Tunneling Rate Constants for H2CO+H on Amorphous Solid Water Surfaces

TL;DR

This study calculates hydrogenation and abstraction rate constants for H2CO on amorphous solid water surfaces at low temperatures, revealing dominant reaction pathways and strong isotope effects relevant for astrochemical modeling.

Contribution

It introduces a combined QM/MM and instanton approach to compute rate constants for H2CO reactions on amorphous water ice, including fits for astrochemical models.

Findings

CH3O formation dominates with 80% branching ratio.

High activation barriers suppress H2CO dissociation into H2+HCO.

Strong kinetic isotope effects observed across channels.

Abstract

Formaldehyde (H2CO) 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 H2CO yielding CH3O, CH2OH, and the hydrogen abstraction resulting in H2+HCO on an amorphous solid water (ASW) surface using a quantum mechanics/molecular mechanics (QM/MM) model. The binding energies of H2CO 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+H2CO surface reactions down to 59 K. We found that the channel producing CH2OH 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 CH3O (branching ratio 80%) and hinders the H2CO dissociation into H2+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.