Descriptors sigma charge

A specific example will clarify the procedure. When the sigma charge descriptor is generated using the following substructure with four differently substituted 12-methyl benz(a)-anthracenes, the differences in electron withdrawing power of the 7 position substituents are reflected in the descriptor value. 

Table II. The substructures used for the generation of the environment descriptors and the sigma charge descriptors are shown in Figure 2. In this figure a terminal X indicates an atom of undetermined type and undetermined connectivity. An X found imbedded in a substructure, such as the X found in substructure 4 is constrained to have the connectivity found in the substructure to obtain a match, but any atom type. For example, the substituent on the benzo ring is substructure 5 can be a methyl group, a chlorine, or an ethyl group, among others. The mean and standard deviation for each descriptor and the values for 7,12-dimethyl benz(a)anthracene are shown in Table III.

Figure 2. The substructures used for calculation of the environment and sigma charge descriptors for the PAH study...

Currently, only saturated positions are analyzable as the lipophilic (f) and electronic descriptors (x) lose meaning for delocalized structures. Electronegativity correlates nicely with sigma charges only in localized structures. Eventual extension of the method to delocalized structures will, of course, use sigma charges as descriptors. Two examples of the method follow. 

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