The ortho-to-para ratio of H₂Cl⁺: Quasi-classical trajectory calculations and new simulations in light of new observations

¹ Departments of Chemistry and Astronomy, University of Virginia, McCormick Road Charlottesville VA 22904 USA
e-mail: romane.le_gal@cfa.harvard.edu
² Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA
³ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, Place Eugène Bataillon, 34095 Montpellier, France
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden

Received: 13 July 2017
Accepted: 22 August 2017

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 H₂Cl⁺ 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 ± 0.13 and 3.1 ± 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 H₂Cl⁺ 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 H₂Cl⁺ 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 H₂Cl⁺ and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.