Haya Kornweitz

The effect of reagent rotation on steric requirements of elementary exchange reactions

Abstract The body-fixed coordinate system appears to be particularly suited for discussing the role of reagent rotation. The simple picture provided by the jz-conserving approximation accords with the available evidence and with specially performed trajectory computations for the O + HCl(v= 0,j) and O + DCl(v= 0,j) reactions. An even simpler, Line-of-centers-type model is also discussed. Both the possible decline in reactivity at low rotational excitation and its increase at higher levels of excitation can be accounted for.

The kinetics of the reaction F + H2 HF + H. A critical review of literature data

Published experimental studies concerning the determination of rate constants for the reaction F + H2 HF + H are reviewed critically and conclusions are presented as to the most accurate results available. Based on these results, the recommended Arrhenius expression for the temperature range 190–376 K is k = (1.1 ± 0.1) × 10−10 exp |-(450 ± 50)/T| cm3 molecule−1 s−1, and the recommended value for the rate constant at 298 K is k = (2.43 ± 0.15) × 10−11 cm3 molecule−1 s−1. The recommended Arrhenius expression for the reaction F + D2 DF + D, for the same temperature range, based on the recommended expression for k and accurate results for the kinetic isotope effect k/k is k = (1.06 ± 0.12) × 10×10 exp |-(635 ± 55)/T|cm3 molecule−1 s−1, and the recommended value for 298 K is k = (1.25 ± 0.10) × 10−11 cm3 molecule−1 s−1.

The exoergic F+CH4 reaction as an example of peripheral dynamics

Abstract Classical trajectory computations for thermal reactants on a six-atom potential show a forward scattering component which is correlated with the HF product being formed with high vibrational excitation. These trajectories are peripheral collisions where the F atom approaches CH4 with a high impact parameter and reaction is through a nearly collinear F–H–C configuration with a stretched F–H bond. Other trajectories are well described by a hard-sphere model whose cutoff is below the range of peripheral collisions. Comparison is made with the F+H2 and other reactions where nearly thermoneutral channels correlate with forward scattering.

Correlations between dynamical properties and features of potential energy surfaces for the light atom transfer reactions O+HCl→OH+Cl AND Cl+HCl→ClH+Cl

Abstract A three-dimensional quasiclassical trajectory study of the dynamics of the light atom transfer reaction O( 3 P) + HCl(ν=0)→ OH + Cl was carried out employing two LEPS potential energy surfaces (I and II). Attention was focused mainly on three-dynamical properties; the oscillatory behavior of partial cross sections as a function of collision energy; the rotational excitation of the products; and the influence of reagent rotation on reactivity. Distinct differences were found between surfaces I and II with respect to these properties. The examination of individual trajectories indicated that there is a significant difference in the nature of these surfaces. While surface I is governed by weak repulsive forces, surface II is governed by strong attractive forces which tend to direct the reactants toward a collinear geometry. The present results confirm conclusions reached from an earlier study of the reaction Cl+HCl→ClH+Cl concerning correlations between dynamical properties and features of potential energy surfaces. For surfaces of the type that we termed HREP, since they are of rep ulsive nature and they lead to h ighly rotationally e xcited p roducts, no significant oscillations of partial cross sections are obtained and reagent rotation promotes the reaction. On the other hand, for surfaces of the type that we termed COLD ( col linearly d irecting), since they tend to direct the reactants toward a collinear geometry and form rotationally “ cold ” products, significant oscillations of partial cross sections are obtained and reagent rotation causes a decline in reactivity.

Steric hindrance can be probed via the dependence of the reactivity on reagent rotation: O+HCl

Abstract Kinematic considerations imply that the steric hindrance due to a “bulky” group can be directly probed using rotationally excited reactants. At typical collision and angular velocities, a shadow due to the bulky group restricts entry into the cone of acceptance for rotationally excited reactants. The conclusion is discussed for the realistic case where the anisotropy of the long range potential can reorient the incoming trajectories. Exact and approximate (reactant jz conserving) classical trajectory computations for the O+HCl and O+DCl reactions, on two different potential energy surfaces are used as an illustration.

The effect of reagent vibrational excitation on the oscillatory behavior and other dynamical properties of the light-atom-transfer reactions Cl + HCl → ClH + Cl and O + HCl → OH + Cl

Abstract A three-dimensional quasiclassical trajectory study was carried out to investigate the effect of reagent vibrational excitation on several dynamical properties of the light-atom-transfer reactions Cl + HCl → ClH + Cl and O + Hcl → OH + Cl for various types of potential energy surfaces. These properties are the oscillatory behavior of partial cross sections as a function of collision energy, the influence of reagent rotation on reaction cross sections, and the rotational excitation of the products. Earlier studies for ground vibrational state reagents (ν = 0) showed that these properties correlate well with features of potential energy surfaces. It was found that vibrational excitation of the reagents causes a significant amplification of the oscillatory behavior for all types of surfaces and leaves the dependence of reaction cross sections on reagent rotation unchanged qualitatively. The differences in the rotational energy of the products for different types of surfaces, while still significant for ν= 1, tend to decrease with reagent vibrational excitation. The present results indicate an obvious advantage in using vibrationally excited reagents in experimental investigations of oscillations in reaction cross sections. They also indicate that studies with rovibrational state selected reagents can provide significant information concerning the nature of potential energy surfaces for heavy+light-heavy→heavy-light+heavy bimolecular reactions.

Correlations between dynamical properties and features of potential energy surfaces for the exothermic light-atom-transfer reaction O+HBr→OH+Br

Abstract A three-dimensional quasiclassical trajectory study of the dynamics of the exothermic reaction O+HBr→OH+Br, which belongs to the heavy+light-heavy class of reactions, was carried out. Two LEPS potential energy surfaces with contrasting features were employed in the calculations. Attention was focused mainly on three dynamical properties: the oscillatory behavior of partial cross sections as a function of collision energy, the influence of reagent rotation on reactivity, and the rotational excitation of the products. It is concluded that correlations between dynamical properties and features of potential energy surfaces found earlier for thermoneutral (Cl+HCl) or nearly thermoneutral (O+HCl) heavy+light-heavy reactions are also correct for the O+HBr exothermic reaction.

Oscillating reactivity in quasiclassical trajectory calculations for the reaction O(3P) + HC1 → OH + Cl

Abstract Strong oscillations in the reactivity as a function of collision energy were found in collinear quasiclassical trajectory calculations for the reaction O(3P)+HCl(v = 0) → OH + Cl. Oscillations were also observed in 3D calculations for HCl(v = 0, J = 0) under the special conditions of near-zero impact parameters b and small and specific values of the azimuthal orientation angle θr, the angle between the molecular axis of HC1 and the initial direction of the relative velocity vector. These oscillations were washed out when θr and b were randomly selected from the appropriate ranges, and were not observed even for the reactive scattering in the angular ranges 170 to 180° and 175 to 180°.

Driving high threshold chemical reactions during the compression interlude in cluster surface impact

Molecular-dynamics simulations of a cluster impacting a hard surface show that, initially, the cluster is rapidly compressed and translationally heated. During this short but distinct stage, the cluster is a suitable medium for chemistry: the number of layers of the cluster is not changing; the constituents of the cluster can collide several times and both bimolecular and collisionally driven unimolecular reactions can occur. Hypersonic velocities of impact are needed for a considerable temperature rise. Following compression, the cluster fragments by expanding into a hemispheroidal plume. For supersonic impact, the cluster expands nearer to the surface forming an oblate, omelet-like, hemispheroid.

Oscillations in product state distributions in the light-atom transfer reactions Cl+HCl→ClH+Cl and O+HCl→OH+Cl

Abstract Three-dimensional quasiclassical trajectory studies of the reactions Cl+HCl→ClH+Cl and O+HCl→OH+Cl, which belong to the class heavy+light-heavy→heavy-light+heavy, reveal the novel phenomenon of oscillations in product state distributions as a function of collision energy. The conditions for obtaining such oscillations are the same as those found in earlier studies for obtaining oscillations in partial cross sections.