Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always go on to form products . The transition state shown below occurs during the SN2 reaction of bromoethane with a hydroxyl anion.
File:Transition State.png

# 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.

# Explanation
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.

# 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.  Femtochemical 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.

# Locating Transition States by Computational Chemistry
Transition state structures can be determined by searching for first-order saddle points on the potential energy surface. Such a saddle point is a point where there is a minimum in all dimensions but one. 
Almost all quantum-chemical methods (DFT, MP2, ...) can be used to find transition states. However, locating them is often problematic as the starting structure must be very close to the actual TS. 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 either the product or the starting material, depending on which is higher in enthalpy.

# The Structure-correlation principle
The structure-correlation principle states that that structural changes which occur along the reaction coordinate can reveal themselves in the ground state as deviations of bond distances and angles from normal values along the reaction coordinate. . According to this theory if one particular bond length on reaching the transition state increases then this bond is already longer in its ground state compared to a compound not sharing this transition state. One demonstration of this principle is found in the two bicyclic compounds depicted below . The one on the left is a bicylcooctene which at 200C extrudes ethylene in a retro-Diels-Alder reaction.
Compared to the compound on the right (which, lacking an alkene group, is unable to give this reaction) the bridgehead carbon-carbon bond length is expected to be shorter if the theory holds because on approaching the transition state this bond gains double bond character. For these two compounds the prediction holds up based on X-ray crystallography and 13C coupling constants (inverse linear relationship with bond length).

# 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.