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
InChI=1/C3H6O/c1-3(2)4/h4H,1H2,2H3/p-1/fC3H5O/h4h/q-1
NARVIWMVBMUEOG-MFCHEKMICA
InChI=1/C3H5O/c1-3(2)4/h1H2,2H3/q-1
QWOJMRHUQHTCJG-UHFFFAOYAL
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
doi:10.1351/goldbook.
