Thermodynamic stabilization delocalization energy

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-... 

According to the JTH scheme, the calculated RE and REPE values are listed in Table 8. Encouragingly, our VB RE values bear an excellent linear correlation with the delocalization energies obtained by Wiberg [59] at the B3LYP/6-311G level (the correlation coefficient=0.994) for the first four members of polyacene and polyphenanthrene series, respectively. Thus it appears that the VB RE is a good measure of the thermodynamic stability for isomeric BHs. 

Let us consider the significance of this stabilization, which is sometimes called the delocalization energy (DE). The stabilization results from the removal of the restriction that the tt electrons be localized between two particular atoms. Comparison of the DE of 1,3,5-hexatriene and benzene would suggest that the triene is stabilized by almost half the extent of benzene, but thermodynamic comparisons do... 

One approach to evaluation of the aromaticity of a molecule is to determine the extent of thermodynamic stabilization. Attempts to describe stabilization of a given aromatic molecule in terms of simple HMO calculations have centered on the delocalization energy. The total rr-electron energy of a molecule is expressed in terms of the energy parameters a and 6 that are used in HMO calculations. This energy value can be compared to that for a hypothetical localized version of the same molecule. The HMO energy for the tt electrons of benzene is 6a + 86. The same quantity for... 

There have been two general approaches to determining the amount of stabilization that results from aromatic delocalization. One is to use experimental thermodynamic measurements. Bond energies, as was mentioned in Chapter 1, are nearly additive when there are no special interactions between the various bond types. Thus, it is possible to calculate such quantities as the heat of combustion or heat of hydrogenation of cyclohexatriene by assuming that it is a compound with no interaction between the conjugated double bonds. For example, a very simple... 

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). 

A carbocation is strongly stabilized by a C substituent (Figure 7.1c) through n-type interactions which involve substantial delocalization into the substituent. The LUMO energy is relatively unchanged, but the reactivity of the electron-deficient center toward attack by nucleophiles is reduced because the orbital coefficients are smaller. Allyl and benzyl carbocations are prototypical of C -substituted carbo-cations. The effects of substitution are cumulative. Thus, the more C -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. A prime example is triphenyl carbocation. 

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