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

Abstract

<jats:p>The rotational barrier RB around C–NH2 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–NH2 bond, increase of flattening of NH2 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 NH2 stretching frequencies, and increase in stabilization energy. The rotational barrier was also found to correlate well with empirical pKa and Hammett σp 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 (NH2) and electron-withdrawing groups.</jats:p>

Rotational Barrier and Quantification of Electron-Donating Substituent Effects: a Computational Study of para-Substituted Benzaldehydes

“…Models other than EDF2/6-31G* in Spartan'14 are not accurately corrected for chemical shifts and they show inconsistency in the relative positions of NMR peaks. The approach using ωB97X-D/6-31G** was used in the current work as it is a preferred computational model for such studies on parasubstituted anilines [14] and disubstituted 1,3-butadienes. [20] Energies of the minimum and transition states in both theoretical methods were corrected for zero-point energies from the measurement of IR vibrational frequencies except in calculation of stabilization energy where they almost cancel out in [Eq.…”

“…The relatively smaller change in the above mentioned parameters in transition states is because the conjugation is the lowest in these states [14] and therefore changing substituents has a smaller effect on their values. Data of the strong electron-accepting CN and NO2 substituents show opposite trends in comparison to all other electron-donating substituents as shown in Tables 1-3.…”

“…[39] Besides, several modifications and corrections of these constants have been reported in literature. [25,40] RB was shown before to be a reliable method to measure π-conjugation in para-substituted anilines [14] and in disubstituted 1,3-butadienes. [20] These studies include a large number of different substituents and rely on many calculated and empirical parameters that are affected by change of substituents.…”

“…The rotational barrier (RB) has been reported to increase with the enhancement of electronic delocalization due to electron donor or accepting groups in parasubstituted anilines, [14] phenols, [15] benzaldehydes, [16,17] acetophenones, [16,18] benzyl cations, anions, and radicals [19] and disubstituted 1,3-butadienes. [20] However, with the exception of electron-withdrawing effects in para-substituted anilines, [14] these studies are limited to small number of substituents. Moreover, no quantitative correlation has been documented in these studies between the effect of substituent on rotational barrier and conjugation.…”

Rotational Barrier and Bond Dissociation Energy and Enthalpy: Computational Study of the Substituent Effects in <i>Para</i>-Substituted Anilines and Phenols

“…Some geometric, atomic, and molecular parameters have been calculated in this study to investigate further the governing reason behind the trend of BDE/BDEt with the nature of the substituent. Previous studies have reported that RB is a valid and valuable measure for the stability in para-substituted anilines [35], parasubstituted benzaldehydes [36], as well as in disubstituted 1,3-butadienes [37] through resonance or inductive effects. Hence, the correlation of the BDEs/BDEts of 27 para-substituted anilines and phenols with RB is a particular focus of interest in the current work.…”