Sally Chapman

Rotational excitation of linear molecules by collisions with atoms: Comparison of classical and quantum methods

Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO–He, CS–H2, OCS–H2, HCl–He, and HCl–Ar at collision energies up to 500 cm−1. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.

Theoretical three-dimensional potential-energy surface for the reaction of Be with HF

Abstract The three-dimensional potential-energy surface for reaction of Be and HF has been computed and fit with an analytic form. Several hundred points on the surface were obtained from a two-configuration MC SCF function using a DZ GTO basis set. Comparisons with much larger calculations at a smalle rnumber of points suggest that this level of approximation gives a good qualitative and probably a reasonable quantitative description. The present surface is in good accord with a previous collinear surface (although the barrier is significantly higher in the present work) and also with a recently published valence-bond calculation. The surface is found to be quite insensitive to orientation for angles of approach between collinear and perpendicular: the minimum-energy path is not collinear. An analytic fit to the surface has been obtained. The general features are reproduced by a three-structure effective hamiltonian which is motivated by valence-bond-like ideas and which accurately reproduces the asymptotic diatomic limits. In the region of strong interaction this model is augmented by a damped fourth-order polynomial in the internuclear separations. Despite the complexity of this fitting function, it provides only a fair quantitative representation.

Theoretical study of collinear Be+FH(v1) →BeF(v2) +H

The potential energy surface for collinear Be+FH→BeF+H has been studied at various levels of ab initio approximation. A final surface was obtained from a first order configuration interaction wavefunction, using the iterative natural orbital method and a medium‐sized basis set of Slater atomic functions; this is expected to give a semiquantitative description of the reactive process. The exothermicity is computed to be 6 kcal/mole which can be compared with the best experimental value of 2±4 kcal/mole. The barrier height is predicted to be 28 kcal/mole at a geometry where both internuclear separations are extended by about 0.4 bohr from their asymptotic equilibrium values. This surface differs qualitatively from simple LEPS models. The curvature of the reaction path is much more abrupt, the atom effecting little distortion of the partner molecule until quite close approach in both entrance and exit channels. The surface was fit with bicubic splines and dynamics was studied by the quasiclassical trajectory...

A classical trajectory study of the reaction Be+HF(v,J)→BeF(v′J′)+H in three dimensions

The reaction Be+HF(v,J)→BeF(v′,J′)+H in three dimensions is studied using the quasiclassical trajectory method. The surface was recently calculated using ab initio techniques. The surface has a high barrier and a noncollinear transition state. The angular dependence of the surface is weak over a fairly wide range of angles. There is a deep potential well representing the stable molecule HBeF. We have explored the effects of reagent translation, vibration, and rotation on the reaction. The surface exhibits a strong preference for product translation, particularly near threshold. Collisions which pass near the deep potential well make a significant contribution to the reaction only when the HF molecule is internally excited. The dynamics of these collisions are in sharp contrast to the more direct ones. These results are related to recent theoretical work on the LiFH system and to experimental work on alkaline earth‐hydrogen halide reactions.

A theoretical study of the effects of vibration on the reaction O3+NO→O2+NO2

A series of model potential energy surfaces for the reaction O3+NO→O2+NO2 is described. Nuclear geometry is unconstrained: The surfaces are nine dimensional. However, the possible interaction of more than one electronic surface has not been included. The model surfaces include various exoergicities, transition state geometries, and overall reaction topologies. Classical trajectories have been used to describe the dynamics on these surfaces. A variety of initial conditions, including selective excitation of vibrational modes of the ozone molecule, has been included. While translational and vibrational energy have distinctive effects on the reaction, no mode‐specific enhancement of the reaction cross section was observed for any of the model functions. This is interpreted to result from the high degree of coupling of the internal coordinates to the reaction path and to the multidimensional curvature of the reaction coordinate. The results suggest caution in using prior studies of triatomic exchange reaction...

Quasiclassical trajectory study of colinear Cl− + HBr → HCl + Br−

Abstract Energy distributions in the products of the ion-molecule reaction Cl − + HBr → HC1 + Br − have been studied using quasiclassical trajectories on a semi-empirical collinear potential energy surface. Vibrational energy is favored in the products. While some trajectories are long-lived, the kinematic factor of the light central atom prevents effective energy redistribution.

Collinear proton transfer in a symmetric bihalide system

Abstract A semiclassical vibrationally adiabatic theory is applied to the collinear gas phase proton transfer reaction Cl − + HCl → ClH + Cl − . It is found that tunneling is negligible in the reaction and that the principal dynamic features are governed by the deep potential for the reactants. Evidence is presented that quasiclassical trajectory dynamics is unreliable for this collinear system, in that the trajectories exhibit marked nonadiabaticity - i.e. complex formation in which there is significant energy transfer between the proton motion and the relative motion of the two chloride ions. It is argued that, by contrast, the quantum collinear reaction is adiabatic and direct in character. Comparison with recent experimental results suggests that development of a full three-dimensional theory may be necessary for an adequate picture of this reaction.

Reactive bands and rebounding trajectories in collinear F + H2

Abstract Bands of reactive and unreactive trajectories have been mapped for classical collinear F + H2 (υ = 0). The edges of the band are characterized by trajectories which undergo several reflections in the corner of the potential energy surface. Multiple reflections are seen to lead to less sharply defined band edges.