Potential-energy surfaces Late barrier

Figure 1 The ubiquitous elbow potential energy surface showing for the dissociation of a diatomic molecule on a surface. This is a function of die molecular bond length and the molecule-surface distance. The reactants are intact molecules, while die products are the atoms chemisorbed separately on the surface. The two extreme cases are shown, an early barrier for which the initial vibration of the molecule is ineffective in overcoming the barrier, and a late barrier for which vibration assists in the dissociation process.

Figure 8. Two-dimensional potential energy surfaces (schematic) for (a) early and (b) late barrier (B) of dissociation of H2 on a transition metal surface.

Of course, this suggests a barrier on the potential energy surface, which prevents further reaction of the complex to form KCl + H. With the classification of Polanyi such a barrier is a late barrier, localized somewhere in the exit channel of the reaction [70, 73], With such a barrier, translational energy is less efficient at promoting the reaction than a comparable increase in the vibrational energy of the HCl reaction partner [269], This anticipation based on the Polanyi rule for barrier location was exactly confirmed by the gas-phase measurements of Brooks and co-workers [123, 124],... 

On the other hand, there are significant zero-point energy effects in the reaction of Mu with H2 and D2, since these reactions are highly endothermic (AH = 32 and 38 kj mol 1, respectively) so the vibrationally adiabatic barrier is late. The potential energy surface of the H3 system is known to a high accuracy, so comparison of the results for these... 

Nonempirical LCAO-MO-SCP study for linear HeH surface demonstrates a late barrier, or potential-energy step characteristic for reactions showing vibrational enhancement (Fig. 61) 453... 

Fig. 14.2. The drainpipe A+BC -> AB+C (for a fictitious collinear system). The surface of the potential energy for the motion of the nuclei is a function of distances 1 aB and 1 bc On the left side, there is the view of the surface, while on the right side, the corresponding maps are shown. The barrier positions are given by the crosses on the right figures. Panels (a) and (b) show the symmetric entrance and exit channels with the separating barrier. Panels (c) and (d) correspond to an exothermic reaction with the barrier in the entrance channel ( an early barrier ). Panels (e) and (f) correspond to an endothermic reaction with the barrier in the exit channel ( a late barrier ). This endothermic reaction will not proceed spontaneously, because due to the equal width of the two channels, the reactant s free energy is lower than the product s free energy. Panels (g) and (h) correspond to a spontaneous endothermic reaction, because due to the much wider exit channel (as compared to the entrance channel), the free energy is lower for the products. Note that there is a van der Waals complex well in the entrance channel just before the barrier. There is no such well in the exit channel.

We notice that the molecule can pass either an early barrier located in the entrance channel (where r the gas phase equilibrium distance) or a late barrier in the exit channel where r > If the barrier is late, then vibrational excitation of the incoming molecule enhances the dissociative sticking process if it is early, the dissociation is enhanced by increasing the translational energy of the molecule. This early and late barrier discussion is therefore identical to the one known from gas-phase reaction dynamics [123]. Thus it is natural to try to use similar model potentials in molecule surface interaction to those which have been used in gas-phase dynamics. Such a model potential is the one due to London, Eyring, Polanyi, and Sato, in short, denoted the LEPS potential. 

Potential energy contours of the eleven surfaces are shown in Figs. 2-5. If the angle 0i (or 82) is minimized, the A, B, C, and D surface types can be characterized by their potential energy contour maps in the r], R plane. Such plots are shown in Fig. 2. In the terminology used for A 4 BC -> AB 4- C reactions, all of the surfaces have late barriers for H-C-C -> H + C=C dissociation. Potential energy contour maps of r vs, 0i for optimized R are shown in Figs. 3-5 for all eleven surfaces. These maps illustrate the... 

Calculations of QCL trajectories have also been carried out on endoergic reactive systems to explore whether vibrational energy transfer is facilitated by this form of intermolecular potential. It was anticipated that collisions in which the line (/ AB/ e.AB) = ( Bc/ e,Bc) was crossed twice (or a larger number of even times) by trajectories which turned the corner on the surface but failed to surmount the late barrier, might be especially effective in transferring energy. This was confirmed, but these encounters were rare. 

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