Conformational features and aromaticity of [18] to [38] 1,2,4,5-cyclophanes

Abstract

This study presents a comprehensive investigation of the conformational properties and aromaticity of thirty-one conformers of [1₈] to [3₈] cyclophanes. The smallest member, [1₈] cyclophane, adopts a rigid basket-like structure with D_4h symmetry, exhibiting pronounced distortions in its benzene rings (43º) compared to the structurally related [8] collarene (4.7º). As the bridges become larger (n = 2, 3) in [2₈] and [3₈] cyclophanes, the number of possible conformers increases to 10 and 20, with energy differences between the most stable and least stable conformers being 23.4 and 10.3 kcal/mol, respectively. All conformers were initially optimized at the B3LYP/6–311 + G** level of density functional theory (DFT) and their properties were subsequently calculated at the same level. The properties of these molecules were evaluated from three perspectives: magnetic, geometric, and energetic. For this purpose, various indices such as nucleus-independent chemical shift (NICS), HOMO–LUMO gap, magnetic susceptibility (χ), harmonic oscillator model for aromaticity (HOMA), and relative energy were employed. Additionally, NICS scans were carried out to elucidate ring current distribution and inter-ring interactions. Based on the obtained results, a Ball-and-Cycle model is proposed to rationalize the structural, property, and stability trends within these series of three-dimensional supramolecular systems.