Pi electron donor-acceptor

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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:

Rπ-totalpEDA
-CH26.5620.571
-NH6.4810.491
-O6.3870.397
-NH26.1360.145
-OH6.1120.121
-F6.0690.078
-Cl 6.0530.062
-Br6.0470.057
-CH36.0050.014
-H5.9910.000
-NH3+5.984-0.007
-SiH35.974-0.017
-Li5.971-0.020
-CF35.967-0.024
-CN5.955-0.035
-CONH25.947-0.044
-BeH5.938-0.052
-COOH5.923-0.068
-NO25.922-0.069
-BF25.914-0.077
-CFO5.910-0.081
-CHO5.903-0.087
-COCN5.874-0.117
-NO5.861-0.129
-BH25.849-0.142
-N2+5.764-0.227
-CH2+5.380-0.611

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The sEDA parameter 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.

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.{{cite book}}: CS1 maint: location (link) CS1 maint: location missing publisher (link)