Theoretical study on the conformation and aromaticity of [13] to [33] double-stranded cyclophanes encompassing three benzene subunits

In the present study, conformations and aromaticity of seventeen [1₃], [2₃] and [3₃] double-stranded cyclophanes composed of three benzene moieties were investigated at B3LYP level of density functional theory. The stability of these conformers is different depending on the type of cycloalkane-like cycles formed by (CH₂) _n bridges and the peripheral benzene rings, and also the curvature of the latter. In [1₃] cyclophane, this leads to only one conformer containing puckered cyclobutane-like cycle, at an angle of about 37^◦. In comparison, in cyclophanes [2₃] and [3₃], the corresponding cycles are cyclohexane-like and cyclooctane-like, respectively. While conformer 2a having TB-like cycles is the most stable member of the [2₃] family, of eight ground-state conformers obtained for [3₃], 3a with two Chair-Chair-like and one Twist-Chair-like cycles is the most stable one, and 3h with three Twist-Boat-like cycles being 6.5 kcal higher in energy than the former is the least stable member of series. In addition to the conformational analysis, the aromaticity of the series has been assessed using different indexes, including magnetic criteria nucleus-independent chemical shifts (NICS-based) at the cage and at the ring centers, HNMR chemical shift, Magnetic Susceptibility, HOMA and relative energy (RE).