Hydrogen atom mobility, kinetic isotope effects and tunneling on interstellar ices (Ih and ASW)

Transitions of a single H atom between local minima on the surfaces of crystalline ice (Ih) and amorphous solid water (ASW) are studied theoretically in the temperature range 4-25 K. Binding energies, barrier heights, transition rate constants and the kinetic isotope effect (KIE) with and without tunneling are calculated. Harmonic transition state theory is used to obtain the transition rate constants and tunneling is treated with the Wigner tunneling correction, Eckart barrier correction and harmonic quantum transition state theory (HQTST). The classical binding energies are smaller on Ih (<47 meV) than on ASW (<89 meV). Also the classical barrier heights are smaller on Ih (<14 meV) than on ASW (<69 meV) and distributed over a range of energies, in line with previous experimental observations. Similarly the vibrationally adiabatic ground state (VAG) barrier heights are smaller on Ih (< 7 meV) than on ASW (<54 meV). The surface morphology strongly influences the well depths. Tunneling increases some of the transition rate constants substantially but has a much smaller effect on others. The average KIE for Ih is higher than for ASW for the same range of barrier heights.