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Multi-hydrogenated species with proper symmetry properties can presentdifferent spin configurations, and thus exist under different spin symmetryforms, labeled as para and ortho for two-hydrogen molecules. We investigatedhere the ortho-to-para ratio (OPR) of H$_2$Cl$^+$ in the light of newobservations performed in the z=0.89 absorber toward the lensed quasar PKS1830-211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Twoindependent 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 statisticalweight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogennuclei can refer to either a statistical result or a high-temperature limitdepending on the reaction mechanism leading to its formation. It is thuscrucial to identify rigorously how OPRs are produced in order to constrain theinformation that these probes can provide. To understand the production of theH$_2$Cl$^+$ OPR, we undertook a careful theoretical study of the reactionmechanisms involved with the aid of quasi-classical trajectory calculations ona new global potential energy surface fit to a large number of high-level abinitio data. Our study shows that the major formation reaction for H$_2$Cl$^+$produces this ion via a hydrogen abstraction rather than a scramblingmechanism. Such a mechanism leads to a 3:1 OPR, which is not changed bydestruction and possible thermalization reactions for H$_2$Cl$^+$ and is thuslikely to be the cause of observed 3:1 OPR ratios, contrary to the normalassumption of scrambling.

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