Pi electron donor-acceptorThe pEDA parameter (pi electron donor-acceptor) is a pi-electron substituent effect scale, described also as mesomeric or resonance effect. There is also a complementary scale - sEDA. The more positive is the value of pEDA the more pi-electron donating is a substituent. The more negative pEDA, the more pi-electron withdrawing is the substituent (see the table below). The pEDA 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 is perpendicular to the z-axis. Then, the 2pz orbital occupations of ring carbon atoms are summed up to give the total pi- occupation. From this value the sum of pi-occupation for unsubstituted benzene (value close to 6 in accord to Huckel rule) is subtracted resulting in original pEDA parameter. For pi-electron donating substituents like -NH2, OH or -F the pEDA parameter is positive, and for pi-electron withdrawing substituents like -NO2, -BH2 or -CN the pEDA is negative. The pEDA scale was invented by Wojciech P. Oziminski and Jan Cz. Dobrowolski and the details are available in the original paper.[1] The pEDA scale linearly correlates with experimental substituent constants like Taft-Topsom σR parameter.[2] For easy calculation of pEDA the free of charge for academic purposes written in Tcl program with graphical user interface AromaTcl is available. Sums of pi-electron occupations and pEDA parameter for substituents of various character are gathered in the following table:
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