The 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:
| R | σ-total | sEDA | |
| -Li|19.826|0.460|-|-BeH | 19.762 | 0.396 | |
| -BF2|19.559|0.193|-|-SiH3 | 19.550 | 0.184 | |
| -BH2|19.539|0.173|-|-CH2+|19.406|0.040|-|-H | 19.366 | 0.000 | |
| -CFO | 19.278 | -0.088 | |
| -CHO | 19.264 | -0.102 | |
| -COOH|19.256|-0.110 | |||
| -COCN|19.247|-0.119 | |||
| -CF3|19.237|-0.130 | |||
| -CONH2|19.226|-0.140 | |||
| -CN|19.207|-0.159 | |||
| -Br|19.169|-0.197 | |||
| -CH3|19.137|-0.229 | |||
| -NO|19.102|-0.264 | |||
| -Cl|19.102|-0.264 | |||
| -NO2|19.046|-0.320 | |||
| -N2+ | 19.034 | -0.332 | |
| -CH2−|18.964|-0.402 | |||
| -NH3+|18.950|-0.416 | |||
| -NH2|18.915|-0.451 | |||
| -NH−|18.825|-0.541 | |||
| -OH|18.805|-0.561 | |||
| -F | 18.745 | -0.621 | |
| -O− | 18.735 | -0.631 |