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, 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
[edit]- ^ 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.
- ^ 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.