Alfredo Aguado

Universidad Autónoma de Madrid

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Octavio Roncero

Instituto de Física Fundamental

Miguel Paniagua

Universidad Autónoma de Madrid

Alexandre Zanchet

Instituto de Física Fundamental

Cristina Sanz‐Sanz

Universidad Autónoma de Madrid

Susana Gómez‐Carrasco

Universidad de Salamanca

Pablo del Mazo‐Sevillano

Universidad Autónoma de Madrid

J. R. Goicoechea

Consejo Superior de Investigaciones Científicas

C. Tablero

Universidad Politécnica de Madrid

J. Cernicharo

Consejo Superior de Investigaciones Científicas

M. Agúndez

Consejo Superior de Investigaciones Científicas

Cooperative Institutions

Centre National de la Recherche Scientifique

Consejo Superior de Investigaciones Científicas

Universidad Autónoma de Madrid

Instituto de Física Fundamental

Max Planck Society

Spanish National Observatory

Leiden University

Instituto de Ciencia de Materiales de Madrid

Centro de Astrobiología

University of Oxford

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Gas-phase Elemental abundances in Molecular cloudS (GEMS)III. Unlocking the CS chemistry: the CS+O reaction

Niyazi Bulut Octavio Roncero Alfredo Aguado Jean‐Christophe Loison D. Navarro-Almaida

Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10−15 cm3 s−1, which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.

Reaction rate

Semiclassical physics

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Citations (7)

Transition state spectroscopy via infrared excitation of Li⋯HF and Li⋯DF van der Waals precursors

The Journal of Chemical Physics (1999)

Miguel Paniagua Alfredo Aguado M. Lara Octavio Roncero

The photoinitiated reactions after infrared excitation from the LiHF and LiDF complexes in the reactant valley are studied as an extension of a recent communication by Paniagua et al. [J. Chem. Phys. 109, 2971 (1998)]. For LiHF two broad bands, associated to Δv=1 and 2 transitions, are obtained at which the probability of forming LiF products is very high, >90%. For LiDF the Δv=1 band consists of several narrow resonances, and some of them are supported by the barrier separating reactant and product valleys. Even at these resonances the reaction probability is relatively high, starting at a value about 30% and increasing rapidly to >90% with increasing energy. This implies the tunneling through the barrier. The reason for the high efficiency in the photoinitiated reaction is that the main excitation corresponds to the HF (or DF) stretch within the complex, which is the “active” mode for the reaction in agreement with the presence of a late barrier. These results are very different from those obtained in Li+HF or Li+DF collisions at the same total energies, the reaction probabilities being much lower in these latter since the excitation of the HF (DF) mode is unlikely to occur during the collision.

10.1063/1.480040

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Citations (42)

Global fit of ab initio potential energy surfaces I. Triatomic systems

Computer Physics Communications (1998)

Alfredo Aguado C. Tablero Miguel Paniagua

Triatomic molecule

Potential energy surface

Fortran

10.1016/s0010-4655(97)00135-5

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Citations (130)

UNDERSTANDING CHEMICAL REACTIONS INVOLVING NON-ADIABATIC TRANSITIONS: PREDISSOCIATION OF THE ELECTRONICALLY EXCITED Li-HF COMPLEX

Progress in theoretical chemistry and physics (2006)

Alfredo Aguado Miguel Paniagua Cristina Sanz‐Sanz Octavio Roncero

Excited electronic state

Molecular electronic transition

Atomic electron transition

Representation

10.1007/1-4020-4528-x_19

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Citations (2)

IAU volume 13 issue S332 Cover and Front matter

Proceedings of the International Astronomical Union (2017)

Martin Cordiner Jin Lai N. A. Teanby Charles W. Nixon Matthew D. Palmer

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Front cover

Content (measure theory)

10.1017/s174392131800090x

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Nonadiabatic State-to-State Reactive Collisions among Open Shell Reactants with Conical Intersections: The OH(²Π) + F(²P) Example

The Journal of Physical Chemistry A (2010)

Alexandre Zanchet Tomás González‐Lezana Alfredo Aguado Susana Gómez‐Carrasco Octavio Roncero

Accurate wave packet calculations on the OH(2Π) + F(2P) → O(3P) + HF(1Σ+) reactive collisions are performed using a recently proposed coupled diabatic states. Adiabatic and nonadiabatic dynamics are compared in detail, analyzing the final state distribution of products. It is found that with the new surfaces a significant increase of the rate constant is obtained, with noticeable nonadiabatic effects. The inclusion of the spin−orbit splittings for the calculation of the electronic partition function produces an important increase of the reaction rate constants, yielding a rather good agreement with the experimental results. It is also concluded that spin−orbit couplings are also necessary in the entrance channel to describe this reaction.

Diabatic

Reaction dynamics

10.1021/jp101914a

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Citations (24)

Understanding Structure, Size, and Charge Effects for the H₂ Dissociation Mechanism on Planar Gold Clusters

The Journal of Physical Chemistry C (2010)

Alexandre Zanchet Anaís Dorta-Urra Alfredo Aguado Octavio Roncero

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 H2 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.

Antibonding molecular orbital

Gold cluster

Charge density

10.1021/jp106733s

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Citations (19)

The HeH2+ potential energy surface revisited

Miguel Paniagua Luis Velilla Alfredo Aguado Manuela Castro Simonetta Cavalli

Source

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Global fit of ab initio potential energy surfaces: II.1. Tetraatomic systems

Computer Physics Communications (2001)

Alfredo Aguado C. Tablero Miguel Paniagua

Fortran

Potential energy surface

10.1016/s0010-4655(00)00181-8

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Citations (40)

Non-adiabatic couplings and dynamics in proton transfer reactions of Hn+ systems: Application to H2+H2+→H+H3+ collisions

The Journal of Chemical Physics (2015)

Cristina Sanz‐Sanz Alfredo Aguado Octavio Roncero Fedor Y. Naumkin

Analytical derivatives and non-adiabatic coupling matrix elements are derived for Hn+ systems (n = 3–5). The method uses a generalized Hellmann-Feynman theorem applied to a multi-state description based on diatomics-in-molecules (for H3+) or triatomics-in-molecules (for H4+ and H5+) formalisms, corrected with a permutationally invariant many-body term to get high accuracy. The analytical non-adiabatic coupling matrix elements are compared with ab initio calculations performed at multi-reference configuration interaction level. These magnitudes are used to calculate H2(v′=0,j′=0)+H2+(v,j=0) collisions, to determine the effect of electronic transitions using a molecular dynamics method with electronic transitions. Cross sections for several initial vibrational states of H2+ are calculated and compared with the available experimental data, yielding an excellent agreement. The effect of vibrational excitation of H2+ reactant and its relation with non-adiabatic processes are discussed. Also, the behavior at low collisional energies, in the 1 meV-0.1 eV interval, of interest in astrophysical environments, is discussed in terms of the long range behaviour of the interaction potential which is properly described within the triatomics-in-molecules formalism.

Diatomic molecule

Adiabatic theorem

Formalism (music)

10.1063/1.4937138

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Citations (26)