Improper (dihedral) functionsΒΆ
Previously, we see that dihedral functions are defined by four atoms connected successively, forming a chain. However, the improper dihedral term is a special form of dihedral where four atoms are specified such that three atoms are connected to the fourth central atom, as shown below.
where \(\alpha\) is the improper angle of bond vector 1-3 relative to the plane consists of atom 2, 3 and 4.
Improper functions are included to restrict the geometry of certain parts of molecules that otherwise would not be able to account for by simple classical bonds. For example, to preserve the planar (flat) conformation of carbonyl groups in a molecule.
Here, the carbon atom is sp2 hybridized where the pi-bonding on the carbonyl C=O bond restricts the rotation of atom A and atom B relative to the carbonyl group. Subsequently, carbonyl groups adopt a planar conformation with \(\alpha\) = 0 degree.