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This paper presents two new adiabatic, global potential energy surfaces (PESs) for the two lowest 3A′ and 3A″ electronic states of the O(3P) + H2 system. For each of these states, ab initio electronic energies were calculated for more than 5000 geometries using internally contracted multireference configuration interaction methods. The calculated points were then fitted using the ansatz by Aguado et al. [Comput. Phys. Commun. 108, 259 (1998)] leading to very accurate analytical potentials well adapted to perform reaction dynamics studies. Overall, the topographies of both PESs are in good agreement with the benchmark potentials of Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)], but those presented in this work reproduce better the height and degeneracy of the two states at the saddle point. Moreover, the long range potential in the entrance channel does not require any cutoff. These features make the new PESs particularly suitable for a comparison of the dynamics on each of them. The new set of PESs was then used to perform quantum mechanics and quasiclassical trajectory calculations to determine differential and integral cross sections, which are compared to the experimental measurements by Garton et al. [J. Chem. Phys. 118, 1585 (2003)]. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A quasi-classical study of the endoergic Au+(1S) + H2(X1Σg+) → AuH+ (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 insertion. [ABSTRACT FROM AUTHOR]

The first calculations of state-to-state reaction probabilities and product state-resolved integral cross sections at selected collision energies (0.05, 0.1, 0.5, and 1.0 eV) for the title reaction on the ab initio potential energy surface of [Zanchet et al. J. Phys. Chem. A 110, 12017 (2006)] with the OH reagent in selected rovibrational states (v=0–2, j=0–5) have been carried out by means of the real wave packet (RWP) and quasiclassical trajectory (QCT) methods. State-selected total reaction probabilities have been calculated for total angular momentum J=0 in a broad range of collision energies. Integral cross sections and state-specific rate coefficients have been obtained from the corresponding J=0 RWP reaction probabilities for initially selected rovibrational states by means of a capture model. The calculated RWP and QCT state-selected rate coefficients are practically temperature independent. Both RWP and QCT reaction probabilities, integral cross sections, and rate coefficients are almost independent of the initial rotational excitation. The RWP results are found to be in an overall good agreement with the corresponding QCT results. The present results have been compared with earlier wave packet calculations carried out on the same potential energy surface. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Π)→CO(X 1Σ+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state X 2A′ of COH. Differential cross sections (DCSs), and product vibrational, rotational and translational distributions have been determined for a wide range of collision energies (0.001–1 eV). The role of excitations (rotation or vibration) of the OH reactant on these quantities has been investigated. Product vibrational, rotational, and translational distributions are found to be almost independent on the rovibrational state of OH, whereas DCSs show a weak dependence on the initial rotational state of OH. We also analyze the results using a study based on the lifetime of the intermediate complex and on the kinematic constraint associated with the mass combination. [ABSTRACT FROM AUTHOR]

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Π)→CO(X 1Σ+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state, X 2A′, of HCO/COH. Total and state-specific integral cross sections have been determined for a wide range of collision energies (0.001–1 eV). Then, thermal and state-specific rate constants have been calculated in the 1–500 K temperature range. The thermal rate constant varies from 1.78×10-10 cm3 s-1 at 1 K down to 5.96×10-11 cm3 s-1 at 500 K with a maximum value of 3.39×10-10 cm3 s-1 obtained at 7 K. Cross sections and rate constants are found to be almost independent of the rovibrational state of OH. [ABSTRACT FROM AUTHOR]

The rate constants for the formation, destruction, and collisional excitation of SH+ are calculated from quantum mechanical approaches using two new SH+2 SH 2 + $ {\rm 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+(3Σ−) + H(2S) channel. The formation of SH+ through the S+ + H2 reaction is endothermic by ≈9860 K, and requires at least two vibrational quanta on the H2 molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H2(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 H2 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. [ABSTRACT FROM AUTHOR]

State-to-state rate constants for the title reaction are calculated using the electronic ground state potential energy surface and an accurate quantum wave-packet method. The calculations are performed for H2 in different rovibrational states, v = 0, 1 and J = 0 and 1. The simulated reaction cross section for v = 0 shows a rather good agreement with the experimental results of Gerlich et al., both with a threshold of 0.36 eV and within the experimental error of 20%. The total reaction rate coefficients simulated for v = 1 are two times smaller than those estimated by Hierl et al. from cross sections measured at different temperatures and neglecting the contribution from v > 1 with an uncertainty factor of two. Thus, part of the disagreement is attributed to the contributions of v > 1. The computed state-to-state rate coefficients are used in our radiative transfer model code applied to the conditions of the Orion Bar photodissociation region, and leads to an increase of the line fluxes of high-J lines of CH+. This result partially explains the discrepancies previously found with measurements and demonstrates that CH+ excitation is mostly driven by chemical pumping. [ABSTRACT FROM AUTHOR]

An analytical potential energy surface for a rigid Rb2 in the 3σu+ state interacting with one helium atom based on accurate ab initio computations is proposed. This 2-dimensional potential is used, together with the pair approximation approach, to investigate Rb2 attached to small helium clusters HeN with N = 1-6, 12, and 20 by means of quantum Monte Carlo studies. The limit of large clusters is approximated by a flat helium surface. The relative orientation of the dialkali axis and the helium surface is found to be parallel. Dynamical investigations of the pendular and of the in-plane rotation of the rigid Rb2 molecule on the surface are presented. [ABSTRACT FROM AUTHOR]

[Display omitted] • Unprecedented exhaustive QCT calculations have been carried out in the range 5–800 K. • Rate coefficients fall rapidly from 5 K to 150 K and then level-off. • The abrupt decrease is mainly due to the electronic degeneration factor. • Cross sections and rate coeffcients are very sensitive to the rotational excitation of NH. • Two capture models based on the long-range interaction have been developed. Rate coefficients of the complex-forming C + NH → H + CN reaction have been calculated at temperatures ranging from 5 K to 800 K using quasi-classical trajectories on a potential energy surface which accurately describes the attractive long-range interaction, along with results using two capture models. In contrast with the constant value recommended in astrochemical databases, a steep decrease of the rates has been found up to 150 K, and then they tends to be nearly constant. Such behavior is analyzed in terms of the rovibrational state-selected rate coefficients and cross sections singling out the role played by the rotational excitation of the initial diatom. The effect of the electronic degeneracy is also discussed. [ABSTRACT FROM AUTHOR]

We present 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(3P) and OH(X 2Π). With OH fixed at its ground vibrational state-averaged distance r0, the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and the angle γ=CG⁁H, where G represents the mass center of OH. The 18×18 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 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, and 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 18×18 potential matrix generates adiabatic long-range PESs in good agreement with their ab initio counterparts. [ABSTRACT FROM AUTHOR]

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)]. 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 H5+ 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 H5+ 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 et al. [J. Chem. Phys. 134, 194311 (2011)] 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 interest. [ABSTRACT FROM AUTHOR]