Tunnelling dominates the reactions of hydrogen atoms with unsaturated alcohols and aldehydes in the dense medium

TL;DR

This study uses quantum chemical methods to analyze hydrogen atom reactions with unsaturated alcohols and aldehydes, revealing reaction rates and mechanisms relevant to interstellar chemistry.

Contribution

It provides detailed reaction rate constants and insights into the influence of functional groups on hydrogen reactions, explaining experimental observations.

Findings

Hydrogen addition to aldehyde groups is slow across molecules.

Hydrogen abstraction from aldehyde groups is relatively fast.

Reaction rates vary significantly depending on functional groups attached.

Abstract

Hydrogen addition and abstraction reactions play an important role as surface reactions in the buildup of complex organic molecules in the dense interstellar medium. Addition reactions allow unsaturated bonds to be fully hydrogenated, while abstraction reactions recreate radicals that may undergo radical-radical recombination reactions. Previous experimental work has indicated that double and triple C--C bonds are easily hydrogenated, but aldehyde -C=O bonds are not. Here, we investigate a total of 29 reactions of the hydrogen atom with propynal, propargyl alcohol, propenal, allyl alcohol, and propanal by means of quantum chemical methods to quantify the reaction rate constants involved. First of all, our results are in good agreement with and can explain the observed experimental findings. The hydrogen addition to the aldehyde group, either on the C or O side, is indeed slow for all molecules considered. Abstraction of the H atom of the aldehyde group, on the other hand, is among the faster reactions. Furthermore, hydrogen addition to C--C double bonds is generally faster than to triple bonds. In both cases, addition on the terminal carbon atom that is not connected to other functional groups is easiest. Finally, we wish to stress that it is not possible to predict rate constants based solely on the type of reaction: the specific functional groups attached to a backbone play a crucial role and can lead to a spread of several orders of magnitude in the rate constant.