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Contributing structures of the carbonate ion. In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or forms, [1] also variously known as resonance structures or canonical structures) into a resonance hybrid (or hybrid structure) in valence bond theory.
A valence bond structure resembles a Lewis structure, but when a molecule cannot be fully represented by a single Lewis structure, multiple valence bond structures are used. Each of these VB structures represents a specific Lewis structure. This combination of valence bond structures is the main point of resonance theory.
Clar's rule states that for a benzenoid polycyclic aromatic hydrocarbon (i.e. one with only hexagonal rings), the resonance structure with the largest number of disjoint aromatic π-sextets is the most important to characterize its chemical and physical properties. Such a resonance structure is called a Clar structure. In other words, a ...
In chemistry, the mesomeric effect (or resonance effect) is a property of substituents or functional groups in a chemical compound.It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom. [1]
In another case, the stereoelectronic effect can result in an increased contribution of one resonance structure over another, which leads to further consequences in reactivity. For 1,4- benzoquinone monoxime, there are significant differences in the physical properties and reactivities between C2-C3 double bond and C5-C6 double bond.
Operationally, there are three ways in which alternative resonance structures may be generated: (1) from the LEWIS option, considering the Wiberg bond indices; (2) from the delocalization list; (3) specified by the user. [1] Below is an example of how NRT may generate a list of resonance structures.
A demonstration that how some well known 1,3-dipoles like ozone, nitro compounds and azides can be shown to have a resonance structure having 1,3 relationship between positive and negative formal charges. Known 1,3-dipoles are: Azides (RN 3) Ozone (O 3) Nitro compounds (RNO 2) Diazo compounds (R 2 CN 2) Some oxides. Azoxide compounds (RN(O)NR)
The 3-dimensional structure of the nitrilium ylide itself may also provide a clue as to the most appropriate resonance structure, with a linear R–C≡N–C unit supportive of the charge distribution indicated for resonance structures 1a & 1b and also consistent with the nomenclature nitrilium ylide.