-OH2 hydrogen bond. If the Cl is solvated by one dangling H2O hydrogen, HCl adsorbs molecularly. If two dangling hydrogens are available in a surface hexagon, HCl dissociates to a Cl--H3O+ contact ion pair. The simulations thus predict a mechanism by which HCl can ionize readily on ice surfaces. This mechanism is consistent with a saturation coverage of 0.33 monolayers for ionized HCl on ice surfaces. As a comparison we have also simulated HCl colliding with a cubic (H2O)8 cluster, in which the whole system was treated by the semiempirical method. Hydrogen chloride adsorbs on the cluster and, depending on the temperature, the (H2O)(8) cube may open up, thereby initiating HCl ionization. The results are discussed in relation with stratospheric heterogeneous ozone chemistry and available experimental and theoretical results.