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.^[1] 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),^[2] 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.

References

^ Organic chemistry 4th. Ed., Morrison and Boyd ISBN 0-205-05838-8
^ Rzepa, Henry S.; Cass, M.E.; Hii, K.K. "Mechanisms that Interchange Axial and Equatorial Atoms in Fluxional processes". Retrieved 2008-03-27.

[1] Organic chemistry 4th. Ed., Morrison and Boyd ISBN 0-205-05838-8

[2] Rzepa, Henry S.; Cass, M.E.; Hii, K.K. "Mechanisms that Interchange Axial and Equatorial Atoms in Fluxional processes". Retrieved 2008-03-27.

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