Energetic and Geometric Characteristics of Substituents, Part 3: The Case of NO2 and NH2 Groups in Their Mono-Substituted Derivatives of Six-Membered Heterocycles

Paweł A. Wieczorkiewicz,Tadeusz M. Krygowski,Halina Szatylowicz

doi:10.3390/sym14010145

Paweł A. Wieczorkiewicz, Tadeusz M. Krygowski + Show 1 more

Open Access

https://doi.org/10.3390/sym14010145

Copy DOI

Journal: Symmetry	Publication Date: Jan 12, 2022
Citations: 2	License type: CC BY 4.0

Affiliation: Warsaw University of Technology, University of Warsaw

Abstract

Substituted heterocyclic arenes play important roles in biochemistry, catalysis, and in the design of functional materials. Exemplary six-membered heteroaromatic molecules, that differ from benzene by inclusion of one heteroatom, are pyridine, phosphorine, arsabenzene, and borabenzene. This theoretical study concerns the influence of the heteroatom present in these molecules on the properties of substituents of two types: electron-donating (ED) NH2 group and electron-accepting (EA) NO2 group, attached at the 2-, 3-, or 4-position. The effect is evaluated by the energy of interaction (Erel) between the substituent and the substituted system and electronic properties of the substituents described by the charge of the substituent active region (cSAR) index. In addition, several geometric descriptors of the substituent and heteroaromatic ring, as well as changes in the aromaticity, are considered. The latter are assessed using the Electron Density of Delocalized Bonds (EDDBs) property of delocalized π electrons. The obtained results show that the electronegativity (EN) of the heteroatom has a profound effect on the EA/ED properties of the substituents. This effect is also reflected in the geometry of studied molecules. The Erel parameter indicates that the relative stability of the molecules is highly related to the electronic interactions between the substituent and the heteroarene. This especially applies to the enhancement or weakening of π-resonance due to the EN of the heteroatom. Additionally, in the 2-heteroarene derivatives, specific through-space ortho interactions contribute to the heteroatom effects.

Highlights

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
The first quantitative approach to the description of the substituent effect was proposed by Hammett [1,2]
The first case can be represented by the different substituent constants for the para and meta positions, while the substituent constants σ+ and σ, characterizing the substituent effects in molecules with positively and negatively charged reaction sites, respectively, are an example of the second case

Summary

Introduction

Hammett substituent constants or alike are used to characterize the substituent effect quantitatively [3]. The first case can be represented by the different substituent constants for the para and meta positions, while the substituent constants σ+ and σ–, characterizing the substituent effects in molecules with positively and negatively charged reaction sites, respectively, are an example of the second case. This is known as the reverse substituent effect [4]. For any substituent in any system, it can be quantified using the charge of the substituent active region (cSAR) concept [5,6]

Objectives

Methods

Results

Conclusion