Quantum mechanical study of the 16O + 18O18O → 16O18O + 18O exchange reaction: Integral cross sections and rate constants.

The isotopic exchange reaction, 16O + 18O18O → 16O18O + 18O, involving excited ozone, O 3 * , as intermediate complex, was investigated by means of a time independent quantum mechanical approach using the TKTHS potential energy surface (PES) [V. G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013)] of ozone. State-to-state integral cross sections were calculated for collision energies lower than 0.4 eV. Then specific and thermal rate constants were computed between 10 K and 350 K using these cross sections. The full quantum thermal rate constant is found to be in better agreement with the most recent experimental data than with previous studies where other O3 PESs were employed, confirming therefore the higher accuracy of the TKTHS PES. However, the present theoretical thermal rate constant still remains below the measured rate, maybe due to the neglect of non-adiabtic couplings.

State-to-state chemistry and rotational excitation of CH+ in photon-dominated regions.

We present a detailed theoretical study of the rotational excitation of CH+ due to reactive and nonreactive collisions involving C+(2P), H2, CH+, H and free electrons. Specifically, the formation of CH+ proceeds through the reaction between C+(2P) and H2(νH2 = 1, 2), while the collisional (de)excitation and destruction of CH+ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000 K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH+ with H atoms at kinetic temperatures below 50 K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H2 has a substantial effect on the excitation of CH+ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH+ toward the Orion Bar and the planetary nebula NGC 7027. Our results further suggest that the population of νH2 = 2 might be significant in the photon-dominated region of NGC 7027.

Quantum dynamics of (16)O + (36)O2 and (18)O + (32)O2 exchange reactions.

We present quantum dynamical investigations of (16)O + (36)O2 and (18)O + (32)O2 exchange reactions using a time-independent quantum mechanical method and an accurate global potential energy surface of ozone [Dawes et al., J. Chem. Phys. 135, 081102 (2011)]. Initial state-selected integral cross sections, rate constants, and Boltzmann averaged thermal rate constants are obtained and compared with earlier experimental and theoretical results. The computed thermal rate constants for the oxygen exchange reactions exhibit a negative temperature dependence, as found experimentally. They are in better agreement with the experiments than the previous studies on the same reactions.

A comparative account of quantum dynamics of the H⁺ + H2 reaction at low temperature on two different potential energy surfaces.

Rotationally resolved reaction probabilities, integral cross sections, and rate constant for the H(+) + H2 (v = 0, j = 0 or 1) → H2 (v' = 0, j') + H(+) reaction are calculated using a time-independent quantum mechanical method and the potential energy surface of Kamisaka et al. [J. Chem. Phys. 116, 654 (2002)] (say KBNN PES). All partial wave contributions of the total angular momentum, J, are included to obtain converged cross sections at low collision energies and rate constants at low temperatures. In order to test the accuracy of the KBNN PES, the results obtained here are compared with those obtained in our earlier work [P. Honvault et al., Phys. Rev. Lett. 107, 023201 (2011)] using the accurate potential energy surface of Velilla et al. [J. Chem. Phys. 129, 084307 (2008)]. Integral cross sections and rate constants obtained on the two potential energy surfaces considered here show remarkable differences in terms of magnitude and dependence on collision energy (or temperature) which can be attributed to the differences observed in the topography of the surfaces near to the entrance channel. This clearly shows the inadequacy of the KBNN PES for calculations at low collision energies.

Time-dependent quantum wave packet dynamics of S + OH reaction on its electronic ground state.

Initial state-selected dynamics of the S((3)P) + OH (X(2)Π) → SO (X(3)Σ(-)) + H ((2)S) reaction on its electronic ground potential energy surface (X̃(2)A") is investigated here by a time-dependent wave packet propagation (TDWP) approach. Total reaction probabilities for the three-body rotational angular momentum up to J = 138 are calculated to obtain converged integral reaction cross sections and state-specific rate constants employing the centrifugal sudden (CS) approximation. The convergence of the latter quantities is checked by varying all parameters used in the numerical calculations. The cross section and rate constant results are compared with those available in the literature, calculated with the aid of the quasi-classical trajectory method on the same potential energy surface. Reaction probabilities obtained with the TDWP approach exhibit dense oscillatory structures, implying formation of a metastable quasi-bound complex during the collision process. The effect of rotational and vibrational excitations of reagent OH on the dynamical attributes is also examined. While the rotational excitation of reagent OH decreases the reactivity, its vibrational excitation enhances the same.

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.

State-to-state quantum mechanical calculations of rate coefficients for the D+ + H2 → HD + H+ reaction at low temperature.

The dynamics of the D(+) + H2 → HD + H(+) reaction on a recent ab initio potential energy surface (Velilla, L.; Lepetit, B.; Aguado, A.; Beswick, J. A.; Paniagua, M. J. Chem. Phys. 2008, 129, 084307) has been investigated by means of a time-independent quantum mechanical approach. Cross-sections and rate coefficients are calculated, respectively, for collision energies below 0.1 eV and temperatures up to 100 K for astrophysical application. An excellent accord is found for collision energy above 5 meV, while a disagreement between theory and experiment is observed below this energy. We show that the rate coefficients reveal a slightly temperature-dependent behavior in the upper part of the temperature range considered here. This is in agreement with the experimental data above 80 K, which give a temperature independent value. However, a significant decrease is found at temperatures below 20 K. This decrease can be related to quantum effects and the decay back to the reactant channel, which are not considered by simple statistical approaches, such as the Langevin model. Our results have been fitted to appropriate analytical expressions in order to be used in astrochemical and cosmological models.

Time-dependent quantum wave packet dynamics of the C + OH reaction on the excited electronic state.

Quantum state-selected dynamics of C((3)P) + OH (X(2)Π) → CO(a(3)Π) + H ((2)S) reaction on its first excited electronic potential energy surface (1(2)A(")) is examined here using a time-dependent wave packet propagation approach. All partial wave contributions for the total angular momentum, J = 0-95, are included to obtain the converged cross sections and initial state-selected rate constants in the temperature range of 10-500 K. The reaction probability, as a function of collision energy, exhibits dense oscillatory structures owing to the formation of resonances during collision. These resonance structures also persist in reaction cross sections. The effect of reagent rotational and vibrational excitation on the dynamical attributes is examined and discussed. Reagent rotational excitation decreases the reactivity whereas, vibrational excitation of the reagent has minor effects on the reactivity. The results presented here are in good accord with those obtained using the time-independent quantum mechanical and quasi-classical trajectory methods.

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.

Effects of the rotational excitation of D2 and of the potential energy surface on the H+ + D2 --> HD + D+ reaction.

The H(+) + D(2) --> HD + D(+) reaction has been theoretically investigated by means of an exact quantum mechanical approach, a quasiclassical trajectory method, and two statistical methods based in the propagation of either wave functions or trajectories. The study addresses the possible changes on the overall dynamics of the title reaction when the D(2) diatom is rotationally excited to its v = 0, j = 1 state. In addition, the reactivity for the ground rotational state on two different potential energy surfaces (PESs), namely, the surface by Aguado et al. [J. Chem. Phys. 112, 1240 (2000)] and the PES by Kamisaka et al. [J. Chem. Phys. 116, 654 (2002)], is examined. Reaction probabilities and cross sections at 0.524 and 0.1 eV collision energies are calculated. The major differences with respect to the reaction initiated with D(2) in its ground rovibrational state are observed for the lowest collision energy E(c) = 0.1 eV. Differential cross sections have been found to depend to some extend on the PES employed. In addition, at E(c) = 0.1 eV further discrepancies in the total and rotational cross sections are noticeable.

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.

Quantum mechanical and quasiclassical trajectory scattering calculations for the C(1D) + H2 reaction on the second excited 1 1A" potential energy surface.

Time-independent quantum mechanical (QM) and quasiclassical trajectory (QCT) scattering calculations have been carried out for the C(1D) + H2 --> CH + H reaction at a collision energy of 80 meV on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., Phys. Chem. Chem. Phys. 7, 1476 (2005)] of 1 1A" symmetry, corresponding to the second singlet state 1 1B1 of CH2. A general good agreement has been found between the QM and QCT rotational distributions and differential cross sections (DCSs). In both cases, DCSs are strongly peaked in the forward direction with a small contribution in the backward direction in contrast with those obtained on the 1 1A' surface, which are nearly symmetric. Rotational distributions obtained on the 1 1A" surface are somewhat colder than those calculated on the 1 1A' surface. The specific dynamics and the contribution of the 1 1A" surface to the overall reactivity of this system are discussed.