Magnetically induced currents in [n]cycloparaphenylenes, n = 6-11.
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.
Taubert
Stefan
S
Department of Chemistry, P.O. Box 55 (A.I. Virtanens plats 1), FIN-00014 University of Helsinki, Helsinki, Finland. stefan.taubert@helsinki.fi
Sundholm
Dage
D
Pichierri
Fabio
F
D001555:Benzene Derivatives / Q000737:chemistry*; D008094:Lithium / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000592:standards; D008280:Magnetics; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D012015:Reference Standards
D001555:Benzene Derivatives; D009942:Organometallic Compounds; D008094:Lithium; D008274:Magnesium