Direct Dynamics Calculations Research Articles

Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

A comprehensive study on unsteady effects of oscillating Gurney flaps, or microflaps, has been conducted. Two-dimensional unsteady airloads for lift, moment, and drag due to an oscillating microflap were computed using a compressible Reynolds-averaged Navier―Stokes flow solver. The computational fluid dynamics results were generated with an overset-mesh approach that captures oscillatory microflap motion. Three microflap configurations were examined to determine the type most suitable in terms of actuation efficiency and practical implementation. Furthermore, a reduced-order model for the unsteady microflaps was developed based on computational fluid dynamics simulations, using the rational function approximation approach. The resulting rational function approximation model is a state-space time-domain aerodynamic model that accounts for unsteadiness, compressibility, and time-varying freestream effect. The agreement between the reduced-order model and direct computational fluid dynamics calculations was found to be excellent for a wide range of flow conditions examined. The approximate model is suitable for incorporation in a comprehensive code, from which the potential of microflaps for active control of vibrations in rotors can be determined. Open-loop control studies indicated that the microflap produces substantial vibration reduction (80% reduction in vertical shear), whereas closed-loop control using combined harmonic inputs consisting of a sum of four harmonics (2―5/rev) reduced the vibration objective by 92%, on a hingeless rotor configuration resembling the Messerschmitt-Bolkow-Blohm BO-105. These results confirmed the control authority and effectiveness of the microflap for vibration reduction in rotorcraft.

High-level direct-dynamics variational transition state theory calculations including multidimensional tunneling of the thermal rate constants, branching ratios, and kinetic isotope effects of the hydrogen abstraction reactions from methanol by atomic hydrogen

We report a detailed theoretical study of the hydrogen abstraction reaction from methanol by atomic hydrogen. The study includes the analysis of thermal rate constants, branching ratios, and kinetic isotope effects. Specifically, we have performed high-level computations at the MC3BB level together with direct dynamics calculations by canonical variational transition state theory (CVT) with the microcanonically optimized multidimensional tunneling (μOMT) transmission coefficient (CVT/μOMT) to study both the CH(3)OH+H→CH(2)OH+H(2) (R1) reaction and the CH(3)OH+H→CH(3)O+H(2) (R2) reaction. The CVT/μOMT calculations show that reaction R1 dominates in the whole range 298≤T (K)≤2500 and that anharmonic effects on the torsional mode about the C-O bond are important, mainly at high temperatures. The activation energy for the total reaction sum of R1 and R2 reactions changes substantially with temperature and, therefore, the use of straight-line Arrhenius plots is not valid. We recommend the use of new expressions for the total R1 + R2 reaction and for the R1 and R2 individual reactions.

Direct MP2 molecular dynamics studies of H atom reaction with CD4 and CH4

The mechanism and dynamics of the H + CD(4) -> CD(3) + HD (I) and H + CH(4) -> CH(3) + H(2) (II) reactions have been investigated by electronic structure methods. The minimum-energy path and vibrational frequencies along the intrinsic reaction coordinate are calculated at MP2/cc-pVDZ level. Energy distributions of the products are also obtained by the direct classical trajectory calculations at the MP2/cc-pVDZ level. It is found that most of the available energy appears as product translational energy, and very little of the available energy is partitioned into internal excitation of the HD (H(2)) product for reaction I (II), which is in agreement with the experimental evidence. The results indicate that the experimental results could be reproduced by the direct MP2 molecular dynamics calculations. The rotational state distributions of the products show the HD (H(2)) products are formed with lower rotational quantum numbers than the CD(3) (CH(3)) products. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 4433-4442, 2011

The rheology of solid glass

As the glass transition is approached from the high temperature side, viewed as a liquid, the properties of the ever more viscous supercooled liquid are continuous functions of temperature and pressure. The point at which we decide to classify the fluid as a solid is therefore subjective. This subjective decision does, however, have discontinuous consequences for how we determine the rheological properties of the glass. We apply the recently discovered relaxation theorem to the time independent, nondissipative, nonergodic glassy state to derive an expression for the phase space distribution of an ensemble of glass samples. This distribution is then used to construct a time dependent linear response theory for aged glassy solids. The theory is verified using molecular dynamics simulations of oscillatory shear for a realistic model glass former with excellent agreement being obtained between the response theory calculations and direct nonequilibrium molecular dynamics calculations. Our numerical results confirm that unlike all the fluid states, including supercooled liquids, a solid glass (in common with crystalline states) has a nonzero value for the zero frequency shear modulus. Of all the states of matter, a supercooled fluid approaching the glass transition has the highest value for the limiting zero frequency shear viscosity. Finally, solid glasses like dilute gases and crystals have a positive temperature coefficient for the shear viscosity whereas supercooled and normal liquids have a negative temperature coefficient.

Theoretical study of the dynamics of F+alkanethiol self-assembled monolayer hydrogen-abstraction reactions

The dynamics of the reactions of F atoms with octanethiol self-assembled monolayers (SAMs) has been studied using theoretical methods. F+SAM classical trajectories have been propagated directly using a quantum-mechanics (QM)/molecular-mechanics scheme in which the QM portion is described using a specific-reaction-parameters (SRP) semiempirical Hamiltonian. This SRP Hamiltonian has been derived using ab initio information of model gas-phase F+alkane reactions and its accuracy has been calibrated via comparison of the result of direct-dynamics calculations with available experiments on the F+CH(4)-->HF+CH(3) and F+C(2)H(6)-->HF+C(2)H(5) reactions. The F+SAM calculations are used to analyze HF product-energy distributions at collision energies ranging from 0.80 to 11.53 kcal mol(-1) and 0 degrees, 30 degrees, and 60 degrees incident angles with respect to the surface normal. The calculations show that while the HF product is vibrationally excited, it desorbs translationally and rotationally cold at all collision energies and incident angles explored. The calculated results shed light into recent experiments of F-atom reactions with liquid alkane surfaces by providing mechanistic understanding of the factors that govern the amount of energy deposited into the various degrees of freedom of the HF product. Specifically, examination of the dynamics of postreaction HF collisions with the surface shows the role that secondary collisions play in quenching rotational and translational excitation of HF before desorption from the surface.

Communications: Classical trajectory study of the postquenching dynamics of OH A ∑2+ by H2 initiated at conical intersections

We report results of a classical trajectory calculation of the postquenching dynamics of OH A (2)Sigma(+) by H(2). This is done by performing roughly 100 000 trajectories at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" model of the dynamics, respectively. The trajectories are propagated on a new ground state ab initio-based potential energy surface. This surface is a permutationally invariant fit to roughly 23,000 electronic energies (multireference configuration interaction/correlation-consistent-augmented-triple-zeta basis) at configurations obtained mostly from direct-dynamics calculations (complete active space second order perturbation theory with correlation-consistent-augment double-zeta basis), also initiated at the CoIs. Final rovibrational state distributions of the ground electronic state OH product and the H-atom translational energy distributions for abstraction and insertion mechanisms are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The calculated H(2) rovibrational distributions (not measured experimentally) are also presented.

Electronic state dependence of the ion–molecule reaction CH3CN+ + CH3CN → CH4CN+ + CH2CN: threshold electron–secondary ion coincidence (TESICO) and direct ab initio molecular dynamics study

The ion-molecule reaction, CH(3)CN(+) + CH(3)CN → CH(3)CNH(+) + CH(2)CN, has been investigated using the threshold electron-secondary ion coincidence (TESICO) technique. Relative reaction cross sections for two microscopic reaction mechanisms, i.e., proton transfer (PT) from the acetonitrile ion CH(3)CN(+) to neutral acetonitrile CH(3)CN and hydrogen atom abstraction (HA) by CH(3)CN(+) from CH(3)CN, have been determined for two low-lying electronic states, (2)E and (2)A(1) of the CH(3)CN(+) primary ion. The cross section for PT of the (2)A(1) state was smaller than that of the (2)E state, whereas that of HA are almost the same in the two states. Ab initio calculations showed that the dissociation of the C-H(+) bond of CH(3)CN(+) is easier in the (2)E state than that in the (2)A(1) state. The direct ab initio molecular dynamics (MD) calculations showed that two mechanisms, direct proton transfer and complex formation, contribute the reaction dynamics.

Exploring the sloped-to-peaked S2/S1 seam of intersection of thymine with electronic structure and direct quantum dynamics calculations

The role of the seam of intersection between the lowest (pi,pi*) and (n,pi*) excited states in the decay of electronically excited singlet thymine has been investigated with ab initio complete active space self-consistent field (CASSCF) calculations and direct dynamics variational multiconfiguration Gaussian (DD-vMCG) quantum dynamics on the full-dimensional CASSCF surface, with 39 degrees of freedom. The seam has a sloped-to-peaked topography, and the dynamics at the different segments of the seam have been studied by varying the initial conditions of the propagation. When the wave packet is directed to the peaked segments, part of it traverses the seam, stays on the (pi,pi*) state and heads towards decay to the ground state. In contrast to this, when the wave packet is driven to sloped seam segments it bounces back to the minimum of the (pi,pi*) state. Significant population transfer to the (n,pi*) state is observed in both cases. The results suggest that a sloped-to-peaked topography can be used to control photochemical reactivity, by driving the wave packet to different regions of the seam where a different outcome of the propagation can be expected.

Theoretical study on the effect of intramolecular hydrogen bonding on OH stretching overtone decay lifetime of ethylene glycol, 1,3-propanediol, and 1,4-butanediol

It is well known that the existence of hydrogen bonds causes the vibrational spectra for the donor OH bond to broaden. Most times this broadening is attributed to inhomogeneous contributions, however in this paper, we studied the homogeneous contribution coming from the decay lifetime of the OH stretching excitation. Alkane diol, which possesses an OH(b)OH(f) (b and f stand for hydrogen bonded and free, respectively) intramolecular hydrogen bond, provides us with a controllable molecule to systematically study the effect of intramolecular hydrogen bond on the decay rate of the OH vibrational excitation. In the present study we performed local mode vibrational calculation of gas phase ethylene glycol (EG), 1-3 propanediol (PD), and 1-4 butanediol (BD) using the potential energy curves calculated using B3LYP/6-31+G(d,p), MPW1PW91/6-311+G(2d,p), M06-2X/6-311+G(2df,2p), and MP2/6-311++G(3df,3pd) methods. In addition, we studied the dynamics of OH overtone excited state (Δv(OH) = 3, 4) for the two most stable conformers of the aforementioned diols using the direct dynamics calculation employing the B3LYP/6-31+G(d,p) method. From the trajectory simulations we showed that the decay lifetime from the donor OH(b) excitation decreases while that from the acceptor OH(f) excitation increases as we increase the intramolecular hydrogen bond strength in going from EG, PD, to BD. These results are consistent with the gas phase experimental results of Kjaergaard et al. where the OH(b) peaks, which are sharp for EG disappear for BD. From the detailed analysis on the trajectories, we observed that the decrease in decay lifetime for the OH(b) bond excitation originates from the increase in the energy flow to the OH(b)O torsion/bend motion, while the increase in lifetime for OH(f) bond is due to the fixing of the OH(f) geometry by accepting a donor hydrogen. Furthermore, by plotting the calculated red shift versus the decay lifetime we obtained the following relationship T(decay)(fs) = 343176(Δω(cm(-1)))(-1.36).

An optimum strategy for solution chemistry using semiempirical molecular orbital method: Importance of description of charge distribution

For the purpose to execute direct dynamics calculation in solution chemistry, we propose an optimum strategy for solution chemistry using semiempirical molecular orbital (MO) method with neglect of diatomic differential overlap (NDDO) approximation with specific solution reaction parameters (SSRP), i.e., the NDDO-SSRP method. In this strategy, the empirical parameters of the semi-empirical MO method were optimized individually for target molecule or ion by reference to the ab initio MO calculation data for many configurations on the potential energy surface near the reaction path. For demonstration, the NDDO-SSRP method was applied to two molecules and two ions (OH(-), H(2)O, NH(3), NH(4)(+)) at their equilibrium states in aqueous solution, respectively. Accordingly, it was verified that both the potential energy surface and the charge distribution of these solutes in aqueous solution are dramatically improved to reproduce themselves accurately at ab initio MO calculation level. In conclusion, it is expected that the NDDO-SSRP method should become quite useful for dynamic and statistical applications to chemical reaction systems in solution.

Direct Dynamics Study of Hydrogen-Transfer Isomerization of 1-Pentyl and 1-Hexyl Radicals

The rate constants of three intramolecular hydrogen-transfer isomerization reactions, namely, 1-4 isomerization of the 1-pentyl radical and 1-4 and 1-5 isomerizations of the 1-hexyl radical, are calculated using variational transition state theory with multidimensional tunneling, in particular by using canonical variational theory (CVT, which is the version of variational transition state theory in which the transition state dividing surface is optimized for a canonical ensemble) with small-curvature tunneling (SCT) for the transmission coefficient. The required potential energy surfaces were obtained implicitly by direct dynamics employing interpolated variational transition state theory with mapping (IVTST-M) and variational transition state theory with interpolated single-point energies (VTST-ISPE). Single-level direct dynamics calculations were performed for all of the reactions by IVTST-M using M06-2X/MG3S or M08-HX/cc-pVTZ+ potential energy surfaces or both. The stationary points of 1-4 isomerization of 1-pentyl and the stationary points for the forward reactions of 1-4 and 1-5 isomerizations of 1-hexyl were also optimized by BMC-CCSD, and for all three reactions we also performed dual-level direct dynamics calculations using VTST-ISPE in which MCG3-MPW single-point energies served as the higher level. The calculated MCG3-MPW//M06-2X/MG3S rate constants agree well with experimental values for 1-4 isomerization of the 1-pentyl radical at high temperature, and this validates the accuracy of this theoretical method for 1-4 isomerization. The MCG3-MPW//M06-2X/MG3S method was therefore used to make a reliable prediction for the rata constants of 1-4 isomerization of the 1-hexyl radical for which a direct experimental measurement is not available. The calculated CVT/SCT/M08-HX/cc-pVTZ+ rate constants agree well with experimental values for 1-5 isomerization of the 1-hexyl radical, and they show that the tunneling effect for these reactions was underestimated in previous work.

DFT and direct MD study of the diffusion of sodium ion on graphenes

The structures and electronic states of sodium ion (Na +) trapped on the graphene have been investigated by means of density functional theory (DFT) calculation to elucidate the nature of interaction between Na + and the graphenes. In addition, direct molecular orbital-molecular dynamics (MO-MD) calculation [Tachikawa, J. Phys. Chem. C, 112 (2008) 10193] was applied to diffusion processes of the Na + ion on graphene. The graphene composed of 37 benzene rings was used as a model of graphene. The B3LYP/LANL2MB calculation showed that the sodium ion is stabilized in hexagonal site and is located at ca. 2.230 Å from the graphene surfaces. The direct MO-MD calculation showed that the Na + ion diffuses freely on the graphene surface, but the ion did not approach the edge region due to the fact that a high potential barrier exists near the edge region. The nature of interaction between Na + and graphene was discussed on the basis of theoretical results.

Ab-Initio and MD Studies on the Interaction of Carbon Nano-Materials with Alkali Ion and Atom

The structures and electronic states of sodium atom (Na) trapped on the graphene surfaces have been investigated by means of density functional theory (DFT) calculation to elucidate the nature of interaction between Na and the graphenes. In addition, direct molecular orbital-molecular dynamics (MO-MD) calculation [Tachikawa, J. Phys. Chem. C, 112 (2008) 10193] was applied to diffusion processes of the Na atom on graphene surfaces. Seven graphenes (n = 7, 14, 19, 29, 37, 44 and 52, where n means numbers of rings in each carbon cluster) were examined in the present study. The B3LYP/LANL2MB calculation showed that the sodium atom is located at ca. 3.0 Å from the graphene surfaces. The direct MO-MD calculation showed that diffusion of Na atom is slower than that of Na+ ion on graphene surface. The nature of the interaction between Na atom and the carbon clusters was discussed on the basis of theoretical results.

Interaction of Lithium Ion (Li+) with Chlorinated Graphene (Cl-Graphene) Surface: A Direct Ab-Initio MD Study

Electronic states of lithium ion (Li+) interacting with a chlorinated graphene surface (Cl-graphene) have been investigated by means of hybrid density functional theory (DFT) to elucidate the effect of chlorination of graphene on the diffusion of lithium ion. Also, direct molecular orbital-molecular dynamics (MO-MD) calculation [H. Tachikawa, J. Phys. Chem. C, 112 (2008) 10193] was applied to diffusion processes of the Li+ ion on Cl-graphene. The potential barrier height for movement of Li+ was estimated to be 2.95 kcal/mol on Cl-graphene. The direct MO-MD calculations showed that the Li+ ion diffuses freely on bulk and edge region of Cl-graphene at 300 K. The nature of the interaction between Li+ and Cl-graphene was discussed on the basis of theoretical results.

Direct ab initio dynamics calculations for rates and the kinetic isotope effects of multiproton transfer in ClONO2+HCl→HNO3+Cl2 reactions with water clusters: Breakdown of the rule of the geometric mean

We performed high-level quantum mechanical calculations and direct ab initio reaction dynamics calculations for multiple proton transfers in ClONO(2)+HCl-->HNO(3)+Cl(2) with water clusters containing one to two water molecules, which can be used as a model of the reactions occurring on ice surface in stratospheric clouds. The energy barriers of these reactions depend on the number of water molecules involved. Two and three protons in these reactions with one and two water molecules, respectively, were transferred concertedly and asynchronously. The potential energy barrier at the MP2/6-311++(3df,3pd)//MP2/6-31G(d,p) level was 4.8 kcal/mol for the triple proton transfer involving two water molecules with a rate constant of 1.6x10(3) s(-1) at 197 K. The potential energy curve near the saddle points was very flat and the tunneling effect on the proton transfer was negligible. The primary HH/DH kinetic isotope effect for the double proton transfer involving one water molecule was lower than unity due to the enhanced force constant at the transition state. The rule of the geometric mean for the concerted proton transfer does not hold in these reactions because the zero-point energy changes of each proton in flight at the transition state are not the same in the highly asynchronous processes.

Experimental and Theoretical Investigations of the Inelastic and Reactive Scattering Dynamics of O(3P) Collisions with Ethane

Detailed experimental and theoretical investigations have been carried out for the reaction of O((3)P) with CH(3)CH(3) at collision energies in the range of 80-100 kcal mol(-1). Experiments were performed on a crossed molecular beams apparatus with a laser breakdown source (which produces beams of O((3)P) with average velocities of 6.5 to 8.5 km s(-1)) and a pulsed supersonic source of CH(3)CH(3) having an average velocity of approximately 0.8 km s(-1). A rotatable quadrupole mass spectrometer allowed universal detection, with angular and velocity resolution of products scattering from the crossing region of the two reagent beams. Theoretical calculations were carried out in two stages, (1) electronic structure calculations to characterize the stationary points associated with the title reaction and (2) direct dynamics calculations employing the MSINDO semiempirical Hamiltonian and density functional theory (B3LYP/6-31G**). The dynamics of O-atom inelastic scattering and H-atom abstraction to form OH + C(2)H(5) were clearly revealed by the experiment and were matched well by theory. Both of these processes favor high-impact parameters, with most of the available energy going into translation, indicating a stripping mechanism for H-atom abstraction. H-atom abstraction was the dominant reactive pathway, but H-atom elimination to form OC(2)H(5) + H was also inferred from the experimental results and observed in the theoretical calculations. This reaction proceeds through small-impact-parameter collisions, and most of the available energy goes into internal excitation of the OC(2)H(5) product, which likely leads to secondary dissociation to H(2)CO + CH(3) or CH(3)CHO + H. A relative excitation function for the H-atom elimination channel was also measured and compared to a calculated absolute excitation function. The theoretical calculations also identified several additional reaction pathways with low relative yields, including C-C bond breakage to form OCH(3) + CH(3). Interference from OC(2)H(5) decomposition products in the experiment inhibited the unambiguous observation of the low-yield reaction pathways that were identified by theory, although an upper limit for the relative yield of C-C bond breakage was determined.

Theoretical Study of the Reaction of CH3 with HOCO Radicals

The reaction of HOCO radicals with CH3 radicals is examined using the coupled cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the CH3 + HOCO reaction can produce both the H2O + CH2CO and the CH4 + CO2 products through acetic acid and enediol intermediates. Direct ab initio dynamics calculations determine the thermal rate coefficients to be k(T/K) = 3.24 x 10(-11)T0.1024 in cm3 x molec(-1) x s(-1) at T </= 1000 K for the overall reaction. In addition, the product branching ratio of (H2O + CH2CO) to (CH4 + CO2) is predicted to be RH2O/CH4(T/K) = 1.52 + (1.95 x 10(-4))T using RRKM theory. Both the thermal rate coefficients and the product branching ratios are weakly temperature dependent.

Direct ab initio dynamics calculations of the reaction rate for the hydrogen abstraction reaction of NCO with CH4 and C2H6

The kinetics of the hydrogen abstraction reactions NCO + CH4 (R1) and NCO + C2H6 (R2) have been studied over a wide temperature range. The minimum energy paths (MEPs) were calculated at the MP2/cc-pVDZ level and single-point calculations were refined at the G3MP2 level. The rate constants for the title reactions were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions. The fitted three-parameter formulae are k 1 = 2.52 × 10−22 T 3.46 exp(2466/T) and k 2 = 9.8 × 10−22 T 3.2 exp(411.8/T) cm3 molecule−1 s−1 for (R1) and (R2), respectively. The calculated rate constants were found to be in good agreement with the available experimental data. Deuterium kinetic isotope effects were also investigated. Both reactions show a significant kinetic isotope effect in the low-temperature range.

Dynamics of Vibrational Overtone Excited Pyruvic Acid in the Gas Phase: Line Broadening through Hydrogen-Atom Chattering

The dynamics of overtone-excited pyruvic acid (PA) is studied using a combination of experimental and theoretical methods. It is experimentally observed that high overtone excitation of the OH-stretching mode of PA in the gas phase leads to a unimolecular decarboxylation reaction. An RRKM analysis of the rate is consistent with previous experiments for the thermal reaction but is inconsistent with the present overtone chemistry; from this it is concluded that the overtone-induced reaction is likely to be a direct reaction. Using a Fourier transform infrared spectrometer and a cavity ring-down spectrometer, the spectrum for the OH-stretch fundamental and overtone transitions is measured. We assign two conformers of PA in the spectrum, the Tc and Tt, corresponding to distinct orientations of the OH-group. The spectral peaks for the Tc-conformer broaden dramatically at the third and fourth overtones while those of the Tt-conformer remain relatively narrow. Using a three-mode quantum mechanical model for the vibrational states, the line positions and intensities are well reproduced by theory. The line widths, and the associated dynamical interpretation, are provided by a direct dynamics calculation, where the potential is computed "on-the-fly" and all degrees of freedom are included. It is found that the line broadening is due to the onset of H-atom chattering between the two O-atoms, an effect that occurs for the Tc-conformer but not the Tt-conformer. This H-atom-transfer process is the first step of the decarboxylation reaction mechanism, which subsequently involves breaking the C-C bond. The theoretical and experimental line widths agree but do not correspond to the full reaction time which is much longer than the initial chattering step.

Energetics and kinetics of the reaction of HOCO with hydrogen atoms

The potential energy surface for the reaction of HOCO radicals with hydrogen atoms has been explored using the CCSD(T)/aug-cc-pVQZ ab initio method. Results show that the reaction occurs via a formic acid (HOC(O)H) intermediate, and produces two types of products: H(2)O+CO and H(2)+CO(2). Reaction enthalpies (0 K) are obtained as -102.0 kcalmol for the H(2)+CO(2) products, and -92.7 kcalmol for H(2)O+CO. Along the reaction pathways, there exists a nearly late transition state for each product channel. However, the transition states locate noticeably below the reactant asymptote. Direct ab initio dynamics calculations are also carried out for studying the kinetics of the H+HOCO reaction. At room temperature, the rate coefficient is predicted to be 1.07x10(-10)cm(3) molec(-1) s(-1) with a negligible activation energy E(a)=0.06 kcalmol, and the branching ratios are estimated to be 0.87 for H(2)+CO(2), and 0.13 for H(2)O+CO. In contrast, the product branching ratios have a strong T dependence. The branching ratio for H(2)O+CO could increase to 0.72 at T=1000 K.