Delocalization energy resonance

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. 

Spectroscopically determined values of P vai y, but they aie usually around —2.4 eV. In the section on resonance stabilization, we saw that thermodynamic measurements of the total resonance stabilization of butadiene yield 11 and 29 kJ mol according to the reference standard chosen. Calculate the delocalization energy of buta-1,3-diene in units of p. Determine two values for the size of the energy unit p from the thermochemical estimates given. Do these agree well or poorly with the spectroscopic values ... 

Two independent molecular orbital calculations (HMO method) of delocalization energies for isoindole and isoindolenine tautomers agree that the isoindole form should possess the more resonance stabilization. The actual difference calculated for isoindole-isoindolenine is about 8 kcal/mole, but increases in favor of the isoindole with phenyl substitution at position 1 (Table VI).Since isoindole and isoindolenine tautomers have roughly comparable thermodynamic stabilities, the tautomeric proce.ss is readily obser-... 

IR VTMCD band in each of the [Fe3S4] spectra, therefore, corresponds to the uniaxial a- a transition of Fe-Fe interaction and the resonance delocalization energy, (3, is determined to be 4290 25 cm for Type 1 clusters and 4350 25 cm for Type 2 clusters. Hence, the Fe-Fe interactions within the valence-delocalized pair are very similar in both locations. 

Resonance or delocalization energy is not a real energy inasmuch as it is not something that can be measured. It is simply the difference between the actual energy of the molecule and the energy of two or more hypothetical resonance structures. 

Triazine has not yet been prepared, and until fairly recently there were no well-documented examples of monocyclic 1,2,3-triazine derivatives. Early estimates of the potential degree of stability of 1,2,3-triazine were made on the basis of the theoretical resonance energy the results obtained indicated a delocalization energy of about 25 kcal/mole, which implies that the ring system should be reasonably stable and certainly amenable to synthesis. Many other related molecular orbital (MO) calculations have been carried out on similar systems more recently. 

The energy of the hybrid, E, is always less than the calculated energy of any hypothetical contributing structure, E. The difference between these energies is the resonance (delocalization) energy, E/. 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

However, it may be assumed that the difference between the energy of one Kekule structure and the actual energy as calculated by MO theory will also be the resonance energy except that, in keeping with the basic concept of MO theory, we shall call it the. delocalization energy. 

Since is the stable MO, the two n electrons will occupy it and their combined energy will be 2/J. Thus each of the pairs of localized n electrons in a Kekule structure contributes 2P to the energy of the molecule, making the total 7i electron energy of the localized, cyclohexatriene structure 6p. But the actual energy is 8/ hence the resonance or delocalization energy is 2 ... 

Hence the resonance or delocalization energy is 1.66/ , which, taking (1 = 20 kcal/mol, comes to about 33 kcal/mol. 

An MO study of all twenty pyranopyrandiones has been reported (73MI22200). Use was made of HMO calculations together with the co technique. Calculations were also performed on some methyl-substituted pyranopyrandiones and afforded values for the delocalization energy, 7r- bond orders and tt- charge densities. Empirical resonance energies were obtained... 

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