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Detailed quasiclassical trajectory calculations of the reaction H+CH4(ν3=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. [ABSTRACT FROM AUTHOR]

A modified and recalibrated potential energy surface for the gas-phase Cl+CH4→HCl+CH3 reaction is reported and tested. It is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to updated experimental and theoretical stationary point properties and experimental forward thermal rate constants. From the kinetics point of view, the forward and reverse thermal rate constants and the activation energies were calculated using the variational transition-state theory with semiclassical transmission coefficients over a wide temperature range of 150–2500 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of tunneling in this hydrogen abstraction reaction. A dynamics study was also performed on this PES using quasiclassical trajectory (QCT) calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found a noticeable internal energy in the coproduct methyl radical, both in the ground-state [CH4 (v=0)] and vibrationally excited [CH4 (v=1)] reactions. This CH3 internal energy was directly precluded in some experiments or oversimplified in previous theoretical studies using pseudotriatomic models. Second, our QCT calculations give HCl rotational distributions slightly hotter than those in experiment, but correctly describing the experimental trend of decreasing the HCl product rotation excitation in going from HCl (v′=0) to HCl (v′=1) for the CH4 (v=1) reaction. Third, the state specific scattering distributions present a reasonable agreement with experiment, although they tend to make the reaction more forward and backward scattered than found experimentally probably because of the hotter rotational distribution and the deficiencies of the QCT methods. [ABSTRACT FROM AUTHOR]

The H2 (v’, J’) partial reaction cross sections arising from collisions of 1.3 eV H atoms with HI have been measured using coherent anti-Stokes Raman scattering (CARS) spectroscopy. Quasiclassical trajectory (QCT) calculations on a representative potential energy surface have been used to simulate the dynamics of this reaction. Comparison of the QCT and experimental results shows good agreement in average energy disposal, total cross section, and rotational state distributions. The only significant disagreement is in the detailed vibrational state distribution, for which the calculations give monotonically decreasing population with increasing v’, while the experimental results show a significant population inversion between v’=0 and v’=1. Based on the QCT-experiment comparison we make suggestions for improving the H+HI potential energy surface. [ABSTRACT FROM AUTHOR]

Full-dimensional quantum and classical calculations have been carried out for inelastic (nonreactive) energy transfer in H[sub 2]+H[sub 2] on the ab initio potential energy surface of Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)]. State-to-state cross sections are determined and compared for transitions from H[sub 2](0,j[sub ab])+H[sub 2](1,j[sub cd]). While there is excellent agreement for transitions involving small Δj, for larger Δj and for vibrational relaxation, significant differences are observed which exhibit no systematic trends. Reasons for this disagreement are discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dynamics of the title reaction is investigated on an ab initio based potential energy surface using a full-dimensional quantum wave packet method within the centrifugal sudden approximation. It is shown that the reaction between H and HCN leads to both the hydrogen exchange and hydrogen abstraction channels. The exchange channel has a lower threshold and larger cross section than the abstraction channel. It also has more oscillations due apparently to quantum resonances. Both channels are affected by long-lived resonances supported by potential wells. Comparison with experimental cross sections indicates underestimation of the abstraction barrier height. [ABSTRACT FROM AUTHOR]

Rotationally inelastic collisions of the CH3 molecule in its ground X2A2′′ electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH3, frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy De was computed to equal 27.0 cm-1. At the global minimum, the helium atom lies in the CH3 plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision-induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Δk = ±3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined. [ABSTRACT FROM AUTHOR]

Real wave packet propagations were carried out on both a single ground electronic state and two-coupled-electronic states of the title reaction to investigate the extent of nonadiabatic effects on the distinguishable-atom reaction cross sections. The latest diabatic potential matrix of Abrol and Kuppermann [J. Chem. Phys. 116, 1035 (2002)] was employed in the present nonadiabatic quantum state-to-state scattering calculations over a total energy range-from threshold (the zero point of the reagent H2) to 3.0 eV. Based on the assumption that the hydrogen atoms are distinguishable in the collisions where the inelastic and elastic ones are excluded, no significant nonadiabatic effects have been found in the calculations of the full state-to-state integral and differential cross sections up to a total energy of 3.0 eV for product vibrational levels v′=0, 1, 2, 3. Our results therefore confirm the recent and the previous studies of the geometric phase effects in H+H2 employing a different diabatic double many-body expansion potential matrix or a different BKMP2 potential energy surface. [ABSTRACT FROM AUTHOR]

We utilize classical trajectory calculations to study the reaction dynamics of the dissociative adsorption of H2 on the stepped Pt(211) surface. The potential-energy surface has been obtained through an accurate interpolation of density-functional theory data at the generalized gradient approximation level, using the corrugation reduction procedure. New techniques for visualizing the collective dynamics of trajectories are introduced to elucidate the reaction mechanisms involved. Reaction exhibits a nonmonotonic dependence on collision energy, first decreasing with energy, and then increasing. A strong component of direct nonactivated reaction exists at the top edge of the step over the entire range of energies. The inverse relationship between reaction and collision energy at low energies is attributed to trapping in weak chemisorption wells. These wells also influence the direct reaction at the step, leading to a strong asymmetric dependence on incidence angle. Reaction on the terrace is activated, and only contributes significantly at high energies. Agreement with experiments on Pt(533) [A. T. Gee, B. E. Hayden, C. Mormiche, and T. S. Nunney, J. Chem. Phys. 112, 7660 (2000); Surf. Sci. 512, 165 (2002)] is good, and we are able to suggest new interpretations of the experimental data. [ABSTRACT FROM AUTHOR]

State-resolved cross beam experiments [H. Udseth et al., J. Chem. Phys. 60, 3051 (1974); J. Krutein and F. Linder, J. Chem. Phys. 71, 599 (1979); G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)], coupled with proton energy loss spectroscopy for the inelastic scattering of H+ from CO in the collision range of 10–30 eV show very low vibrational excitation of the target molecule. Stimulated by the experimentally observed low vibrational inelasticity in the system the ground and the first two low-lying excited electronic potential-energy surfaces have been computed using the ab initio multireference configuration interaction method. Quantum dynamics has been performed on the ground potential energy surface in the framework of vibrational close-coupling rotational infinite-order sudden approximation. The various computed dynamical attributes such as differential and integral cross sections, and average vibrational energy transfer are analyzed in detail, and compared successfully with the available experimental results. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We develop a quantal model for studying four-center reactions, A2+B2→2AB, and collision induced dissociation A2+B2→A+B2+A. The method involves using hyperspherical coordinates to describe vibrations of the A2 and B2 bonds and a global vibration and rotation of the exchange products. Application to the H4 system is presented, using a realistic potential energy surface. The reaction goes through a four-center linear transition state located just above the dissociation threshold. In the energy range studied (5–5.5 eV), collision induced dissociation competes with the four-center reaction and is the dominant process. It is found that vibrational energy, originally deposited in one of the diatomic partners, is much more efficient than translational energy in promoting reaction. Vibrational and rotational final distributions show that the products are internally hot. This simple quantal model, yet very demanding computationally, illustrates in detail many features of the H4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dynamical resonances in collinear He+H2+ → HeH++H reaction on the McLaughlin–Thompson–Joseph–Sathyamurthy potential energy surface are identified in terms of the transition state spectrum I(E), computed by Fourier transforming the autocorrelation function C(t) for the system. Eigenenergies of quasibound states corresponding to the resonances, starting from the nonreactive regime up to the dissociation threshold are reported. The nature of some of the resonance states is investigated by calculating the eigenfunctions for the corresponding eigenenergies and also their lifetimes. The relation to the corresponding resonant periodic orbits in classical mechanics at those energies is also established. In addition we predict several resonances at higher energies not reported in earlier calculations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies a nonreactive vibrational transition, atom exchange reaction and dissociation processes in a collinear helium+hydrogen collision using a time-independent quantum mechanical method. Replacement to the algorithm based on three-point finite difference formula; Interaction potential surface; Energy dependence of probabilities.

The Cs(7p)+(X 1∑+g, v=0) H2→(X 2∑+, v=0) CsH+H reactive collision was recently experimentally observed from a crossed beam experiment by Crépin et al. This reaction is rather unusual since it starts from a highly excited state of the system (11th potential surface) and must reach the ground state surface in the product channel without energy loss. Accurate nonempirical CI calculations of the potential energy surface for the collinear collision are reported, using large basis sets and a nonempirical relativistic pseudopotential for the Cs atom. The adiabatic potential surfaces, which exhibit irregular behavior, are reinterpreted in terms of an ab initio nearly diabatic effective Hamiltonian spanned by five neutral repulsive channels Cs(6s)×H2, Cs(6p)×H2,..., Cs(7p)×H2 and an ionic very flat Cs+H-2 channel which tends to the product wave function (Cs+H-)×H. The intersection of these diabatic potential surfaces are accessible from the entrance energy and this picture supports a harpooning mechanism, the 7p electron jumping on the H2 system at RCsH ≊6 bohr, the reactive systems being those which remain diabatically on this ionic surface through the avoided crossings with the lower Cs(7s)×H2,...,Cs(6s)×H2 neutral surfaces. The electronic couplings between the diabatic states are given as functions of the RCsH and RHH coordinates and their behavior is qualitatively understood. The existence of accidental conical intersections between some neutral diabatic states and the ionic state potential surfaces is demonstrated; these conical intersections should increase the reaction cross section. This diabatic study confirms, therefore, the harpooning mechanism previously proposed to interpret this paradoxical photochemical reaction. [ABSTRACT FROM AUTHOR]

We have calculated state-to-state reaction cross sections for two new energies and state-to-state vibrationally inelastic cross sections for four energies for H+D2 collisions on the most accurate available potential energy surface. The dynamics calculations are based on the quasiclassical trajectory quadratic smooth sampling method. We present a detailed analysis of the theoretical inelastic and reaction cross sections and compare the trends in the reactive scattering results to those calculated from Gerrity and Valentini’s experiments and extrapolations thereof. The agreement between theory and experiment for the reactive scattering at 0.98–1.3 eV relative translational energy is stupendously good. [ABSTRACT FROM AUTHOR]

Differential cross sections (DCSs) for the H + D2 → HD(v' = 4, j') + D reaction at 3.26 eV collision energy have been measured using the photoloc technique, and the results have been compared with those from quantum and quasiclassical scattering calculations. The quantum mechanical DCSs are in good overall agreement with the experimental measurements. In common with previous results at 1.97 eV, clear interference patterns which appear as fingerlike structures have been found at 3.26 eV but in this case for vibrational states as high as v' = 4. The oscillatory structure is prominent for low rotational states and progressively disappears as j' increases. A detailed analysis, similar to that carried out at 1.97 eV, shows that the origin of these structures could be traced to interferences between well defined classical mechanisms. In addition, at this energy, we do not observe the anomalous positive j'-θ trend found for the v' = 4 manifold at lower collision energies, thus reinforcing our explanation that the anomalous distribution for HD(v' = 4, j') at 1.97 eV only takes place for those states associated with low product recoil energies. [ABSTRACT FROM AUTHOR]

We report the methyl radical product state distributions for the reactions of H and Cl with CHD3(ν1=1,2) at collision energies of 1.53 and 0.18 eV, respectively. Both reactions demonstrate mode selectivity. The resulting state distributions from the H+CHD3(ν1=1,2) reactions are well described by a spectator model. The reactions Cl+CHD3(ν1=1,2) exhibit similar behavior, but in some aspects the spectator model breaks down. We attribute this breakdown to enhanced intramolecular vibrational redistribution in the Cl+CHD3(ν1=1,2) reactions compared to the H+CHD3(ν1=1,2) reactions, caused by the interaction of the slower Cl atom with the vibrationally excited CHD3, which is promoted either by its longer collision duration, its stronger coupling, or both. [ABSTRACT FROM AUTHOR]

Quantum-mechanical scattering calculations were performed for the rovibrational relaxation of CO in collisions with H atoms using the close-coupling approach for collision energies between 10-6 and 1500 cm-1. We adopted the H–CO interaction potential of Werner, Keller, and Schinke and computed the state-to-state and total cross sections for the quenching of the υ=1, j=0–2 levels of CO. Numerous resonances, as a consequence of the van der Waals potential, are observed and the cross sections are found to approach the Wigner limit at low energies. Also, by averaging the cross sections over a Boltzmann distribution of velocities of the incoming atom, quenching rate coefficients are obtained and found to be consistent with previous infinite-order sudden approximation calculations for temperatures between 100 and 300 K. [ABSTRACT FROM AUTHOR]

We have investigated, both theoretically and experimentally, the reactions of naphthylium C10H7+ and d-naphthylium C10D7+ ions with H2 and D2. Cross sections as functions of the collision energy have been measured for a variety of reaction channels. Theoretical calculations have been carried out at the density functional theory level which utilizes the hybrid functional B3LYP and the split-valence 6-31G* basis set. The key features of the potential energy surfaces and the relevant thermochemical parameters have been calculated and they provide insights on the reaction mechanisms. The bimolecular reactivity of C10H7+ with H2 is dominated by the production of naphthalene cation C10H8+. The reaction is not a direct atom-abstraction process, but instead it proceeds via the formation of a stable intermediate complex C10H9+ of σ type geometry, with a significant mobility of hydrogen along the ring. This mobility allows the scrambling of the hydrogen atoms and causes the successive statistical fragmentation of the complex into a variety of product channels. Elimination of one H(D) atom appears to be favored over elimination of one H2 or HD molecule. Alternatively, the intermediate complex can be stabilized either by collision with a third body or by emission of a photon. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present single-photon spectroscopy in molecular hydrogen starting from the metastable c3Πu- state to a number of triplet nd-Rydberg states (v=0–4, n=12–20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i3Πg state is assumed to be responsible for field-free predissociation of the v=0 Rydberg levels. The stronger observed predissociation channel of the v=1 Rydberg levels is due to the nonadiabatic interaction with the h3Σg+ state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H2+ molecular ion. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time domain tunable diode laser absorption spectroscopy has been used to measure rotationally resolved transient absorption line shapes and nascent rotational populations for CO2 molecules excited into the (0002) vibrational state by collisions with translationally hot hydrogen atoms. The 0002 rotational population distribution and rotationally resolved linewidths are remarkably similar to those previously obtained for 0001. Within the context of a simple physical model used to interpret the data, the similar rotational distributions and translational recoils for 0001 and 0002 suggest that these two states are excited by similar collision trajectories, wherein asymmetric stretching excitation is optimized when H strikes near the end of the O–C–O molecule. The magnitude of population scattered into 0002 is ∼21 times smaller than that scattered into 0001. [ABSTRACT FROM AUTHOR]

The energies for the interaction of a hydrogen atom with a nitrogen molecule have been calculated for large separation distances using a complete-active-space self-consistent-field/externally contracted configuration interaction method. H–N2 transport cross sections and collision integrals have been calculated using sudden approximations and a semiclassical description of the scattering. The values of these quantities are found to be close to the corresponding values determined from the average (isotropic) potential energy. The collision integrals are applied to determine diffusion and viscosity coefficients; the theoretical diffusion agrees well with the measured data available from experiments at low temperatures. [ABSTRACT FROM AUTHOR]

Reactions of N+4 ions with H2, HD, and D2 are studied from thermal to 5 eV kinetic energy under single-collision conditions in a guided-ion beam mass spectrometer. Reactant ions are formed in a flow tube source to ensure thermalization. Despite being exothermic by 1.5 eV, formation of N2H+ (N2D+) is observed to proceed with an activation barrier that we measure to be 0.09±0.03 eV at 0 K, independent of the hydrogen isotopomer used. Possible reaction mechanisms are discussed, and the present results are compared to previous flow and drift tube measurements. Effects of collisional reheating on the derivation of thermochemistry in drift-tube experiments are discussed. We also discuss the difficulty of deriving reliable thermochemistry from pseudo-Arrhenius plots when the mathematical form of the cross section excitation function is unknown. [ABSTRACT FROM AUTHOR]

Investigates the collisional parameters of pure rotational transitions of H[sub 2] self-perturbed and He perturbed. Use of stimulated gain Raman spectroscopy; Exhibition of a linear behavior by collisional frequency shift; Analysis of elastic and inelastic contributions as function of temperature and rotational quantum number.

Accurate 3D quantum state-to-state reaction probabilities and collision lifetimes for the H+O2→OH+O combustion reaction for total angular momentum J=0 are reported. The reaction probabilities are dominated by resonances, many of which overlap. The total reaction probability is not enhanced by vibrational or rotational excitation of the reactants. The first accurate final state distributions are presented, and probability is found to spread out into all open product vibrational and rotational channels, with a rather strong tendency to produce highly excited product rotational states. The first calculated collision lifetimes are presented, and resonances with lifetimes of 1–10 ps are found at most energies. Whether the reaction behaves statistically is discussed, and future extensions needed are also discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have carried out classical dynamical simulations of collisions of Ar12H2 clusters with a rigid Si(111)(1×1) solid surface for a variety of collision speeds and two different cluster geometries. At low cluster temperatures, the most stable cluster geometry is that with the H2 on the outside of the cluster. It is found that dissociative chemisorption of the hydrogen can occur with this cluster geometry at H2 collision energies as low as 0.05 eV; this is markedly lower than the 0.4 eV threshold for ‘‘bare’’ hydrogen on this surface. The reactivity increases with collision energy until a steric limit of roughly 33% reaction is attained. This corresponds to that fraction of collisions in which the H2 can be caged by the Ar cluster on the surface. The other isomer, with the H2 inside the Ar cluster, is less reactive at low energies, since the H2 does not have free access to the surface. However, the reactivity begins to rise steeply away from threshold as the H2 becomes sufficiently energetic to rearrange the Ar ‘‘solvent’’ shell. Studies of the details of the dynamics reveal that the kinetic energy of the Ar cluster can be ‘‘pooled’’ to allow access to relatively high energy channels, such as chemisorption, dissociative trapping, and even collision-induced dissociation. [ABSTRACT FROM AUTHOR]

Simulations of the available experimental results for the H+D2→HD+D reaction at 2.2 eV collision energy, have been carried out using quasiclassical trajectory (QCT) calculations on ab initio potential energy surfaces. Although there are some differences between experiment and theory, the global description of the measurements provided by the QCT approach is reasonably good considering the uncertainty of the data. Higher resolution studies would be desirable in order to clarify the apparent discrepancies. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]