Potential-energy surfaces Early barrier

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

Fig. 3.1.4 Contour plot of a potential energy surface for the reaction A + BC —> AB + C. The surface is shown as a function of the two internuclear distances Rab and Rbc at a fixed approach angle. The barrier (marked with an arrow) occurs in the entrance channel, i.e., an early 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.

The H + HI abstraction reaction is very exothermic and proceeds upon a potential energy surface with an early barrier and low activation energy. In such a system, there is a propensity to convert the translational energy of the reaction into... 

It is not entirely clear that both HF HCl and HCl HF represent minima on the potential energy surface of this mixed complex. An early calculation by Hobza et al. [109] indicated that the barrier separating these two putative minima will disappear when dispersion and zero-point vibrations are considered. Later calculations by Latajka and Scheiner [103] using a larger basis set and explicit inclusion of correlation arrived at a barrier of only 0.6 kJ/mol after zero-point correction. While the question of whether both of these minima indeed exist remains open to higher levels of theory, it seems clear that any barrier which might exist will be exceedingly small. 

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