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

Abstract Transitions of a single H atom between local minima on the surfaces of crystalline ice (I h ) 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 I h ( h ( h ( h is higher than for ASW for the same range of barrier heights.