Extended LEPS Surface Research Articles

Effect of vibrational excitation of HBr on the H+HBr abstraction and exchange reactions

The effect of vibrational excitation of HBr on the H+HBr→H2+Br and H+H′Br→H′+HBr reactions has been investigated on the extended LEPS surface (ELEPS) constructed on the basis of quantum chemically calculated points of PES. Together with this surface the LEPS surface of Sudhakaran and Raff [1] was used for comparison at two relative translational energies. A quasiclassical trajectory method was used to study the abstraction and exchange reaction dynamics. The reactive cross section was calculated as a function of the relative collision energy and the vibrational state of HBr. The following conclusions can be drawn from the results of the study: (i) vibrational excitation v=0→ v=2 more than doubles the reaction cross section, (ii) the increase in the collision energy is most effectively channelled into the product translational energy.

Variational transition state theory with least-action tunneling calculations for the kinetic isotope effects in the Cl+H2 reaction: Tests of extended-LEPS, information-theoretic, and diatomics-in-molecules potential energy surfaces

We apply conventional and variational transition state theory with least-action-ground-state and other transmission coefficients to calculate the rate constants and kinetic isotope effects for the reaction Cl+H2→HCl+H. First we consider collinear reactions and compare the calculated results to accurate quantal results for a given potential energy surface. This tests the dynamical methods and shows that they are reliable enough for testing potential energy surfaces. We then make calculations for the three-dimensional reactions employing 11 potential energy surfaces that have been proposed in previous work. Seven of the surfaces are extended LEPS surfaces, as proposed by Persky, Klein, and Stern; Truhlar, Magnuson, and Garrett; and Valencich and co-workers; one is an information-theoretic-bond-order-plus-anti-Morse-bend surface (called AL/AB) proposed by Agmon, Levine, Truhlar, Magnuson, and Garrett; and the final three surfaces are a diatomics-in-molecules-plus-three-center-terms surface proposed by Baer and Last and two diatomics-in-molecules surfaces proposed by Isaacson and Muckerman. Three of the surfaces (the final surface of Persky, Klein, and Stern; the first surface of Truhlar, Magnuson, and Garrett; and the AL/AB surface—all of which have relatively symmetric saddle points) are shown to be more reasonable than the others for predicting the rate constants and the H2/D2 and the HD/DH kinetic isotope effects. The calculations also indicate that the room temperature rate constants are dominated by quantum mechanical tunneling.

Semiclassical and quasiclassical calculation of reaction probabilities for collinear X + F2→XF + F (X = Mu, H, D, T)

Semiclassical calculations of reaction probabilities have been carried out for the collinear H + F2 (n = 0, 1) reaction using the best extended LEPS surface No. II of Jonathan et al. Both real and complex valued classical trajectories have been included in the calculations for an energy range where the quasiclassical total reaction probability is unity. Comparison with quantum results shows the semiclassical reaction probabilities are accurate to about ± 0·05 provided only two real or complex stationary phase points make an important contribution to the S matrix element, so that the uniform Airy or integer Bessel approximations are valid. Real semiclassical calculations are also reported for the collinear Mu, D, T + F2 (n = 0) reactions. For the D and T reactions, the semiclassical reaction probabilities are estimated to be accurate to ± 0·05, except close to the reaction threshold, but for the Mu reaction the estimated errors are much larger. In addition, quasiclassical calculations for the reaction prob...

Uni- and bimodal product energy distributions for the reactions H + Cl 2 (υ = 1) and D + Cl 2 (υ = 1)

Vibrotational reaction probabilities for the reactions H + Cl 2 (υ = 1) and D + Cl 2 (υ = 1) at translational energies close to 0.1 eV are predicted to be bimodal, with the D reaction showing the more pronounced bimodality. The corresponding rotational and vibrational product distributions may be either uni- or bimodal. The results are from accurate quantum calculations for the collinear reactions, together with an information theoretic 1D → 3D transformation. An optimized, extended LEPS surface is used.

On the potential surface dependence of the H + F 2 reaction. I. Quantum mechanical and information theoretic investigations for various extended LEPS-surfaces

Effects of potential surface variations for the H + F 2 (ν = O) → HF + F reaction have been investigated by a combination of collinear quantum calculations and an information theoretic 1D to 3D transformation. First collinear quantum calculations using the state path sum method have been performed for five extended LEPS surfaces with different combinations of Sato parameters. Total reaction probabilities at E trans = 0.109 eV increase with decreasing barrier height. The average vibrational product excitation (ƒ ν′) is found to be a linear function of the attractivity index α ⊥. The collinear vibrational distributions have been subjected to an information theoretic 1D to 3D transformation. Resulting 3D vibrational product distributions show approximately the same breadth for all surfaces, with (ƒ ν′) again linearly related to α ⊥. The breadth of the calculated distributions is narrower than the experimental one. Deviations from experiment, and the problem of selecting an “optimal” LEPS surface, are discussed. It is argued that the deviations might stem from a failure of the LEPS surfaces to account for some distinctive non-collinear reactive collisions.

Muonium chemistry: quantum mechanical calculations for the collinear reaction Mu + F 2(ν = 0) → MuF(ν′ ⩽ 3) + F

Quantum mechanical transition probabilities have been calculated for the collinear reaction Mu + F 2(ν = 0) → MuF(ν′ ⩽ 3) + F by the state path sum method. The “best” extended LEPS surface no. II of Jonathan et al. for the H + F 2 reaction has been used. Approximately 40% of the available reaction energy goes into the product vibrational degree of freedom, with the most populated state being ν′ = 1. Collinear rate constants have also been calculated. At 300 K tunnelling is very important, but at 900 K classically allowed processes are dominant.