A Novel Electrostatic Approach to Substituent Constants:  Doubly Substituted Benzenes

Abstract

The most negative-valued molecular electrostatic potential (MESP) minimum (V min) observed over the benzene ring is proposed as a sensitive quantity for the analysis of the electronic perturbations due to the substituents attached to it. MESP topography of 45 doubly substituted benzenes is mapped at HF/6-31G** level for an appraisal of this proposition. The V min values are seen to clearly reflect the changes due to the different orientations (para, meta, and ortho) and conformations of the substituents and different mechanisms of electron donation or withdrawal. Good linear correlations are obtained with V min and the experimentally determined σ values of the ortho- and meta-disubstituted benzenes. New quantities, D p, D m, and D o, introduced in this work as the substituent pair-constants respectively for the para, meta, and ortho arrangements, provide a quantitative measure of the simultaneous effect of two substituents over the aromatic nucleus. The predictive power of these quantities is checked in the case of some triply substituted benzenes using an equation V min = V ben + ∑ΔV min(mono) − ∑D p − ∑D m − ∑D o where V ben is the MESP minimum of benzene and ΔV min(mono) is the difference between the monosubstituted benzene V min and V ben. These predicted values are in fairly good agreement with the MESP values obtained at HF/6-31G** level.

Application of Electron Delocalization Indicators in the Study of Electrophilic Aromatic Substitution Reactions

“…Then, the interactions of an electrophile and an activated ring at their minimum of electrostatic energy will be stronger than those involving benzene or deactivated rings, leading to a more prompt formation of complexes and Wheland intermediates. Similar considerations apply also to the analysis of di-and tri-substituted benzenes [144]. The consideration of the 45 disubstituted benzenes that can be formed with the substituents NO 2 , Cl, CH 3 , OCH 3 and NH 2 at the RHF/6-31G(d,p) approximation revealed that the position of the most negative critical points of the MESP is dictated by the strongest activating group over the benzene ring.…”

“…The activating or deactivating character of a group as well as its orientation properties is generally rationalized in terms of mesomeric and inductive effects [1][2][3]101]. There are a number of works which have dealt with the effect of a substituent or a heteroatom in an EAS reaction using the topological analysis of scalar fields [51,109,[140][141][142][143][144][145][146]. This section is aimed to summarize the most relevant information provided by these studies.…”

“…The topography of the MESP has also been used to study the changes in the electronic charge distribution as a consequence of a substitution in benzene [143][144][145]. Due to the long range of the electrostatic interaction, Gadre and Suresh [143] suggested that the minima of the MESP are the initial atractors for an arriving electrophile prior to the formation of a complex.…”

Ab initiostudy of the thermopower of biphenyl-based single-molecule junctions

“…13,28 Furthermore, M7 shows the largest substituent-related shifting effect on the biphenyl backbone in our family of molecules due to the electrondonating nature of the four attached methyl side groups. 13,38,43 Its thermopower hence arises from a detailed interplay between the substituent-related shifting and the large torsion angle, as explained in Ref. 13.…”

An electrostatic scale of substituent resonance effect

“…Previous work has shown that MESP is an excellent descriptor to interpret the substituent effects. [21][22][23][24][25][26] Since MESP is an experimentally observable property 27 and intimately related to the electron distribution, the use of MESP will definitely change the status of resonance effect from a qualitative concept to a quantitative concept.…”