State-to-state reactive scattering in six dimensions using reactant-product decoupling: OH + H2 → H2O + H (J = 0)

We extend to full dimensionality a recently developed wave packet method [M. T. Cvitas? and S. C. Althorpe, J. Phys. Chem. A 113, 4557 (2009)] for computing the state-to-state quantum dynamics of AB + CD → ABC + D reactions and also increase the computational efficiency of the method. This is done by introducing a new set of product coordinates, by applying the Crank-Nicholson approximation to the angular kinetic energy part of the split-operator propagator and by using a symmetry-adapted basis-to-grid transformation to evaluate integrals over the potential energy surface. The newly extended method is tested on the benchmark OH + H(2) → H(2)O + H reaction, where it allows us to obtain accurately converged state-to-state reaction probabilities (on the Wu-Schatz-Fang-Lendvay-Harding potential energy surface) with modest computational effort. These methodological advances will make possible efficient calculations of state-to-state differential cross sections on this system in the near future.     

Acceleration of the reaction OH + CO → H + CO2 by vibrational excitation of OH

The collision complex formed from a vibrationally excited reactant undergoes redissociation to the reactant, intramolecular vibrational relaxation (randomization of vibrational energy), or chemical reaction to the products. If attractive interaction between the reactants is large, efficient vibrational relaxation in the complex prevents redissociation to the reactants with the initial vibrational energy, and the complex decomposes to the reactants with low vibrational energy or converts to the products. In this paper, we have studied the branching ratios between the intramolecular vibrational relaxation and chemical reaction of an adduct HO(v)-CO formed from OH(X(2)Π(i)) in different vibrational levels v = 0-4 and CO. OH(v = 0-4) generated in a gaseous mixture of O(3)/H(2)/CO/He irradiated at 266 nm was detected with laser-induced fluorescence (LIF) via the A(2)Σ(+)-X(2)Π(i) transition, and H atoms were probed by the two-photon excited LIF technique. From the kinetic analysis of the time-resolved LIF intensities of OH(v) and H, we have found that the intramolecular vibrational relaxation is mainly governed by a single quantum change, HO(v)-CO → HO(v-1)-CO, followed by redissociation to OH(v-1) and CO. With the vibrational quantum number v, chemical process from the adduct to H + CO(2) is accelerated, and vibrational relaxation is decelerated. The countertrend is elucidated by the competition between chemical reaction and vibrational relaxation in the adduct HOCO.     

Time-dependent wave packet theory for state-to-state differential cross sections of four-atom reactions in full dimensions: application to the HD + OH → H2O + D reaction

Time-dependent wave packet method has been developed to calculate differential cross section for four-atom reactions in full dimension, utilizing an improved version of reactant-product-decoupling scheme. Differential cross sections for the title reaction were calculated for collision energy up to 0.4 eV. It is found that the differential cross sections for the reaction are all peaked in the backward direction. The majority of H(2)O is produced in the first stretch excited state, with a large fraction of available energy for the reaction going into H(2)O internal motion. As compared in a previous report by Xiao et al. [Science 333, 440 (2011)], the differential cross section at E(c) = 0.3 eV and the differential cross section at the backward direction as a function of collision energy agree with experiment very well, indicating it is possible now to calculate complete dynamical information for some simple four-atom reactions, as have been done for three-atom reactions in the past decades.     

Quantum dynamics of the S+OH→SO+H reaction

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.     

Nonadiabatic quantum dynamics in O(3P)+H2→OH+H: a revisited study

To investigate the extent of nonadiabatic effects in the title reaction, quasi-classical trajectory and nonadiabatic quantum scattering as well as the nonadiabatic quantum-classical trajectory calculations were performed on the accurate ab initio benchmark potential energy surfaces of the lowest (3)A' and (3)A" electronic states [Rogers et al., J Phys Chem A 2000, 104, 2308], together with the spin-orbit coupling matrix [Maiti and Schatz, J Chem Phys 2003, 119, 12360] and the lowest singlet (1) A' potential energy surface [Dobby and Knowles, Faraday Discuss 1998, 110, 247]. Comparison of the calculated total cross sections from both adiabatic and nonadiabatic calculations has demonstrated that for adiabatic channels including (3)A'→(3)A' and (3)A"→(3)A", difference does exist between the two kinds of adiabatic and nonadiabatic calculations, showing nonadiabatic effects to some extent. Such nonadiabatic effects tend to become more conspicuous at high collision energies and are found to be more pronounced with trajectories/quantum wave packet initiated on (3)A' than on (3)A". Furthermore, the present study also showed that nonadiabatic effects can bring the component of forward-scattering in the product angular distributions.     

State-to-state quantum dynamics of the N(4S) + OH(X2Π) → H(2S) + NO(X 2Π) reaction

Quantum state-to-state dynamics for the N((4)S) + OH(X(?2)Π) → H((2)S) + NO(X(?2)Π) reaction is reported on an accurate ab initio potential energy surface of the lowest triplet electronic state (a(3)A(")) of HNO∕HON. It was found that the reaction is dominated by long-lived resonances supported by the HNO and HON wells. Significant non-reactive scattering was observed, indicating substantial deviations from the statistical limit. Due to the large exothermicity of the reaction, the NO product has hot internal state distributions: its rotational state distribution is inverted and peaks near the highest accessible rotational level; and its vibrational state distribution extends to υ = 10 and decays monotonically with the vibrational quantum number. In particular, the predicted product vibrational distribution is in reasonably good agreement with experiment. The calculated differential cross section is dominated by scattering in both the forward and backward directions, consistent with the formation of reaction intermediates.     

Rotationally correlated reactivity in the CH (v = 0, J, F(i)) + O2 → OH (A) + CO reaction

The rotational-state-selected CH (v = 0, J, F(i)) beam has been prepared by using an electric hexapole and applied to the crossed beam reaction of CH (v = 0, J, F(i)) + O(2) → OH (A) + CO at different O(2) beam conditions. The rotational state selected reactive cross sections of CH (RSSRCS-CH) turn out to depend remarkably on the rotational state distribution of O(2) molecules at a collision energy of ～?0.19 eV. The reactivity of CH molecules in the N = 1 rotational states (namely ∣J = 1∕2, F(2)> and ∣J = 3∕2, F(1)> states, N designates the angular momentum excluding spin) becomes strongly enhanced upon a lowering of the rotational temperature of the O(2) beam. The RSSRCS-CH in these two rotational states correlate linearly with the population of O(2) molecule in the specific K(O(2)) frame rotation number states: CH(|J = 1/2,F(2)>) with O(2)(|K(O(2)) = 1>);CH(|J = 3/2,F(1)>) with O(2)(|K(O(2)) = 3>). These linear correlations mean that the rotational-state-selected CH molecules are selectively reactive upon the incoming O(2) molecules in a specific rotational state; here, we use the term "rotationally correlated reactivity" to such specific reactivity depending on the combination of the rotational states between two molecular reactants. In addition, the steric asymmetry in the oriented CH (∣J = 1∕2,?F(2),?M = 1∕2>) + O(2) (|K(O(2)) = 1>) reaction turns out to be negligible (< ±1%). This observation supports the reaction mechanism as theoretically predicted by Huang et al. [J. Phys. Chem. A 106, 5490 (2002)] that the first step is an intermediate formation with no energy barrier in which C-atom of CH molecule attacks on one O-atom of O(2) molecule at a sideways configuration.     

An ab initio-based global potential energy surface for the SH3 system and full-dimensional state-to-state quantum dynamics study for the H2 + HS → H2S + H reaction

An accurate potential energy surface for the ground electronic state of SH₃ system has been constructed with 41,882 high level ab initio energy points and the neural network fitting method. The time-dependent wave packet method has been used to calculate the first state-to-state differential cross sections for the title reaction up to 1.2 eV in full dimensions, based on the reactant–product decoupling scheme. It is found that the majority of H₂S are produced in the ground vibrational state, with a large fraction of available energy for the reaction ending up as product translational motion. The differential cross sections at the threshold energy are dominated by a very narrow peak in the backward direction. With the increase of collision energy, the width of the angular distribution increases considerably, which is a typical feature of a direct reaction via abstract mechanism, similar to the H₂ + OH → H₂O + H reaction. © 2018 Wiley Periodicals, Inc.     

Abstraction versus insertion in the reaction O(1D2)+H2→OH(υ″ = 2) + H

The product hydroxyl radical arising from the reaction O(¹D₂)+H₂→OH+H was detected by LIF following excitation of the off-diagonal transition OH(A²Σ⁺, υ′=1←X²Π, υ″=2) in the region 348–357 nm. The rotational population distribution in υ″=2 appears to be inverted and quite similar to that previously reported for υ″=0 and 1. Because rotationally cool OH was not observed, there is no evidence for the existence of an abstractive pathway in which the subject reaction occurs without the initial formation, via insertion, of a chemically activated HOH collision complex.     

Communication: a chemically accurate global potential energy surface for the HO + CO → H + CO2 reaction

We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ~35,000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data.     

Dynamic aspects of the reaction O(1D) + H2S → OH + SH

The energy distribution of nascent OH(²Π, υ, J) produced in the reaction of O(¹D) with H₂S has been measured by laser-induced fluorescence. The rotational distributions in υ″ = 0 and υ″ = 1 are Boltzmannian with temperature parameters T_r″-0 = 2300 ± 100 K and T_r⁻-1 = 2650 ± 150 K. A population ratio N(υ″ = 1)/N(υ″ = 0) = 0.17 is observed. The product-state distribution over the different spin and A components is statistically within the experimental uncertainty of 20%. A comparison of the OH product populations from the title reaction with the well known OH yield from the O(¹D)+H₂O reaction shows that 25% of the reactive encounters follow the reaction channel which produces OH in υ″ = 0 and υ″ = 1.     

Influence of collision energy on the dynamics of the reaction H (2S) + NH (X3Σ−) → N (4S) + H2 (X1Σ g + ) by the state-to-state quantum mechanical study

The conversion of cholesterol to pregnenolone is a physiologically essential process which initiates with two sequential hydroxylation processes catalyzed by cytochrome P450 side-chain cleavage enzyme (P450_SCC). Extensive efforts have been exerted; however, the mechanistic details remain obscure. In this work, we employed the dispersion-corrected density functional theoretical (DFT-D) calculations to investigate the mechanistic details of such hydroxylation processes. Calculated results reveal that the active intermediate Compound I (CpdI) of P450_SCC hydroxylates cholesterol efficiently, which coincides with previous spectrometric observations. The hydrogen bond effect of water molecule within the active site lowers the energy barrier significantly. Intriguingly, the adjacent hydrogen bond (H-bond) between the hydroxyl group of the substrate and the oxo group of CpdI in the second hydroxylation affects the H-abstraction significantly. Such H-bond was weakened during the C–H bond activation process, increasing the energy barriers by approximately 2 kcal/mol, which is different to the intermolecular H-bond effect of water₉₀₃ found by Shaik et al. that decreases the barrier by about 4 kcal/mol. Such adjacent H-bond also affects the transition state by bending the alignment of the C–H–O moiety, and consequently lowering the kinetic isotope effect values. Besides, a series of DFT-D calculations (Grimme’s D2, D3-zero, and D3-BJ methods) were performed and accessed to find out an appropriate protocol for H-bond containing hydroxylation process. Our results show that DFT-D single-point energies (SPE) based on geometries optimized with non-dispersion-corrected DFT varies drastically and sometime presents unreasonable results. DFT-D SPE calculations on DFT-D optimized geometries present stable and reasonable results.     

Quasiclassical trajectory study of the reaction O(3P) + HI → OH + I

Three-dimensional quasiclassical trajectory calculations were carried out for the reaction of oxygen atoms O(³P) with hydrogen iodide molecules (HI and DI) for the temperature range 200–550 K, using a LEPS potential-energy surface. The calculated results include reaction cross sections, rate constants, kinetic isotope effects, the influence of vibrational and rotational excitation of the reactants on the dynamics, and the product energy partitioning and angular distribution. The calculated results are in good agreement with the available experimental results. The dynamics of the O + HI reaction is discussed in view of the associated mass combination H + LH′ (H and H′ are heavy atoms and L is a light atom), and in relation to earlier trajectory results for the reactions O + HCl and O + HBr.     

Quasi-classical trajectory study of the S + OH → SO + H reaction: from reaction probability to thermal rate constant

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.     

Comment on the possible role of the reaction H+H2O→H2+OH in the radiolysis of water at high temperatures

In a recent paper (Radiation Physics and Chemistry, 2005, vol. 74, pp. 210) it was suggested that the anomalous increase of molecular hydrogen radiolysis yields observed in high-temperature water is explained by a high activation energy for the reaction H+H₂O→H₂+OH. In this comment we present thermodynamic arguments to demonstrate that this reaction cannot be as fast as suggested. A best estimate for the rate constant is 2.2×10³ M⁻¹ s⁻¹ at 300 °C. Central to this argument is an estimate of the OH radical hydration free energy vs. temperature, ΔG_hyd(OH)=0.0278t−18.4 kJ/mole (t in °C, equidensity standard states), which is based on analogy with the hydration free energy of water and of hydrogen peroxide.     

Ab initio study on the dynamical properties of the hydrogen abstraction reaction NH2 + OH → NH + H2O

 The geometry of the transition state of the title reaction was optimized at the unrestricted Hartree–Fock, the spin-unrestricted second-order M?ller–Plesset, and the spin-unrestricted quadratic configuration interaction with all single and double substitutions levels of theory. The changes in the geometry, the bound vibrational modes, and the potential energy along the minimum energy path are discussed. Variational transition-state theory rate constants calculated with the tunneling and curvature effect correction agree very well with the experimental values. Received: 23 April 1999 / Accepted: 9 June 1999 / Published online: 15 December 1999     

A quantum scattering study of collinear state-to-state reaction probabilities for the F+H_2(ν)→-HF(v'''')+H system

A new quantum scattering approach (linear combination of arrangement channels-scattering wavefunction,LCAC-SW) proposed by Deng and his co-workers is used to calculate collinear state-to-state reaction probabilities for the F + H-2(v)→HF(v')+H system.Several interesting problems such as threshold energy,compound states and enhance by translational energy of the reactants and the vibration excitation of products are discussed and they are compared with other theoretical investigations reported in the literature.It is shown that the LCAC-SW approach is the successful one of quantum scattering methods.     

Ab initio potential energy surface and quantum dynamics for the H + CH4 → H2 + CH3 reaction

A new full-dimensional potential energy surface for the title reaction has been constructed using the modified Shepard interpolation scheme. Energies and derivatives were calculated using the UCCSD(T) method with aug-cc-pVTZ and 6-311++G(3df,2pd) basis sets, respectively. A total number of 30,000 data points were selected from a huge number of molecular configurations sampled by trajectory method. Quantum dynamical calculations showed that the potential energy surface is well converged for the number of data points for collision energy up to 2.5 eV. Total reaction probabilities and integral cross sections were calculated on the present surface, as well as on the ZBB3 and EG-2008 surfaces for the title reaction. Satisfactory agreements were achieved between the present and the ZBB3 potential energy surfaces, indicating we are approaching the final stage to obtain a global potential energy surface of quantitative accuracy for this benchmark polyatomic system. Our calculations also showed that the EG-2008 surface is less accurate than the present and ZBB3 surfaces, particularly in high energy region.