Van der Waals strain

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.

See also

• Van der Waals force
• Van der Waals molecule
• Van der Waals radius
• Van der Waals surface

References

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.