Pi electron donor-acceptor pEDA parameter 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.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 2p z 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 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.Wojciech P. Oziminski and Jan Cz . Dobrowolski and the details are available in the original paper.experimental substituent constants like Taft-Topsom σR parameter.free of charge for academic purposes written in Tcl program with Graphical User Interface is available.character are gathered in the following table:R π-total pEDA -CH 2 − 6.562 0.571 -NH − 6.481 0.491 -O − 6.387 0.397 -NH 2 6.136 0.145 -OH 6.112 0.121 -F 6.069 0.078 -Cl 6.053 0.062 -Br 6.047 0.057 -CH 3 6.005 0.014 -H 5.991 0.000 -NH 3 + 5.984 -0.007 -SiH 3 5.974 -0.017 -Li 5.971 -0.020 -CF 3 5.967 -0.024 -CN 5.955 -0.035 -CONH 2 5.947 -0.044 -BeH 5.938 -0.052 -COOH 5.923 -0.068 -NO 2 5.922 -0.069 -BF 2 5.914 -0.077 -CFO 5.910 -0.081 -CHO 5.903 -0.087 -COCN 5.874 -0.117 -NO 5.861 -0.129 -BH 2 5.849 -0.142 -N 2 + 5.764 -0.227 -CH 2 + 5.380 -0.611
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