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ABSTRACT: We report quasiclassical trajectory dynamical calculations for the C(3P) + OH(X2)→CO(a3) + H(2S) using a recently developed ab initio potential energy surface for the first electronic state of HCO of 12A" symmetry. The dependence of integral cross sections on the collision energy is determined. Product energy and angular distributions have also been calculated. Integral cross sections show no energy threshold and decrease as the collision energy increases. The comparison with results obtained from a statistical quantum method seems to confirm that the reaction is mainly dominated by an indirect mechanism in which a long-lived intermediate complex is involved.
The Journal of Physical Chemistry A 02/2013; · 2.95 Impact Factor
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ABSTRACT: More than 100 reactions between stable molecules and free radicals have been shown to remain rapid at low temperatures. In contrast, reactions between two unstable radicals have received much less attention due to the added complexity of producing and measuring excess radical concentrations. We performed kinetic experiments on the barrierless N((4)S) + OH((2)Π) → H((2)S) + NO((2)Π) reaction in a supersonic flow (Laval nozzle) reactor. We used a microwave-discharge method to generate atomic nitrogen and a relative-rate method to follow the reaction kinetics. The measured rates agreed well with the results of exact and approximate quantum mechanical calculations. These results also provide insight into the gas-phase formation mechanisms of molecular nitrogen in interstellar clouds.
Science 12/2011; 334(6062):1538-41. · 31.20 Impact Factor
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ABSTRACT: We present accurate quantum calculations of state-to-state cross sections for the N + OH → NO + H reaction performed on the ground (3)A'' global adiabatic potential energy surface of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)]. The OH reagent is initially considered in the rovibrational state ν = 0, j = 0 and wave packet calculations have been performed for selected total angular momentum, J = 0, 10, 20, 30, 40,...,120. Converged integral state-to-state cross sections are obtained up to a collision energy of 0.5 eV, considering a maximum number of eight helicity components, Ω = 0,...,7. Reaction probabilities for J = 0 obtained as a function of collision energy, using the wave packet method, are compared with the recently published time-independent quantum mechanical one. Total reaction cross sections, state-specific rate constants, opacity functions, and product state-resolved integral cross-sections have been obtained by means of the wave packet method for several collision energies and compared with recent quasi-classical trajectory results obtained with the same potential energy surface. The rate constant for OH(ν = 0, j = 0) is in good agreement with the previous theoretical values, but in disagreement with the experimental data, except at 300 K.
The Journal of chemical physics 09/2011; 135(10):104307. · 3.09 Impact Factor
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ABSTRACT: First quasi-classical trajectory calculations have been carried out for the S((3)P) + OH(X (2)Π) → SO(X (3)Σ(-)) + H((2)S) reaction on an ab initio global potential energy surface for the ground electronic state, X (2)A'', of HSO. Cross sections, computed for collision energies up to 1 eV, show no energy threshold and decrease with the increasing collision energy. Rate constants have been calculated in the 5-500 K temperature range. The thermal rate constant is in good agreement with approximate quantum results, while a disagreement is found at 298 K with the experimental data. Product energy distributions have also been reported at four collision energies from 0.001 to 0.5 eV. The shapes of the rovibrational and angular distributions suggest the formation of an intermediate complex that is more and more long-lived as the collision energy increases.
Physical Chemistry Chemical Physics 02/2011; 13(18):8414-21. · 3.57 Impact Factor
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ABSTRACT: The C(3 P)+OD(X 2Π) reaction has been studied by means of quantum mechanical real wave packet (RWP) and quasiclassical trajectory (QCT) methodologies on the ground potential energy surface of Zanchet et al. [J. Phys. Chem. A 110, 12017 (2006)]. Initial state selected total reaction probabilities at J = 0 total angular momentum have been calculated for a wide range of collision energies. Product state-resolved integral cross-sections at selected collision energies and excitation functions have been determined from the RWP calculations using the J-shifting approximation and from QCT calculations. State-specific and thermal rate coefficients have been calculated using both methodologies up to 500 K. The effect of reagent rotational excitation on the dynamics for the C(3 P)+OH(X 2Π) and C(3 P)+OD(X 2Π) reactions has been investigated and interesting discrepancies between the QCT and RWP results have been found. The RWP results are found to be in an overall good agreement with the corresponding QCT results, although the QCT integral cross-section and rate coefficients are slightly smaller than those obtained from the RWP calculations.
Molecular Physics. 02/2011; 109(4):543-550.
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ABSTRACT: First accurate quantum mechanical scattering calculations have been carried out for the S((3)P)+OH(X (2)Π)→SO(X (3)Σ(-))+H((2)S) reaction using a recent ab initio potential energy surface for the ground electronic state, X (2)A("), of HSO. Total and state-to-state reaction probabilities for a total angular momentum J=0 have been determined for collision energies up to 0.5 eV. A rate constant has been calculated by means of the J-shifting approach in the 10-400 K temperature range. Vibrational and rotational product distributions show no specific behavior and are consistent with a mixture of direct and indirect reaction mechanisms.
The Journal of chemical physics 10/2010; 133(14):144315. · 3.09 Impact Factor
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ABSTRACT: Faced with the lack of experimental data on the C(3P) + OH(X2Pi) --> CO(X1Sigma+) + H(2S) reaction, we propose here to compare rate constant values and their behavior with temperature following various dynamical models and, in particular, to check the sensivity of these quantities with the long-range part of the potential energy surface. For that, we have evaluated the C + OH rate constant using the quasiclassical trajectory (QCT) method, the adiabatic capture centrifugal sudden approximation (ACCSA), and the mean potential capture theory (MPCT) based on a full ab initio potential energy surface fitted with q12,5 kernels or on a perturbative multipolar expansion (MPE) potential including the monomer spin orbit splittings (MPE-SO) or not. Despite the various approximations involved in the different methods and PESs, an excellent agreement is obtained in a subset of three models: the ACCSA method with PME-SO or ab initio PESs and the QCT method with the latter PES. This suggests that the reaction takes place once the system enters the deep valley of products. In that case, the errors due to these approximate methods and PESs are small and, consequently, the rate constants are accurately calculated. Furthermore, these findings provide evidence of preponderance of the entrance channel in the reactivity of this system.
The Journal of Physical Chemistry A 07/2010; 114(28):7494-9. · 2.95 Impact Factor
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ABSTRACT: Faced with the lack of experimental data on the C(3P) + OH(X2Π) → CO(X1Σ+) + H(2S) reaction, we propose here to compare rate constant values and their behavior with temperature following various dynamical models and, in particular, to check the sensivity of these quantities with the long-range part of the potential energy surface. For that, we have evaluated the C + OH rate constant using the quasiclassical trajectory (QCT) method, the adiabatic capture centrifugal sudden approximation (ACCSA), and the mean potential capture theory (MPCT) based on a full ab initio potential energy surface fitted with q12, 5 kernels or on a perturbative multipolar expansion (MPE) potential including the monomer spin orbit splittings (MPE-SO) or not. Despite the various approximations involved in the different methods and PESs, an excellent agreement is obtained in a subset of three models: the ACCSA method with PME-SO or ab initio PESs and the QCT method with the latter PES. This suggests that the reaction takes place once the system enters the deep valley of products. In that case, the errors due to these approximate methods and PESs are small and, consequently, the rate constants are accurately calculated. Furthermore, these findings provide evidence of preponderance of the entrance channel in the reactivity of this system.
06/2010;
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ABSTRACT: A detailed quasiclassical trajectory on the N + OH(v=0, j=0,1,5,10) --> NO + H reaction is reported at four collision energies from 0.01 to 0.5 eV. The vibrational distributions which are statistical and the angular distributions, which present a forward/backward symmetry, are consistent with the formation of a long-lived intermediate complex. Our results show globally a weak dependence of the angular and rovibrational distributions on the rotational excitation of OH, but a more pronounced effect of the collision energy.
The Journal of chemical physics 09/2009; 131(9):094302. · 3.09 Impact Factor
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ABSTRACT: We report in this paper ab initio calculations of the potential energy surfaces (PESs) for the four states involved in the C((3)P) + OH(X(2)Pi)--> CO(a(3)Pi) + H((2)S) reaction as well as numerical values of the rate constants for two states, 1(2)A'' and 1(4)A'' which show no potential barriers during the reaction. In contrast, the other two states, i.e. the 2(2)A' and 1(4)A' states, are energetically not favourable to the reaction as the first state has a potential barrier of 0.2 eV in the entrance channel and the former one presents long range potential wells and repulsive wall for carbon approaches near OH. The ab initio calculations of the potential energies have been performed at the multireference internally contracted single and double configuration interaction (MR-SDCI) level corrected for its size-inconsistency by the Davidson method (+Q), and using Dunning aug-cc-pVQZ atomic basis sets. Global PESs have then been generated for the two A'' states from an analytical fit obtained with the reproducing kernel Hilbert space method on a large number of ab initio points located on a regular grid in Jacobi coordinates. The title reaction is much less exoergic (-0.41 eV) than the one on the ground state and each state presents many extrema (four for the 1(2)A'' and eight for the 1(4)A''). From the configuration and energy of these extrema, different reaction mechanisms are suggested depending on the collision energy. Quasi-classical trajectory calculations on these global PESs have been used to estimate reactive cross-sections as functions of the collision energy and thermal rate constant as a function of the temperature. The weighted rate constant for each state, i.e. including the spin-orbit population factor, increases with the temperature contrary to the ground state one. Nevertheless, a decreasing behaviour with the temperature remains between 10 and 500 K if we consider the total rate constant of C((3)P) + OH(X(2)Pi), sum of the three reactive states rate constants.
Physical Chemistry Chemical Physics 08/2009; 11(29):6182-91. · 3.57 Impact Factor
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ABSTRACT: The dynamics of the O + OH reaction on the ground state potential energy surface (PES) is investigated by means of the quasiclassical trajectory method and two statistical methods: phase space theory and statistical quantum method. Preliminary calculations with an exact quantum method are also reported. The quasiclassical trajectory calculations show evidence for a phase space bottleneck inhibiting the intramolecular energy transfer between the O-H and O-O bonds. As a result, the probability of the intermediate complex dissociating back toward the reactants is high, thereby yielding a reaction probability significantly lower than expected for a barrierless and exothermic reaction. The features of the PES, which are the cause of this dynamical effect, are identified. This is essentially the conservation of the equilibrium distance of the O-H bond, hardly changed by a close encounter with an oxygen atom. The statistical calculations, which do not take into account the PES in the complex region, yield a high reaction probability, much larger than the probability calculated from the dynamical methods, both classical and quantum. If the statistical cross sections are corrected by a scaling factor, which corresponds actually to scaling the capture probability, then a good agreement is observed between dynamical and statistical calculations of the product state distributions. The differential cross sections calculated with all the methods show a backward-forward symmetry, with sharp polarization peaks. The complex lifetime is divided into two parts by the bottleneck. During the first part, the system remains trapped in a small region of the phase space and has a high probability to dissociate back toward the reactants. This is a nonstatistical effect due to the PES shape. During the second part, fast intramolecular vibrational energy redistribution takes place, leading to a statistical distribution of energy on the rovibrational states of the products. These findings indicate that the O + OH reaction has mixed dynamics, both with statistical and nonstatistical aspects.
The Journal of chemical physics 06/2009; 130(18):184301. · 3.09 Impact Factor
