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

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±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 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.

Deuterated forms of H 3 + and their importance in astrochemistry

“…In particular, (1) the deuteration ratios within the central few thousand astronomical units predicted by the proton-hop model are in very good agreement with the observed values ( 0.4, 0.2 and 0.06 for NH 2 D/NH 3 , NHD 2 /NH 2 D and ND 3 /NHD 2 , respectively; see §2); (2) the ortho-to-para ratios of NH 2 D and NHD 2 are also in better agreement with observations, with the first one increasing and the second one decreasing, as required by the analysis of Harju et al [46]. Although a proper line radiative transfer analysis needs to be carried out to quantify the agreement between model predictions and observations, overall we can conclude that proton hop appears to be a better representation of nuclear spin-state chemistry, in agreement with the conclusion of Le Gal et al [47].…”

“…This disagreement between model predictions and observations indicates that our basic assumption of complete scrambling may not be correct. Indeed, Le Gal et al [47] recently performed an observational and theoretical study of H 2 Cl + , finding that its major formation reaction (HCl + + H 2 → H 2 Cl + + H) produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Inspired by these recent findings, we modified our model to eliminate the full scrambling assumption from Sipilä et al [37] model and include instead the proton hop mechanism.…”

The ortho-to-para ratio of water in interstellar clouds

“…Second, the existence of nuclear-spin selection rules in chemical reactions, as predicted theoretically by Quack (1977), has been demonstrated experimentally in ion-neutral reactions involving H + 3 (Uy, Cordonnier & Oka 1997;Crabtree et al 2011b). The nuclear-spin chemistry of interstellar molecules has gained interest in recent years and detailed models have been dedicated to the OPR of NH 3 (Faure et al 2013;Le Gal et al 2014), H + 3 and its deuterated isotopologues (Albertsson et al 2014;Harju et al 2017b), deuterated ammonia (Harju et al 2017a), H 2 O + (Herbst 2015) and H 2 Cl + (Neufeld et al 2015;Le Gal et al 2017). These studies have shown that the OPR of molecules formed in the gas phase can be significantly lower than the statistical (hightemperature limit) values and is entirely controlled by chemical selection rules.…”