Magnetic Aromaticity of Cycloporphyrin Nanorings

Abstract

The ascertainment of magnetic aromaticity is not necessarily straightforward, especially for large and bent systems, such as the cycloporphyrin nanorings recently synthesized by the group of Anderson. Six of these cycloporphyrin nanorings were studied here computationally. Indirect methods, based on nuclear shielding and magnetizabilities, and direct methods, based on standard quantum mechanics, were both used effectively to determine their magnetically induced current strength, which mostly confirmed Anderson’s classification. However, in the case of hexanions, and in particular for cyclohexaporphyrin hexacations, a significant cancellation of delocalized diatropic and paratropic flow occurred, showing that the resultant faint aromatic character was a result of competing aromatic and antiaromatic contributions, as also evidenced by the ipsocentric method. A warning is renewed on the use of isotropic shielding to determine the tropicity of the magnetically induced current.

Reply to the Correspondence on "How Globally Aromatic Are Six-Porphyrin Nanorings?"

“…This high sensitivity of the ring current strengths to the amount of HF exchange suggests this molecule is susceptible to large delocalization errors and calls for the use of DFAs that minimize its impact. The conclusions about the aromaticity of c-P6[b 6 ] • T6 6+ are also backed up by the study of Monaco and co-workers, 40 who employed BHandHLYP -which was the DFA recommended for the calculation of magnetizabilities in the recent study of Sundholm and co-workers 34 -and obtained J=2.0 nA/T. Although the RCM provides an excellent tool to assess the aromaticity of many classical organic molecules, 36,41 the results in Table 3 suggest we can overestimate the actual ring current strength using it.…”

On the JAP Method for the Indirect Determination of Delocalized Currents from Experimental Chemical Shifts**

“…The sketches reported in the SI show that in several cases the current is reminiscent of in‐plane σ currents, although these are cases where the currents have low intensity. A more significant problem has already been discussed in the case of cycloporphyrin nanorings, where the in‐plane currents can be contra‐rotating heterotropic [33] . This can substantially change the values modeled in a coarse‐grained approach, because the nuclei can feel a sizable current, because one of the two heterotropic loops is closer to them, even if the overall current is very small. Weighing by the net projected cross‐section area, Eq.…”

“…[30]. From this analysis, we excluded cycloporphyrin nanorings, which have been object of a previous study [33] …”

Reply to the Correspondence on “How Aromatic Are Molecular Nanorings? The Case of a Six‐Porphyrin Nanoring”**

“…This high sensitivity of the ring current strengths to the amount of HF exchange suggests this molecule is susceptible to large delocalization errors and calls for the use of DFAs that minimize its impact. The conclusions about the aromaticity of c-P6[b 6 ]•T6 6 + are also backed up by the study of Monaco and co-workers, [40] who employed BHandHLYP -which was the DFA recommended for the calculation of magnetizabilities in the recent study of Sundholm and coworkers [34] -and obtained J = 2.0 nA T À 1 .…”