Energy aromaticity from

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

FIGURE 1.22. Solvent reorganization energies derived from the standard rate constants of the electrochemical reduction of aromatic hydrocarbons in DMF (with n-Bu4N+ as the cation of the supporting electrolyte) uncorrected from double-layer effects. Variation with the equivalent hard-sphere radii. Dotted line, Hush s prediction. Adapted from Figure 4 in reference 13, with permission from the American Chemical Society. 

A. Hydroxyfuran (also analogous thiophene, pyrrole) - Tautomer non aromatic but some additional resonance energy arises from ester etc. group... 

Besides aromatization, the energy resulting from relief of cyclic strain can be a driving force of the dienone-phenol rearrangement. Thus, it was reported that dienone 302 is... 

Recentiy published crystal structures of antibody 4C6, an antibody that catalyzes another cationic cyclization reaction (Figure 6), revealed that this antibody has exquisite shape complementarity to its eliciting hapten 5. The active site contains multiple aromatic residues which shield the high-energy intermediate from solvent and stabilize the carbocation intermediates through cation-7r interactions. 

According to the MNDO calculations (88MI4), the planar structure of the trigermacyclopropenyl cation (296) also corresponds to a minimum on the PES, whereas the analogous structure of (SnH)3+ (297) is a third-order saddle point. The MNDO calculation of the ISE (291) [isodesmic reaction (88)], with a correction for the strain energy determined from isodesmic reaction (89), shows the aromatic stabilization of (291) to be insignificant (1.8 kcal/mol) ... 

Bond separation reactions and heats of formation obtained from bond separation energies suffer from two serious problems. The first is that bond types in reactants and products for some types of processes may not actually be the same or even similar . The bond separation reaction for benzene is an obvious example. Here to reactant (benzene) incorporates six equivalent aromatic carbon-carbon bonds, midway between single and double bonds, while the products (three ethanes and three ethylenes) incorporate three distinct carbon-carbon single bonds and three distinct carbon-carbon double bonds. 

With nonpolar aromatic molecules such as benzene, naphthalene and anthracene, cohesion is almost entirely due to van der Waals interaction, but the cohesive energies are significantly larger than in aliphatic nonpolar molecules. The larger cohesive energies arise from the greater polarizability of the n-clouds. Arrangement of molecules... 

Finally, if we abandon Hiickel s topological approach altogether and consider more elaborate quantum-mechanical approaches, the concept of aromaticity derived purely from a consideration of -electrons becomes blurred and tends to disappear completely. In fact, allelectron methods allow the calculation of aromatic properties (Section V,B) of a given substance without introducing explicitly the concept of aromaticity. Certain authors, notably Dewar,19 have published resonance energies derived from self-consistent field molecular-orbital (SCF-MO) calculations, and these could be used as a measure of aromaticity. 

A linear relationship has been shown to exist between structural aromaticity indices and resonance energies (92T335). From this so-called unified aromaticity index, IA, has been proposed. It makes for a more appropriate comparison the aromaticity of heterocycles of different size. 

Several cases have been reported of sensitized chemical reaction from the triplet states of aromatic molecules. Anthracene triplets formed by triplet energy transfer from coronene react with carbon tetrachloride to yield, among other products, 9-chloroanthracene and hydrogen chloride,214 the same reaction which occurs upon direct excitation.215... 

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