Tunneling Rate Constants for H 2 CO+H on Amorphous Solid Water Surfaces

Abstract

Formaldehyde (H 2 CO) is one of the most abundant molecules observed in the icy mantle covering interstellar grains. Studying its evolution can contribute to our understanding of the formation of complex organic molecules in various interstellar environments. In this work, we investigated the hydrogenation reactions of H 2 CO yielding CH 3 O, CH 2 OH, and the hydrogen abstraction resulting in H 2 +HCO on an amorphous solid water (ASW) surface using a quantum mechanics/molecular mechanics (QM/MM) model. The binding energies of H 2 CO on the ASW surface vary broadly, from 1000 to 9370 K. No correlation was found between binding energies and activation energies of hydrogenation reactions. Combining instanton theory with QM/MM modeling, we calculated rate constants for the Langmuir–Hinshelwood and the Eley–Rideal mechanisms for the three product channels of H+H 2 CO surface reactions down to 59 K. We found that the channel producing CH 2 OH can be ignored, owing to its high activation barrier leading to significantly lower rates than the other two channels. The ASW surface influences the reactivity in favor of formation of CH 3 O (branching ratio ∼80%) and hinders the H 2 CO dissociation into H 2 +HCO. In addition, kinetic isotope effects are strong in all reaction channels and vary strongly between the channels. Finally, we provide fits of the rate constants to be used in astrochemical models.