The direction of attack and separation
Figure 24.19 shows the results of a calculation of the potential energy as an H atom approaches an H2 molecule from different angles, the H2 bond being allowed to relax to the optimum length in each case. The potential barrier is least for collinear attack, as we assumed earlier. (But we must be aware that other lines of attack are feasible and contribute to the overall rate.) In contrast, Fig. 24.20 shows the potential energy changes that occur as a Cl atom approaches an HI molecule. The lowest barrier occurs for approaches within a cone of half-angle 30° surrounding the H atom. The relevance of this result to the calculation of the steric factor of collision theory should be noted: not every collision is successful, because not everyone lies within the reactive cone. If the collision is sticky, so that when the reactants collide, they orbit around each other, the products can be expected to emerge in random directions because all
Fig. 24.19 An indication of the anisotropy of the potential energy changes as H approaches H2with different angles of attack. The collinear attack has the lowest potential barrier to reaction. The surface indicates the potential energy profile along the reaction coordinate for each configuration.
Fig. 24.20 The potential energy barrier for the approach of Cl to HI. In this case, successful encounters occur only when Cl approaches within a cone surrounding the H atom.
memory of the approach direction has been lost. A rotation takes about 1 ps, so if the collision is over in less than that time the complex will not have had time to rotate and the products will be thrown off in a specific direction. In the collision of K and I2, for example, most of the products are thrown off in the forward direction. This product distribution is consistent with the harpoon mechanism (Section 24.1c) because the transition takes place at long range. In contrast, the collision of K with CH3I leads to reaction only if the molecules approach each other very closely. In this mechanism, K effectively bumps into a brick wall, and the KI product bounces out in the backward direction. The detection of this anisotropy in the angular distribution of products gives an indication of the distance and orientation of approach needed for reaction, as well as showing that the event is complete in less than 1 ps.
