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[References: 66 tit.]

Magnetizabilities and magnetically induced current densities have been calculated and analyzed for a series of antiaromatic cyclo[4k]carbons (k = 2−11), iso[n]phlorins (n = 4−8), expanded porphyrinoids, and meso−meso, β−β, β−β triple-linked porphyrin and isophlorin arrays. The cyclo[4k]carbons with k = 2−6 are predicted to be closed-shell paramagnetic molecules due to the very strong paratropic ring current combined with its large radius. Larger cyclo[4k]carbons with k = 6−11 are diamagnetic because they sustain a paratropic ring current whose strength is weaker than −20 nA T−1 , which seems to be the lower threshold value for closed-shell paramagnetism. This holds not only for cyclo[4k]carbons but also for other organic molecules like expanded porphyrinoids and oligomers of porphyrinoids. The present study shows that meso− meso, β−β, β−β triple-linked linear porphyrin and isophlorin arrays have a domainlike distribution of alternating diatropic and paratropic ring currents. The strength of their local paratropic ring currents is weaker than −20 nA T−1 in each domain. Therefore, linear porphyrin and isophlorin arrays become more diamagnetic with increasing length of the ribbon. For the same reason, squareshaped meso−meso, β−β, β−β triple-linked free-base porphyrin and isophlorin tetramers as well as the Zn(II) complex of the porphyrin tetramer are diamagnetic. We show that closed-shell molecules with large positive magnetizabilities can be designed by following the principle that a strong paratropic current ring combined with a large ring-current radius leads to closed-shell paramagnetism.