Transition state

Source: Wikipedia, the free encyclopedia.
Not to be confused with
Transition of state
.

In

double dagger
(‡) symbol.

As an example, the transition state shown below occurs during the SN2 reaction of bromoethane with a hydroxide anion:

angstroms. Note the elongated C-Br and C-O bonds, and the trigonal bipyramidal structure
.

The

reactants and products, especially those close to the transition state.[3]

According to the transition state theory, once the reactants have passed through the transition state configuration, they always continue to form products.[3]

History of concept

The concept of a transition state has been important in many theories of the rates 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 by Eyring, Evans and Polanyi, and introduced basic concepts in chemical kinetics that are still used today.[citation needed]

Explanation

A

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.[citation needed]

Observing transition states

Because the structure of the transition state is a first-order

IR spectroscopy was developed for 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.

Determining the geometry of a transition state

Transition state structures can be determined by searching for first-order saddle points on the potential energy surface (PES) of the chemical species of interest.

geometry optimization
.

The Hammond–Leffler postulate

The

Hammond–Leffler Postulate
predicts a late transition state for an endothermic reaction and an early transition state for an exothermic reaction.

A dimensionless reaction coordinate that quantifies the lateness of a transition state can be used to test the validity of the

Hammond–Leffler postulate for a particular reaction.[5]

The structure–correlation principle

The structure–correlation principle states that structural changes that 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.

bicyclic compounds depicted below.[7] The one on the left is a bicyclo[2.2.2]octene, which, at 200 °C, extrudes ethylene in a retro-Diels–Alder reaction
.

Structure Correlation Principle

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.

Implications for enzymatic catalysis

One way that 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.

See also

References

  1. ISBN 0-471-41799-8
    .
  2. B3LYP functional and a 6-31+G* basis set
    .
  3. ^
    ISBN 0-7167-8759-8
  4. ^ Frank Jensen (1999). Introduction to Computational Chemistry. England: John Wiley and Sons Ltd.
  5. doi:10.1002/jcc.21440
    .
  6. doi:10.1021/ar00089a002
    .
  7. PMID 17371072
    .
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