Suchergebnisse

8 pags., 5 figs. ; Is the rise of the rate constant measured in laval expansion experiments of OH with organic molecules at low temperatures due to the reaction between the reactants or due to the formation of complexes with the buffer gas? This question has importance for understanding the evolution of prebiotic molecules observed in different astrophysical objects. Among these molecules methanol is one of the most widely observed, and its reaction with OH has been studied by several groups showing a fast increase in the rate constant under 100 K. Transition state theory doesn't reproduce this behavior and here dynamical calculations are performed on a new full dimensional potential energy surface developed for this purpose. The calculated classical reactive cross sections show an increase at low collision energies due to a complex forming mechanism. However, the calculated rate constant at temperatures below 100 K remains lower than the observed one. Quantum effects are likely responsible for the measured behavior at low temperatures. ; We acknowledge the support of the Ministerio de Economía y Competitividad (SPAIN), under grants No. FIS2014-52172-C2 and 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). The authors thankfully acknowledges the computer resources at finisterrae2 (CESGA) and technical support provided by CESGA (AECT-2018-2-0001). Finally, the COST action CM- 1401, "Our Astro-Chemical History: Past, Present, and Future" is also acknowledged. ; Peer Reviewed

10 págs; 14 figs.; 2 tabs. ; State-to-state cross-sections for the S + H(v,j) → SH(v′,j′) + H endothermic reaction are obtained using quantum wave packet (WP) and quasi-classical (QCT) methods for different initial ro-vibrational H(v,j) over a wide range of translation energies. The final state distribution as a function of the initial quantum number is obtained and discussed. Additionally, the effect of the internal excitation of H on the reactivity is carefully studied. It appears that energy transfer among modes is very inefficient that vibrational energy is the most favorable for the reaction, and rotational excitation significantly enhances the reactivity when vibrational energy is sufficient to reach the product. Special attention is also paid to an unusual discrepancy between classical and quantum dynamics for low rotational levels while agreement improves with rotational excitation of H. An interesting resonant behaviour found in WP calculations is also discussed and associated with the existence of roaming classical trajectories that enhance the reactivity of the title reaction. Finally, a comparison with the experimental results of Stowe et al. for S + HD and S + D reactions exhibits a reasonably good agreement with those results. ; O. R. and N. B. acknowledge CSIC for the travelling grant I-LINK0775. Financial support from the Scientific and Technological Research Council of TURKEY (TUBITAK) (Project No. TBAG- 112T827) and the Ministerio de Economía e Innovación (Spain), for grants CSD2009-00038 and FIS2014-52172-C2, is gratefully acknowledged. The computations have been performed on the High Performance and Grid Computing Center (TR-Grid) at ULAKBIM/TURKEY and CESGA computer center. We also thank 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 COST action CM1401 'Our Astrochemical History'. ; Peer Reviewed

10 pags., 7 figs., 2 tabs., 2 apps. ; The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross sections of HCN and HNC isomers have been calculated in the 7-13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need to consider non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental value. It is found that HNC is photodissociated faster than HCN by a factor of 2.2 for the local interstellar radiation field and 9.2 for the solar radiation field, at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field, it is necessary to use different photodissociation rates for each of the two isomers, which are obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range. ; The research leading to these results has received funding 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 support by the Ministerio de Economía e Innovación under grants Nos. CSD2009-00038, AYA2012-32032, AYA2016-75066-C2-1-P and FIS2014-52172-C2. M.A. thanks funding support from the Ramón y Cajal programme of Spanish MINECO (RyC-2014- 16277). The calculations have been performed in the CCC-UAM and trueno-CSIC computing centers. ; Peer Reviewed

9 págs.; 6 figs.; 3 tabs. ; The ground and some excited electronic states of the methyl radical have been characterized by means of highly correlated ab intio techniques. The specific excited states investigated are those involved in the dissociation of the radical, namely the 3s and 3pz Rydberg states, and the A1 and B1 valence states crossing them, respectively. The C¿H dissociative coordinate and the HCH bending angle were considered in order to generate the first two¿dimensional ab initio representation of the potential surfaces of the above electronic states of CH3, along with the nonadiabatic couplings between them. Spectroscopic constants and frequencies calculated for the ground and bound excited states agree well with most of the available experimental data. Implications of the shape of the excited potential surfaces and couplings for the dissociation pathways of CH3 are discussed in the light of recent experimental results for dissociation from low¿lying vibrational states of CH3. Based on the ab initio data some predictions are made regarding methyl photodissociation from higher initial vibrational states. ; This work has been financed by the Spanish MINECO (grants CTQ2012-37404-C02-01, CTQ2015-65033-P, and FIS2013-40626-P) and EU COST Actions CM1401 and CM1405 as well as the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). This research has been carried out within the Unidad Asociada Quımica Fısica Molecular between the Departamento de Quımica Fısica of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Cientıficas (CSIC). The Centro de Supercomputacion de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed

8 págs.; 5 figs.; 1 tab. ; Open Access funded by Creative Commons Atribution Licence 4.0 ; In the last decade, the development of theoretical methods has allowed chemists to reproduce and explain almost all of the experimental data associated with elementary atom plus diatom collisions. However, there are still a few examples where theory cannot account yet for experimental results. This is the case for the preferential population of one of the Λ-doublet states produced by chemical reactions. In particular, recent measurements of the OD( Π) product of the O(P)+D reaction have shown a clear preference for the Π (A′) Λ-doublet states, in apparent contradiction with ab initio calculations, which predict a larger reactivity on the A″ potential energy surface. Here we present a method to calculate the Λ-doublet ratio when concurrent potential energy surfaces participate in the reaction. It accounts for the experimental Λ-doublet populations via explicit consideration of the stereodynamics of the process. Furthermore, our results demonstrate that the propensity of the π (A′) state is a consequence of the different mechanisms of the reaction on the two concurrent potential energy surfaces. ; We acknowledge funding by the Spanish Ministry of Economy and Competitiveness (grants CTQ2012-37404-C02, CTQ2015-65033-P and Consolider Ingenio 2010 CSD2009-00038). M.B. gratefully acknowledges the support of the U.K. EPSRC (via Programme Grant EP/L005913/1). P.G.J. acknowledges the Spanish Ministry of Economy and Competitiveness for the Juan de la Cierva fellowship (IJCI-2014-20615). A.Z. acknowledges the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement number 610256 (NANOCOSMOS). ; Peer Reviewed

The correlation between chemical structure and predissociation dynamics has been evaluated for a series of linear and branched alkyl iodides with increasing structural complexity by means of femtosecond time-resolved velocity map imaging experiments following excitation on the second absorption band (B-band) at around 201 nm. The time-resolved images for the iodine fragment are reported and analyzed in order to extract electronic predissociation lifetimes and the temporal evolution of the anisotropy while the experimental results are supported by ab initio calculations of the potential energy curves as a function of the C-I distance. Remarkable similarities are observed for all molecules consistent with a major predissociation of the initially populated bound Rydberg states 6A″ and 7A′ through a crossing with the purely repulsive states 7A″, 8A′ and 8A″ leading to a major R + I*(2P1/2) (R = CH3, C2H5, n-C3H7, n-C4H9, i-C3H7 and t-C4H9) dissociation channel. The reported electronic predissociation lifetimes are found to decrease for an increasing size of the linear radical, reflecting the shifts observed in the position of the crossings in the potential energy curves, and very likely a greater non-adiabatic coupling between the initially populated Rydberg states and the repulsive states leading to dissociation induced by other coordinates associated to key vibrational normal modes. The loss of anisotropy is fully accounted for by the parent molecular rotation during predissociation and the rotational temperature of the parent molecule in the molecular beam is reasonably derived ; S.M.P. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, grant agreement No 842539. This work has been financed by the Spanish MINECO (grant CTQ2015-65033-P) and Ministerio de Ciencia, Innovación y Universidades (grant PGC2018-096444-B-I00)

11 pags., 4 figs., 2 tabs. ; The correlation between chemical structure and predissociation dynamics has been evaluated for a series of linear and branched alkyl iodides with increasing structural complexity by means of femtosecond time-resolved velocity map imaging experiments following excitation on the second absorption band (B-band) at around 201 nm. The time-resolved images for the iodine fragment are reported and analyzed in order to extract electronic predissociation lifetimes and the temporal evolution of the anisotropy while the experimental results are supported by ab initio calculations of the potential energy curves as a function of the C-I distance. Remarkable similarities are observed for all molecules consistent with a major predissociation of the initially populated bound Rydberg states 6A″ and 7A′ through a crossing with the purely repulsive states 7A″, 8A′ and 8A″ leading to a major R + I*(P) (R = CH, CH, n-CH, n-CH, i-CH and t-CH) dissociation channel. The reported electronic predissociation lifetimes are found to decrease for an increasing size of the linear radical, reflecting the shifts observed in the position of the crossings in the potential energy curves, and very likely a greater non-adiabatic coupling between the initially populated Rydberg states and the repulsive states leading to dissociation induced by other coordinates associated to key vibrational normal modes. The loss of anisotropy is fully accounted for by the parent molecular rotation during predissociation and the rotational temperature of the parent molecule in the molecular beam is reasonably derived. ; M.L.M.S. acknowledges fnancial support through a predoctoral contract from Universidad Complutense de Madrid. S.M.P. acknowledges funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, grant agreement No 842539. Tis work has been fnanced by the Spanish MINECO (grant CTQ2015-65033-P) and Ministerio de Ciencia, Innovación y Universidades (grant PGC2018-096444-B-I00). Te facilities provided by the Center for Ultrafast Lasers of Universidad Complutense de Madrid are acknowledged.

10 pags., 9 figs., 4 tabs. ; The electronic states and the spin–orbit couplings between them involved in the photodissociation process of the radical molecules CH3X, CH3X → CH3 + X (X = O, S), taking place after the Ã(2A1) ← [X with combining tilde](2E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C–X dissociative mode and the CH3 umbrella mode. Spin–orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH3S case where the spin–orbit couplings are more than twice more intense than in CH3O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained. ; This work was supported by MINECO (Spain) (grants No. CTQ2015-65033-P, FIS2013-40626-P, and FIS2016-76418-P) and EU COST Action CM1401 as well as the European Research Council under the European Union's Seventh Framework Programme (FP/ 2007–2013)/ERC Grant Agreement No. 610256 (NANOCOSMOS). This research was carried out within the Unidad Asociada Quımica Fsica Molecular between the Departamento de Quımica Fısica of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Cientıficas (CSIC). AB acknowledges the financial support from the Tunisian Ministry of Higher Education, Scientific Research, and Technology, of the Short Term Scientific Mission (STSM) program of the COST Action CM1401, and of LSAMA at the Universite´ de Tunis El Manar that made possible research visits to the Instituto de Fısica Fundamental (CSIC). The Centro de Supercomputación de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed

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. ; Peer Reviewed

9 pags., 4 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0 ; The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at 201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial rovibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Ca elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed. ; D. V. C. and S. M. P. contributed equally to the paper. D. V. C. thanks a contract from Spanish MINECO under the FPI predoctoral program. S. M. P. acknowledges nancial support from a postdoctoral contract of the Regional Government of Madrid (Spain) (Grant PEJD-2016/IND-3217). A. B. acknowledges the financial support of the Tunisian Ministry of Higher Education, Scientific Research, and Technology, of the Short Term Scientific Mission (STSM) program of the COST Action CM1401, and of LSAMA at the Universite de Tunis El Manar, that made possible research visits to the Instituto de Fısica Fundamental (CSIC, Spain). This work has been nanced by the Spanish MINECO (Grants No. CTQ2015-65033-P, FIS2016- 76418-P and FIS-2016-77889-R) and EU COST Action CM1401. This research has been carried out within the Unidad Asociada Quımica Fısica Molecular between the Departamento de Quiımica Fisica of Universidad Complutense de Madrid and CSIC. The facilities provided by the Centro de Laseres Ultrarrapidos at Universidad Complutense de Madrid are gratefully acknowledged. The Centro de Supercomputacion de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources

8 págs.; 6 figs.; 1 tab. ; The photodissociation dynamics of the methyl radical from the 3s and 3pz Rydberg states have been studied using the velocity map and slice ion imaging in combination with pump–probe nanosecond laser pulses. The reported translational energy and angular distributions of the H(2S) photofragment detected by (2+1) REMPI highlight different dissociation mechanisms for the 3s and 3pz Rydberg states. A narrow peak in the translational energy distribution and an anisotropic angular distribution characterize the fast 3s photodissociation, while for the 3pz state Boltzmann-type translational energy and isotropic angular distributions are found. High level ab initio calculations have been performed in order to elucidate the photodissociation mechanisms from the two Rydberg states and to rationalize the experimental results. The calculated potential energy curves highlight a typical predissociation mechanism for the 3s state, characterized by the coupling between the 3s Rydberg state and a valence repulsive state. On the other hand, the photodissociation on the 3pz state is initiated by a predissociation process due to the coupling between the 3pz Rydberg state and a valence repulsive state and constrained, later on, by two conical intersections that allow the system to relax to lower electronic states. Such a mechanism opens up different reaction pathways leading to CH2 photofragments in different electronic states and inducing a transfer of energy between translational and internal modes. ; S. M. P. acknowledges financial support from Campus de Excelencia Internacional Moncloa and LASING S. A. D. V. C. acknowledges a contract from MINECO under the Fondo de Garantıía Juvenil. A. Z. thanks the support from the European Research Council under the European Union's 7th Framework Program (FP7/2007–2013)/ERC Grant agreement 610256 (NANOCOSMOS). M. G. G. is grateful to Spanish MINECO for a contract through Programa de Técnicos de Apoyo a Infraestructuras. This work has been financed by the Spanish MINECO (grants FIS2011- 29596-C02-01, CTQ2012-37404-C02-01, FIS2013-40626-P and CTQ2015-65033-P) and by COST Actions CM1401 and CM05. This research has been carried out within the Unidad Asociada Química Física Molecular between Departamento de Química Física of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Científicas (CSIC). The facilities provided by the Centro de Láseres Ultrarrápidos at UCM are acknowledged. The Centro de Supercomputación de Galicia (CESGA, Spain) and CTI (CSIC) are acknowledged for the use of their resources. ; Peer Reviewed

21 pags., 5 figs. ; Subnanometer-sized metal clusters often feature a molecule-like electronic structure, which makes their physical and chemical properties significantly different from those of nanoparticles and bulk material. Considering potential applications, there is a major concern about their thermal stability and susceptibility towards oxidation. Cu clusters of only 5 atoms (Cu5 clusters) are first synthesized in high concentration using a new-generation wet chemical method. Next, it is shown that, contrary to what is currently assumed, Cu5 clusters display nobility, beyond resistance to irreversible oxidation, at a broad range of temperatures and oxygen pressures. The outstanding nobility arises from an unusual reversible oxidation which is observed by in situ X-ray Absorption Spectroscopy and X-ray Photoelectron Spectroscopy on Cu5 clusters deposited onto highly oriented pyrolitic graphite at different oxygen pressures and up to 773 K. This atypical property is explained by a theoretical approach combining different state-of-the-art first principles theories. It reveals the essential role of collective quantum effects in the physical mechanism responsible for the nobility of Cu5 clusters, encompassing a structural 'breathing' through concerted Cu–Cu elongations/contractions upon O2 uptake/release, and collective charge transfer as well. A predictive phase diagram of their reversible oxidation states is also delivered, agreeing with the experimental observations. The collective quantum effects responsible of the observed nobility are expected to be general in subanometer-sized metal clusters, pushing this new generation of materials to an upper level. ; This work has been partly supported by the European Union's Horizon 2020 Research and Innovation Programme un-der Grant Agreement No. 825999; the Spanish Agencia Estatal de Investigación (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER, UE) under Grant No. MAT2016-75354-P; the Austrian Science Fund (FWF) under Grant P29893-N36; the CMST COST Action CM1405 "Molecules in Motion" (MOLIM); the Xunta de Galicia, Spain (Grupos Ref. Comp.ED431C 2017/22 and AEMAT ED431E 2018/08); Obra Social Fundación La Caixa: Ref.LCF/PR/PR12/11070003; ANPCyT PICT (2017-1220 and 2017-3944) and UNLP (Project11/X790), Argentina. ; No