Carbon atoms single/double bonds, alternation between

Kekule proposed that the structure for benzene resonated between two alternate structures in which the position of the double and single bonds switched positions. In the figure, benzene is depicted as changing back and forth between two structures in which the position of the double bonds shifts between adjacent carbon atoms. The two structures are called resonance structures. 

Unsaturated hydrocarbons contain one or more double or triple bonds between carbon atoms and belong to one of three classes alkenes, alkynes, or aromatic hydrocarbons. Alkenes contain one or more double bonds, alkynes contain one or more triple bonds, and aromatic hydrocarbons contain three double bonds alternating with three single bonds in a six-carbon ring. Ethylene (the simplest alkene), acetylene (the simplest alkyne), and benzene (the simplest aromatic) are represented by the following structural formulas ... 

Notice the alternating double and single bonds between the carbon atoms. These are called conjugated bonds, or resonance bonds. The electrons in those bonds are not locked on to one atom—instead they spend their time bouncing from atom to atom. This gives the effect of something in between a double bond and a single bond, more of a one-and-a-half bond. 

Finally, synthetic metals made of polymeric organic molecules may also show the property of ferromagnetism. Organic materials of this kind were first demonstrated in 1987 by Ovchinnikov and his co-workers at the Institute of Chemical Physics in Moscow. The polymer they used was based on a polydiacetylene backbone, which contains alternating double-single and triple-single bonds between the carbon atoms of the molecule (10.2). 

An arrangement of alternate single and double bonds between the carbon atoms in diolefinic compounds. 

In a molecule such as formaldehyde, the bonding and non-bonding orbitals are localized (like the bonds) between pairs of atoms. Such a picture of localized orbitals is valid for the a orbitals of single bonds and for the n orbitals of isolated double bonds, but it is no longer adequate in the case of alternate single and double carbon-carbon bonds (in so-called conjugated systems). In fact, overlap of the n orbitals allows the electrons to be delocalized over the whole system (resonance... 

Since the two structures are chemically and energetically equivalent, it is possible that in long chains one part tends to assume one version of the structure and another part the other version with a transition zone in between. This zone may extend over a few carbon atoms and one may assume that from one side the double bond gradually become longer and the single bond accordingly shorter, in order progressively to reverse the alternation of the bond sequence ... 

In contrast, in the SSH model, the electrical bandgap arises because of the alternation between single and double carbon-carbon bonds, a signature of the Peierls distortion in a ID system. When a perfect ID chain of equidistant carbon atoms is considered, the electronic structure resulting from the electronic coupling between the atomic Pz-orbitals is that of a half-filled n band, implying a metallic... 

Another classic example of resonance is the benzene molecule. The localized resonance forms are termed Kekule forms (after Friedrich August Kekule, who first deduced the structure of benzene) and have alternating single and double bonds between carbon atoms. The actual benzene molecule is a resonance hybrid of the contributing resonance forms as the bond lengths are equal (single and double bonds have different lengths). 

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