INDEX ring substitution pattern

The second structural property described by the 4ypc index is the substitution pattern on the benzene ring. The value of the 4ypc index increases sharply with the degree of substitution, while in the isomeric classes of substituted benzenes it increases with the proximity of substituents. Thus, this structural parameter has also been found to be very useful in describing activities and properties of polysubstituted benzenes [103], chlorinated benzenes [279], and polychlorinated biphenyls [286]. 

Another analytical (and toxicological) problem was that only a few of the 209 PCB congeners required for confirmation of identity and calibration had been synthesized. Various alternative strategies were devised to identify individual PCBs. The most successful of these was a GC retention time index (RI) prediction based on the additivity of the indices for the two rings. These half-RI values were obtained from symmetrically substituted PCBs and a few asymmetrical PCBs in which one ring had a substitution pattern with a known half-RI. There were still problems, however. One does not need to look hard in recent literature to come up with two or three alternative identities for some major GC peaks in PCB chromatograms. 

A reasonable QSAR was also developed between the formula weight and valences third-order path chi index of a series of triphenyl- and tricyclohexyltin benzoates and the LC50 values of the An. stephensi larvae. The chi index is a parameter which encodes branching pattern information of the substitution on the ring. Cross validation of the training set also indicated that the constructed model could be used to predict the LC50 value for this series of compounds. 

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