Editing Activated complex (section)

{{Short description|Unstable configurations of atoms formed while a chemical reaction progresses}}
[[File:Ac com.svg|thumb|[[Reaction coordinate]] diagram showing the activated complex in the region with highest [[potential energy]].]] 

In [[chemistry]], an '''activated complex''' represents a collection of [[Reaction intermediate|intermediate structures]] in a [[chemical reaction]] when [[Chemical bond|bonds]] are breaking and forming. The activated complex is an arrangement of atoms in an arbitrary region near the [[saddle point]] of a [[potential energy surface]].<ref>{{GoldBookRef|file=A00092|title=Activated complex}}</ref> The region represents not one defined state, but a range of unstable configurations that a collection of atoms pass through between the [[reactant]]s and [[Product (chemistry)|products]] of a reaction. Activated complexes have partial reactant and product character, which can significantly impact their behaviour in chemical reactions.<ref>[[Peter Atkins]] and Julio de Paula, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.809 {{ISBN|0-7167-8759-8}}</ref> 

The terms activated complex and [[transition state]] are often used interchangeably, but they represent different concepts.<ref>{{Citation |last=Tuñón |first=Iñaki |title=The transition state and cognate concepts |date=2019 |url=https://linkinghub.elsevier.com/retrieve/pii/S0065316019300036 |work=Advances in Physical Organic Chemistry |volume=53 |pages=29–68 |publisher=Elsevier |doi=10.1016/bs.apoc.2019.09.001 |isbn=978-0-08-102900-8 |last2=Williams |first2=Ian H.}}</ref> Transition states only represent the highest potential energy configuration of the atoms during the reaction, while activated complex refers to a range of configurations near the transition state. In a [[reaction coordinate]], the transition state is the configuration at the maximum of the diagram while the activated complex can refer to any point near the maximum.

[[Transition state theory]] (also known as activated complex theory) studies the [[Chemical kinetics|kinetics]] of reactions that pass through a defined intermediate state with [[Gibbs free energy|standard Gibbs energy of activation]] {{math|Δ''G''°<sup>‡</sup>}}.<ref>{{GoldBookRef|file=T06470|title=Transition State Theory}}</ref> The transition state, represented by the double dagger symbol represents the exact configuration of atoms that has an equal probability of forming either the reactants or products of the given reaction.<ref>{{GoldBookRef|file=T06468|title=Transition State}}</ref>

The [[activation energy]] is the minimum amount of energy to initiate a chemical reaction and form the activated complex.<ref name=":0">{{Cite book |url=http://link.springer.com/10.1007/978-1-4020-4547-9 |title=Chemical Kinetics and Reaction Dynamics |date=2006 |publisher=Springer Netherlands |isbn=978-1-4020-4546-2 |location=Dordrecht |language=en |doi=10.1007/978-1-4020-4547-9}}</ref> The energy serves as a threshold that [[Reagent|reactant]] molecules must surpass to overcome the [[Activation energy|energy barrier]] and transition into the activated complex. [[Endothermic process|Endothermic reactions]] absorb energy from the surroundings, while [[Exothermic process|exothermic reactions]] release energy. Some reactions occur spontaneously, while others necessitate an external energy input. The reaction can be visualized using a [[reaction coordinate]] diagram to show the activation energy and potential energy throughout the reaction.

Activated complexes were first discussed in transition state theory (also called activated complex theory), which was first developed by [[Henry Eyring (chemist)|Eyring]], [[Meredith Gwynne Evans|Evans]], and [[Michael Polanyi|Polanyi]] in 1935.<ref>{{Cite journal |last=Eyring |first=Henry |date=1935 |title=The Activated Complex in Chemical Reactions |url=https://pubs.aip.org/jcp/article/3/2/107/203352/The-Activated-Complex-in-Chemical-Reactions |journal=The Journal of Chemical Physics |language=en |volume=3 |issue=2 |pages=107–115 |doi=10.1063/1.1749604 |issn=0021-9606|url-access=subscription }}</ref>