Localisation energy

Most correlations of rates with localisation energies have used values for the latter derived from the Hiickel approximation. More advanced methods of m.o. theory can, of course, be used, and fig. 7.1 illustrates plots correlating data for the nitration of polynuclear hydrocarbons in acetic anhydride -" with localisation energies derived from self-... 

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

Numerous m.o.-theoretical calculations have been made on quinoline and quinolinium. Comparisons of the experimental results with the theoretical predictions reveals that, as expected (see 7.2), localisation energies give the best correlation. jr-Electron densities are a poor criterion of reactivity in electrophilic substitution the most reactive sites for both the quinolinium ion and the neutral molecule are predicted to be the 3-, 6- and 8-positions. ... 

Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation (electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone is rather more susceptible to electrophilic attack than is the 2-position, as indicated by jt-electron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonic acid (6.3). The severity of the reaction conditions ensures that the thermodynamically favoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group, is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained by sulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). It appears that mercuration first takes place at the 1-position followed by displacement. Some disulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8-disulphonic acids, respectively. Separation of the various compounds can be achieved without too much difficulty. Sulphonation of anthraquinone derivatives is also of some importance. 

Hiickel m.o. calculations fail badly with benzimidazole. Localisation energies for the free base and the cation indicated C(4) to be the most reactive position towards electrophilic attack,156157 and led to the false conclusion that substitution involved the free base, the orientation being controlled by charge57 densities. 

This is probably related to the deep localised energy states which were caused by the In composition fluctuations of the InGaN active layer due to a phase separation during growth [20-23],... 

The rates of attack of radicals on aromatic rings correlate with ionisation potential, with localisation energy and with superdelocalisability (see page 130), a picture reminiscent of the situation in aromatic electrophilic substitution. As in that field, there are evidently a number of related factors affecting reactivity. Frontier orbitals provide useful explanations for a number of observations in the field. 

We could at least argue, therefore, that if oc were large, so that it was, in a sense, quite hard work restricting most of the re-electrons so that they did not get onto Cr, then the transition state would have a high energy. The most-favourable position of attack would be the one in which the localisation energy were the lowest. 

Let us consider nitrobenzene as an example. Table 8-1 gives the localisation energies, in units of J , for the three types of attack, nucleophilic, radical and electrophilic, in each of the ortho-, meta- and para-positions. 

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