• Hammett sigma constants;
  • quantum chemical parameters;
  • substituent parameters;
  • substituent constants;
  • QSAR;
  • AM1


The Hansch-Fujita correlation analysis proved to be one of the major breakthroughs in the process of understanding relation between molecular properties and biological activity. In this approach the Hammett σ constants are used to characterize electronic effects of substituents. Experimentally determined σ constants have, despite their numerous very successful applications in QSAR studies, also some disadvantages, most notably availability only for limited set of substituents and not very good quality for uncommon functional groups. As an alternative the use of quantum chemical parameters is possible. In this article a methodology for theoretical estimation of Hammett σ constants is described, based on correlation with semiempirical atomic charges on the simple group attached to the substituent. The agreement with experiment for 63 most common organic substituents is good (r2 = 0.89 for σmeta and 0.93 for σpara respectively, two-parameters equation). As an example of possible application of this theory a World Wide Web-based program for interactive calculation of electronic substituent constants is described.