A Langevin Canonical Approach to the Study of Quantum Stochastic Resonance in Chiral Molecules

A Langevin canonical framework for a chiral two-level system coupled to a bath of harmonic oscillators is used within a coupling scheme different from the well-known spin-boson model to study the quantum stochastic resonance for chiral molecules. This process refers to the amplification of the response to an external periodic signal at a certain value of the noise strength, being a cooperative effect of friction, noise, and periodic driving occurring in a bistable system. Furthermore, from this stochastic dynamics within the Markovian regime and Ohmic friction, the competing process between tunneling and the parity violating energy difference present in this type of chiral systems plays a fundamental role. This mechanism is finally proposed to observe the so-far elusive parity-violating energy difference in chiral molecules.Helen Clara Peñate-Rodríguez and Germán Rojas-Lorenzo acknowledge a scientific project from InSTEC. Pedro Bargueño acknowledge the support from the Faculty of Science and Vicerrectoría de Investigaciones of Universidad de Los Andes, Bogotá, Colombia. Salvador Miret-Artés acknowledges a grant with Ref. FIS2014-52172-C2-1-P from the Ministerio de Economía y Competitividad (Spain). We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

Reactividad de sistemas de oro con hidrógeno molecular: del átomo aislado a agregados

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A comparative study of the Au + H2, Au+ + H2, and Au− + H2 systems: Potential energy surfaces and dynamics of reactive collisions

10 pags.; 8 figs.; 3 tabs.In order to study the Au− + H2 collision, a new global potential energy surface (PES) describing the ground electronic state of AuH−2 system is developed and compared with the PESs of the neutral [Zanchet et al., J. Chem. Phys. 132, 034301 (2010)] and cationic systems [Anaís et al., J. Chem. Phys. 135, 091102 (2011)]. We found that Au− − H2 presents a H-Au-H insertion minimum attributed to the stabilization of the LUMO 3b2 orbital, which can be considered as the preamble of the chemisorption well appearing in larger gold clusters. While the LUMO orbital is stabilized, the HOMO 6a1 is destabilized, creating a barrier at the geometry where the energy orbitals’ curves are crossing. In the anion, this HOMO is doubly occupied, while in the neutral system is half-filled and completely empty in the cation, explaining the gradual disappearance of the well and the barrier as the number of electrons decreases. The cation presents a well in the entrance channel partially explained by electrostatic interactions. The three systems’ reactions are highly endothermic, by 1.66, 2.79, and 3.23 eV for AuH, AuH+, and AuH− products, respectively. The reaction dynamics is studied using quasi-classical trajectory method for the three systems. The one corresponding to the anionic system is new in this work. Collision energies between 1.00 and 8.00 eV, measured for the cation, are in good agreement with the simulated cross section for the AuH+. It was also found that the total fragmentation, in three atoms, competes becoming dominant at sufficiently high energy. Here, we study the competition between the two different reaction pathways for the anionic, cationic, and neutral species, explaining the differences using a simple model based on the topology of the potential energy surfaces. C 2015 AIP Publishing LLCThis work has been supported by Comunidad Autónoma de Madrid (CAM) under Grant No. S2009/MAT/1467 and by Ministerio de Economía e Innovación under Project Nos. CSD2009-00038 and FIS2011-29596-C02. We also acknowledge the ICTS grants for computing time at CESGA supercomputer center. The authors want to thank the reviewers for their suggestions to improve the paper quality.Peer Reviewe

Communication: Theoretical exploration of AuH2, D2, and HD reactive collisions

A quasi-classical study of the endoergic Au(1S) H 2(X1g+) → AuH (2) H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)].10.1063/1.3514899 For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, AuHH, is the dominant reaction channel for energies above the H 2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion. © 2011 American Institute of Physics.This work is supported by Comunidad Autónoma de Madrid, Grant No. S-2009/MAT/1467, and by Ministerio de Ciencia e Innovación, Grant Nos. CSD2009-00038 and FIS2010-18132.Peer Reviewe

Understanding structure, size, and charge effects for the H2 dissociation mechanism on planar gold clusters

The H2 dissociation on several planar Aunq gold clusters, with n = 4,..., 10 and charges q = 0, ±1, has been studied in detail as a function of the nuclear configuration of the cluster and at different sites of attack. It is found that the formation of a well in the entrance channel is a necessary condition for the dissociation to occur. This well always appears in sites of Aunq where there is a defect in the electronic density with respect to that of the n neutral and isolated gold atoms or, in other words, where there is a positive charge due to the polarization of the electronic density associated to the electronic correlation. When H 2 attacks on linear sides of three atoms, on the middle gold atom, the reactivity is fully determined by this entrance well. On the contrary, when attacking corners there is a second step, in which a b2 antibonding orbital crosses the a1 HOMO orbital. The b2 orbital is strongly stabilized by an important bonding overlap between the H2 and the two neighboring gold atoms orbitals. For obtuse corners, with atoms of coordination 3, the stabilization due to this H-Au bonding overlap occurs at shorter distances than for acute angles, of coordination 2, simply because the neighboring gold atoms are structurally closer. Thus, the crossing occurs at shorter H-H distances for the obtuse angle, yielding lower dissociation barriers, while for the acute case the barrier is always high. The height of the barrier as a function of the charge is explained by the occupation of the frontier orbitals. For those cases in which the Aun- anion presents the entrance well, the stabilizing b2 orbital has typically the maximum occupation yielding the lower reaction barriers. The relaxation of the gold cluster in the reaction is analyzed by optimizing the total system at the stationary points. For the relaxed case, the MEP's obtained are nearly parallel to those obtained for the frozen gold cluster, which validates the main conclusions of this work. © 2010 American Chemical Society.This work has been supported by Comunidad Auto´noma de Madrid (CAM) under Grant No. S-0505/MAT/0303 and by Ministerio de Ciencia e Innovacio´n under Project Nos. CTQ2007-62898 and CTQ2007- 63332. The calculations have been performed at CESGA and IFF computing centers.Peer Reviewe

Dissipative geometric phase and decoherence in parity-violating chiral molecules

Within a generalized Langevin framework for open quantum systems, the cyclic evolution of a two-level system is analyzed in terms of the geometric phase extended to dissipative systems for Ohmic friction. This proposal is applied to the dynamics of chiral molecules where the tunneling and parity violating effects are competing. The effect of different system-bath coupling functions in the dissipated energy is shown to be crucial to understand the behavior of the geometric phase as well as the decoherence displayed by the corresponding interference patterns. © 2012 American Institute of Physics.This work has been funded by the MICINN (Spain) through Grant Nos. CTQ2008-02578, FIS2010-18132, and by the Comunidad Autónoma de Madrid, Grant No. S-2009/MAT/1467. P.B. acknowledges a Juan de la Cierva fellowship from the MICINN and A.D.-U. acknowledges a JAE fellowship from CSIC. H.C.P.-R. and G.R.-L. acknowledge a scientific project from INSTEC.Peer Reviewe

Electron transport signature of H2 dissociation on atomic gold wires

5 pags.; 4 figs. PACS number(s): 73.63.Nm, 31.15.xv, 71.15.MbNonequilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of H2 dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H2 dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wire. © 2014 American Physical Society.This work has been supported by Comunidad Autonoma ´ de Madrid (CAM) under Grant No. S-2009/MAT/1467, by the Ministerio de Ciencia e Innovacion under Grant No. FIS2011- ´ 29596-C02, and by the European-Union Integrated Project AtMol.Peer Reviewe

Mechanism of molecular hydrogen dissociation on gold chains and clusters as model prototypes of nanostructures

The reactivity of H2 on several gold clusters is studied using density functional theory with generalized gradient approximation methods, as model systems designed to study the main effects determining their catalytic properties under controlled conditions. Border effects are studied in finite linear gold chains of increasing size and compared with the corresponding periodic systems. In these linear chains, the reaction can proceed with no barrier along the minimum energy path, presenting a deep chemisorption well of E1.4 eV. The mechanism presents an important dependence on the initial attacking site of the chain. Linear Au4 chains joined to model-nanocontacts, formed by 2 or 3 gold atoms, in a planar triangle or in a pyramid, respectively, are also studied. The reaction barriers found in these two cases are E0.24 and 0.16 eV, respectively, corresponding to H2 attacking the more coordinated edge atom of the linear chain. The study is extended to planar clusters with coordinations IV and VI, for which higher H2 dissociation barriers are found. However, when the planar gold clusters are folded, and the Au–Au distances elongated, the reactivity increases considerably. This is not due to a change of coordination, but to a larger flexibility of the gold orbitals to form bonds with hydrogen atoms, when the planar sd-hybridization is broken. Finally, it is concluded that the major factor determining the reactivity of gold clusters is not strictly the coordination of gold atoms but their binding structure and some border effects.This work has been supported by CAM (Comunidad Auto´noma de Madrid) under grant S-0505/MAT/0303 and by the Ministerio de Ciencia e Innovacio´ n under projects CTQ2007-62898, CTQ2007-63332 and MAT2007-60966.Peer reviewe