Energy of intermediates

As with the phase diagrams and Pourbaix diagrams, the theoretical standard hydrogen electrode also allows us to calculate the relative energies of intermediates in electrochemical reactions. As an example, we investigate the oxygen reduction reaction (ORR). We look at the four proton and electron transfer elementary steps ... 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

Figure 7. Energies of intermediates and TS of ethene epoxidation by various rhenium peroxo complexes. Barrier heights (in kcal/mol) given relative to the corresponding direct precursors.

One should note that the surface coverage can affect the standard free energy of intermediate states and thus also the activation-energy barrier and in this indirect way, the current density of the reaction. 

The problem of calculating reaction rate is as yet unsolved for almost all chemical reactions. The problem is harder for heterogeneous reactions, where so little is known of the structures and energies of intermediates. Advances in this area will come slowly, but at least the partial knowledge that exists is of value. Rates, if free from diffusion or adsorption effects, are governed by the Arrhenius equation. Rates for a particular catalyst composition are proportional to surface area. Empirical kinetic equations often describe effects of concentrations, pressure, and conversion level in a manner which is valuable for technical applications. 

Activation energies of intermediate compounds such as acetic acid, methanol, and other low-molecular-weight compounds (170 to 350 kj/mol) are greater than high-molecular-weight compounds (20 to 100 kj/mol). 

Besides, on closing the cycle the energy of intermediate MOs changes in accordance with their pseudosymmetry the energy of T A MO increases and that of WS MO decreases (Fig. 27). 

FIGURE 4. Reaction centre mechanism. (A) Room temperature light-driven transmembrane electron transfer in the Rb. sphaeroides reaction centre. (B) The energetics of electron transfer in the Rb. sphaeroides reaction centre at room temperature, showing approximate free energies of intermediate states in forward electron transfer. Solid arrows show forward electron transfer, and dotted arrows show charge recombination to the ground state when forward electron transfer is blocked. 

Table 8.9 Energies of Intermediate Species in the Photochemistry of Enone 43...

Fig. 3.13. Relative energies of intermediates for bromination of methoxybenzene, benzene, and nitrobenzene indicating the effect of substituents on energy of intermediates.

Figure 1. Free energy of intermediates of Embden-Myerhoff glycolytic sequence in human erythrocytes. Reproduced, with permission, from Ref. 6. Copyright 1970,...

Figure 10.15 Adsorption energies of intermediates (crosses x = l circles x = 2 triangles x = 3), intermediates (circles x=l triangles x = 2), OH, and SH intermediates plotted against adsorption energies of C, N, O, and S, respectively. The adsorption energy of molecule A is defined as the total energy of A adsorbed in the lowest energy position outside the surface minus...

Scheme 14.3 Comparison of the energy of intermediates involved in arylation reactions.

Siegel DP (2008) The Gaussian curvature elastic energy of intermediates in membrane fusion. Biophys J 95(11) 5200-5215... 

Draw the reaction curve for an exothermic reaction with two intermediates (the energy of intermediate 2 is lower in energy than intermediate 1) where the energy of transition state 2 is lower in energy than transition state 1, but the energy of transition state 3 is higher than transition state 2 and lower than 1. 

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