Electronic structure of mesoionic compounds. The classification problem
Abstract
Mesoionic compounds are conventionally categorized into two types based on the primary origin of electrons within the conjugated system, specifically determined by the arrangement of heteroatoms in the five-membered ring. An examination of diverse mesoionic compounds has been undertaken to address the pivotal query surrounding their classification: does the primary origin of electrons hold significance, and does this criterion demarcate a definitive boundary between the two types of compounds? To comprehensively address this issue, the DFT calculations were performed for a set of mesoionic molecules. The electronic properties of the molecules were studied within the frameworks of quantum theory of atoms in molecules (QTAIM) and the Nuclear Independent Chemical Shift (NICS).
To comprehend the topological distinctions among representatives of the two types, we describe a set of indices designed to characterize the spatial distribution of electronic parameters within the molecular frameworks. The results obtained show that the existing classification is to a certain extent justified, with the main distinguishing factor between the two types being the nature of the distribution of the resulting ellipticities of bonds in five-membered ring and the structure of the molecular orbitals. At the same time, based on NICS calculations we concluded that both classes are not characterized by pronounced aromaticity of the mesoionic ring for the selected set of molecules.
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