Exploring Aromaticity and Antiaromaticity Using Magnetic Shielding Calculations

This thesis investigates aromaticity and antiaromaticity using magnetic shielding, with a particular focus on extending the conclusions that can be drawn from nucleus-independent chemical shifts (NICS) by using three-dimensional isotropic magnetic shielding volume data to visualise magnetic shielding with isosurfaces. In particular, the extent of aromatic reversal upon excitation from the ground to the lowest triplet excited state is explored for a range of molecular systems.

Through the case study of thiophene and its condensed forms, it is demonstrated that excited-state aromaticity is highly sensitive to geometry, with isomers of condensed heterocycles experiencing significantly different levels of aromaticity reduction upon excitation, while a select few show true Baird-type aromaticity reversals. Condensed macrocycles show a variety of behaviours when excited from the ground to the lowest energy triplet state: the degree of aromatic reversal in heterocirculenes is highly dependent on ring strain, and any changes in aromaticity are relatively modest; aromaticity in norcorrole is dominated by large $\pi$ networks, so the molecule experiences a significant aromatic reversal upon excitation.

Significant contributions include the proposal of novel structures, such as sulfinfinitenes, and the examination of boron clusters, where $\sigma$-aromaticity dominates magnetic shielding over $\pi$-aromaticity. Additionally, deshielding in the T$_{\text{1}}$ state of ethyne highlights that the central deshielded region in antiaromatic molecules is not solely due to electron delocalisation. Model chemistry dependence is highlighted, with varying computational methods yielding different results, emphasising the need for chemical intuition alongside computational insights. The study of annulenes reveals that excited-state and geometry-dependent effects must be carefully considered, particularly when charged systems are involved.

Ultimately, this work reinforces the complexity of aromaticity as a continuum, rather than a binary property, and suggests that magnetic shielding can provide a robust tool for investigating electronic structure, provided it is paired with a comprehensive chemical understanding.