First-principles theoretical study of molecular HCl adsorption on a hexagonal ice (0001) surface

A key step in the mechanism of polar ozone depletion involves the interacti on of HCl with the particles that constitute polar stratospheric clouds. Th ese particles exist as hexagonal ice Ih crystals below approximately 185 K. As a first step, we examine the binding of molecular HCl to a number of ex tended surface models of the basal (0001) face of hexagonal ice in addition to a cluster model representative of an unreconstructed adsorption site. C alculations are performed using gradient-corrected density-functional theor y and high-order correlation methods. After correcting for zero-point energ ies, we estimate the molecular binding energy and enthalpy at 185 K to be 2 3 and 25 kJ/mol, respectively, for low (single-molecule) HCl coverage. Thes e estimates decrease by a few kJ/mol at higher HCl coverage and are based p artially on a low density of dangling surface OH groups. Our results set th e stage for future studies addressing the possible ionization of molecularl y adsorbed HCl using first-principles molecular dynamics.