Editing Transition state (section)

== 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]].