Van der Waals strain is
strain resulting from
Van der Waals repulsion when two
substituent
In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule.
The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...
s in a
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
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, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...
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 isomerization, isomerize by exchanging the two axial ligands (see the figure) for two of the equatorial ones. It is the most ...
or the
Bartell mechanism),
[
] Van der Waals strain can cause significant differences in
potential energy
In physics, potential energy is the energy of an object or system due to the body's position relative to other objects, or the configuration of its particles. The energy is equal to the work done against any restoring forces, such as gravity ...
, 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; it has Symbol (chemistry), 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 ...
as compared to
fluorine
Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...
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 and chemistry, the van der Waals force (sometimes 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 elec ...
*
Van der Waals molecule
*
Van der Waals radius
*
Van der Waals surface
References
Stereochemistry
Strain
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