Resonance energy and aromaticity

Write an essay of approximately 2000 words on the history and the various definitions of the concept of resonance, resonance energy, and aromaticity. 

Resonance Energies and Aromaticities of Furan, Pyrrole, and Thiophene... 

Relating resonance energy and aromaticity of benzenoid molecules directly with the number of Kekule structures has a long history [9, 39, 40]. It was generally believed (and accepted as self-evident) that among benzenoid isomers, stability/aromaticity increases with their K-values [41]. Of a variety of K-dependent expressions for resonance energy, we mention here that of Swinbome-Sheldrake [42]... 

Moreover, the combination of an SCF treatment for n-systems and a classical force field allows the calculation of resonance energies and aromatic stabilization energies in delocalized systems. The MMP2 resonance energies agree well with values obtained by isodesmic equations. A resonance stabilization of 53.2 kcal mol is found for the parent allyl cation. 

Resonance energies and tautomerism of substituted aromatic heterocycles and their benzo derivatives Reaction-field-supermolecule approach to calculation of solvent effects... 

In agreement with the Hiickel rule those annulenes and dehydroannulenes which contain (4 n + 2) 77 electrons and a reasonably planar carbon skeleton appear to be aromatic. Aromaticity in annulenes is usually equated with positive resonance energy and the absence of bond alternation. The most direct method of measuring bond alternation is by single crystal X-ray diffraction. Unfortunately this method has been applied in only a few cases. 

The difference -4602 - (-4581) =-21 kJ/moI shows small (negative) resonance energy and no aromaticity. 

Vertical Resonance Energies of Aromatic and Antiaromatic Species... 

A simple, consistent and highly-visual model emerges for the behavior of correlated Jt electrons in all of the aromatic systems that we have studied. The special stability of such molecules arises from the mode of coupling of the electron spins, as shown by the magnitudes of the resonance energies, and not from any supposed delocalization of the orbitals. We have thus developed a theoretical framework on which to build a coherent picture of the structure and properties of aromatic molecules. 

The Crick-Watson base pairing with the complementarity C-G and A-T (for DNA) or A-U (for RNA) is a consequence of the delicate balance between aromatic resonance energies and bond energies for CC, CO and CN single and double bonds. 

Applications of the SHM are discussed in great detail in several books [21] here we will deal only with those applications which are needed to appreciate the utility of the method and to smooth the way for the discussion of certain topics (like bond orders and atomic charges) in later chapters. We will discuss the nodal properties of the MOs stability as indicated by energy levels and aromaticity (the An + 2 rule) resonance energies and bond orders and atomic charges. 

The a-tocotrienol has a number of isolated double bonds on the alkyl chain and also has the double bonds of the aromatic ring. The isolated double bonds are easier to reduce by catalytic hydrogenation as the aromatic ring would prefer to retain its resonance energy from aromaticity. Thus, low-pressure hydrogenation and the use of simple catalysts like Pt will selectively reduce the isolated double bonds of a-tocotrienol to a-tocopherol. 

The enhanced stability arising from the property of aromaticity is known as the resonance energy and for benzene this is 150.6 kJ mok. ... 

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