Evaluating Aromaticity in Large Macrocycles

Aromaticity is one of the most pivotal concepts in chemistry, especially in organic chemistry. It is associated with cyclic electron delocalization (or conjugation) in closed circuits, giving rise to bond-length equalization, energy stabilization, large magnetic anisotropies, and abnormal chemical shifts, among other well-known effects. Density functional theory (DFT) is the usual method of choice for performing a computational study of large aromatic systems due to its balance between accuracy and computational cost.

In the last two decades, large macrocycles with unique optical and aromatic properties have been synthesized. Many of these studies include a computational section, using the hybrid functional B3LYP as the standard density functional approximation, where the systems are computed in isolation and gas phase.

In recent projects, we have demonstrated that the study of aromaticity and other molecular properties in large conjugated rings, such as nanorings, is strongly influenced by the amount of delocalization error (DE) present in the density functional approximation (DFA). Hybrid functionals, like B3LYP, often yield optimal geometries that are overly symmetric, exaggerating electron delocalization over the ring, i.e., overestimating the aromatic character of the system. Correctly treating DE reveals that the reported classifications of aromatic and antiaromatic large systems become more subtle.

In this internship, the candidate will learn how to quantify aromaticity using computational chemistry. The aromaticity of selected macrocycles will be studied using different DFAs, and its modification on the aggregation process will be investigated.

The candidate should have a basic knowledge of quantum chemistry (assumed in chemistry and physics BSc. students) and be eager to learn the basics of electronic structure theory and DFT.

[1] I. Casademont-Reig et al. Phys. Chem. Chem. Phys., 20, 2787 (2018); I. Casademont-Reig et al. Molecules, 25, 711 (2020); I. Casademont-Reig et al. Angew. Chem. Int. Ed., 60, 24080 (2021); I. Casademont-Reig et al. Angew. Chem. Int. Ed., 61, 06836 (2022); I. Casademont-Reig et al. Chem. Eur. J., 29, e202202264 (2023).