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15 pags., 4 figs., 1 tab., An ab initio study of the interaction of O2 the most abundant radical and oxidant species in the atmosphere, with the Cu5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for using multi-reference perturbation theory, allowing to investigate the experimentally confirmed resistivity of Cu5 clusters towards oxidation. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multi-reference approach, the transition barrier for O2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu5 arranged in a bipyramidal structure, the O-O bond cleavage is confirmed as the rate-determining step. However, for planar Cu5, the high energy barrier for O2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case., This work has been partly supported by the Spanish Agencia Estatal de Investigacion (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P, by the Austrian Science Fund (FWF) under Grant P29893-N36, MINECO, Spain (MAT2015-67458-P - co nanced with FEDER Funds - and CTQ2013-44762-R), and by the COST Action CM1405 \Molecules in Motion" (MOLIM)., The CESGA super-computer center (Spain) is acknowledged for having provided the computational resources used in this work. P.L.C. expresses her gratitude for a contract for graduated students from the \Garant a Juvenil" program from the Comunidad de Madrid. D.B. expresses his gratitude for a postdoctoral grant from Xunta de Galicia, Spain (ED481D 2017/021).

8 pags., 7 figs., 4 tabs., The rate constants for the formation, destruction, and collisional excitation of SH are calculated from quantum mechanical approaches using two new SH potential energy surfaces (PESs) of A″ and A″ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH(ς) + H(S) channel. The formation of SH through the S + H reaction is endothermic by ≈9860 K, and requires at least two vibrational quanta on the H molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H(v? =? 2) are in very good agreement with the quantum results above 100 K. Further quasi-classical calculations are then performed for v? =? 3, 4, and 5 to cover all vibrationally excited H levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH column densities, in agreement with those inferred from recent ALMA observations of the Orion bar., The research leading to these results has received funding from MICIU under grants No. FIS2017-83473-C2 and AYA2017- 85111-P. FL acknowledges financial support from the Institut Universitaire de France. NB acknowledges the computing facilities by TUBITAK-TRUBA.

12 pag.s., 11 figs., 3 tabs. -- This article replaces the version published on 8th September 2017, to provide an updated citation for ref. 19., A new method is proposed to analytically represent the potential energy surface of reactions involving polyatomic molecules capable of accurately describing long-range interactions and saddle points, needed to describe low-temperature collisions. It is based on two terms, a reactive force field term and a many-body term. The reactive force field term accurately describes the fragments, long-range interactions among them and the saddle points for reactions. The many-body term increases the desired accuracy everywhere else. This method has been applied to the OH + HCO → HO + HCO reaction, giving a barrier of 27.4 meV. The simulated classical rate constants with this potential are in good agreement with recent experimental results [Ocaña et al., Astrophys. J., 2017, submitted], showing an important increase at temperatures below 100 K. The reaction mechanism is analyzed in detail here, and explains the observed behavior at low energy by the formation of long-lived collision complexes, with roaming trajectories, with a capture observed for very long impact parameters, >100 a.u., determined by the long-range dipole-dipole interaction., We acknowledge the support from Ministerio de Economıa e Innovacion (Spain), for grants CSD2009-00038, AYA2016-75066-C2-1-P, RyC-2014-16277 and FIS2014-52172-C2 and from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). E. Jimenez acknowledges the Spanish Ministry of Economy and Competitiveness for supporting this work under the GASSOL project (CGL2013-43227-R). AC is indebted to the French National Programme ‘‘Physique et Chimie du Milieu Interstellaire’’ (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. We also acknwoledge the finantial support of the European COST program, CM1401, ‘‘Our Astrochemical History’’. The calculations have been performed at the CSIC computing centers, which are acknowledged.

9 pags., 8 figs., We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and non-reactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2=1,2)⁠, while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10–3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ towards the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2=2 might be significant in the photon-dominated region of NGC 7027., This work has been supported by the Agence Nationale de la Recherche (ANR-HYDRIDES), contract ANR-12-BS05-0011-01 and by the CNRS national programme ‘Physico-Chimie du Milieu Interstellaire’. IFS acknowledges generous financial support from Région Haute-Normandie, the GRR Electronique, Energie et Matériaux, the ‘Fédération de Recherche Energie, Propulsion, Environnement’, the fund Bioengine and the LabEx EMC, via the project PicoLIBS. OR and AZ acknowledge the financial support of Ministerio de Economia e Innovación under grant FIS2014-52172-C2 and the European Research Council under ERC Grant Agreement n. 610256 (NANOCOSMOS). François Lique and Pierre Hily-Blant are acknowledged for useful discussions.