Van der Waals strain

In chemistry, van der Waals strain is strain resulting from van der Waals repulsion when two substituents in a molecule approach each other with a distance less than the sum of their van der Waals radii. Van der Waals strain is also called van der Waals repulsion and is related to steric hindrance. ["Organic chemistry" 4th. Ed., Morrison and Boyd ISBN 0-205-05838-8] One of the most common forms of this strain is eclipsing hydrogen, in Alkanes.

In rotational and pseudorotational mechanisms

In molecules whose vibrational mode involves a rotational or pseudorotational mechanism (such as the Berry mechanism or the Bartell mechanism) [cite web|url=http://www.ch.ic.ac.uk/rzepa/bpr/ |last= Rzepa |first= Henry S. |authorlink= Henry Rzepa |coauthors= Cass, M.E., Hii, K.K |title=Mechanisms that Interchange Axial and Equatorial Atoms in Fluxional processes |accessdate=2008-03-27] , van der Waals strain can cause significant differences in potential energy, even between molecules with identical geometry. PF₅, for example, has significantly lower potential energy than PCl₅. Despite their identical trigonal bipyramidal molecular geometry, the higher electron count of chlorine as compared to fluorine causes a potential energy spike as the molecule enters its intermediate in the mechanism and the substituents draw nearer to each other.

External links

* [http://www.chem.ed.ac.uk/teaching/undergrad/chemistry1_icb/supplement/appendix11.pdf More reading]

References