Product Rovibrational Distributions Research Articles

Differential cross sections and product ro-vibrational distributions for 16O+36O2 and 18O+32O2 exchange reactions

We report the rotational-state resolved integral cross sections and differential cross sections for 18O + 16O16O (v = 0, j = 1) →18O16O (v′ = 0, j′) + 16O and 16O + 18O18O (v = 0, j = 1) →18O16O (v′ = 0, j′) + 18O reactions obtained using time-independent quantum mechanical method on an ab initio potential energy surface of ozone [Dawes et al. J. Chem. Phys.135, 081102(2011)]. The results for both the reactions in the collision energy range of 0.001 – 0.1 eV are presented and discussed the isotopic effects found. The present results are compared with the earlier experimental and theoretical results reported in the literature.

Differential Cross Sections and Product Rovibrational Distributions for (16)O + (32)O2 and (18)O + (36)O2 Collisions.

We report rotationally resolved opacity functions, product rotational distributions, and differential cross sections for the (16)O + (16)O(16)O (v = 0,j = 1) → (16)O(16)O (v' = 0,j') + (16)O and (18)O + (18)O(18)O (v = 0,j = 1) → (18)O(18)O (v' = 0,j') + (18)O collisions calculated by a time-independent quantum mechanical method employing one of the latest potential energy surface of ozone [ Dawes ; et al. J. Chem. Phys. 2013 , 139 , 201103 ]. The results obtained for both collisional systems in the energy range 0.001-0.2 eV are examined, and interesting mass scaling effects have been discovered. The shapes of product angular distributions suggest a transition from an indirect to a direct scattering, via an osculating intermediate complex.

Influence of the Reactants Rotational Excitation on the H + D2(v = 0, j) Reactivity.

We have analyzed the influence of the rotational excitation on the H + D2(v = 0, j) reaction through quantum mechanical (QM) and quasiclassical trajectories (QCT) calculations at a wide range of total energies. The agreement between both types of calculations is excellent. We have found that the rotational excitation largely increases the reactivity at large values of the total energy. Such an increase cannot be attributed to a stereodynamical effect but to the existence of recrossing trajectories that become reactive as the target molecule gets rotationally excited. At low total energies, however, recrossing is not significant and the reactivity evolution is dominated by changes in the collision energy; the reactivity decreases with the collision energy as it shrinks the acceptance cone. When state-to-state results are considered, rotational excitation leads to cold product's rovibrational distributions, so that most of the energy is released as recoil energy.

State-to-State Reaction Dynamics in Collision of Deuterium Molecule with Excited-State Nitrogen Atom

Abstract The collision dynamics of deuterium molecule with excited-state nitrogen atom has been investigated at a state-to-state dynamic level by using the real wave packet method. The energy-dependent behavior of the differential and integral cross sections, especially from the reactant quantum states of v = 0, j = 0 to the product quantum states of j′ = 0–7, v′ = 0–5, is presented and discussed for total energies up to 0.4 eV. The present study predicted noninverted vibrational populations and inverted rotational populations in product rovibrational distributions. Comparison of the theoretical total differential cross section with the previous experimental measurement has also been presented and discussed.

Isotope effects on the dynamics properties and reaction mechanism in the Cl(2P) + NH3 reaction: a QCT and QM study

Isotope effects on dynamics properties and reaction mechanism in the title reaction, which evolve through deep wells in the entry and exit channels, were analysed using both quasi-classical trajectory and reduced dimensional quantum mechanical calculations, for collision energies in the range 3.0–10.0 kcal mol−1, on an analytical potential energy surface (PES-2010) recently developed by our group. The analysis of different dynamics properties (reaction probability, product energy partitioning, and product rovibrational distributions) shows the reaction behaviour of the two reactions, Cl + NH3 and Cl + ND3, to be similar: direct mechanism, with a small percentage of indirect mechanisms, that is, trapping complex mediated. We find that the only dynamics property dependent on the isotope effect is the product scattering distribution. In particular, while the perdeuterated reaction favours backward scattering, the perprotio analogue favours forward scattering. This behaviour is related to the smaller maximum impact parameter for the perdeuterated reaction.

Crossed jet reactive scattering dynamics of F+H2O→HF(v,J)+OH:HF(v,J) product quantum state distributions under single-collision conditions

Reactive scattering dynamics of F+H(2)O-->HF+OH have been investigated under single-collision, crossed, supersonic jet conditions at 5.4(1.3) kcalmol, and nascent HF(v,J) rovibrational populations (v<or=2, J<or=17) have been observed via high resolution IR laser absorption methods. Vibrational populations are highly inverted, with 75(2)% and 21(5)% of the observed HF product formed in v=1 and 0, respectively. Small but finite branching [5(1)%] into the v=2 manifold is observed, which is accessible only via the additional center of mass collision energy in the crossed jets. Despite energy constraints associated with substantial vibrational excitation, the reaction dynamics also lead to rotationally hyperthermal HF, with populations observed up to near the energetic limit for each of the v=0,1,2 vibrational manifolds. The trends in these nascent product rovibrational distributions are interpreted in terms of ab initio predictions of a strongly bent, early-barrier transition state and long-range dipole-dipole interactions in the exit channel.

Correlated vH2 and jCO product states from formaldehyde photodissociation: Dynamics of molecular elimination

A detailed study of the photoinduced molecular elimination pathway of formaldehyde on the ground state surface was carried out using high-resolution dc slice ion imaging. Detailed correlated H(2) rovibrational and CO rotational product quantum state distributions were measured by imaging spectroscopically selected CO velocity distributions following photodissociation at energies from approximately 1800 to approximately 4100 cm(-1) above the barrier to molecular elimination. Excitation to the 2(1)4(1), 2(1)4(3), 2(2)4(1), 2(2)4(3), and 2(3)4(1) bands of H(2)CO are reported here. The dependence of the product rovibrational distributions on excitation energy are discussed in light of a dynamical model which has been formulated to describe the strong product state correlations observed.

Quantitative Calculation of Product Rovibrational Distributions from Atom−Diatom Exchange Reactions

We describe a rapid, accurate method for calculating rovibrational distributions in diatomic products from elementary chemical reactions. The basis of the model is momentum interconversion at a critical configuration defined in terms of molecular dimensions of the species involved. This approach shares common elements with recent models of inelastic processes and the kinematic reactive model of Elsum and Gordon. We point out that these and related approaches represent a development of Newtonian mechanics equivalent to that followed in the conventional formulation of classical mechanics, but one in which motive force for change at the molecular level is attributed to dp/dt rather than to dV/dq. This leads to a particularly transparent form of mechanics that uses only familiar data such as bond length, mass, spectroscopic constants, and velocity, yet may be applied to the highly resolved single collision experiments of molecular reaction dynamics. We describe key aspects of the computational method, e.g., the definition of the critical configuration, the disposal of reaction enthalpy, the manner of assigning product vibrational states, and the way in which conservation of energy is ensured. Examples are chosen to illustrate the range of reactions to which the method may be applied. Each would represent a challenge to conventional theory. We show that velocity-angular momentum diagrams may be used to interpret data and to give physical insight into the origins of observed rotational distributions. Good agreement is obtained between experimental and calculated (v,j) distributions for a wide range of elementary reactions suggesting that our model, despite its simplicity, captures the principal physics of chemical change at the molecular level

CHEMICAL REACTIONS IN THE O(1D) + HCl SYSTEM II.

Using the accurate global potential energy surfaces for the 11A′, 11A′′, and 21A′ states reported in the previous sister Paper I, detailed quantum dynamics calculations are performed for these three adiabatic surfaces separately for J = 0 (J: total angular momentum quantum number). Overall reaction probabilities for O + HCl → OH + Cl and H + ClO, the branching ratio between the two reactions, effects of the initial rovibrational excitation, and product rovibrational distributions are evaluated in the total energy region E tot ≤ 0.9 eV. Significant contributions to the overall reaction dynamics are found from the two excited 11A′′ and 21A′ potential energy surfaces, clearly indicating the insufficiency of the dynamics only on the ground 11A′ surface. The detailed dynamics on the excited surfaces are reported in the third paper of this series.

A Quasiclassical Trajectory Study of the Cl + HCN → HCl + CN Reaction Dynamics. Microscopic Reaction Mechanism of the H(Cl) + HCN → H2(HCl) + CN Reactions

The Cl + HCN → HCl + CN reaction dynamics has been studied using the quasiclassical trajectory method. The potential energy surface is taken from an accurate global surface for the HHCN system. Cl + HCN and H + HCN have very similar energetics, so the present calculation provides a test of whether the Cl + HCN dynamics is captured by a model in which the only difference is provided by the mass of the attacking atom. We find generally good agreement with experimental studies of the Cl + HCN reaction, including CN product rovibrational distributions and the relative rate coefficients for HCN initially in highly excited vibrational states. The results correctly describe the differences between Cl and H attack, so apparently the differences in the reactivity of these two reactions are a kinematic effect. A detailed analysis of the microscopic reaction mechanism of the H + HCN → H2 + CN and Cl + HCN → HCl + CN reactions is also provided. This shows that the H and Cl reactions are both dominated by direct dynam...

Vibrational–rotational distributions of NO formed from N+O 2 reactive collisions

We present the results of quasi-classical trajectory (QCT) calculations for the N( 4 S)+ O 2 reaction over a broad range of collision energies with scaled ab initio potential energy surfaces of the 2 A ′ and 4 A ′ electronic states. The analysis of product rovibrational distributions support the suggestion that this reaction is a source of highly excited NO in the upper atmosphere. The computed NO vibrational distribution is found to be in good agreement with a recent measurement.

A simple model for product rovibrational distributions in elementary chemical reactions

We explore the application of a simple model of collisional processes, developed initially for inelastic collisions, to the analysis of product rovibrational states in elementary chemical reactions. The model depicts collisional transfer as a process of momentum exchange (predominantly linear-to-angular momentum) and is modified to take account of change in center-of-mass and enthalpy change that accompany reaction. The kinematics of center-of-mass shift derived by Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)] lead to two limiting cases based on the parameter β. The kinematic extremes alternatively may be specified in terms of the molecular torque arm about which interconversion of linear and angular momentum is effected. This torque arm length approximates to the product bond length when β≃0 and the reactant bond length when β≃90°. Our approach shares elements in common with the classical kinematic model of Elsum and Gordon but is somewhat simpler and more transparent. The method is shown to give accurate peak values of v, j states of the products of a wide range of elementary reactions for which experimental data is available. Monte Carlo trajectory calculations based on the physical principles described here give excellent fits to experimental v, j distributions in F+I2→IF+I, H+D2→HD+D, and Cl+H2→HCl+H using input data consisting of atomic radii, atomic masses, velocities, and reaction enthalpies.

What is the best semiclassical method for photochemical dynamics of systems with conical intersections?

We present a systematic test of four general semiclassical procedures for the theoretical treatment of multistate molecular processes such as electronically nonadiabatic photochemical reactions. The methods are tested by comparing their predictions to accurate quantal results for three two-state model reactions involving conical intersections. The four methods tested are Tully’s fewest-switches version of trajectory surface hopping (1990), the Blais–Truhlar trajectory surface hopping method (1983), the Ehrenfest scheme (1975–1979), and the Meyer–Miller method (1979). We test the ability of the classical path methods to predict both electronic probabilities and product rovibrational distributions. For each of the four basic approaches we test six options for extracting final-state information from the calculated dynamics. We find that, although in most cases there is qualitative agreement between average quantum mechanical and trajectory results, the overall average error is about 50% for Tully’s fewest-switches method, the Ehrenfest method, and the Meyer–Miller method, and even higher, about 60%, for the Blais–Truhlar method. These values do not include additional errors in the below-threshold regions, which are especially large for the Meyer–Miller method because of the electronic zero-point energy in the Meyer–Miller classical analog Hamiltonian.

Unusual Insertion Mechanism in the Reaction C(3P) + H2 → CH + H

We use quasiclassical trajectories to study the reaction C(3P) + H2 → CH + H using an accurate ab initio potential surface, making comparisons with recent product distribution measurements. An analysis of the reactive trajectories indicates that the formation of CH + H is completely dominated by insertion to form a 3CH2 intermediate. However, this C insertion is initiated from nearly linear C−H−H geometries rather than by perpendicular attack as has been found for other insertion reactions (like O(1D) + H2). Perpendicular insertion collisions do occur, but they are almost always nonreactive due to poor coupling between modes initially excited in 3CH2 and the product reaction coordinate (C−H stretch). The calculated product rovibrational distributions are in excellent agreement with experiment, but we find that they are not sensitive to the presence of this modified insertion mechanism. Reactions like O(1D) + H2 may also show the modified insertion mechanism at high energies. Results from C + HD and C + HC...

Influence of transition state resonances on integral cross sections and product rovibrational distributions for the Cl+HCl→ClH+Cl reaction

An accurate quantum scattering calculation for the Cl+HCl→ClH+Cl reaction has been performed. In particular, we study the influence of the lowest transition state resonance on the energy dependence of the state-to-state integral cross sections and product rovibrational distributions. The calculations use a recently developed centrifugal sudden hyperspherical (CSH) coordinate reactive scattering code. The Bondi–Connor–Manz–Römelt semiempirical potential energy surface is employed. All 161 partial waves needed for the convergence of the cross sections are included in the calculations. We find that the resonance perturbs certain reagent and product rotational levels of the vibrational ground state (v=0, j=14–16), as well as all open rotational levels (j=0–8) of the first vibrationally excited state (v=1). Transitions from the ground reagent to the ground product vibrational state, such as v=0, j=15→v′=0, j ′=15, show almost no resonance structure in the integral cross sections; rather direct scattering dominates the partial wave sum. On the other hand, transitions between perturbed v=0 rotational states and any v′=1 rotational state, or between any v=1 state and perturbed v′=0 states, or between any v=1 and any v′=1 state, show a novel resonance feature in the integral cross sections. This novel feature is a sudden smooth ‘‘step’’ in the integral cross section, centered at the resonance energy for the partial wave with zero total angular momentum quantum number (J=0). The step has a width equal to the J=0 resonance width. Sometimes this step is superimposed on a slowly varying background which arises from direct scattering. A quantitative description of these resonant steps in the integral cross sections is developed using a J-shift approximation. Because the resonance influences all rotational states for v=v′=1 in a similar way, there is no significant effect on the product rotational distributions due to the resonance. However, the resonance does produce detectable stepping behavior in the product vibrational distribution.

On the direct vibrational spectroscopy of transition states

A technique for optically exciting and detecting vibrational resonances in transition state (TS) configurations of chemical reactions is described. By radiation-dressing the TS with tunable picosecond pulses under appropriate initial conditions, resonant TS interactions can become manifested by measurable changes in reaction rates and product rovibrational distributions. The essential features illustrating the TS spectral lineshapes and experimental conditions were modeled by adopting a radiation-dressed potential surface and evaluating the vibrational transition probability by an exact density-matrix solution. Results are presented describing (1) the conditions under which significant TS transition probability occurs, and (2) how a frequency-resolved vibrational spectrum can be interpreted in terms of the shape and dynamics of the reactive surface.

A test of surprisal functions: C + + H 2 reaction product rovibrational distributions

The technique of surprisal inference, proposed by Zamir et al., is examined. In many cases the inferred single-parameter surprisals do not reproduce the experimental data adequately. Two-parameter functions are described which give a marked improvement, although they are still inferior to the distributions derived directly from experiment.