The definition is based on the
valence-bond formulation of the quantum mechanical idea of the wavefunction of a molecule
as composed of a linear combination of wavefunctions, each representative of a formula
containing bonds that are only single, double or triple with a particular pairing
of electron spins. Each such formula represents a contributing structure, also called
'
resonance structure' to the total wavefunction, and the degree to which each contributes is indicated
by the square of its
coefficient in the linear combination. The contributing structures, also called '
canonical forms', themselves thus have a purely formal significance: they are the components from
which wavefunctions can be built. Structures may be covalent (or non-polar) or ionic
(or polar). The representation is frequently kept qualitative so that we speak of
important or major contributing structures and minor contributing structures. For
example, two major non-equivalent contributing structures for the conjugate base of
acetone are:
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1100
