Theoretical reevaluation of the π-donating effect of electron-withdrawing substituents in the conjugated cationic systems

The pi-donating effects of acceptor substituents are investigated theoretically using the high-level ab initio calculations for the isodesmic reactions, X-(CH=CH)(n)-CH3 + (CH2=CH)(n)-CH2+ --> X-(CH=CH)(n)-CH2+ + (CH2=CH)(n)-CH3 where n = 1 and 2 and X = NH2, OCH3, F, H, Cl, CHO, CF3, CN, and NO2. Electron delocalization is enhanced in the cationic systems and also by accounting for electron-correlation effect. The reaction energies (DeltaHdegrees), bond-length changes (Ad), and electronic charge shifts (Deltaq(pi)) do not provide a reliable measure of the pi-donating ability of the acceptor substituents. In contrast, the percentage contribution of resonance structures (wt %) by the natural resonance theory (NRT) analysis and the energy changes due to deletion of the relevant pi orbitals (DeltaE(D)) are the better methods of evaluating the pi-donating behaviors. Proper accounting of the stabilization energies due to pi-donation by the electron acceptors in the conjugated cationic species is only possible with the high-level correlated methods. In contrast to the earlier reports that the two strong acceptors, X = CF3 and NO2, have some pi-donating ability under strong pi-electron demand conditions, the NRT and AED analyses do not support such an effect. We conclude that the pi-donating effect of 7r-acceptors increases in the order X = CHO < CN < Cl. Stabilization of the correlated energies with X = CF3 and NO2 is almost entirely due to n-sigma*-type charge-transfer delocalization.