The ortho-to-para ratio of H$_2$Cl$^+$: Quasi-classical trajectory calculations and new simulations in light of new observations

TL;DR

This study combines new astronomical observations and advanced theoretical calculations to determine that the ortho-to-para ratio of H$_2$Cl$^+$ is primarily set by a hydrogen abstraction formation mechanism, explaining the observed 3:1 ratio.

Contribution

It provides the first detailed theoretical analysis of the formation mechanism of H$_2$Cl$^+$, linking the observed OPR to a hydrogen abstraction process rather than scrambling.

Findings

Observed OPR of H$_2$Cl$^+$ is approximately 3:1 in two different sightlines.

Theoretical calculations show the formation occurs via hydrogen abstraction, not scrambling.

The formation mechanism explains the consistent 3:1 OPR observed in space.

Abstract

Multi-hydrogenated species with proper symmetry properties can present different spin configurations, and thus exist under different spin symmetry forms, labeled as para and ortho for two-hydrogen molecules. We investigated here the ortho-to-para ratio (OPR) of H$_2$Cl$^+$ in the light of new observations performed in the z=0.89 absorber toward the lensed quasar PKS 1830-211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Two independent lines of sight were observed, to the southwest (SW) and northeast (NE) images of the quasar, with OPR values found to be $3.15 \pm 0.13$ and $3.1 \pm 0.5$ in each region, respectively, in agreement with a spin statistical weight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogen nuclei can refer to either a statistical result or a high-temperature limit depending on the reaction mechanism leading to its formation. It is thus crucial to identify rigorously how OPRs are produced in order to constrain the information that these probes can provide. To understand the production of the H$_2$Cl$^+$ OPR, we undertook a careful theoretical study of the reaction mechanisms involved with the aid of quasi-classical trajectory calculations on a new global potential energy surface fit to a large number of high-level ab initio data. Our study shows that the major formation reaction for H$_2$Cl$^+$ produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Such a mechanism leads to a 3:1 OPR, which is not changed by destruction and possible thermalization reactions for H$_2$Cl$^+$ and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.