In a chemical reaction, an unstable arrangement of atoms characteristic of the highest energy through which the atoms must pass during the reaction.
History of concept
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 (also referred to as the Activated Complex Theory), which was first developed around 1935 and which introduced basic concepts in chemical kinetics which are still used today.
Observing transition states
Because of the rules of quantum mechanics, the transition state cannot be captured or directly observed; 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.
Locating Transition States by Computational Chemistry
Transition state structures can be determined by searching for first-order saddle points on the potential energy surface. Almost all quantum-chemical methods (DFT, MP2, ...) can be used to find transition states. Methods for locating transition states are QST2 or QST3 where the starting structure is determined from the substrate and product geometries. It is often easier (especially for large systems) to optimize to a transition state geometry using semiempirical methods such as AM1 or PM3, and then use the geometries obtained as input for better methods.
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.
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