Transition state
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Introduction
A collision between reactant molecules may or may not result in a successful reaction. The outcome depends on factors such as the relative kinetic energy, relative orientation and internal energy of the molecules. Even if the collision partners form an activated complex they are not bound to go on and form products, and instead the complex may fall apart back to the reactants.
Formal definition
The transition state of a reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along the reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always go on to form products.
History
The concept of a transition state has been important in many theories of the rate at which chemical reactions occur. This started with the transition state theory, which was first developed around 1935 and which introduced basic concepts in chemical kinetics which are still used today.
Problems in observing transition states
Because of the rules of quantum mechanics, the transition state cannot be captured or directly observed; the population at that point is zero. However, cleverly manipulated spectroscopic techniques can get us as close as the timescale of the technique will allow us. Femtosecond IR spectroscopy was developed for precisely that reason, and it is possible to probe molecular structure extremely close to the transition point. Often along the reaction coordinate reactive intermediates are present not much lower in energy from a transition state making it difficult to distinguish between the two.
The Hammond-Leffler postulate
The Hammond-Leffler Postulate states that the structure of the transition state more closely resembles the product or the starting material, depending on which is higher in enthalpy.
Implications for enzymatic catalysis
One way in which enzymatic catalysis proceeds is by stabilizing the transition state through electrostatics. By lowering the energy of the transition state, it allows a greater population of the starting material to attain the energy needed to overcome the transition energy and proceed to product.
References
Solomons, T.W. Graham & Fryhle, Craig B. (2004). Organic Chemistry (8th ed.). John Wiley & Sons, Inc. ISBN 0-471-41799-8.