Full Reaction Pathway Dynamics for Atmospheric Decomposition Reactions: The Photodissociation of H$_2$COO

TL;DR

This study uses advanced machine learning potentials to analyze the detailed energy distribution and pathways in the photodissociation of the atmospheric Criegee intermediate H$_2$COO, revealing complex bifurcations and non-RRKM effects.

Contribution

It provides a comprehensive, full-dimensional dynamical analysis of H$_2$COO decomposition using a machine learned potential energy surface, highlighting bifurcation pathways and non-RRKM behavior.

Findings

CO$_2$ + H$_2$ forms via two bifurcating pathways.

Energy partitioning varies between direct and indirect pathways.

Formic acid intermediates show stretched exponential lifetime distributions.

Abstract

Branching ratios for fragmentation channels of important meta- and unstable species are essential for a molecular-level characterization of atmospheric chemistry. Here, the molecular product channels for the decomposition dynamics of the smallest Criegee intermediate, H$_2$COO, are quantitatively investigated. Using a high-quality, full-dimensional machine learned potential energy surface (CASPT2/aug-cc-pVTZ), the translational, rotational, and vibrational energy distributions of the CO$_2$+H$_2$, H$_2$O+CO, and HCO+OH fragmentation channels were analyzed to elucidate partitioning of the available energy. The CO$_2$ + H$_2$ product forms through two different pathways that bifurcate after formation of the OCH$_2$O intermediate. Along the direct pathway, CO$_2$ is preferentially vibrationally excited with H$_2$in its vibrational ground state, whereas for the indirect pathway going through formic acid, H$_2$ can populate levels with $v > 0$. For all product channels passing through energized formic acid, the lifetime distributions are described by stretched exponentials with $\beta$ ranging from 1.1 to 1.7. This is a clear signature of non-RRKM effects and suggests that the explicit molecular dynamics needs to be followed for a quantitative and realistic description of the photodissociation dynamics.