Delocalization energy term

The only system for which a satisfactory set of estimates of the four main effects (A)—(D) has been made is that of ice. The most appropriate values for this situation are shown in the table. Justification for these particular values can be found in a recent article by the present writer [30]. It will be recognized that these values are those appropriate to a fairly long Ox 02 bond, of length 2-76 A. Shorter bonds would require larger values for all the terms of this table. Thus, on the basis of Coulson and Danielsson s calculations [23] it seems probable that the delocalization energy doubles itself when the Oj 02 distance is reduced from 2 76 A to 2 50 A. 

Is it not true that the most naive interpretation of the estimates of electrostatic and delocalization energies for ice (+ 8 and + Skoal, respectively) suggest, not that the interaction is essentially electrostatic, but rather that the electrostatic and delocalization terms are of about equal importance ... 

Within the HMO framework, simple arithmetic leads from the values of orbital energies to the values of total w-electronic energy (W), delocalization energy (DE), excitation energy of the N- -Vl transition [E(N —Fi)], and other quantities.1-4 In our case, these values are W = 6ac + 6.2l07/3cc DE = 1.2107/3CC and E N-+ Fx) = 1.6053]3CC. It is a frequent practice to lead discussions in terms of the factors k( which appear in the expressions for orbital energies, Et = a.c + kiPcc. 

Substituents in the ortho position, almost without exception, reduce the reactivity of benzoic acid derivatives a significant rate increase is a prima facie case for intramolecular catalysis (see p. 201). This effect is associated in the case of the esterification of or/fio-substited benzoic acids with a somewhat lower enthalpy of activation, and a greater decrease in the entropy of activation. The first factor may be due to a secondary steric effect145, whereby the energy of the initial state is raised because the carboxyl group is twisted out of the plane of the ring, to reduce non-bonded interaction with the or/fio-substituent (see p. 178), and delocalization energy is lost. The lower entropy term is presumably accounted for by the bulk effect of the substituent. 

Those familiar with the language of the molecular orbital picture of bonding may be surprised that no parallel to the delocalization energy seems present in our description. That effect would occur in the VB treatment only if ionic terms axe included. We thus conclude that delocalization is less important than the exchange attraction in bonding. 

This term allows for delocalization and is therefore called the delocalization energy. 

Methoxyazocines exhibit a two-electron polarographic wave corresponding to conversion to the lOir-electron aromatic dianion (7UA161). The HMO delocalization energy for the planar azocinyl dianion has been calculated to be -5.1 /S, compared to -3.7 /S for cyclotetraenide dianion. Voltammetric data for 2-methoxy-3,8-dimethylazocine in acetonitrile-tetramethylammonium fluoroborate have been interpreted in terms of the direct formation of the dianion at the electrode followed by rapid protonation of the strongly basic dianion (76ACS(B)773). In contrast, the reduction of cyclooctatetraene under these conditions takes place in two steps. 

Simon, Z., Baiaban, A.T., Ciubotariu, D. and Balaban, T.-S. (1985a). QSAR for Carcinogenesis by Polycyclic Aromatic Hydrocarbons and Derivatives in Terms of Delocalization Energy, Minimal Sterical Differences and Topological Indices. Rev.Roum.Chim.,30,985-1000. 

The weak reactivity of benzene is in part attributed to this additional stabiliza- j tion. That stabilization energy is often called the delocalization energy or reso- nance energy of benzene and is taken as a measure of the aromaticity of the j molecule. The expression resonance energy comes from the description of] the delocalized tt system in valence-bond theory in terms of the resonant hybrid... 

Since the levels are nearly continuous, this can be done by integration. This energy should be compared with the energy of an electron in the two-atom case, = a- - There is a delocalization energy of 0.27 / /electron. Note that in Huckel theory, there is no repulsion energy term. The Huckel P is one-half of the 3 values in Table 5.2. 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

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