Formation of interstellar SH+^+ from vibrationally excited H2_2: Quantum study of S+^+ + H2_2 ⇄\rightleftarrows SH+^+ + H reactions and inelastic collisions

The rate constants for the formation, destruction, and collisional excitation of SH$^+$ are calculated from quantum mechanical approaches using two new SH$_2^+$ potential energy surfaces (PESs) of $^4A''$ and $^2A''$ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH$^+$ ($^3\Sigma^-$) + H($^2S$) channel. The formation of SH$^+$ through the S$^+$ + H$_2$ reaction is endothermic by $\approx$ 9860 K, and requires at least two vibrational quanta on the H$_2$ molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H$_2$($v=2$) are in very good agreement with the quantum results above 100K. Further quasi-classical calculations are then performed for $v=3$, 4, and 5 to cover all vibrationally excited H$_2$ 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.Comment: 8 pages, 7 figure

Quantum and quasi-classical calculations for the S+ + H2(v,j) → SH+(v′,j′) + H reactive collisions

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 Reviewe

Long-range multipolar potentials of the 18 spin-orbit states arising from the C(3^3P) + OH(X2Π^2\Pi) interaction

31 pages (texte+figures)We present the multipolar potentials at large intermolecular distances for the 18 doubly-degenerate spin-orbit states arising from the interaction between the two open-shell systems, C($^3$P) and OH(X$^2\Pi$). With OH fixed at its ground vibrational state averaged distance $r_0$, the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance $R$ and the angle $\gamma$ between $R$ and $r$. The 18 $\times$ 18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the coulombic interaction potential, resulting in a multipolar expansion of the potential matrix expressed as a series of terms varying in $R^{-n}$. The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18 $\times$ 18 potential matrix generates adiabatic long-range PESs in good agreement with their {\it ab initio} counterparts

Non-adiabatic quantum dynamics of the electronic quenching OH(A(2)sigma(+)) + Kr

We present the dynamics of the electronic quenching OH(A2S+) + Kr(1S)-OH(X2P) + Kr(1S), withOH(A2S+) in the ground ro-vibrational state. This study relies on a new non-adiabatic quantum theorythat uses three diabatic electronic statesS+,P0, andP00, coupled by one conical-intersection and nineRenner-Teller matrix elements, all of which are explicitly considered in the equation of the motion. Thetime-dependent mechanism and initial-state-resolved quenching probabilities, integral cross sections,thermal rate constants, and thermally-averaged cross sections are calculatedviathe real wavepacketmethod. The results point out a competition among three non-adiabatic pathways:S+2P0,S+2P00,andP02P00. In particular, the conical-intersection effectsS+-P0are more important than theRenner-Teller couplingsS+-P0,S+-P00, andP0-P00. Therefore,P0is the preferred product channel.The quenching occursviaan indirect insertion mechanism, opening many collision complexes, and theprobabilities thus present many oscillations. Some resonances are still observable in the cross sections,which are in good agreement with previous experimental and quasi-classical findings. We also discussthe validity of more approximate quantum methods

Communication: theoretical exploration of Au+H2, D2, and HD reactive collisions

The following article appeared in Journal of Chemical Physic 135.9 (2011): 091102 and may be found at http://scitation.aip.org/content/aip/journal/jcp/135/9/10.1063/1.3635772A quasi-classical study of the endoergic Au(1S)+ H2(X1Σg+) → AuHAuH+(2Σ+)+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)]. 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, Au+H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertionA.D.-U. acknowledges a JAE fellowship supported by CSIC. This work is supported by Comunidad Autónoma de Madrid, Grant No. S2009/MAT/1467, and by Ministerio de Ciencia e Innovación, Grant Nos. CSD2009-00038 and FIS2010-18132. The calculations have been performed at CESGA and IFF computing centers. P.B.A. thanks the National Science Foundation for suppor

Electron transport signature of H2 dissociation on atomic gold wires

Nonequilibrium 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 wireThis work has been supported by Comunidad Autónoma de Madrid (CAM) under Grant No. S-2009/MAT/1467, by the Ministerio de Ciencia e Innovación under Grant No. FIS2011-29596-C02, and by the European-Union Integrated Project AtMol (http://www.atmol.eu). We would like to thank as well the CESGA computing center for the computing time under the ICTS grant

Dynamically biased statistical model for the ortho/para conversion in the H2 + H3+ → H3+ + H2 reaction

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)10.1063/1.2430711]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H 5+ complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H 5+ complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree [J. Chem. Phys. 134, 194311 (2011)10.1063/1.3587246] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interestThis work has been supported by the program CONSOLIDER-INGENIO 2010 of Ministerio de Ciencia e Innovación under Grant No. CSD2009-00038, entitled “Molecular strophysics: the Herschel and Alma era,” and by Grant Nos. FIS2011-29596-C02 and QCQ2008-02578, and by Comunidad Autónoma de Madrid (CAM) under Grant No. S-2009/MAT/146

DpgC-Catalyzed Peroxidation of 3,5-Dihydroxyphenylacetyl-CoA (DPA-CoA): Insights into the Spin-Forbidden Transition and Charge Transfer Mechanisms

[EN]Despite being a very strong oxidizing agent, most organic molecules are not oxidized in the presence of O2 at room temperature because O2 is a diradical whereas most organic molecules are closed-shell. Oxidation then requires a change in the spin state of the system, which is forbidden according to non-relativistic quantum theory. To overcome this limitation, oxygenases usually rely on metal or redox cofactors to catalyze the incorporation of, at least, one oxygen atom into an organic substrate. However, some oxygenases do not require any cofactor, and the detailed mechanism followed by these enzymes remains elusive. To fill this gap, here the mechanism for the enzymatic cofactor-independent oxidation of 3,5-dihydroxyphenylacetyl-CoA (DPA-CoA) is studied by combining multireference calculations on a model system with QM/MM calculations. Our results reveal that intersystem crossing takes place without requiring the previous protonation of molecular oxygen. The characterization of the electronic states reveals that electron transfer is concomitant with the triplet–singlet transition. The enzyme plays a passive role in promoting the intersystem crossing, although spontaneous reorganization of the water wire connecting the active site with the bulk presets the substrate for subsequent chemical transformations. The results show that the stabilization of the singlet radical-pair between dioxygen and enolate is enough to promote spin-forbidden reaction without the need for neither metal cofactors nor basic residues in the active site

Physical and spectroscopic properties of pure C2H4 and CH4:C2H4 ices

[EN] Physical and spectroscopic properties of ices of C2H4 and CH4:C2H4 mixtures with 3:1, 1:1 and 1:3 ratios have been investigated at 30 K. Two laboratories are involved in this work. In one of them, the density and refractive index of the samples have been measured by using a cryogenic quartz microbalance and laser interferometric techniques. In the other one, IR spectra have been recorded in the near- and mid-infrared regions, and band shifts with respect to the pure species, band strengths of the main bands, and the optical constants in both regions have been determined. Previous data on ethylene and the mixtures studied here were scarce. For methane, both the wavenumbers and band strengths have been found to follow a regular pattern of decrease with increasing dilution, but no pattern has been detected for ethylene vibrations. The method employed for the preparation of the samples, by vapour deposition under vacuum, is thought to be adequate to mimic the structure of astrophysical ices. Possible astrophysical implications, especially by means of the optical constants reported here, have been discussed.This work has been funded by the Ministerio de Ciencia y Competitividad (MINECO) of Spain under grants FIS2013-48087-C2-1P, FIS2013-48087-C2-2P and AYA2015-71975-REDT 'Polvo Cosmico' by the Ministerio de Ciencia e Innovacion of Spain under grant CDS2009-00038 and by the European Research Council project ERC-2013-Syg 610256 'Nanocosmos'. GM acknowledges MINECO PhD grant BES-2014-069355. Our skillful technicians C. Santonja, M. A. Moreno, A. Gonzalez and J. Rodriguez are also gratefully acknowledged.Molpeceres, G.; Satorre Aznar, MÁ.; Ortigoso, J.; Zanchet, A.; Luna Molina, R.; Millán Verdú, C.; Escribano, R.... (2017). Physical and spectroscopic properties of pure C2H4 and CH4:C2H4 ices. Monthly Notices of the Royal Astronomical Society. 466(2):1894-1902. https://doi.org/10.1093/mnras/stw3166S18941902466