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( Resonance (chemistry))
Resonance in chemistry is a theory used to represent and model certain types of non-classical molecular structures. Resonance is a key component of valence bond theory and arises when no single conventional model using only even number of electrons shared exclusively by two atoms can actually represent the observed molecule. There are two closely related but useful-to-distinguish meanings given to the term resonance. One of these has to do with diagrammatic representation of molecules using Lewis structures while the other has to do with the mathematical description of a molecule using valence bond theory. In both cases, resonance involves representing or modeling the structure of a molecule as an intermediate, average (a resonance hybrid) between several simpler but incorrect structures. A single Lewis structure often cannot represent the true electronic structure of a molecule. While one can only show an integral number of covalent bonds between two and only two atoms using these diagrams, one often finds that the experimentally deduced or calculated (from Quantum mechanics) structure of the molecule does not match any of the possible Lewis structures but rather has properties in some sense intermediate to these. Resonance structures are then employed to approximate the true electronic structure. Take the example of benzene (shown above, right). In a Lewis diagram, two carbons can be connected by one or two covalent bonds, but in the observed benzene molecule the carbon-carbon bond lengths are 139 pm, longer than typical C=C double bonds (133 pm) yet shorter than typical C-C single bonds (154 pm). More importantly, they are all equivalent, a fact no Lewis structure can explain. Therefore one calls the two Lewis structures canonical, contributing or resonating structures and the real molecule is considered to be their average, called a resonance hybrid. Resonance structures of the same molecule are connected with a double-headed arrow. This form of resonance is simply a way of representing the structure graphically. It is only a notation and does not represent a real phenomenon. The individual resonance structures do not exist in reality the molecule does not inter-convert between them. Instead, the molecule exists in a single unchanging state, intermediate between the resonance structures and only partially described by any one of them. This sharply distinguishes resonance from tautomerism. When it is said that a molecule is stabilized by resonance or that amides are less basic because the lone pair on nitrogen is involved in resonance with the carbonyl pi electron, no phenomenon is implied. What is meant is simply that the molecule behaves differently from what we expect by looking at its Lewis structure because the structure diagrammed does not represent the actual structure of the molecule. From this viewpoint, the terminology treating resonance as something that 'happens' is perhaps an unfortunate historical burden. It is also not correct to say that resonance occurs because electrons "flow", "circulate", or change their place within the molecules. Such a behavior would produce a magnetic field, an effect that is not observed in reality. However, a phenomenon of this sort may be induced by the application of an external magnetic field perpendicular to the plane of an aromatic ring this causes the appearance of an opposing magnetic field, demonstrating that that the delocalised pi electrons are truly flowing. The applied magnetic field induces a current density ("ring current") of circulating electrons in the pi system; this current in turn induces a magnetic field. A common manifestation of this effect is the large chemical shift observed in the NMR spectrum of aromatic structures.
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