Sigma electron donor-acceptor

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, 2p_x 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.