When are Antiaromatic Molecules Paramagnetic?

Abstract

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 domain-like 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, square-shaped meso-meso, β-β, β-β triple linked free-base porphyrin and isophlorin tetramers as well as 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.of the ring current. The largest member of the series namely iso[8]phlorin has a magnetizability of 3532 a.u., because it sustains a very strong ring current of -617 nA T -1 whose radius is 5.3 Å. Calculations on expanded porphyrins fulfilling Hückel's rule for antiaromaticity show that orangarin, rosarin, amethyrin are diamagnetic, since they sustain paratropic ring currents that a weaker than -20 nA T -1 . The largest member of that series is the hypothetic circle-shaped [40]octaphyrin(1.1.1.1.1.1.1.1) that sustains a strong paratropic ring current of -561 nA T -1 whose radius is 5.3 Å. The circle-shaped [40]octaphyrin(1.1.1.1.1.1.1.1) molecule is therefore strongly paramagnetic with a magnetizability of 3244 a.u.

Benchmarking Magnetizabilities with Recent Density Functionals

“…Previous studies have also suggested that the M06-2X functional sometimes underestimates magnetizabilities and ring-current strengths. 128 , 129 , 132 Revised versions of Minnesota functionals have been studied in this work and found to yield somewhat more accurate magnetizabilities than the original parameterizations. However, the revised versions also still appear on the second half of the ranking list.…”

Remarkable Size Effect on Photophysical and Nonlinear Optical Properties of All‐Carboatomic Rings, Cyclo[18]carbon and Its Analogues

“…For carbon rings with different sizes, electronic properties including the gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), singlet‐triplet splitting, ionization potential, and electron affinity for cyclo[ n ]carbons ( n =10–100) were previously studied using thermally‐assisted‐occupation density functional theory (TAO‐DFT) [25] . Aromatic behavior and molecular magnetizability of some cyclo[ n ]carbons ( n up to 100) were also reported by Baryshnikov, Valiev, and their collaborators [26,27] . Optical properties, such as electronic absorption spectrum and (hyper)polarizability, are important molecular characteristics because they determine the possibility that molecules will eventually be used as practical optical materials.…”

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

“…and C 24 . 41 Furthermore, after the recent experimental generation of cyclo[18]carbon by Anderson and co-workers, 42 , 43 the aromaticity of cyclo[4 n + 2]carbons has gained increased attention, 39 , 44 yet these are outside our explicit focus herein.…”