Delta chi indexes

The chi indexes may be viewed as a basic set of variables from which useful combinations may be developed to emphasize certain structure features. With such transformed chi indexes, it is also possible to set up QSAR equations, often with fewer variables, which also permit structure comparisons among different classes of molecules. To assist in that comparison, we introduce a transformed set of chi indexes. The delta chi index is the difference between a simple index and the corresponding valence index. In this example, A % is used and defined as follows ... 

The simple index encodes information only about the sigma electron network, whereas the valence index also encodes information about the nonsigma electrons. By virtue of raking the difference, the delta chi index emphasizes the contribution of the nonsigma electrons. For the alkenes and alkylbenzenes, A x... 

Because the delta chi indexes, A x> emphasize the role of the nonsigma electrons, they have become important in relating to properties which are more dependent on lone pair and 7t electrons. The delta chi indexes introduced above have also been found useful in an analysis of the ionization potentials of a set of 24 alkyl amines, alcohols, and ethers. The delta chi for the first three orders are defined as above. 

The variable A x in QSAR Eq. [33] provides discrimination only among the three molecular classes because the atom contribution to A x is zero for saturated carbon atoms. The A°x provides very little structure information with respect to skeletal variation, but it does encode the atom identities and some of the skeletal environment immediately surrounding the heteroatom. The addition of the A x variable greatly increases the discrimination among the three classes because it encodes skeletal information about the carbon atoms a to the heteroatom. The QSAR is improved considerably by the addition of A x. Finally, the addition of A x further improves the QSAR by adding information about the broader reaches of the skeletal environment of each heteroatom, namely, atoms p to the heteroatoms. It can also be seen that the effects of atoms P to the heteroatom are much less important than the heteroatom itself or the a carbon atoms. By the introduction of the delta-chi indexes, an atom level interpretation is made possible. 

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See also in sourсe #XX -- [ , ]

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