Conjugated atom

Knott s rule concerns the importance of the place of the nitrogen atom replacing a methine carbon in the conjugated chain when the atom is separated from the active auxochromic atoms by an odd number of conjugated atoms, the shift is bathochromic. It is hypsochromic when there is an even number, Tne importance of the shift could establish a measure of M effect of various heterocyclic nuclei (79. 124). Many papers have been published, and examples have been given to verify these rules (79-84). 

Such cross-conjugation (where two of the vinyls are conjugated with each other through boron but not simultaneously with the third vinyl35) always lowers the total delocalization energy (DE) of the conjugated atomic set In simple Hiickel MO theory, for example, ( )-l,3,5-hexatriene (37) has a DE of 0.988/3, while 2-vinyl-1,3-butadiene (38) yields a DE of 0.900/3. 

Applying these rules in pericyclic reactions it has been shown and a generalization given that thermal reactiom occur via aromatic transition states while photochemical reactions proceed via antiaromatic transition state. A cyclic transition state is considered to be aromatic or isoconjugate with the corresponding aromatic system if the member of conjugated atoms and that of the n... 

The discussion will now be specialized to parent molecules which are even alternant hydrocarbons. All the analytical properties to be referred to originate from the basic properties of these molecules. Thus, for an even alternant hydrocarbon, with n = 2m conjugated atoms, A e) is a polynomial with even powers of e in which successive non-zero terms alternate in sign, so that... 

When the two conjugated atoms approach each other, the leptons might in principle annihilate before the hadrons do. We have found that this is not the case. Even though the annihilation reaction constant for para-positronium is larger than that for protonium, the probability of e+ — e annihilation at any given interhadronic distance R is weighted by the hadronic probability density at that distance. Because of that, the e+ — e annihilation occurs mainly at R 1 whereas the hadrons annihilate basically at R = 0. 

S imply summation over one or other of the two sets (starred or unstarred) into which the conjugated atoms can be divided, no two atoms of the same set being directly linked. 

The method outlined in the last section can be applied to any reaction involving the conversion of an even AH to an odd AH with one conjugated atom less, or of an odd AH to an even AH with one conjugated atom more. The method can be extended to alternant systems containing heteroatoms by the following procedure, due to Coulson and Longuet-Higgins.19... 

In a pericyclic reaction, electron counting can be effected in several ways, all equivalent. For example, in the Diels-Alder reaction, one can count the number of conjugated atoms in butadiene and in ethylene, or the number of bonds made (two o and one n bonds) or broken (three n bonds) in the process. In all cases, a total of six intervening electrons are obtained. 

As the number of conjugated atoms increases, the energy gaps between the MOs decrease. 

Do you see a pattern forming The important point is not the number of conjugated atoms but the number of electrons in the n system. When they have 4 or 8 7t electrons, both benzene and cyclooctatetraene adopt nonplanar structures when they have 6 or 10 7t electrons, a planar structure is preferred. 

Fig. 11. Plot of log PAB (or ffad) vs number of conjugated atoms in the bridging ligand for complexes of the type [ (NH l)5Ru 2( -L)l5. [Reproduced with permission from Ref. (771. Copyright (1983) American Chemical Society.]...

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