Abstract:
Substituent effects in organic chemistry are generally described in terms of experimentally derived
Hammett parameters whereas a convenient theoretical tool to study these effects in p-conjugated
molecular systems is molecular electrostatic potential (MESP) analysis. The present study shows that the
difference between MESP at the nucleus of the para carbon of substituted benzene and a carbon atom
in benzene, designated as DVC, is very useful to quantify and classify substituent effects. On the basis of
positive and negative DVC values, a broad classification of around 381 substituents into electron
withdrawing and donating categories is made. Each category is again sorted based on the magnitude of
DVC into subcategories such as very strong, strong, medium, and weak electron donating/withdrawing.
Furthermore, the data are used to show the transferability and additivity of substituent effects in
p-conjugated organic molecules such as condensed aromatic, olefinic, acetylenic, and heterocyclic
systems. The transferability properties hold good for DVC in all these molecular systems. The additive
properties of substituent effects are strongly reflected on DVC and the predictive power of the data to
assign the total substituent effects of multi-substituted systems is verified. The DVC data and the present
classification of substituents are very useful to design p-conjugated organic molecular systems with
desired electron rich/poor character.
