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10 pags., 8 figs., 2 tabs. -- Published as part of The Journal of Physical Chemistry virtual special issue “F. Javier Aoiz Festschrift”., The H + HeH+→ He + H2+ reaction has been studied by means of a combination of theoretical approaches: a statistical quantum method (SQM), ring polymer molecular dynamics (RPMD), and the quasiclassical trajectory (QCT) method. Cross sections and rate constants have been calculated in an attempt to investigate the dynamics of the process. The comparison with previous calculations and experimental results reveals that despite the fact that statistical predictions seem to reproduce some of the overall observed features, the analysis at a more detailed state-to-state level shows noticeable deviations from a complex-forming dynamics. We find some differences in cross sections and rate constants obtained in the QCT calculation with a Gaussian binning procedure with respect to previous works in which the standard histogram binning was employed., T.G.-L. would like to express thanks for support from the MINECO with Grant No. FIS2017-83157-P. This work was supported by the “Programme National Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP cofunded by CEA and CNES. Y.S. would like to express thanks for support from the High Performance Computing Platform MESO@LR at the University of Montpellier and from the Computing Center of the “Institut National de Physique Nucleaire ́ ” (IN2P3). Y.V.S. and S.B. acknowledge the support of the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Projects: INFRASTRUCTURE/1216/0070 and Cy-Tera NEA YΠOΔOMH/ΣTPATH/0308/31).

11 pags., 7 figs., Microscopic-level understanding of the separation mechanism for two-dimensional (2D) membranes is an active area of research due to potential implications of this class of membranes for various technological processes. Helium (He) purification from the natural resources is of particular interest due to the shortfall in its production. In this work, we applied the ring polymer molecular dynamics (RPMD) method to graphdiyne (Gr2) and graphtriyne (Gr3) 2D membranes having variable pore sizes for the separation of He isotopes, and compare for the first time with rigorous quantum calculations. We found that the transmission rate through Gr3 is many orders of magnitude greater than Gr2. The selectivity of either isotope at low temperatures is a consequence of a delicate balance between the zero-point energy effect and tunneling of 4He and 3He. In particular, a remarkable tunneling effect is reported on the Gr2 membrane at 10 K, leading to a much larger permeation of the lighter species as compared to the heavier isotope. RPMD provides an efficient approach for studying the separation of He isotopes, taking into account quantum effects of light nuclei motions at low temperatures, which classical methods fail to capture., Y. V. S. and S. B. acknowledge the support of the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Projects: INFRASTRUCTURE/ 1216/0070 and Cy-Tera NEA YPODOMH/STPATH/0308/31). Y. V. S. was also supported by the Russian Foundation for Basic Research grant number 20-03-00833. J. C.-M. and M. I. H. were supported by Spanish MICINN with Grant FIS2017-84391-C2-2-P and PID2020- 114957GB-I00. We are also indebted to ‘‘Centro de Supercomputacio´n de Galicia’’ (CESGA) and specially to the people in the help desk.

10 pags., 7 figs., 1 tab., This work addresses the kinetics and dynamics of the gas-phase reaction between O(1D) and HD molecules down to low temperature. Here, measurements were performed by using a supersonic flow (Laval nozzle) reactor coupled with pulsed laser photolysis for O(1D) production and pulsed-laser-induced fluorescence for O(1D) detection to obtain rate constants over the 50-300 K range. Additionally, temperature-dependent branching ratios (OD + H/OH + D) were obtained experimentally by comparison of the H/D atom atom yields with those of a reference reaction. In parallel, theoretical rate constants and branching ratios were calculated by using three different techniques; mean potential phase space theory (MPPST), the statistical quantum mechanical method (SQM), and ring polymer molecular dynamics (RPMD). Although the agreement between experimental and theoretical rate constants is reasonably good, with differences not exceeding 30% over the entire temperature range, the theoretical branching ratios derived by the MPPST and SQM methods are as much as 50% larger than the experimental ones. These results are presented in the context of earlier work, while the possible origins of the discrepancies between experiment and theory are discussed., T.G.L. acknowledges support from MICINN with Grant FIS2017-83157-P. K.M.H. acknowledges support from the French programs “Physique et Chimie du Milieu Interstellaire” (PCMI) and “Programme National de Planetologie ́ ” (PNP) funded by the Centre National de la Recherche Scientifique (CNRS) and Centre National d’Etudes Spatiales (CNES). Y.V.S. acknowledges the support of the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Project: INFRASTRUCTURE/1216/00/0 and Cy-Tera NEA YΠOΔOMH/ΣTPATH/0308/31). Computer time was provided by the Pôle Modelisation HPC facilities of ́ the Institut des Sciences Moleculaires UMR 5255 CNRS ́ − Universitéde Bordeaux, cofunded by the Nouvelle Aquitaine region.

8 pags., 5 figs., 1 tab., The quantum dynamics of the title reactions are studied using the ring polymer molecular dynamics (RPMD) method from 20 to 1200 K using recently proposed full dimensional potential energy surfaces which include long-range dipole-dipole interactions. A V-shaped dependence of the reaction rate constants is found with a minimum at 200-300 K, in rather good agreement with the current experimental data. For temperatures above 300 K the reaction proceeds following a direct H-abstraction mechanism. However, below 100 K the reaction proceeds via organic-molecule···OH collision complexes, with very long lifetimes, longer than 10 s, associated with quantum roaming arising from the inclusion of quantum effects by the use of RPMD. The long lifetimes of these complexes are comparable to the time scale of the tunnelling to form reaction products. These complexes are formed at zero pressure because of quantum effects and not only at high pressure as suggested by transition state theory (TST) calculations for OH + methanol and other OH reactions. The zero-pressure rate constants reproduce quite well measured ones below 200 K, and this agreement opens the question of how important the pressure effects on the reaction rate constants are, as implied in TST-like formalisms. The zero-pressure mechanism is applicable only to very low gas density environments, such as the interstellar medium, which are not repeatable by experiments., P.d.M.-S., A.A., E.J., and O.R. acknowledge the support of the Ministerio de Economıa y Competitividad (SPAIN) under ́ Grant No. FIS2017-83473-C2 and from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). Y.V.S. acknowledges the support of the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Project: INFRASTRUCTURE/1216/0070). The calculations have been performed at CESGA (finisterrae2), CENITS (lusitania2), and SCAYLE (calendula) supercomputer centers under RES (Red Española de Supercomputacion) computational Grant Nos. AECT-2018-2-0001, AECT- ́2018-3-0001, and AECT-2019-1-0001.

Because of its fundamental importance in astrochemistry, the H + H → H + H reaction has been studied experimentally in a wide temperature range. Theoretical studies of the title reaction significantly lag primarily because of the challenges associated with the proper treatment of the zero-point energy (ZPE). As a result, all previous theoretical estimates for the ratio between a direct proton-hop and indirect exchange (via the H complex) channels deviate from the experiment, in particular, at lower temperatures where the quantum effects dominate. In this work, the ring polymer molecular dynamics (RPMD) method is applied to study this reaction, providing very good agreement with the experiment. RPMD is immune to the shortcomings associated with the ZPE leakage and is able to describe the transition from direct to indirect mechanisms below room temperature. We argue that RPMD represents a useful tool for further studies of numerous ZPE-sensitive chemical reactions that are of high interest in astrochemistry., Y.V.S. thanks the European Regional Development Fund and the Republic of Cyprus for support through the Research Promotion Foundation (Project Cy-Tera NEA YΠOΔOMH/ ΣTPATH/0308/31). S.G.-C., A.A., and O.R. acknowledge the support of the Ministerio de Economıá y Competitividad (SPAIN) under Grants FIS2014-52172-C2 and FIS2017- 83473-C2 and the support from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). We also acknowledge the financial support of the European COST program, CM1401, “Our Astrochemical History”. The calculations have been performed at the CyI (Cy-Tera), CESGA (finisterrae2), UAM (CCC), and CSIC (trueno) supercomputer centers.