in theoretical organic chemistry
Redistribution of the valence-shell electron density throughout a molecular entity as compared with some localized models (individual atoms in their valence states, separated bonds or fragments). Different topological modes of the electron delocalization include:
  1. ribbon delocalization of either π- or σ-electrons (i.e., electrons occupying respectively π- and σ-orbitals)
  2. surface delocalization of σ-electrons occurring through an overlap of radially oriented σ-orbitals of a cyclic molecule, as is the case of cyclopropane; and
  3. volume delocalization of σ-electrons through an overlap of σ-orbitals directed inside a molecular polyhedron, as is the case in tetrahedrane.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1933 (