Sigma electron donor-acceptorThe sEDA parameter (sigma electron donor-acceptor) is a sigma-electron substituent effect scale, described also as inductive and electronegativity related effect. There is also a complementary scale - pEDA. The more positive is the value of sEDA the more sigma-electron donating is a substituent. The more negative sEDA, the more sigma-electron withdrawing is the substituent (see the table below). The sEDA parameter for a given substituent is calculated by means of quantum chemistry methods. The model molecule is the monosubstituted benzene. First the geometry should be optimized at a suitable model of theory, then the natural population analysis within the framework of Natural Bond Orbital theory is performed. The molecule have to be oriented in such a way that the aromatic benzene ring lays in the xy plane and is perpendicular to the z-axis. Then, the 2s, 2px and 2py orbital occupations of ring carbon atoms are summed up to give the total sigma system occupation. From this value the sum of sigma-occupation for unsubstituted benzene is subtracted resulting in original sEDA parameter. For sigma-electron donating substituents like -Li, -BH2, -SiH3, the sEDA parameter is positive, and for sigma-electron withdrawing substituents like -F, -OH, -NH2, -NO2, -COOH the sEDA is negative. The sEDA scale was invented by Wojciech P. Oziminski and Jan Cz. Dobrowolski and the details are available in the original paper.[1] The sEDA scale linearly correlates with experimental substituent constants like Taft-Topsom σR parameter.[2] For easy calculation of sEDA the free of charge for academic purposes written in Tcl program with graphical user interface AromaTcl is available. Sums of sigma-electron occupations and sEDA parameter for substituents of various character are gathered in the following table:
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