Self-polarizability

Isolated molecule STrif Ft charge self-polarizability free valence heterolytio homolytic... 

It is now possible to interpret more clearly the roles of the charges g, and self-polarizabilities 7t,, of the unperturbed heterocycle in equation (18). Although the values of tt,, are not often available, it is permissible for qualitative discussion to invoke those of the parent hydrocarbon,... 

It is also clear (e.g. from the inactive 4 position in quinoline) that there is in general no more justification for using 7Tr, by itself than for using g, it is only for alternant hydrocarbons that the self-polarizability is the discriminating factor, and in all other cases tt,, should be considered only to the extent that it modifies conclusions based on the leading term g Sa,. in (18). Moreover, the relative importance of the two terms depends on the value of 8[Pg.87]

Most of these developments may be applied most directly within the framework of the isolated molecule method, in which the reactivity indices are the charges and self-polarizabilities of the unperturbed ground state of a given molecule calculations based on the localization model (e.g. Nesbet, 1962) have made less progress, and will not be considered. It is therefore natural to enquire whether indices similar to and tt,, in Hiickel theory can still be defined, and calculated more precisely, in self-consistent field theory. The obvious questions are... 

Overall, a large variability in Koc values was observed, especially for the more polar compounds. For relatively nonpolar esters, Koc was considered a useful model for sorption to soil with high organic carbon and low clay content, with variation by a factor of 3-5. Significant correlations between Koc and molecular size, self polarizability, and MCIs were observed. For polar acids and amines, Koc values varied up to two orders of magnitude, and poor correlations were obtained for most of the parameters examined. 

We have already considered one criterion of this kind, the free valence number here we shall consider three other properties of molecules which have been used in similar connections. These are the charge density, self-polarizability, and frontier orbital density. All of these properties show correlations with reactivity, at least in certain ranges of compounds, but these correlations are due in each case to fortuitous correspondences between them and the quantities 6ED or 6E, —the same situation that arises in the case of free valence number. 

There is therefore a correlation between the rate and the reciprocal of the self-polarizability, due to this accidental first-order correspondence between it and the reactivity number. The correlation does not in any way support the physical model on which it was originally based. 

These free valence numbers in the S method run parallel to the self-polarizability from the M.O. description which has still to be discussed. A larger free valence therefore also means a smaller activation energy for (electrophilic) substitution and probably for radical substitution (p. 284). A high bond order runs parallel to a low bond localization energy of a tu electron pair in this bond and thus with a low activation energy for molecular addition and ozonization. 

In most isocyclic hydrocarbons (alternating hydrocarbons, see below) the charge distribution is uniform, so that qk = 1 for all atoms. Then there occurs in place of these quantities the so-called self-polarizability ... 

Isolated molecule ffr n r r Fr charge self-polarizability free valence heterolytic homolytie... 

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