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 2p_z 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.{{Cite journal|last=Ozimiński|first=Wojciech P.|last2=Dobrowolski|first2=Jan C.|date=2009-08-01|title=σ- and π-electron contributions to the substituent effect: natural population analysis|journal=Journal of Physical Organic Chemistry|language=en|volume=22|issue=8|pages=769–778|doi=10.1002/poc.1530|issn=1099-1395}}

The pEDA scale linearly correlates with experimental substituent constants like Taft-Topsom σR parameter.{{Cite book|title=Prog. Phys. Org. Chem.|last=R. W. Taft|first=R. D. Topsom|year=1987|location=16|pages=1–83}}

For easy calculation of pEDA the free of charge for academic purposes written in Tcl program with graphical user interface [https://www.researchgate.net/project/AromaTcl-a-program-for-calculation-of-sEDA-pEDA-and-HOMA-indices AromaTcl] is available.

Sums of pi-electron occupations and pEDA parameter for substituents of various character are gathered in the following table:

class="wikitable" |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

Organochloride

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