Entrance valley

In the entrance valley rAB is large and decreases towards the col. At the entrance to the exit valley rBC is large, and decreases towards the col. At the col rAB = rBC, and this holds for all points along the line, PQ, as drawn. On the profile the activated complex lies at a maximum where rAB = rBC-... 

Other situations can occur where the activated complex lies in the entrance valley, an early barrier, or in the exit valley, a late barrier. 

The activated complex and PE barrier are in the entrance valley, where rAH > rBC,... 

Reactions on attractive surfaces with early barriers are promoted by high translational energy in the reactants, with vibrational energy playing a minor role. Selective enhancement by translational energy is easiest when there is a straight run up the entrance valley to the critical configuration. 

Vibrational energy is less important, because high vibrational energy would encourage the reaction unit to hit the barrier wall perpendicular to the reaction coordinate at the end of the straight run up the entrance valley, from which it would be bounced back down the entrance valley. 

If the well is in the entrance valley the reaction unit can often need vibrational energy, but if it is in the exit valley translational energy is often more effective. Such features are indicative of a collision complex which lasts sufficiently long for many vibrations and some rotations to occur. The lifetime of a collision complex is long... 

Since the M—N distance in the activated complex is only slightly greater than the intemuclear distance in the reactant molecule, then the reaction entity has only moved very slightly along the reactant valley. This means that the activated complex lies in the entrance valley, giving an early barrier. This conclusion is verified by the large P—M distance in the activated complex, which indicates that P is still far from MN in the activated complex. 

The next step is to make a reasonable choice of the surface separating reactants from products. It is clear that it should be near the saddle point, if such a point exists. To make the geometrical factor in the expression for the rate constant simple, we choose the dividing surface to be perpendicular to the rc,AB-coordinate spanning the entrance valley from some minimum distance tab,min to some maximum distance r AB.max- That is, we identify coordinate q with / c ab, and the surface is given by the equation... 

These results show that one could represent the motion of the complex by one-dimensional modes on surfaces correlated to excited atomic calcium (see Figure 4-7), and the spectra are characteristic of the entrance valley of this reaction. On the other hand, the chemiluminescent reaction of excited calcium with HCl is known to occur with very high cross sections 25 A2 ( D2) (Brinckman et al. 1980) to 68 A2 (1P) (Rettner and Zare 1982). These cross sections agree with a passage of the ionic covalent crossing without a barrier at 3.5 A. Hence, there should be a smooth passage from the reagents to the products and the observation of action spectra with distinct features, not a continuum, can be interpreted as the excitation of local modes perpendicular to the reaction coordinate. 

F+H7. We see this sort of behavior In the reaction F+H2(v 0) + HF(v 2)+H on a modification (41) of the M5 potential surface. This modified surface Is called surface 3 In reference 41 It differs from the M5 surface In the following three qualitative ways (1) the new surface has a lower colllnear barrier to reaction, (11) It has a softer bending potential In the entrance valley, and (111) has a much lower adiabatic barrier In the exit valley for reaction Into the HF(v =3) final state. 

He-t-H . Ue performed a set of BCRLM calculations for the reaction HefH2 HeH +H using the DIM representation (42) of the potential surface. A great many resonance features have been seen (43) In the colllnear reactive dynamics of this system, and many have been Identified (44) with the attractive well which Is present on this surface In t1 e entrance valley. Because the bending potential predicted by the DIM surface Is very weak, the BCRLM dynamics (for the 1 0 partial wave) resembles that of the colllnear calculation. Reaction probabilities for the reaction He4-H2 (v 3) HeH (v °0)+H are shown In Figure 15 for several partial waves. The predominance of many resonances In each partial wave Is evident In the figure. 

Figure 16. Angular distributions for the reaction He+H2 (v 3) HeH (v 0)+H. Total energies are measured from the minimum of the H2 entrance valley well.

The computational approach of Kuppermann (1971) has recently been applied by Baer (1974) to H + Cl2 and D + Cl2 reactions, partly to improve upon previous calculations that neglected some closed channels. He used a LEPS surface with a barrier of 0-108eV in the entrance valley. Reaction probabilities for H + Cl2 from i = 0,1. 2 to rf < 7 showed that the 0 - 4 transition dominated at low energies, while 0 - 5 and then 0 -> 6 dominated as energies increased. The general trend was the same for vt = 1 or 2, but, in detail, the distributions of v depended on ly These dependencies were discussed in terms of a model of vertical non-adiabatic transitions between two displaced vibrational wells. Results with t,- = 0 for D + Cl2 showed that 0 — 5 dominated at low energies. Total transition probabilities were weakly dependent on both vt and isotopic masses. 

For collisions at large impact parameters, these observations have been interpreted44,45 in terms of reaction dynamics which are initiated by an electron jump in the entrance valley of the potential surface. A mutual ion dissociation reaction follows, in which the halogen molecule anion dissociates... 

The previous examples (Section II.A) of facile four-centre reactions appear to be enabled by the intrusion of ionic interactions into the electronic structure of the potential energy surfaces. The alkali halide-alkali halide exchange reactions involve purely electrostatic interactions. The alkali dimer-halogen molecule reactions involve an electron jump in the entrance valley of the potential surface and thereafter proceed as ion recombination reactions. The present alkali halide-halogen molecule reactions also appear to be permitted by virtue of ionic interactions. The reaction complex, as illustrated in Fig. 26, has the structure of an alkali trihalide molecule. The observed... 

The electron jump mechanism, (see 1), has long been invoked71,2 in a highly simplified manner to explain the stripping dynamics of alkali atom-halogen molecule reactions M + X2. Electron transfer occurs in the entrance valley of the covalent M + X2 potential surface near the intersection with the ionic M+ + X2 potential surface. The M - X2 internuclear distance R at the intersection Rc is roughly estimated... 

Another system where accurate microcanonical rate constants have been calculated is Li + HF - LiF + H with 7 = 0 (172). This reaction has variational transition states in the exit valley. Variational transition state theory agrees very well with accurate quantum dynamical calculations up to about 0.15 eV above threshold. After that, deviations are observed, increasing to about a factor of 2 about 0.3 eV above threshold. These deviations were attributed to effective barriers in the entrance valley these are supernumerary transition states. After Gaussian convolution of the accurate results, only a hint of step structure due to the variational transition states remains. Densities of reactive states, which would make the transition state spectrum more visible, were not published (172). 

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