Application Resonance Stabilization Energies

The term resonance energy has been used in several w ays in the literature, but it is generally used to mean the difference between an experimentally determined energy of some relatively complicated molecule and the experimental energy 

The actual value of the enthalpy of hydrogenation of 1,3-butadiene is —243 k,l rnol Both are hydrogenated to the same product, u-biitaiie hence the enthalpy diagram (Fig. 7-4) shows that bnta-1,3-diene is 11 kJ rnol. lower in enthalpy than it ought to be on the basis of the reference standard, bnt-l-ene. 

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

Using cyclohexene as the reference standard, calculate the resonance energies of cyclohexa-1,3-diene = —224 kJ moU ), cyclohexa-1,4-diene = 

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We have also proposed a general definition of the concept of stabilization energy applying to any compound, as shown by the numerous applications described in this survey article. Using this concept, we have been able to carry out a critical analysis and to offer a unified definition of various former notions such as hyperconjugation, resonance, ring strain, and steric repulsion energies. 

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. 

Kurihara etal. (90BCJ2531) discussed the stability and resonance energy of troponoids by application of the topological charge stabilization (TCS) rule. Many synthetic and natural troponoids prove to be energetically very stable molecules. The TSC rule can be applied to the formation mechanism of heterocyclotropones. Many benzazinotropone products are completely consistent with this rule, for example, tri- and higher-cyclic quinoxalotro-pones 27 and 166. 

Thermochemical stabilization is probably the most generally applicable of the simple criteria for "aromaticity". Pauling made use of heat of combustion of benzene and a set of average bond energies to derive a resonance energy of 37 kcal/mol.21 The most useful measure of this quantity is derived from the heats of hydrogenation (kcal/mol) obtained by Kistiakowsky and his coworkers 22... 

For metal crystals of appreciable size, the number of possible contributions to the alternative bond distributions is extremely large and this results in a considerable contribution to crystal stability from resonance energy. Because orbitals are incompletely occupied, application of an electric field results in electron migration. Solids containing different electronic structures, such that all available orbitals are doubly occupied, are insulators because free migration of electrons is not possible. This is the situation in covalent crystals. Orbital overlap with the production of covalent shared, but incompletely filled, electron bonds does not occur in molecular or ionic crystals, which are therefore not electronic conductors. 

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