Magnetically induced currents in bianthraquinodimethane-stabilized Möbius and Hückel [16]annulenes

The ring currents, NMR chemical shifts, topology of the chemical bonding, and UV-vis spectra of bianthraquinodimethane-stabilized [16]annulenes possessing Möbius and Hückel topology are investigated. The aromatic character of the title compounds is discussed on the basis of the magnetically induced current density obtained using the gauge-including magnetically induced current (GIMIC) approach. Numerical integration of the current density circling around the [16]annulene ring shows that both the Hückel and the Möbius isomers are non-aromatic. The [16]annulene ring of both isomers sustains a net ring current whose strength is only 0.3 nA/T. The ring current consists of a diamagnetic flow on the outside of the [16]annulene ring and a paramagnetic current inside it. Since the net ring-current strength of the [16]annulene is less than 5% of the ring current strength for benzene, both isomers must be considered non-aromatic by the ring current criterion. The similar bond length alternation of the [16]annulene rings also points to a similarity in aromatic character of the two isomers. The shape of the ring current of the Möbius isomer shows that the current density is somewhat more outspread than that of the Hückel isomer. Spatially separated diatropic and paratropic currents of equal strength follow the annulene bonds. The atoms-in-molecules (AIM) analysis reveals a cage critical point in the region of the outspread current density of the Möbius isomer. Intramolecular CH...pi and pi-pi interactions identified by AIM analysis, in combination with the outspread current density, stabilizes the Möbius isomer relative to the Hückel one. The molecule is characterized by calculating the (13)C and (1)H NMR chemical shifts and the UV-vis spectrum and comparing these to experimental spectra. The (13)C NMR and (1)H NMR chemical shifts are rather similar for the two isomers. The UV-vis spectra are compared with the excitation energies calculated at the time-dependent density functional theory (TDDFT) level using Becke's three-parameter hybrid functional together with the LYP correlation functional (B3LYP), as well as at the approximate coupled cluster singles (CCS) and at the approximate coupled cluster singles and doubles (CC2) levels of theory. The CC2 calculations yield excitation energies in fairly good agreement with experimental data.