Transition state
Encyclopedia
The transition state of a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 is a particular configuration along the reaction coordinate
Reaction coordinate
In chemistry, a reaction coordinate is an abstract one-dimensional coordinate which represents progress along a reaction pathway. It is usually a geometric parameter that changes during the conversion of one or more molecular entities....

. 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. It is often marked with the double dagger
Double Dagger
Double Dagger were a post-punk trio from Baltimore, Maryland composed of only drums, vocals, and a very loud bass guitar which fills the space a guitar would normally take. Vocalist Nolen Strals and bassist Bruce Willen also comprised the graphic design team Post Typography, which has done work for...

 ‡ symbol.

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

History of concept

The concept of a transition state has been important in many theories of the rate at which chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s occur. This started with the transition state theory
Transition state theory
Transition state theory explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium between reactants and activated transition state complexes....

 (also referred to as the activated complex theory), which was first developed around 1935 and which introduced basic concepts in chemical kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...

 which are still used today.

Explanation

A collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...

 between reactant molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s may or may not result in a successful reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

.
The outcome depends on factors such as the relative kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

, relative orientation and internal energy
Internal energy
In thermodynamics, the internal energy is the total energy contained by a thermodynamic system. It is the energy needed to create the system, but excludes the energy to displace the system's surroundings, any energy associated with a move as a whole, or due to external force fields. Internal...

 of the molecules.
Even if the collision partners form an activated complex
Activated complex
In chemistry an activated complex is defined by the International Union of Pure and Applied Chemistry as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col of a potential energy surface"...

 they are not bound to go on and form
products
Product (chemistry)
Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...

, and instead the complex may fall apart back to the reactants.

Observing transition states

Because of the rules of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

, the transition state cannot be captured or directly observed; the population at that point is zero. This is sometimes expressed by stating that the transition state has a 'fleeting existence'. However, cleverly manipulated spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 techniques can get us as close as the timescale of the technique will allow us. Femtochemical
Femtochemistry
Femtochemistry is the science that studies chemical reactions on extremely short timescales, approximately 10–15 seconds .-Introduction:...

 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 intermediate
Reactive intermediate
In chemistry a reactive intermediate is a short-lived, high energy, highly reactive molecule. When generated in a chemical reaction it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures, matrix isolation...

s 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
Potential energy surface
A potential energy surface is generally used within the adiabatic or Born–Oppenheimer approximation in quantum mechanics and statistical mechanics to model chemical reactions and interactions in simple chemical and physical systems...

 (PES). Such a saddle point is a point where there is a minimum in all dimensions but one.
Almost all quantum-chemical methods (DFT
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...

, MP2 etc.) can be used to find transition states. However, locating them is often difficult and there is no method guaranteed to find the right transition state. There are many different methods of searching for transition states and different quantum chemistry program packages include different ones. Many methods of locating transition states also aim to find the minimum energy pathway (MEP) along the PES. Each method has its advantages and disadvantages depending on the particular reaction under investigation. Summaries of some of the main methods are given below.

Synchronous transit

There are several types of synchronous transit type methods with the most common being the linear synchronous transit (LST) method and the quadratic synchronous transit (QST). The LST method generates an estimate of the transition state by finding the highest point along shortest line connecting two minima. The QST method extends this further by subsequently searching for a minimum along a line perpendicular to the previous one. The path connecting minima and the found point may then be searched for a saddle point (a maximum).

Nudged elastic band

There are many variations on the NEB (nudged elastic band) method, including the climbing image nudged elastic band and the elastic band. This method works by guessing the MEP which connects the two stable structures. A discrete number of structures (called images) are placed along the guessed-MEP. These images are moved according to: (A) the force acting on them perpendicular to the path and (B) an artificial spring force keeping the images spaced along the MEP. The highest energy image gives a good estimate of the transition state.

String method

The string method for locating transition states is similar to the NEB in many ways. It also involves a series of images along a guess of the MEP, but in this case the images are moved in two steps. Firstly, the images are moved according to the force acting on them perpendicular to the path. Using an interpolated path, the images are moved short distances along the MEP to make sure they are evenly space. Variations on the string method include the growing string method, in which the guess of the pathway is generated as the program progresses.

Dimer method

The dimer method can be used to find possible transition states without knowledge of the final structure or to refine a good guess of a transition structure. The “dimer” is formed by two images very close to each other on the PES. The method works by moving the dimer uphill from the starting position whilst rotating the dimer to find the direction of lowest curvature (ultimately negative).

The Hammond–Leffler postulate

The Hammond–Leffler Postulate states that the structure of the transition state more closely resembles either the products or the starting material, depending on which is higher in enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...

. A transition state that resembles the reactants more than the products is said to be early, while a transition state that resembles the products more than the reactants is said to be late. Thus, 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 quantifying the lateness of a transition state can be used to test the validity of the Hammond–Leffler Postulate for a particular reaction.

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
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...

 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 bicylco[2.2.2]octene which at 200°C extrudes ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 in a retro-Diels–Alder reaction.


Compared to the compound on the right (which, lacking an alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

 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
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

.

Implications for enzymatic catalysis

One way in which enzymatic
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 proceeds is by stabilizing the transition state through electrostatics
Electrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....

. 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

  • Transition state theory
    Transition state theory
    Transition state theory explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium between reactants and activated transition state complexes....

  • Transition state analogs, chemical compounds mimicking the substrate's transition state and act as enzyme inhibitors
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