Distribution of Reaction Products. V. Reactions Forming an Ionic Bond, M+XC (3 d)

Publisher Website
Google Scholar
Abstract
In the previous paper (Part IV) a preliminary investigation was made of the electron-jump model for reactions M+XC[→M++(XC)−]→products, in which an alkali-metal atom M reacts with a halogen molecule (XC≡XY) or with a halide, to give alkali halide. The method used was the classical trajectory method in two dimensions (2d). In the present work some of these early calculations have been extended into 3d, and comparison has been made between theory and recent molecular-beam experiments. The 12 principal potential-energy hypersurfaces used in this 3d work comprised six which permitted charge migration and six which, though identical in other respects, did not permit charge migration. The groups of six exemplified the effects of separate variation in electron-jump distance, r1x, in the magnitude of the B·C repulsion in (BC)−, Ri, and in the total energy released, QC. All but the last of these variables had important effects on the product energy and angular distributions. It was found that with a proper choice of r1x and Ri, the crude electron-jump model used here was able to account very satisfactorily for the major features of the experimental findings. Sharp forward scattering of the molecular product was one of the features of reactions M+XY, both experimentally and theoretically. The mechanics of this forward scattering, according to the present work consist of an initial M+- -X− attraction, followed by charge migration to give M+Y−+X. The rejected atom is not, therefore, a “spectator,” but is subject to strong forces which enhance its translational energy. A qualitatively similar correlation between forward scattering and increased relative translational energy of the products has been observed experimentally by Birely and Herschbach. In addition, the calculations predict enhanced rotational excitation in the forward-scattered portion of the molecular product; experimental evidence is lacking on this point. The experiments show a striking change from rebound to forward scattering in going from M+XR (where R is an alkyl radical) to M+XY. This change in behavior stems, according to the present work, from the occurrence of charge migration coupled with markedly diminished product repulsion in the M+XY as compared with the M+XR family. A simple analytical model termed the DIPR (Direct Interaction with Product Repulsion) model was shown to account quantitatively for product energy and angular distributions in direct trajectories without secondary encounters. The model provides insight into the transition (on nonmigration surfaces) from backward to sideways to forward scattering as product repulsion is decreased, and shows the utility of the concept of a “stripping threshold energy.”