Fig. 2: Periphery overcrowding tunes the balance of steric strain and aromatic stabilization.

a,b, Cations 1–4 synthesized in this work (a) were selected on the basis of the calculated Gibbs energy differences, ΔG, between tropylium and the Dewar tropylium forms of (CH)₇⁺, Naph, Phen, 1–4 and a series of derivatives of 2 (B3LYP(GD3BJ)/6-31G(d)/CH₂Cl₂) (b). The most overcrowded derivative, 4, is predicted to lose aromaticity in favour of relieving strain by rearranging to its Dewar tropylium form. In b, the indicated positions of the different substituents of the derivatives of 2 correspond to the labelling shown in the structure of 2 in a. Positive ΔG values (orange bars) indicate an energetically favourable tropylium isomer, while negative ΔG values (purple bar) denote a bias towards Dewar tropylium. For asymmetric substitution patterns that break the C₂ symmetry, the lower energy of the two possible Dewar tropylium isomers has been used to estimate ΔG.