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 H 2 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.

The state-to-state differential cross sections for some atom + diatom reactions have been calculated using a new wave packet code, MAD-WAVE3, which is described in some detail and uses either reactant or product Jacobi coordinates along the propagation. In order to show the accuracy and efficiency of the coordinate transformation required when using reactant Jacobi coordinates, as recently proposed [ J. Chem. Phys. 2006 , 125 , 054102 ], the method is first applied to the H + D(2) reaction as a benchmark, for which exact time-independent calculations are also performed. It is found that the use of reactant coordinates yields accurate results, with a computational effort slightly lower than that when using product coordinates. The H(+) + D(2) reaction, with the same masses but a much deeper insertion well, is also studied and exhibits a completely different mechanism, a complex-forming one which can be treated by statistical methods. Due to the longer range of the potential, product Jacobi coordinates are more efficient in this case. Differential cross sections for individual final rotational states of the products are obtained based on exact dynamical calculations for some selected total angular momenta, combined with the random phase approximation to save the high computational time required to calculate all partial waves with very long propagations. The results obtained are in excellent agreement with available exact time-independent calculations. Finally, the method is applied to the Li + HF system for which reactant coordinates are very well suited, and quantum differential cross sections are not available. The results are compared with recent quasiclassical simulations and experimental results [J. Chem. Phys. 2005, 122, 244304]. Furthermore, the polarization of the product angular momenta is also analyzed as a function of the scattering angle.

Chemical kinetics of neutral-neutral gas-phase reactions at ultralow temperatures is a fascinating research subject with important implications on the chemistry of complex organic molecules in the interstellar medium (T∼10-100K). Scarce kinetic information is currently available for this kind of reactions at T<200 K. In this work we use the CRESU (, which means Reaction Kinetics in a Uniform Supersonic Flow) technique to measure for the first time the rate coefficients () of the gas-phase OH+HCO reaction between 22 and 107 K. values greatly increase from 2.1×10 cm s at 107 K to 1.2×10 cm s at 22 K. This is also confirmed by quasi-classical trajectories (QCT) at collision energies down to 0.1 meV performed using a new full dimension and potential energy surface, recently developed which generates highly accurate potential and includes long range dipole-dipole interactions. QCT calculations indicate that at low temperatures HCO is the exclusive product for the OH+HCO reaction. In order to revisit the chemistry of HCO in cold dense clouds, is reasonably extrapolated from the experimental results at 10K (2.6×10 cm s). The modeled abundances of HCO are in agreement with the observations in cold dark clouds for an evolving time of 10-10 yrs. The different sources of production of HCO are presented and the uncertainties in the chemical networks discussed. This reaction can be expected to be a competitive process in the chemistry of prestellar cores. The present reaction is shown to account for a few percent of the total HCO production rate. Extensions to photodissociation regions and diffuse clouds environments are also commented.

Is the rise of the rate constant measured in laval expansion experiments of OH with organic molecules at low temperatures due to the reaction between reactants or to the formation of complexes with the buffer gas?. This question has importance for understanding the evolution of prebiotic molecules observed in different astrophysical objects. Among these molecules methanol is one of the most widely observed, and its reaction with OH has been measured by several groups showing a fast increase of the rate constant under 100K. Transition state theory doesn’t reproduce this behavior and here dynamical calculations are performed on a new full dimensional potential energy surface developed for this purpose. The calculated classical reactive cross sections show an increase at low collision energies due to a complex forming mechanism. However, the calculated rate constant at temperatures below 100 K remains lower than the observed one. Quantum effects are likely responsible for the measured behavior at low temperatures.

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+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 1.78x10(-10) cm3 s-1 at 1 K down to 5.96x10(-11) cm3 s-1 at 500 K with a maximum value of 3.39x10(-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.

Specific rate constants for the S + +H 2 reaction are calculated using the ground quartet state potential energy surface and quasi-classical trajectories method. The calculations are performed for H 2 in different vibrational states v = 0-4 and thermal conditions for rotational and translational energies. The calculations lead to slow rate constants for the H 2 vibrational levels v = 0, 1, but a significant enhancement of reactivity is observed when v > 1. The inverse reaction is also studied and rate constants for v = 0 are presented. For comparison, we also recompile previous results of state-to-state rate constants of the C + +H 2 for H 2 in rovibrational state v, j = (0,0), (1,0), (1,1), and (2,0). The calculated rate coefficients are fitted using an improved form of the standard three-parameter Arrhenius-like equation, which is found to be very accurate in fitting rate constants over a wide range of temperatures (10-4000 K). We investigate the impact of the calculated rate coefficients on the formation of SH + in the photon-dominated region Orion Bar and find an abundance enhancement of nearly three orders of magnitude when the reaction of S + with vibrationally excited H 2 is taken into account. The title reaction is thus one of the principal mechanisms in forming SH + in interstellar clouds.

Quasiclassical trajectory calculations have been carried out for the C((3)P)+OH(X (2)Pi)-->CO(X (1)Sigma(+))+H((2)S) reaction using a recent ab initio potential energy surface for the ground electronic state X (2)A(') 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.

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.