Resonance in conjugated systems

Resonance in Conjugated Systems Containing Aromatic Nuclei... 

Pauling, L. Sherman, J. The Nature of the Chemical Bond. VII. The Calculation of Resonance Energy in Conjugated Systems J. Chem. Phys. 1933,1, 679-686. 

V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals," J.Chem.Physics 1 (1933) 362374 Linus Pauling and J. Sherman, "The Nature of the Chemical Bond. VI. Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures," J.Chem.Physics 1 (1933) 606617 and Pauling and Sherman, "The Nature of the Chemical Bond. VH. The Calculation of Resonance Energy in Conjugated Systems," J.Chem.Physics 1 (1933) 679686. 

CTp) Hammett Polar and resonance effects in conjugated systems (for meta and para substituents)... 

It may be noted that in conjugated systems (having alternate single and double bonds) resonance causes displacement of electrons from one part of the system to another part creating centres of high and low electron density. This is called resonance effect. 

Delocalization energy. To describe quantitatively the interaction of Jt bonds in conjugated systems, the concept of delocalization energy (resonance) En is frequently introduced. Usually it is defined as follows. 

Both hole and charge are not necessary to be localized together on one atom and they can be delocalized over the whole molecule. In fact, the delocalization of the unpaired electron in conjugated system can lead to stable radical cations such as the Wurster salt. This compound is isolable and the chemical structure including its resonance forms are shown in Scheme 1. Aryl amine moieties are thought to be a main core structure in HTMs because amine atom is relatively easy to lose one electron and the resulting radical cation can be stabilized by resonance effect of adjacent aryl substituent. It is worth to note that the Wurster salt mentioned above is stabilized by two factors. One is a resonance effect by aryl substituents and the other is stabilized by counter ion, perchlorate. 

J. Gastciger and H, Sailer, Angeir. Chem. Ini. Ed. Engi, 24, 687 (1985). Calculation of the Charge Distribution in Conjugated Systems by a Quantification of the Resonance Concept. 

Resonance considerations become particularly important in conjugated systems—that is, molecules in which a series of contiguous atoms have orbitals that permit strong electronic interactions over an array of three or more atoms. In the allyl cation, for example, the 7r-system extends over three atoms ... 

The nature of the chemical bond. VII. The calculation of resonance energy in conjugated systems. J. Chem. Phys. 1 (1933) 679—686. (Linus Pauling and J. Sherman). SP 10 ... 

The stability of a radical depends on the rate of its disappearance or formation according to the reaction conditions. The thermodynamic stability of a radical is related to the ease of dissociation of the R-H bond to give R and H or the R-H dissociation energy. The thermodynamic stability of a radical depends on double bond conjugation and electron delocalization stabilized by resonance in allylic systems produced by hydrogen abstraction in polyunsaturated lipids (see Chapter 2). 

---