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

References

  1. ↑ Ozimiński, Wojciech P.; Dobrowolski, Jan C. (2009-08-01). "σ- and π-electron contributions to the substituent effect: natural population analysis". Journal of Physical Organic Chemistry. 22 (8): 769–778. doi:10.1002/poc.1530. ISSN 1099-1395.
  2. ↑ Boyd, Russell J.; Edgecombe, Kenneth E. (1988-06-01). "Atomic and group electronegativities from the electron-density distributions of molecules". Journal of the American Chemical Society. 110 (13): 4182–4186. doi:10.1021/ja00221a014. ISSN 0002-7863.
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