Effect of size, charge, and spin state on Hückel and Baird aromaticity in [N]annulenes

Description

Aromaticity is intricately linked to properties such as conductance and diradical character, underscoring its broad significance.1,2 Understanding the principles that govern aromaticity across different electronic states is essential for rationalising its impact. The Hückel and Baird rules provide a conceptual framework for monocyclic π-conjugated systems in their singlet ground and lowest-lying triplet states, respectively, particularly in the context of [N]annulenes. According to Hückel’s rule, a ground-state molecule is aromatic when it contains (4n+2) π-electrons, whereas Baird’s rule predicts aromaticity in the lowest-lying triplet state for systems with (4n) π-electrons (n = 0, 1, 2, …).

A previous study explored how aromaticity evolves in singlet ground-state [N]annulenes as ring size increases from N = 12 to 66.3 However, no systematic investigation has yet been conducted for the lowest-lying triplet state of [N]annulenes or their charged counterparts. In this work, we addressed this gap by performing DFT calculations across several aromaticity descriptors, with a particular focus on aromatic stabilisation energies (ASEs).4 Our findings reveal that both neutral and charged [N]annulenes adhere to the Hückel and Baird rules. Nevertheless, for larger rings, these rules diminish in significance,5 and the distinction between ASEs (and other indices of aromaticity) of [N]annulenes with (4n) and (4n+2) π-electrons becomes progressively less pronounced.

1 Quintero, S.M.; Van Nyvel, L.; Roig, N.; Casado, J.; Alonso M. J. Phys. Chem. A 2024, 128, 6140-6157.
2Van Nyvel, L.; Casademont-Reig, I.; Eeckhoudt, J.; Quintero, S. M.; De Proft, F.; Geerlings, P.; Alonso, M. Chem. Sci. 2026, accepted manuscript.
3 Jirásek, M.; Rickhaus, M.; Tejerina, L.; Anderson, H. L. J. Am. Chem. Soc. 2021, 143, 2403-2412.
4 Van Nyvel, L.; Alonso, M.; Solà, M. Chem. Sci. 2025, 16, 5613-5622.
5 Casademont-Reig, I.; Ramos-Cordoba, E.; Torrent-Sucarrat, M.; Matito, E. Molecules, 2020, 25, 711.

Periode	13 jul. 2026
Evenementstitel	10th EuChemS Chemistry Congress (ECC10)
Evenementstype	Conference
Locatie	Antwerpen, BelgiumToon op kaart