Quantum dynamical study of the O((1)D) + CH4 → CH3 + OH atmospheric reaction.

Time independent quantum mechanical (TIQM) scattering calculations have been carried out for the O((1)D) + CH4(X(1)A1) → CH3(X(2)A2″) + OH(X(2)Π) atmospheric reaction, using an ab initio ground potential energy surface where the CH3 group is described as a pseudo-atom. 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. The vibrational and rotational state OH product distributions show no specific behavior. The rate coefficient has been calculated by means of the J-shifting approach in the 10-500 K temperature range and slightly depends on T at ordinary temperatures (as expected for a barrierless reaction). Quantum effects do not influence the vibrational populations and rate coefficient in an important way, and a rather good agreement has been found between the TIQM results and the quasiclassical trajectory and experimental ones. This reinforces somewhat the reliability of the pseudo-triatomic approach under the reaction conditions explored.

Quasiclassical trajectory and statistical quantum calculations for the C + OH → CO + H reaction on the first excited 1(2)A″ potential energy surface.

We report quasiclassical trajectory dynamical calculations for the C((3)P) + OH(X(2)Π) → CO(a(3)Π) + H((2)S) using a recently developed ab initio potential energy surface for the first electronic state of HCO of 1(2)A″ symmetry. The dependence of integral cross sections on the collision energy was 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.

H2, H3+ and the age of molecular clouds and prestellar cores.

Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H(3)(+) into H(2)D(+) to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H(2) that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H(2) phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO(+) and N(2)D(+) argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H(2) remains comparable to para-H(2) in abundance outside the dense cores.

An accurate study of the dynamics of the C+OH reaction on the second excited 1(4)A" potential energy surface.

The dynamics of the C((3)P)+OH(X(2)Π) → CO(a(3)Π)+H((2)S) on its second excited potential energy surface, 1(4)A", have been investigated in detail by means of an accurate quantum mechanical (QM) time-dependent wave packet (TDWP) approach. Reaction probabilities for values of the total angular momentum J up to 50 are calculated and integral cross sections for a collision energy range which extends up to 0.1 eV are shown. The comparison with quasi-classical trajectory (QCT) and statistical methods reveals the important role played by the double well structure existing in the potential energy surface. The TDWP differential cross sections exhibit a forward-backward symmetry which could be interpreted as indicative of a complex-forming mechanism governing the dynamics of the process. The QM statistical method employed in this study, however, is not capable to reproduce the main features of the possible insertion nature in the reactive collision. The ability to stop individual trajectories selectively at specific locations inside the potential energy surface makes the QCT version of the statistical approach a better option to understand the overall dynamics of the process.

Quantum mechanical study of the proton exchange in the ortho-para H2 conversion reaction at low temperature.

Ortho-para H(2) conversion reactions mediated by the exchange of a H(+) proton have been investigated at very low energy for the first time by means of a time independent quantum mechanical (TIQM) approach. State-to-state probabilities and cross sections for H(+) + H(2) (v = 0, j = 0,1) processes have been calculated for a collision energy, E(c), ranging between 10(-6) eV and 0.1 eV. Differential cross sections (DCSs) for H(+) + H(2) (v = 0, j = 1) → H(+) + H(2) (v' = 0, j' = 0) for very low energies only start to develop a proper global minimum around the sideways scattering direction (θ≈ 90°) at E(c) = 10(-3) eV. Rate coefficients, a crucial information required for astrophysical models, are provided between 10 K and 100 K. The relaxation ortho-para process j = 1 → j' = 0 is found to be more efficient than the j = 0 → j' = 1 conversion at low temperatures, in line with the extremely small ratio between the ortho and para species of molecular hydrogen predicted at the temperature of interstellar cold molecular clouds. The results obtained by means of a statistical quantum mechanical (SQM) model, which has previously proved to provide an adequate description of the dynamics of the title reactions at a higher collision energy regime, have been compared with the TIQM results. A reasonable good agreement has been found with the only exception of the DCSs for the H(+) + H(2) (v = 0, j = 1) → H(+) + H(2) (v' = 0, j' = 0) process at very low energy. SQM cross sections are also slightly below the quantum results. Estimates for the rate coefficients, in good accord with the TIQM values, are a clear improvement with respect to pioneering statistical studies on the reaction.

Ortho-para H2 conversion by proton exchange at low temperature: an accurate quantum mechanical study.

We report extensive, accurate fully quantum, time-independent calculations of cross sections at low collision energies, and rate coefficients at low temperatures for the H⁺ + H2(v = 0, j) → H⁺ + H2(v = 0, j') reaction. Different transitions are considered, especially the ortho-para conversion (j = 1 → j' = 0) which is of key importance in astrophysics. This conversion process appears to be very efficient and dominant at low temperature, with a rate coefficient of 4.15 × 10⁻¹° cm³ molecule⁻¹ s⁻¹ at 10 K. The quantum mechanical results are also compared with statistical quantum predictions and the reaction is found to be statistical in the low temperature regime (T < 100 K).

State-to-state quantum dynamics calculations of the C + OH reaction on the second excited potential energy surface.

Accurate three-dimensional quantum-mechanical scattering calculations using a time-indepedent hyperspherical method have been performed for the C((3)P) + OH(X(2)Π) → CO(a(3)Π) + H((2)S) reaction on the second excited potential energy surface of 1(4)A″ symmetry. State-to-state reaction probabilities at a total angular momentum J = 0 have been computed in a wide range of collision energies. Many pronounced resonances have been found, espcially at low energy. The product vibrational distributions are noninverted. The present results therefore suggest that the title reaction proceeds via a long-lived intermediate complex. An approximate quantum-mechanical rate constant has also been calculated, and large differences are observed with the quasi-classical trajectory prediction.

Quantum dynamics at the state-to-state level of the C + OH reaction on the first excited potential energy surface.

Total and state-to-state reaction probabilities for the C((3)P) + OH(X(2)Pi) --> CO(a(3)Pi) + H((2)S) reaction on the first excited potential energy surface of 1(2)A'' symmetry have been calculated using an accurate time-independent quantum-mechanical method at a total angular momentum J = 0. The total reaction probability presents a dense resonance structure that was not observed on the ground potential energy surface. The vibrational distributions appear flat or inverted, depending on the collision energy. The rotational distributions show no specific behavior. The rate constant calculated in the J-shifting approach is in good agreement with a previous theoretical result obtained using a quasi-classical trajectory method.

O+OH-->O(2)+H: A key reaction for interstellar chemistry. New theoretical results and comparison with experiment.

We report extensive, fully quantum, time-independent (TID) calculations of cross sections at low collision energies and rate constants at low temperatures for the O+OH reaction, of key importance in the production of molecular oxygen in cold, dark, interstellar clouds and in the chemistry of the Earth's atmosphere. Our calculations are compared with TID calculations within the J-shifting approximation, with wave-packet calculations, and with quasiclassical trajectory calculations. The fully quantum TID calculations yield rate constants higher than those from the more approximate methods and are qualitatively consistent with a low-temperature extrapolation of earlier experimental values but not with the most recent experiments at the lowest temperatures.

State-to-state quantum reactive scattering calculations and rate constant for nitrogen atoms in collision with NO radicals at low temperatures.

Total and state-to-state probabilities have been determined for the N + NO --> N(2) + O reaction for collision energies up to 0.6 eV using a time-independent quantum mechanical method. The probabilities as a function of collision energy show broad oscillations, in strong contrast with previous theoretical results obtained by means of a time-dependent wave packet method that show a dense resonance structure. The rate constant has been calculated in the J-shifting approach for temperatures between 10 and 400 K. It is in good agreement with previous theoretical results obtained only at 100 K and above 200 K and experiments in a wide temperature range.

State-to-state quantum dynamical study of the N + OH --> NO + H reaction.

We have studied the quantum dynamics of the N + OH --> NO + H reaction for collision energies up to 0.7 eV. The hyperspherical method has been used in a time-independent formalism. State-to-state reaction probabilities for a total angular momentum J = 0 have been computed. The results show a high reactivity below 0.45 eV and a very small one above this collision energy. Rotational and vibrational product distributions are presented for three collision energies (0.05, 0.1, and 0.5 eV). The vibrational distributions are found to be noninverted at 0.1 eV and inverted peaking at other energies. Rotational distributions are rather hot even if some low rotational states are strongly populated. These features are consistent with both direct and indirect reaction mechanisms.