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The rotational barrier RB around C–NH 2  bond between the minimum and maximum states of 84 electron-withdrawing groups at  para -position in aniline were studied at the density functional wB97X-D/6-31G** level. The rotational barrier was found to correlate strongly with shortening of the C–NH 2  bond, increase of flattening of NH 2  group, decrease in negative natural charge on amino nitrogen, increase in minimum ionization potential around lone pair of amino nitrogen, increase in maximum (positive) electrostatic potential on amino hydrogens, increase in NH 2  stretching frequencies, and increase in stabilization energy. The rotational barrier was also found to correlate well with empirical p K  a  and Hammett  σ  p  constants. The rotational barrier is shown to be a reliable quantum mechanical approach to measure p-conjugation in para-substituted anilines.  Based on RB a quantitative scale is constructed for the ability of electron-withdrawing substituents to resonate with aniline. A quinone-like structure has been proposed for stronger electron-withdrawing substituents where an extension of resonance stabilization requires the simultaneous presence of electron donor (NH 2 ) and electron-withdrawing groups.

Rotational barrier and electron-withdrawing substituent effects: Theoretical study of -conjugation in para-substituted anilines