Van der Waals strain is
strain resulting from
Van der Waals repulsion when two
substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and '' functional group'', as well as '' ...
s in a
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
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. One of the most common forms of this strain is
eclipsing hydrogen, in
alkane
In organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms tha ...
s.
In rotational and pseudorotational mechanisms
In molecules whose
vibrational mode involves a rotational or pseudorotational mechanism (such as the
Berry mechanism
The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands (see Figure at right) for two of the equatorial ones. It is the most widely ac ...
or the
Bartell mechanism),
[
] Van der Waals strain can cause significant differences in
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potentia ...
, 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
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
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
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and t ...
*
Van der Waals molecule
A Van der Waals molecule is a weakly bound complex of atoms or molecules held together by intermolecular attractions such as Van der Waals forces or by hydrogen bonds.
The name originated in the beginning of the 1970s when stable molecular clus ...
*
Van der Waals radius
*
Van der Waals surface
References
{{reflist
Stereochemistry
Strain