Hydrogen transfer reactions of interstellar Complex Organic Molecules

TL;DR

This study uses quantum tunneling calculations to analyze hydrogen transfer reactions of complex organic molecules in interstellar ices, revealing how reaction shape and reactant specifics influence rates at low temperatures.

Contribution

It provides the first detailed quantum mechanical rate constants for hydrogen transfer reactions of key interstellar organic molecules, highlighting the importance of barrier shape and reactant structure.

Findings

Rate constants depend on barrier shape, not just height.

Hydrogen abstraction from aldehydes is faster than addition.

Heavy-atom tunneling reactions have significantly lower rates.

Abstract

Radical recombination has been proposed to lead to the formation of complex organic molecules (COMs) in CO-rich ices in the early stages of star formation. These COMs can then undergo hydrogen addition and abstraction reactions leading to a higher or lower degree of saturation. Here, we have studied 14 hydrogen transfer reactions for the molecules glyoxal, glycoaldehyde, ethylene glycol, and methylformate and an additional three reactions where \ce{CH_nO} fragments are involved. Over-the-barrier reactions are possible only if tunneling is invoked in the description at low temperature. Therefore the rate constants for the studied reactions are calculated using instanton theory that takes quantum effects into account inherently. The reactions were characterized in the gas phase, but this is expected to yield meaningful results for CO-rich ices due to the minimal alteration of reaction landscapes by the CO molecules. We found that rate constants should not be extrapolated based on the height of the barrier alone, since the shape of the barrier plays an increasingly larger role at decreasing temperature. It is neither possible to predict rate constants based only on considering the type of reaction, the specific reactants and functional groups play a crucial role. Within a single molecule, though, hydrogen abstraction from an aldehyde group seems to be always faster than hydrogen addition to the same carbon atom. Reactions that involve heavy-atom tunneling, e.g., breaking or forming a C--C or C--O bond, have rate constants that are much lower than those where H transfer is involved.