Resonance

Name:
Resonance
Detailed information:
(1) In chemistry, resonance (or mesomerism) is a mathematical concept based on quantum mechanical considerations (i.e., the wave functions of electrons); it is used to describe or express the true chemical structure of certain compounds that cannot be accurately represented by any one valence-bond structure. It was originally applied to aromatic compounds such as benzene, for which there are many possible approximate structures, none of which is completely satisfactory.See Benzene.The resonance concept indicates that the actual molecular structure lies somewhere between these various approximations, but is not capable of objective representation. This idea can be applied to any molecule, organic or inorganic, in which an electron pair bond is present. The term resonance hybrid denotes a molecule that has this property. Such molecules do not vibrate back and forth between two or more structures, nor are they isotopes or mixtures; the resonance phenomenon is rather an idealized expression of an actual molecule that cannot be accurately pictured by any graphic device.(2) In the terminology of spectroscopy, resonance is the condition in which the energy state of the incident radiation is identical with that of the absorbing atoms, molecules or other chemical entities. Resonance is applied in various types of instrumental analysis such as nuclear resonance absorption and nuclear magnetic resonance. See Absorption Spectroscopy.Note: The multiple meanings of nucleus and resonance can be a source of confusion, especially when these terms are closely associated, as in nuclear magnetic resonanceand resonance of a molecular nucleus. In the first of these expressions, nucleus is used in the sense of (1) under “nucleus”, and resonance in sense of (2) under“resonance”. In the second expression, nucleus is used in the sense of (3) under “nucleus” and resonance in the sense of (1) under resonance.