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 |
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.
- ^ R. W. Taft, R. D. Topsom (1987). Prog. Phys. Org. Chem. 16. pp. 1–83.
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