We report calculations of the gauge-independent magnetically induced current densities in [n]cycloparaphenylenes ([n]CP), n = 6-11. In addition to the neutral [n]CPs, the dianion of CP and the current densities of the corresponding metal complexes Li2CP and MgCP are also investigated. By the ring current criterion, the CP with 4n π electrons has a slight antiaromatic character, while CP has (4n + 2) π electrons and is weakly aromatic with a ring current susceptibility strength that is about 25% of the ring current of benzene. The larger neutral [n]CPs, n = 8-11, do not sustain any net ring current around the nanohoop and are essentially nonaromatic. The weak paramagnetic ring current susceptibility of CP flows along a 4n π pathway on either edge of the phenylene rings. For the dianions, the ring current susceptibility strengths are 24-35 nA/T diatropic and thus the addition of two electrons induces an electron delocalization and an aromatic character of the nanohoops. The dilithium complex of CP with (4n + 2) π electrons is aromatic with a net ring current strength of 28 nA/T or 2.4 times the ring current strength of benzene, involving all 62 π electrons in the current pathway. The 1H NMR chemical shieldings and the nucleus-independent chemical shifts correlate with the strengths of the magnetically induced currents. The aromatic [n]cycloparaphenylenes have a quinoid structure, whereas the weakly aromatic or nonaromatic ones are benzoidic.
ASJC Scopus subject areas
- Organic Chemistry