The 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:
| R | π-total | pEDA | |
| -CH2−|6.562|0.571|-|-NH− | 6.481 | 0.491 | |
| -O− | 6.387 | 0.397 | |
| -NH2|6.136|0.145|-|-OH | 6.112 | 0.121 | |
| -F | 6.069 | 0.078 | |
| -Cl | 6.053 | 0.062 | |
| -Br|6.047|0.057|-|-CH3 | 6.005 | 0.014 | |
| -H | 5.991 | 0.000 | |
| -NH3+|5.984|-0.007|-|-SiH3|5.974|-0.017|-|-Li | 5.971 | -0.020 | |
| -CF3|5.967|-0.024|-|-CN | 5.955 | -0.035 | |
| -CONH2|5.947|-0.044|-|-BeH|5.938|-0.052|-|-COOH | 5.923 | -0.068 | |
| -NO2|5.922|-0.069|-|-BF2 | 5.914 | -0.077 | |
| -CFO | 5.910 | -0.081 | |
| -CHO | 5.903 | -0.087 | |
| -COCN|5.874|-0.117|-|-NO | 5.861 | -0.129 | |
| -BH2|5.849|-0.142|-|-N2+|5.764|-0.227|-|-CH2+ | 5.380 | -0.611 |