Magnetically induced current densities in planar, twisted and extended conjugated molecules
Year: 2025
Type: dissertation
Abstract: The fundamental chemical concept “aromaticity” can be traced back to Faraday’s isolated benzene in 1825. Aromaticity refers to a special kind of chemical stability observed in large conjugated planar molecules with delocalized π electrons. The traditional H¨uckel rule is the most widely used criterion for identifying aromaticity. Geometric and thermodynamic criteria, such as bond length equalization and resonance stabilization energies, can also provide useful qualitative insights. However, the molecular response to magnetic pertubations is a better quantitative approach. In many cases, magnetic response properties, and in particular the magnetically induced ring current, can be interpreted as a quantitative measure of the aromaticity of molecules. Experimentally, ring current strengths can be probed by measuring the diamagnetic susceptibility exaltation and proton nuclear magnetic resonance chemical shifts or by probing the nuclear magnetic chemical shift of lithium ions in π-complexes. The related nucleus-independent chemical shift method is a popular computational method to assess aromaticity. However, these approaches may become less reliable in special cases, such as for twisted or lemniscular molecules, where the tropicity of the ring current is difficult to determine from experimental data. Calculating the magnetically induced current density susceptibility using the gauge-including magnetically-induced current density approach is a more reliable way to identify molecular aromaticity and provide a clear and intuitive understanding of aromaticity, as it enables direct determination of the tropicity of ring currents by classifying current-density vortices into diatropic or paratropic ring-current contributions. Studies of lemniscular porphyrinoids and [12]infinitene demonstrate that these molecular structures with M¨obius topologies are aromatic regardless of the number of occupied conjugated orbitals, whereas traditional aromaticity criteria such as H¨uckel’s rule, nuclear magnetic resonance chemical shifts, or nucleus-independent chemical shifts are less reliable for this kind of molecules. Molecular orbitals with M¨obius topology can arise not only from twisting molecular rings but also from d orbitals of transition metals in the ring that induce d–p conjugated bonding and a phase shift. However, our studies of Os- and Pt-conjugated metallacycles show that these systems do not sustain Craig-M¨obius aromatic or antiaromatic ring currents, as previously suggested. The bonding between two stacked antiaromatic Ni-norcorroles consists of a multicenter double bond involving two intermolecular orbitals, resulting in a short stacking distance of less than 3.0 A. While the monomer is antiaromatic, the face-to-face stacked dimer becomes aromatic due to reorganization of the frontier orbitals. An aim of the current-density investigations of benzene, coronene and a series of circum[n]coronene molecules was to touch the boundaries of aromaticity. The study reveals an odd–even alternation in the pattern of the magnetically induced current density susceptibility. Molecules with odd n sustain weakly aromatic Clar rings in the center, whereas those with even n lack them. The circum[n]coronene study shows that the strength of the global diatropic ring current increases with increasing size of the molecule without a clear upper limit.
Source: Työväentutkimus Vuosikirja
Author: Wang, Qian
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7197983948