[5] The difference in potential energy between the actual species and the (computed) energy of the contributing structure with the lowest potential energy is called the resonance energy[6] or delocalization energy. As both Kekulé structures have equal energy, they are equal contributors to the overall structure – the superposition is an equally weighted average, or a 1:1 linear combination of the two in the case of benzene. Therefore, three valid resonance structures can be drawn. The diborane molecule is described by contributing structures, each with electron-deficiency on different atoms. − This reduces the electron-deficiency on each atom and stabilizes the molecule. Isomers are molecules with the same chemical formula but are distinct chemical species with different arrangements of atomic nuclei in space. ; see below for details on usage). Molecules with an extended π system such as linear polyenes and polyaromatic compounds are well described by resonance hybrids as well as by delocalised orbitals in molecular orbital theory. Structures with a low value of potential energy are more stable than those with high values and resemble the actual structure more. [14][15] The alternative term mesomerism[16] popular in German and French publications with the same meaning was introduced by C. K. Ingold in 1938, but did not catch on in the English literature. In a hybrid structure, pi bonds that are involved in resonance are usually pictured as curves Linus Pauling used this mechanism to explain the partial valence of molecules in 1928, and developed it further in a series of papers in 1931-1933. Another example is methanium (CH+5). The current concept of mesomeric effect has taken on a related but different meaning. CC BY-SA 3.0. http://en.wikipedia.org/wiki/Resonance_(chemistry) The allyl cation has two contributing structures with a positive charge on the terminal carbon atoms. In benzene the two cyclohexatriene Kekulé structures, first proposed by Kekulé, are taken together as contributing structures to represent the total structure. It is incorrect to regard resonance hybrids as rapidly interconverting isomers, even though the term "resonance" might evoke such an image. For example, in cyclohexane the bond order is 1 while that in benzene is 1 + (3 ÷ 6) = 1 1⁄2. B {\displaystyle {\ce {A<->B}}} With an electron-withdrawing group qi < 1 on the ortho and para C atoms and qi > 1 for an electron-releasing group. CC BY-SA. Again, in reality, the electronic configuration does not change between the three structures; rather, it has one structure in which the extra electrons are distributed evenly. It has particular value for describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. Quantum mechanical description in VB theory, Comparison with molecular orbital (MO) theory, Practicing chemists familiar with the concepts of resonance and delocalization will often draw just one major contributing structure to implicitly represent a molecule whose structure should be described by invoking a resonance hybrid. Resonance structures have the same number of electrons and therefore have the same overall charge. Resonance (or delocalization) energy is the amount of energy needed to convert the true delocalized structure into that of the most stable contributing structure. Bond lengths can be compared using bond orders. ↽ A Regarded as a formalism that does not necessarily reflect the true electronic structure, such depictions are preferred by the IUPAC over structures featuring partial bonds, charge separation, or dative bonds.[19]. However, the consensus opinion is that while they may make a marginal contribution, the participation of d orbitals is unimportant, and the bonding of so-called hypervalent molecules are, for the most part, better explained by charge-separated contributing forms that depict three-center four-electron bonding. Charge delocalization in anions is an important factor determining their reactivity (generally: the higher the extent of delocalization the lower the reactivity) and, specifically, the acid strength of their conjugate acids. In benzene and other aromatic rings, the delocalized pi-electrons are sometimes pictured as a solid circle.[10]. The curved arrows depict the permutation of delocalized π electrons, which results in different contributors. This cation (an allylic cation) can be represented using resonance, as shown above. For example, in benzene, valence bond theory begins with the two Kekulé structures which do not individually possess the sixfold symmetry of the real molecule. Alternatively to the use of contributing structures in diagrams, a hybrid structure can be used.
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