Quasiclassical trajectory scattering calculations for the OH+O→H+O2 reaction: Cross sections and rate constants

Influence of Singlet Oxygen in H + O2 Collision Reaction

The collision reaction of H + O2 = OH + O is a pivotal step in combustion. To investigate the influence of singlet oxygen on this reaction, we computed potential energy surfaces (PESs) for all six lowest states using high-level ab initio methods and coupled them with embedded atom neural network (EANN) fitting. By integrating quasi-classical trajectory (QCT) with trajectory surface hopping (TSH) based on the fitted PESs, we simulated the dynamics of both ground- and excited-states to derive the reaction rate constants for the forward and reverse processes. The results reveal that the forward reaction facilitates radical generation, promoting combustion reactions. Furthermore, calculations of reverse reaction rate constants indicate that all electronic states ultimately yield ground-state oxygen, leading to radical deactivation and exerting an inhibitory effect on combustion processes.

Combined Quantum Mechanical and Quasi-Classical State-to-State Dynamical Study on the Isotopic Effect in H/D + LiH+/LiD+ → H2/HD/D2 + Li+ Reactions

Coriolis-coupled quantum mechanical (QM-CC) and quasi-classical trajectory (QCT) calculations are carried out to investigate the dynamics of the H(D) + LiH+(v = 0, j = 0) → H2(HD) (v', j') + Li+ reactions on the ground electronic state potential energy surface reported by Martinazzo et al. (Martinazzo et al., J. Chem. Phys. 2003, 119, 11241). The QM-CC and QCT results at the initial state-selected and state-to-state levels are used to investigate the validity and accuracy of the QCT method for these exoergic barrierless reactions. Furthermore, the QCT method is used to understand the isotopic effects on reaction observables like total and state-to-state integral cross section, differential cross section, product energy disposal, and rate constants of H(D) + LiH+(v = 0, j = 0) → H2(HD) (v', j') + Li+ and H(D) + LiD+(v = 0, j = 0) → HD(D2) (v', j') + Li+ reactions. Attempts are also made to understand the impact of the isotopic substitution on the reaction mechanism. It is observed that QM-CC and QCT results closely follow each other at the initial state-selected and state-to-state levels. Noticeable kinematic effects of reagents on the reactivity and mechanism of the reactions are also observed.

Dynamical calculations of O(3P) + OH(2Π) reaction on the CHIPR potential energy surface using the fully coupled time-dependent wave-packet approach in hyperspherical coordinates

We have carried out quantum dynamics calculations for the O + OH → H + O₂ reaction on the CHIPR [A. J. C. Varandas, J. Chem. Phys., 2013, 138, 134117] potential energy surface (PES) for ground state HO₂ using the fully coupled 3D time-dependent wavepacket formalism [S. Adhikari and A. J. C. Varandas, Comput. Phys. Commun., 2013, 184, 270] in hyperspherical coordinates. Reaction probabilities for J > 0 are calculated for different initial rotational states of the OH radical (v = 0; j = 0, 1). State-to-state as well as total integral cross sections and rate-coefficients are evaluated and compared with previous theoretical calculations and available experimental studies. Using the rate constant for the forward (hereinafter considered to be H + O₂ → O + OH) and backward (O + OH → H + O₂) reactions of this reactive system, the equilibrium constant of the reversible process [H + O₂ ⇌ O + OH] is calculated as a function of temperature and compared with previous experimental measurements.

Molecular Dynamics of Combustion Reactions in Supercritical Carbon Dioxide. 6. Computational Kinetics of Reactions between Hydrogen Atom and Oxygen Molecule H + O2 ⇌ HO + O and H + O2 ⇌ HO2

Reactions of the hydrogen atom and the oxygen molecule are among the most important ones in the hydrogen and hydrocarbon oxidation mechanisms, including combustion in a supercritical CO₂ (sCO₂) environment, known as oxy-combustion or the Allam cycle. Development of these energy technologies requires understanding of chemical kinetics of H + O₂ ⇌ HO + O and H + O₂ ⇌ HO₂ in high pressures and concentrations of CO₂. Here, we combine quantum treatment of the reaction system by the transition state theory with classical molecular dynamics simulation and the multistate empirical valence bonding method to treat environmental effects. Potential of mean force in the sCO₂ solvent at various temperatures 1000-2000 K and pressures 100-400 atm was obtained. The reaction rate for H + O₂ ⇌ HO + O was found to be pressure-independent and described by the extended Arrhenius equation 4.23 × 10^-7 T^-0.73 exp(-21 855.2 cal/mol/RT) cm³/molecule/s, while the reaction rate H + O₂ ⇌ HO₂ is pressure-dependent and can be expressed as 5.22 × 10^-2 T^-2.86 exp(-7247.4 cal/mol/RT) cm³/molecule/s at 300 atm.

Time-Dependent Quantum Wave Packet Study of the Si + OH → SiO + H Reaction: Cross Sections and Rate Constants

The dynamics of the Si(³P) + OH(X²Π) → SiO(X¹Σ⁺) + H(²S) reaction is investigated by means of the time-dependent wave packet (TDWP) approach using an ab initio potential energy surface recently developed by Dayou et al. ( J. Chem. Phys. 2013 , 139 , 204305 ) for the ground X²A' electronic state. Total reaction probabilities have been calculated for the first 15 rotational states j = 0-14 of OH(v=0,j) at a total angular momentum J = 0 up to a collision energy of 1 eV. Integral cross sections and state-selected rate constants for the temperature range 10-500 K were obtained within the J-shifting approximation. The reaction probabilities display highly oscillatory structures indicating the contribution of long-lived quasibound states supported by the deep SiOH/HSiO wells. The cross sections behave with collision energies as expected for a barrierless reaction and are slightly sensitive to the initial rotational excitation of OH. The thermal rate constants show a marked temperature dependence below 200 K with a maximum value around 15 K. The TDWP results globally agree with the results of earlier quasi-classical trajectory (QCT) calculations carried out by Rivero-Santamaria et al. ( Chem. Phys. Lett. 2014 , 610-611 , 335 - 340 ) with the same potential energy surface. In particular, the thermal rate constants display a similar temperature dependence, with TDWP values smaller than the QCT ones over the whole temperature range.

Quantum dynamics study on the CHIPR potential energy surface for the hydroperoxyl radical: the reactions O + OH⇋O2 + H

Quantum scattering calculations of the O((3)P)+OH((2)Π)⇌O2((3)Σg (-))+H((2)S) reactions are presented using the combined-hyperbolic-inverse-power-representation potential energy surface [A. J. C. Varandas, J. Chem. Phys. 138, 134117 (2013)], which employs a realistic, ab initio-based, description of both the valence and long-range interactions. The calculations have been performed with the ABC time-independent quantum reactive scattering computer program based on hyperspherical coordinates. The reactivity of both arrangements has been investigated, with particular attention paid to the effects of vibrational excitation. By using the J-shifting approximation, rate constants are also reported for both the title reactions.

Vibrational excitation influence on the H + BrO → HBr + O reaction: a quasi-classical trajectory investigation

Quasi-classical trajectory calculations are employed to investigate the vibrational excitation effect on the scalar and vector properties of the H + BrO → HBr + O reaction using a X1A′ state ab initio potential energy surface (J. Chem. Phys. 2000, 113, 4598). The reaction probability, cross section, and rate constant are carried out with the effect of the collision energy (Ecol = 0.1–6 kcal/mol) and vibrational levels (v = 0–3). A significant vibrational dependency has been observed in the reaction probability and cross section at a relatively low collision energy area and has also been found in a low-temperature (T < 150 K) region of the rate constant. In addition, two product angular distributions, P(θr) and P(ϕr), and two generalized polarization-dependent differential cross sections, PDDCS00 and PDDCS20, are calculated as well. All of these scalar and vector properties have shown sensitive behaviors to the vibrational levels.

Chemical reaction versus vibrational quenching in low energy collisions of vibrationally excited OH with O

Quantum scattering calculations are reported for state-to-state vibrational relaxation and reactive scattering in O + OH(v = 2 - 3, j = 0) collisions on the electronically adiabatic ground state (2)A'' potential energy surface of the HO2 molecule. The time-independent Schrödinger equation in hyperspherical coordinates is solved to determine energy dependent probabilities and cross sections over collision energies ranging from ultracold to 0.35 eV and for total angular momentum quantum number J = 0. A J-shifting approximation is then used to compute initial state selected reactive rate coefficients in the temperature range T = 1 - 400 K. Results are found to be in reasonable agreement with available quasiclassical trajectory calculations. Results indicate that rate coefficients for O2 formation increase with increasing the OH vibrational level except at low and ultralow temperatures where OH(v = 0) exhibits a slightly different trend. It is found that vibrational relaxation of OH in v = 2 and v = 3 vibrational levels is dominated by a multi-quantum process.

A QUASICLASSICAL TRAJECTORY STUDY OF REACTIVE SCATTERING ON AN ANALYTICAL POTENTIAL ENERGY SURFACE FOR GeH2 SYSTEM

A quasiclassical trajectory method was employed to study reaction Ge+H 2 (v=0, j=0) and reverse reaction H+GeH (v=0, j=0) on an analytical potential energy surface obtained from simplified many-body expansion method with fitting to B3P86/CC-pVTZ calculations around a global minimum and a long-range van de Waals well plus spectroscopy data for diatomic molecules GeH and H2 . Reaction probabilities from both reaction and reverse reaction were calculated. Dominant reaction is complex-forming reaction Ge+H2 (v=0, j=0) → GeH2 , and its cross section is 10 times bigger than that of complex-forming reaction from the reverse reaction. There is no threshold effect for complex-forming reaction and the cross sections for both complex-forming reactions decrease with the increase of collision energy. Life time of complex is shown to be decreasing with increase of collision energy. Dominant reverse reaction is reaction H + GeH (v=0,j=0) → Ge+H2 ; the reaction probability decreases with the increase of collision energy and differential cross section shows that this reverse reaction has almost equal angular distribution at low collision energy and mostly forward scattering at high collision energy.

THEORETICAL STUDY OF STEREODYNAMICS AND ISOTOPIC EFFECTS FOR THE REACTION C(3P) + OD(X2Π) → CO(X1Σ+) + D(2S)

Theoretical study on stereodynamics for the title reaction as well as its isotopic effects has been studied via QCT calculations on the ground X2A′ state of ab initio potential energy surface according to the study by Zanchet et al. Four polarization-dependent generalized differential cross-sections PDDCSs ((2π/σ) (dσ00/dωt), (2π/σ)(dσ20/dωt)), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt), and the distributions of P(θr) and P(φr) that denotes the correlations of k-j′ and k-k′-j′ are presented in this work. Product angular distribution and rotational polarization have been analyzed at different collision energies and compared with C+OH reaction. Product angular distribution shows strong forward scattering at low collision energy and becomes more symmetric with forward and backward scattering with the increasing collision energy. The alignment and orientation of product angular momentum presents a different behavior with collision energy, the former one increases monotonically with collision energy, whereas the latter one shows first decreasing and then increasing behavior, which have been analyzed in the present paper. Product rotational polarization for C+OD is weaker than that for C+OH , which is mainly due to the mass factor and zero point energy of C+OD .

Study of the C(3P) + OH(X2Pi) --> CO(a3Pi) + H(2S) reaction: fully global ab initio potential energy surfaces of the 12A'' and 14A'' excited states and non adiabatic couplings

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.