Chemical reaction coordinate diagrams

Quantum tunnelling in chemical reactions can be visualised in terms of a reaction coordinate diagram (Figure 2.4). As we have seen, classical transitions are achieved by thermal activation - nuclear (i.e. atomic position) displacement along the R curve distorts the geometry so that the... 

Figure 2.4. Reaction coordinate diagram for a simple chemical reaction. The reactant A is converted to product B. The R curve represents the potential energy surface of the reactant and the P curve the potential energy surface of the product. Thermal activation leads to an over-the-barrier process at transition state X. The vibrational states have been shown for the reactant A. As temperature increases, the higher energy vibrational states are occupied leading to increased penetration of the P curve below the classical transition state, and therefore increased tunnelling probability.

Fig. 1. Reaction coordinate diagrams for chemical excitation processes 4>...

A FREE-ENERGY REACTION COORDINATE DIAGRAM shows the free energy of the substrate, product, and transition state of a chemical reaction. It tells you how favorable the overall reaction is (AGeq) and how fast (AG1). 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

Figure 1. A hypothetical reaction coordinate diagram for an enzyme-catalyzed chemical reaction.

Diagrams like this ( reaction coordinate diagrams) provide essential connections between important chemical observables - structure, stability, reactivity and selectivity - and energy. These connections are explored in the following sections. 

Real chemical reactions need not occur in a single step, but rather may involve several distinct steps and one or more intermediates . The overall sequence of steps is termed a mechanism and may be represented by a reaction coordinate diagram. 

The usual picture of a chemical reaction is in terms of a onedimensional potential energy (or reaction coordinate) diagram. 

FIGURE 6-2 Reaction coordinate diagram for a chemical reaction. 

FIGURE 1. Reaction coordinate diagram for the bromination of fnms-p-methylstilbene (a) and fra u-p-p -bis(trifluoromethyl)stilbene. (b). Reprinted with permission from G. Bellucci, R. Bianchini, C. Chiappe, R. S. Brown and H. Slebocka-Tilk, J. Am. Chem. Soc., 113, 8012 (1991). Copyright (1991) American Chemical Society... 

Another chemical analogy is nseful in elncidating the meaning of band bending diagrams. Chemists are quite comfortable with reaction coordinate diagrams, in which the free energy of the system is plotted versus some (typically... 

Figure 2. Proposed triple-minimum potential energy vs. reaction coordinate diagram for the reactions of PhN with carbonyl-containing molecules. Reproduced from reference 10. Copyright 1983 American Chemical Society.

Energy surfaces are also used to describe chemical reactions, albeit usually in vastly simplified form, that of the energy reaction-coordinate diagram (or reaction profile for short), examples of which are to be found in every general chemistry textbook (Figure 5.6). Reaction profiles are a convenient way of summarizing many kinds of experimental data and for discussing reactions from a molecular viewpoint. 

Before describing TST it is necessary to develop the concept of the reaction coordinate. We use a two-body reaction example to calculate and illustrate a potential energy surface from which a reaction coordinate diagram is constructed. Elementary reaction kinetics is a rich field 26,21,40 and the material outlined here Is far from comprehensive — it is intended to provide a framework for developing a physical picture and mathematical model of a chemical reaction. 

Figure 1. Schematic reaction coordinate diagram comparing the difference between metal atoms and metal ions inserting into a bond as a first step in a chemical reaction. The attractive well of the M(AB) complex can completely offset the insertion barrier if E >E. ...

A) Ahmed H. ZewaiVs arrow of time" depicting the evolution in the timescale of chemical reacticms accessible to measurement over the course of roughly 130 years. B) Schematic diagram illustrating how femtosecond reactions are monitored. C) A reaction coordinate diagram illustrating the discrete formation of the [I-CN] activated complex on the femtosecond timescale. D) Data for the determination of the lifetime of an activated complex. 

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