Quantum calculations of the O(3P)+H2-->OH+H reaction

Quantum scattering calculations are reported for the O(3P)+H2(v=0,1) reaction using chemically accurate potential energy surfaces of 3A' and 3A" symmetry. We present state-to-state reaction cross sections and rate coefficients as well as thermal rate coefficients for the title reaction using accurate quantum calculations. Our calculations yield reaction cross sections that are in quantitative accord with results of recent crossed molecular beam experiments. Comparisons with results obtained using the J-shifting calculations show that the J-shifting approximation is quite reliable for this system. Thermal rate coefficients from the exact calculations and the J-shifting approximation agree remarkably well with experimental results. Our calculations also reproduce the markedly different OH(v'=0)/OH(v'=1) branching in O(3P)+H2(v=1) reaction, observed in experiments that use different O(3P) atom sources. In particular, we show that the branching ratio is a strong function of the kinetic energy of the O(3P) atom.     

A time-dependent wave packet quantum scattering study of the reaction HD+ (v = 0 - 3;j0 = 1) + He --> HeH+(HeD+) + D(H)

Time-dependent wave packet quantum scattering (TWQS) calculations are presented for HD(+) (v = 0 - 3;j(0)=1) + He collisions in the center-of-mass collision energy (E(T)) range of 0.0-2.0 eV. The present TWQS approach accounts for Coriolis coupling and uses the ab initio potential energy surface of Palmieri et al. [Mol. Phys. 98, 1839 (2000)]. For a fixed total angular momentum J, the energy dependence of reaction probabilities exhibits quantum resonance structure. The resonances are more pronounced for low J values and for the HeH(+) + D channel than for the HeD(+) + H channel and are particularly prominent near threshold. The quantum effects are no longer discernable in the integral cross sections, which compare closely to quasiclassical trajectory calculations conducted on the same potential energy surface. The integral cross sections also compare well to recent state-selected experimental values over the same reactant and translational energy range. Classical impulsive dynamics and steric arguments can account for the significant isotope effect in favor of the deuteron transfer channel observed for HD(+)(v<3) and low translational energies. At higher reactant energies, angular momentum constraints favor the proton-transfer channel, and isotopic differences in the integral cross sections are no longer significant. The integral cross sections as well as the J dependence of partial cross sections exhibit a significant alignment effect in favor of collisions with the HD(+) rotational angular momentum vector perpendicular to the Jacobi R coordinate. This effect is most pronounced for the proton-transfer channel at low vibrational and translational energies.     

Planar study of H2O+Cl<-->HO+HCl reactions

The first four dimensional (4D) quantum scattering calculations on the tetra-atomic H2O+Cl<-->HO+HCl reactions are reported. With respect to a full (6D) treatment, only the planar constraint and a fixed length for the HO spectator bond are imposed. This work explicitly accounts for the bending and local HO stretching vibrations in H2O, for the vibration of HCl and for the in-plane rotation of the H2O, HO and HCl molecules. The calculations are performed with the potential energy surface of Clary et al. and use a Born-Oppenheimer type separation between the motions of the light and the heavy nuclei. State-to-state cross sections are reported for a collision energy range 0-1.8 eV measured with respect to H2O+Cl. For the H2O+Cl reaction, present results agree with previous (3D) non planar calculations and confirm that excitation of the H2O stretching promotes more reactivity than excitation of the bending. New results are related to the rotation of the H2O molecule: the cross sections are maximal for planar rotational states corresponding to 10相似文献   

Quantum and classical studies of the O(3P) + H2(v = 0-3,j = 0) --> OH + H reaction using benchmark potential surfaces

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P) + H2(v = 0-3,j = 0) --> OH + H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v = 0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v = 0 to 6 for v = 1, 5 for v = 2 and 4 for v = 3, suggesting a much slower increase in rate constant with vibrational excitation above v = 1 than below. For H2(v > 0), the classical results are larger than the quantum results by a factor approximately 2 near threshold, but the agreement monotonically improves until they are within approximately 10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v',j') state-resolved cross sections and angular distributions. The QCT state-resolved OH(v') cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.     

Quasiclassical trajectory study of the CH3++HD-->CH2D++H2 Reaction

A full dimensional ab initio potential energy surface for the CH5+ system based on coupled cluster electronic structure calculations and capable of describing the dissociation of methonium ion into methyl cation and molecular hydrogen (J. Phys. Chem. A 2006, 110, 1569) is used in quasiclassical trajectory calculations of the reaction CH3++HD-->CH2D++H2 for low collision energies of relevance to astrochemistry. Cross sections for the exchange are obtained at several relative translational energies and a fit to the energy dependence of the cross sections is used to obtain the rate constant at temperatures between 10 and 50 K. The calculated rate constant at 10 K agrees well with the previously reported experimental value. Internal energy distributions of the products are presented and discussed in the context of zero-point energy "noncompliance".     

Study of the H+O2 reaction by means of quantum mechanical and statistical approaches: the dynamics on two different potential energy surfaces

The possible existence of a complex-forming pathway for the H+O(2) reaction has been investigated by means of both quantum mechanical and statistical techniques. Reaction probabilities, integral cross sections, and differential cross sections have been obtained with a statistical quantum method and the mean potential phase space theory. The statistical predictions are compared to exact results calculated by means of time dependent wave packet methods and a previously reported time independent exact quantum mechanical approach using the double many-body expansion (DMBE IV) potential energy surface (PES) [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)] and the recently developed surface (denoted XXZLG) by Xu et al. [J. Chem. Phys. 122, 244305 (2005)]. The statistical approaches are found to reproduce only some of the exact total reaction probabilities for low total angular momenta obtained with the DMBE IV PES and some of the cross sections calculated at energy values close to the reaction threshold for the XXZLG surface. Serious discrepancies with the exact integral cross sections at higher energy put into question the possible statistical nature of the title reaction. However, at a collision energy of 1.6 eV, statistical rotationally resolved cross sections managed to reproduce the experimental cross sections for the H+O(2)(v=0,j=1)-->OH(v(')=1,j('))+O process reasonably well.     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

Fully Coriolis-coupled quantum studies of the H + O2 (upsilon i = 0-2, j i = 0,1) --> OH + O reaction on an accurate potential energy surface: integral cross sections and rate constants

We present accurate quantum calculations of the integral cross section and rate constant for the H + O2 --> OH + O combustion reaction on a recently developed ab initio potential energy surface using parallelized time-dependent and Chebyshev wavepacket methods. Partial wave contributions up to J = 70 were computed with full Coriolis coupling, which enabled us to obtain the initial state-specified integral cross sections up to 2.0 eV of the collision energy and thermal rate constants up to 3000 K. The integral cross sections show a large reaction threshold due to the quantum endothermicity of the reaction, and they monotonically increase with the collision energy. As a result, the temperature dependence of the rate constant is of the Arrhenius type. In addition, it was found that reactivity is enhanced by reactant vibrational excitation. The calculated thermal rate constant shows a significant improvement over that obtained on the DMBE IV potential, but it still underestimates the experimental consensus.     

State-to-state dynamics of H + O2 reaction, evidence for nonstatistical behavior

Converged differential and integral cross sections are reported for the H + O2 --> OH + O reaction on an improved potential energy surface of HO2(X2A') using a dynamically exact quantum wave packet method and Gaussian weighted quasi-classical trajectory method. The complex-forming mechanism is confirmed by strong forward and backward scattering peaks and by highly inverted OH rotational state distributions. Both the quantum and classical results provide strong evidence for nonstatistical behavior in this important reaction.     

Full-dimensional quantum dynamics study of the H + H2O and H + HOD exchange reactions

The initial state-selected time-dependent wave packet approach is employed to study the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H, HOH' + D exchange reactions with both OH bonds in the H(2)O reactant and OH(D) bond in the HOD reactant treated as reactive bonds. The total reaction probabilities for different partial waves, as well as the integral cross sections, which are the exact CC (coupled-channel) results, are first obtained in this study for the H(2)O(HOD) reactant initially in the ground rovibrational state. Because of the shallow C(3v) minimum along the reaction path, the reaction probabilities for the three reactions present several resonance peaks, with one dominant resonance peak just above the threshold. The cross sections for the H' + HOD → HOH' + D reaction are substantially smaller than those for the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H reactions, indicating that the H'/H exchange reactions are much more favored. In the CC calculations, the resonance peaks in the reaction probabilities diminish quickly with the increase in total angular momenta J, resulting in the existence of a clear step-like feature just above the threshold in the cross sections for the title reactions, which manifests the signature of shape resonances in these reactions. In the CS calculations, the resonance peaks on reaction probabilities persist in many partial waves, and thus the resonance structures can no longer survive the partial-wave summation and are washed out completely in the CS cross sections for the title reactions.     

Quantum and quasiclassical studies of the O(3P)+HCl-->OH+Cl(2P) reaction using benchmark potential surfaces

We have performed quantum mechanical (QM) dynamics calculations within the independent-state approximation with new benchmark triplet A" and A' surfaces [B. Ramachandran et al., J. Chem. Phys. 119, 9590 (2003)] for the rovibronic state-to-state measurements of the reaction O(3P)+HCl(v=2,j=1,6,9)-->OH(v'j')+Cl(2P) [Zhang et al., J. Chem. Phys. 94, 2704 (1991)]. The QM and experimental rotational distributions peak at similar OH(j') levels, but the QM distributions are significantly narrower than the measurements and previous quasiclassical dynamics studies. The OH(low j) populations observed in the measurements are nearly absent in the QM results. We have also performed quasiclassical trajectory with histogram binning (QCT-HB) calculations on these same benchmark surfaces. The QCT-HB rotational distributions, which are qualitatively consistent with measurements and classical dynamics studies using other surfaces, are much broader than the QM results. Application of a Gaussian binning correction (QCT-GB) dramatically narrows and shifts the QCT-HB rotational distributions to be in very good agreement with the QM results. The large QCT-GB correction stems from the special shape of the joint distribution of the classical rotational/vibrational action of OH products. We have also performed QM and QCT calculations for the transition, O+HCl(v=0,T=300 K)-->OH(v'j')+Cl from threshold to approximately 130 kcal mol(-1) collision energy as a guide for possible future hyperthermal O-atom measurements. We find in general a mixed energy release into translation and rotation consistent with a late barrier to reaction. Angular distributions at high collision energy are forward peaked, consistent with a stripping mechanism. Direct collisional excitation channel cross sections, O+HCl(v=0,T=300 K)-->O+HCl(v'=1), in the same energy range are large, comparable in magnitude to the reactive channel cross sections. Although the (3)A" state dominates most collision processes, above approximately 48 kcal mol(-1), the (3)A' state plays the major role in collisional excitation.     

Discrete variable approaches to tetratomic molecules: part II: application to H2O2 and H2CO

We have carried out large-scale calculations for accurate vibrational energy levels of formaldehyde and hydrogen peroxide. The discrete variable representations of the radial and angular coordinates are employed together with the contraction scheme resulting from several diagonalization/truncation steps. The global potential energy surface due to Carter et al. [J. Mol. Spectrosc. 90 (1997) 729] is used for H2CO and due to Koput et al. [J. Phys. Chem. A 102 (1998) 6325] for H2O2. For both molecules, the calculated vibrational energy levels are characterized by combining vibrationally averaged geometries and expectation values of rotational constants with several adiabatic projection schemes for automatic quantum number assignments. The energy levels of H2CO involving the excited v2 and v3 vibrations appear as resonances beyond the zero-order picture consisting of uncoupled 3D stretching and 2D bending modes. The torsional energy levels of H2O2 are studied in great detail and different energy patterns occurring below and above the cis barrier are discussed. Our full dimensional calculations for H2O2 have shown that the OH triad levels, 2vOH, are symmetry adapted local mode states.     

State-to-state reactive differential cross sections for the H+H2-->H2+H reaction on five different potential energy surfaces employing a new quantum wavepacket computer code: DIFFREALWAVE

State-to-state differential cross sections have been calculated for the hydrogen exchange reaction, H+H2-->H2+H, using five different high quality potential energy surfaces with the objective of examining the sensitivity of these detailed cross sections to the underlying potential energy surfaces. The calculations were performed using a new parallel computer code, DIFFREALWAVE. The code is based on the real wavepacket approach of Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)]. The calculations are parallelized over the helicity quantum number Omega' (i.e., the quantum number for the body-fixed z component of the total angular momentum) and wavepackets for each J,Omega' set are assigned to different processors, similar in spirit to the Coriolis-coupled processors approach of Goldfield and Gray [Comput. Phys. Commun. 84, 1 (1996)]. Calculations for J=0-24 have been performed to obtain converged state-to-state differential cross sections in the energy range from 0.4 to 1.2 eV. The calculations employ five different potential energy surfaces, the BKMP2 surface and a hierarchical family of four new ab initio surfaces [S. L. Mielke, et al., J. Chem. Phys. 116, 4142 (2002)]. This family of four surfaces has been calculated using three different hierarchical sets of basis functions and also an extrapolation to the complete basis set limit, the so called CCI surface. The CCI surface is the most accurate surface for the H3 system reported to date. Our calculations of differential cross sections are the first to be reported for the A2, A3, A4, and CCI surfaces. They show that there are some small differences in the cross sections obtained from the five different surfaces, particularly at higher energies. The calculations also show that the BKMP2 performs well and gives cross sections in very good agreement with the results from the CCI surface, displaying only small divergences at higher energies.     

Quasiclassical trajectory study of the reaction H+CH4(nu3 = 0,1)-->CH3+H2 using a new ab initio potential energy surface

Detailed quasiclassical trajectory calculations of the reaction H+CH4(nu3 = 0,1)-->CH3 + H2 using a slightly updated version of a recent ab initio-based CH5 potential energy surface [X. Zhang et al., J. Chem. Phys. 124, 021104 (2006)] are reported. The reaction cross sections are calculated at initial relative translational energies of 1.52, 1.85, and 2.20 eV in order to make direct comparison with experiment. The relative reaction cross section enhancement ratio due to the excitation of the C-H antisymmetric stretch varies from 2.2 to 3.0 over this energy range, in good agreement with the experimental result of 3.0 +/- 1.5 [J. P. Camden et al., J. Chem. Phys. 123, 134301 (2005)]. The laboratory-frame speed and center-of-mass angular distributions of CH3 are calculated as are the vibrational and rotational distributions of H2 and CH3. We confirm that this reaction occurs with a combination of stripping and rebound mechanisms by presenting the impact parameter dependence of these distributions and also by direct examination of trajectories.     

Differential and integral cross sections for the H + O2 --> OH + O combustion reaction

We present accurate differential and integral cross sections for the H + O2 --> OH + O reaction obtained on a newly developed ab initio potential energy surface using time-independent and time-dependent quantum mechanical methods. The product angular distributions near the reaction threshold show pronounced forward and backward peaks, reflecting the complex-forming mechanism. However, the asymmetry of these peaks suggests certain nonstatistical behaviors, presumably due to some relatively short-lived resonances. The integral cross section increases monotonically with the collision energy above a reaction threshold.     

Quantum and quasiclassical study of the collinear reaction O(3P) + H2 → OH + H using a LEPS and fitted AB initio potential energy surface

Accurate quantum calculations of reaction probabilities P^T_ν′←ν have been carried out for the collinear reaction O(³P) + H₂ (ν = 0,1) → OH(ν′) + H using a LEPS and fitted ab initio potential energy surface. The energy dependence of the P^T_{ν′ ← ν} is similar for both surfaces. Collinear quasiclassical trajectory calculations have also been carried out, for comparison, on the LEPS surface for ν = 0, 1 and 2.     

Potential surfaces and dynamics of the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction

We present global potential energy surfaces for the three lowest triplet states in O(3P)+H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction using these surfaces. The surfaces are spline-based fits of approximately 20,000 fixed geometry ab initio calculations at the complete-active-space self-consistent field+second-order perturbation theory (CASSCF+MP2) level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1000-2500 K range using these surfaces. We also compute the total, rovibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approximately 4 km s(-1) (16.9 kcal mol(-1) collision energy) to 11 km s(-1) (122.5 kcal mol(-1) collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approximately 40 kcal mol(-1) collision energy rovibrationally excited OH(X2pi) products are a significant and perhaps dominant contributor to the observed 1-5 micro spectral emission from O(3P)+H2O(X1A1) collisions. Another important result is that OH(X2pi) products are formed in two distinct rovibrational distributions. The "active" OH products are formed with the reagent O atom, and their rovibrational distributions are extremely hot. The remaining "spectator" OH is relatively rovibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approximately 50 kcal mol(-1) collision energy, vibration dominates the OH internal energy, and above approximately 50 kcal mol(-1) rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2pi)+OH(X2pi) collisions.     

Exact quantum dynamics study of the O(+)+H2(v=0,j=0)-->OH(+)+H ion-molecule reaction and comparison with quasiclassical trajectory calculations

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (E(T)) of 0.75 eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with E(T) and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2-0.6 eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH(+) vibrational distribution, where no inversion of population was found. For the OH(+) rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk(') angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.     

Communication: quasiclassical trajectory calculations of correlated product-state distributions for the dissociation of (H2O)2 and (D2O)2

Stimulated by recent experiments [B. E. Rocher-Casterline, L. C. Ch'ng, A. K. Mollner, and H. Reisler, J. Chem. Phys. 134, 211101 (2011)], we report quasiclassical trajectory calculations of the dissociation dynamics of the water dimer, (H(2)O)(2) (and also (D(2)O)(2)) using a full-dimensional ab initio potential energy surface. The dissociation is initiated by exciting the H-bonded OH(OD)-stretch, as done experimentally for (H(2)O)(2). Normal mode analysis of the fragment pairs is done and the correlated vibrational populations are obtained by (a) standard histogram binning (HB), (b) harmonic normal-mode energy-based Gaussian binning (GB), and (c) a modified version of (b) using accurate vibrational energies obtained in the Cartesian space. We show that HB allows opening quantum mechanically closed states, whereas GB, especially via (c), gives physically correct results. Dissociation of both (H(2)O)(2) and (D(2)O)(2) mainly produces either fragment in the bending excited (010) state. The H(2)O(J) and D(2)O(J) rotational distributions are similar, peaking at J = 3-5. The computations do not show significant difference between the ro-vibrational distributions of the donor and acceptor fragments. Diffusion Monte Carlo computations are performed for (D(2)O)(2) providing an accurate zero-point energy of 7247 cm(-1), and thus, a benchmark D(0) of 1244 ± 5 cm(-1).