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Ortho/para conversion of H$_3^+$ in collisions with H$_2$ and H

AutorRoncero, Octavio
Fecha de publicación2-may-2017
Citación2nd Workshop Nuclear spin effects in astrochemistry (2017)
ResumenH$_3^+$ plays a fundamental role in the formation of hydrides in cold environments. The ortho/para transformation of H$_3^+$ is mainly due to its collisions with H$_2$. In these collisions there are two reaction mechanisms, the proton hop and the full-scrambling-exchange, and nuclear spin symmetry introduces differences among them. This fact allowed the experimental determination of the exchange/hop ratio ($\alpha$) at different temperatures[1,2], varying from 0.5 at low temperatures (135K) to a value of $\approx$ 2 for T $>$ 350K. Statistical approaches[3,4] including both mechanisms lead to $\alpha=0.5$. The increase of $\alpha$ implies a transition from a statistical limit, valid at low temperatures, to a direct mechanism at high temperature. In this last circumstances, the indirect exchange mechanism cannot takes place, leading to the dominance of the hop direct mechanism. In order to study this transition, the statistical model was further generalized including a dynamical bias using Quasi-classical trajectories[5]. It was found that the $\alpha$ ratio was significantly overestimated within the Quasi-classical method used, because of the zero-point energy (ZPE) leakage. To avoid this problem, a portion of the zero-point energy was eliminated following an RRKM argument, doing a shift to get a classical density of states approximately equal to the desired quantum one. This crude approach yields to hop/exchange ratio, $\alpha$, similar to the experimental ones[5]. This demonstrate the need of including quantum effects in the simulation of this reaction. Recent preliminary results will be presented using the Ring Polymer Molecular dynamics (RPMD) method[6]. Finally, some preliminary Quasi-Classical results on the H+H$_3^+$ exchange reaction on a recent full dimensional potential[7] will also be presented.
DescripciónConferencia invitada. -- Presentación con 35 diapositivas. -- The second workshop "Nuclear Spin Effects in Astrochemistry" will be held in Grenoble, France, May 2-4, 2017. -- https://nuclearspin2017.sciencesconf.org/
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