Isomeric stabilization energy

Schleyer proposed one last alternative method for ganging ASE. Noting that many of these better methods (especially those analogons to Reaction 3.24) require computation of many compounds, he developed the isomerization stabilization energy (ISE) method, particularly useful for strained aromatic systems. ISE measures the energy realized when an isomeric compound converts into its aromatic analog. Benzene itself cannot be analyzed by the ISE method, however, toluene can, and the ASE values of toluene and benzene are expected to be quite similar. The conversions of two different isomers into toluene provide the ISE for toluene (Reactions 3.27a and 3.27b). Both of these reactions do not conserve s-cis/s-trans diene conformations. Reaction 3.28 can be added once to Reaction 3.27a and twice to Reaction 3.27b to give the corrected ISE values of -32.0 and -28.9 kcal mol , respectively. 

Fig. 5 Isomerization stabilization energy, an isodesmic reaction applied to evaluate the aromaticity of imidazole-ylidenes...

Figure 5.3 Isomeric FH- -CO and FH- -OC complexes (left) and leading donor-acceptor interactions (right), with estimated second-order stabilization energies in parentheses.

Figure 5.26 (a) The isomeric anticooperative open (HF)3 structure (fully optimized), and (b) the leading np interaction with one of the two equivalent Lewis-acid monomers (with the second-order stabilization energy in parentheses). The net binding energy is 7.92 kcal mol-1. 

Isomerizations in which C—C bonds are cleaved homolytically have been chosen several times as probes for the study of substituent effects on radical stabilization. The nature of the intermediates—in some cases there may not even be intermediates but only biradical-like transition states—is often not known in detail. It may thus be uncertain whether the radicals include fully evolved radical centres, especially in the case of intramolecular isomerizations where biradicaloids might be involved. On this basis it is not expected that stabilization energies which derive from rate measurements for isomerizations will be identical to those obtained by other procedures. 

Although it is generally agreed that the thermal isomerization of bicyclo[2.2.0]hexanes to hexa-l,5-dienes takes place via diradical intermediates,113 118 121,123 125 experimental evidence has been obtained which implies otherwise.115,116 While a radical stabilization energy of approximately 4 kcal mol"1 was obtained for the pyrolysis of methyl 4-chlorobicy-clo[2.2.0]hexane-l-carboxylate (28 b) to methyl 5-chlorohexa-l,5-diene-2-carboxylate (29b),115116 as related to the parent 2-chlorohexa-1,5-diene (29a),115-l16-118 kinetic studies have indicated that there is a small but significant increase in activation energy of about 1 kcal mol-1 for the gas-phase and solution pyrolysis of l-chloro-4-methylbicy-clo[2.2.0]hexane (28c), as compared to l-chlorobicyclo[2.2.0]hexane (28a).115-116 In the light of this result, the commonly accepted diradical mechanism must be questioned and it is likely that the isomerization of these compounds occurs via a concerted process. 

Resonance Stabilization Energies of Isomeric Pyrrolizines in fl Units 14... 

Using the appropriate bond-separation reactions, the UF/3-21G aromatic stabilization energies are calculated to be 47.2, 36.4 and 22.5 kcal mol-1 for 22,11 and 21, respectively, compared to 59.0 kcal mol-1 for benzene503 thus the meta-, para- and ortho-isomers have 80, 62 and 38% of the aromaticity of benzene. The different orders of the thermodynamic stability of the three isomeric disilabenzene and of their aromatic stabilization energies... 

Considering what was said about stabilization energies in our previous discussion of thermochemical mimics, we now turn to aryl halides. There are several conceptual approaches to their thermochemistry one can take. The first is to consider halogenated derivatives of benzene, then of naphthalene, then of the isomeric anthracene and phenan-threne, etc. This approach, perhaps more appropriate for a study of generally substituted aromatic hydrocarbons, is immediately thwarted. Although there are many appropriate derivatives of benzene worthy of discussion, thermochemical data on halogenated naphthalenes are limited to the isomeric 1- and 2-monosubstituted derivatives, and halogenated derivatives of other aromatics remain thermochemically unstudied. 

A. Isomeric Energy Differences, Bond Energies, and Divalent State Stabilization Energies... 

Fig. 3. Isomeric energy differences of ethane and derived species in terms of bond dissociation energies (D), n bond energies ( > ), and divalent state stabilization energies (DSSE).

The divalent state stabilization energy (DSSE) is seen to be a particularly useful concept. It arises naturally from the comparison of X2M=MX2 XjMMX energy differences and leads to simple and reasonably accurate approximation schemes for the determination of doublebond dissociation energies and isomeric energy differences. 

With pentacene, the stabilization energy is less than that of the two naphthalene units formed by 1,4 addition across the central ring, and this corresponds to its high reactivity. On the other hand, the isomeric picene would have an energy change comparable to that of phenanthrene, thus accounting for its greater stability. 

As indicated above, the barrier to prolyl cis—trans isomerization is the resonance stabilization energy that is possessed by the C-N imide bond. The task of a prolyl isomerase is, therefore, to develop an enzymatic-chemical strategy that will result in the lowering of this barrier. When one reflects on the strategies that might be used by an enzyme, one realizes that there are two general mechanisms catalysis by distortion and nucleophilic catalysis (see Scheme V). 

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