Editing Transition state

{{Short description|Configuration of a chemical reaction when potential energy is greatest}}
{{Distinguish|Transition of state}}

In [[chemistry]], 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 [[Chemical energy|potential energy]] along this reaction coordinate.<ref>{{cite book|author1=Solomons, T.W. Graham|author2=Fryhle, Craig B.|name-list-style=amp|year=2004|title=Organic Chemistry|edition=8th|publisher=John Wiley & Sons, Inc.|isbn=0-471-41799-8|url-access=registration|url=https://archive.org/details/organicchemistry00solo_0}}</ref>  It is often marked with the [[double dagger]] (‡) symbol.

As an example, the transition state shown below occurs during the [[SN2 reaction|S<sub>N</sub>2]] reaction of [[bromoethane]] with a [[hydroxide]] anion:

[[File:Bromoethane SN2 reaction.svg|650px|center]]

[[File:Base hydrolysis of bromoethane, TS.png|thumb|right|The [[Density functional theory|DFT]]-determined geometry for the transition state of the above reaction.<ref>The calculation used a [[B3LYP]] functional and a 6-31+G* [[basis set (chemistry)|basis set]].</ref> Distances are listed in [[angstroms]]. Note the elongated C-Br and C-O bonds, and the [[Trigonal bipyramidal molecular geometry|trigonal bipyramidal structure]].]]

The [[activated complex]] of a reaction can refer to either the transition state or to other states along the reaction coordinate between [[reactants]] and [[Product (chemistry)|products]], especially those close to the transition state.<ref name=Atkins>[[Peter Atkins]] and Julio de Paula, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.809 {{ISBN|0-7167-8759-8}}</ref>

According to the [[transition state theory]], once the reactants have passed through the transition state configuration, they always continue to form products.<ref name=Atkins/>

== History of concept ==

The concept of a transition state has been important in many theories of the rates at which [[chemical reaction]]s occur. This started with the [[transition state theory]] (also referred to as the activated complex theory), developed independently in 1935 by [[Henry Eyring (chemist)|Eyring]], [[Meredith Gwynne Evans|Evans]] and [[Michael Polanyi|Polanyi]], and introduced basic concepts in [[chemical kinetics]] that are still used today.<ref>{{cite web |title=Theories of reaction rates |url=https://www.britannica.com/science/chemical-kinetics/Theories-of-reaction-rates |website=Encyclopedia Britannica |access-date=30 January 2025}}</ref>

== Explanation ==

A [[collision]] between [[reactant]] [[molecule]]s may or may not result in a successful [[Chemical reaction|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 [[product (chemistry)|products]], and instead the complex may fall apart back to the reactants.{{cn|date=March 2024}}

== Observing transition states ==

Because the structure of the transition state is a first-order [[saddle point]] along a [[potential energy surface]], the population of species in a reaction that are at the transition state is negligible. Since being at a saddle point along the potential energy surface means that a force is acting along the bonds to the molecule, there will always be a lower energy structure that the transition state can decompose into. This is sometimes expressed by stating that the transition state has a ''fleeting existence'', with species only maintaining the transition state structure for the time-scale of vibrations of chemical bonds (femtoseconds). However, cleverly manipulated [[spectroscopy|spectroscopic]] techniques can get us as close as the timescale of the technique allows. [[Femtochemistry|Femtochemical]] [[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 intermediate]]s 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.<ref>{{Cite book|title=Introduction to Computational Chemistry|publisher= John Wiley and Sons Ltd|location= England|year=1999|author= Frank Jensen}}</ref> A first-order saddle point is a [[critical point (mathematics)|critical point]] of index one, that is, a position on the PES corresponding to a minimum in all directions except one. This is further described in the article [[geometry optimization]].

== 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]]. 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 process|endothermic reaction]] and an early transition state for an [[Exothermic process|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.<ref>{{cite journal|title=A dimensionless reaction coordinate for quantifying the lateness of transition states |journal= Journal of Computational Chemistry|year= 2009|doi=10.1002/jcc.21440|author1=Thomas A. Manz |author2=David S. Sholl |volume= 31|issue= 7|pages= 1528–1541|pmid= 19908292}}</ref>

== 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''.<ref>{{cite journal|doi=10.1021/ar00089a002|title=From crystal statics to chemical dynamics|year=1983|last1=Buergi|first1=Hans Beat|last2=Dunitz|first2=Jack D.|journal=Accounts of Chemical Research|volume=16|issue=5|pages=153}}</ref> 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.<ref>{{cite journal |doi=10.1021/jo0625610 |title=Manifestations of the Alder−Rickert Reaction in the Structures of Bicyclo[2.2.2]octadiene and Bicyclo[2.2.2]octene Derivatives |year=2007 |last1=Goh |first1=Yit Wooi |last2=Danczak |first2=Stephen M. |last3=Lim |first3=Tang Kuan |last4=White |first4=Jonathan M. |journal=The Journal of Organic Chemistry |volume=72 |issue=8 |pages=2929–35 |pmid=17371072}}</ref> The one on the left is a bicyclo[2.2.2]octene, which, at 200&nbsp;°C, extrudes [[ethylene]] in a [[retro-Diels–Alder reaction]].

: [[Image:Structure Correlation Principle.png|400px|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 [[enzyme|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 ==
* [[Transition state theory]]
* [[Transition state analog]]s, chemical compounds mimicking the substrate's transition state and act as enzyme inhibitors
* [[Reaction intermediate]]
* [[Reactive intermediate]]
* [[Activated complex]]

== References ==
{{reflist}}

{{DEFAULTSORT:Transition State}}
[[Category:Chemical kinetics]]