Pi electron donor-acceptor

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
  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. R. W. Taft, R. D. Topsom (1987). Prog. Phys. Org. Chem. 16. pp. 1–83.CS1 maint: location (link)
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