Editing Activated complex (section)

== Reaction rate ==

=== Transition state theory ===
Transition state theory explains the dynamics of reactions. The theory is based on the idea that there is an equilibrium between the activated complex and reactant molecules. The theory incorporates concepts from [[collision theory]], which states that for a reaction to occur, reacting molecules must collide with a minimum energy and correct orientation. The reactants are first transformed into the activated complex before breaking into the products.<ref name=":0" /> From the properties of the activated complex and reactants, the reaction rate constant is

<math display="block">k = K \frac{k_\text{B} T}{h}</math>

where ''K'' is the [[equilibrium constant]], <math display="inline">k_\text{B}</math> is the [[Boltzmann constant]], T is the [[thermodynamic temperature]], and ''h'' is the [[Planck constant]].<ref name=":2">{{Cite journal |last=Eyring |first=Henry. |date=1935 |title=The Activated Complex and the Absolute Rate of Chemical Reactions. |url=https://pubs.acs.org/doi/abs/10.1021/cr60056a006 |journal=Chemical Reviews |language=en |volume=17 |issue=1 |pages=65–77 |doi=10.1021/cr60056a006 |issn=0009-2665|url-access=subscription }}</ref> Transition state theory is based on [[classical mechanics]], as it assumes that as the reaction proceeds, the molecules will never return to the transition state.<ref>{{Cite journal |last=Pechukas |first=P |date=1981 |title=Transition State Theory |journal=[[Annual Review of Physical Chemistry]] |language=en |volume=32 |issue=1 |pages=159–177 |doi=10.1146/annurev.pc.32.100181.001111 |issn=0066-426X}}</ref>

=== Symmetry ===
An activated complex with high symmetry can decrease the accuracy of rate expressions.<ref name=":1">{{Cite journal |last=Murrell |first=J. N. |last2=Laidler |first2=K. J. |date=1968 |title=Symmetries of activated complexes |url=https://pubs.rsc.org/en/content/articlelanding/1968/tf/tf9686400371 |journal=Transactions of the Faraday Society |language=en |volume=64 |issue=0 |pages=371–377 |doi=10.1039/TF9686400371 |issn=0014-7672|url-access=subscription }}</ref> Error can arise from introducing [[symmetry number]]s into the [[rotational partition function]]s for the reactants and activated complexes. To reduce errors, symmetry numbers can by omitted by multiplying the rate expression by a statistical factor:

<math display="block">k = l^\ddagger \frac{k_\text{B} T}{h} \frac{Q_\ddagger}{Q_A Q_B} e^{-\frac{\varepsilon}{k_\text{B} T}}</math>

where the statistical factor <math display="inline">l^\ddagger</math> is the number of equivalent activated complexes that can be formed, and the ''Q'' are the [[Partition function (statistical mechanics)|partition functions]] from the symmetry numbers that have been omitted.<ref name=":1" />

The activated complex is a collection of molecules that forms and then explodes along a particular internal normal coordinate. Ordinary molecules have three translational [[Degrees of freedom (physics and chemistry)|degrees of freedom]], and their properties are similar to activated complexes. However, activated complexed have an extra degree of translation associated with their approach to the energy barrier, crossing it, and then dissociating.<ref name=":2" />