Magnetically Induced Currents in [n]Cycloparaphenylenes, n = 6−11

Abstract

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 [6]CP and the current densities of the corresponding metal complexes Li(2)[6]CP and Mg[6]CP are also investigated. By the ring current criterion, the [6]CP with 4n pi electrons has a slight antiaromatic character, while [7]CP has (4n + 2) pi 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 [6]CP flows along a 4n pi 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 [6]CP with (4n + 2) pi 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 pi electrons in the current pathway. The (1)H 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.