Dynamically biased statistical model for the ortho/para conversion in the ${\rm H}_2 + {\rm H}_3^+$H2+H3+ → ${\rm H}_3^{+} +$H3++ H2 reaction

Abstract

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H + 5 complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasiclassical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H + 5 complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree et al. [J. Chem. Phys. 134, 194311 (2011)] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interest.

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

“…For example if only rotational levels with quantum numbers J = 0 (para) and J=1 (ortho) can be produced, there are nine times as many ortho states as para states. These different types of mechanisms have been explored to interpret the synthesis of the OPR of H + 3 (Crabtree et al 2011b,c) and subsequently studied in more detail for the specific system H 2 + H + 3 (Gomez-Carrasco et al 2012). More recently, this distinction of mechanisms has been revived in order to explain the observed OPRs of H 2 O + (Herbst 2015) and H 2 Cl + (Neufeld et al 2015).…”

“…The mechanistic distinctions were also recently considered for observations of the OPRs of H 2 O + (Herbst 2015) and H 2 Cl + (Neufeld et al 2015), providing new insights to explore. A rigorous determination of the mechanisms in a particular system requires a full quantum scattering treatment, but as pointed out by Gomez-Carrasco et al (2012), full quantal computations are rarely achievable because of their complexity and the computational cost. The next best approach is the semi-classical trajectory method, in which atoms and molecules undergo classical trajectories on a quantum mechanical PES.…”

Physical conditions in the central molecular zone inferred by H3+

“…In Sect. 3, we present an analysis of H + 3 excitation based on new H + 3 excitation collisional rates with H 2 computed by Gómez-Carrasco et al (2012). We compare these new results to the pioneering study of Oka & Epp (2004).…”

“…Radiative data come from Lindsay & McCall (2001) and are available electronically on the website 5 maintained by B. McCall. To discuss the uncertainties arising from the uncertainties in the collision rates, we implemented the two sets of collision rates by Oka & Epp (2004) and Gómez-Carrasco et al (2012). Whereas Oka & Epp (2004) do not discriminate between ortho and para levels, Gómez-Carrasco et al (2012) consider the appropriate nuclear spin restrictions and provide state-to-state collision rates for the 24 first levels of H + 3 with H 2 .…”

“…First, Hugo et al (2009) computed reactive and nonreactive collision rates for temperatures below 50 K with a strong ergodicity and full nuclear spin scrambling hypothesis. Second, Gómez-Carrasco et al (2012) extended the temperature range up to 500 K and used a dynamically biased statistical model coupled to a recent global potential energy surface of the H + 5 system (Aguado et al 2010). Finally, Park & Light (2007) studied the ortho-para conversion in the H + 3 + H 2 reaction.…”