How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring

Abstract

<div> <div> <div> <p>Large conjugated rings give rise to novel promising structures that can sustain persistent currents at low temperatures even in the presence of strong magnetic fields. One of the most interesting such molecules was recently synthesized [Anderson et al., Nature, 2017, 541, 3512] in the form of a six-porphyrin nanoring structure, which, according to the authors, in its +6-oxidation state (c-P66+) sustained an aromatic ring current involving 78π electrons; one of the largest aromatic rings ever produced. In this paper, we have provided compelling evidence that this molecule is not aromatic, as it was incorrectly inferred from computational calculations that suffer from large delocalization errors. A thorough analysis of four oxidation states of the six-porphyrin nanoring re- veals that the main reason behind the poor aromaticity of these nanorings is the low delocalization in the transition from the porphyrins to the bridging butadiyne linkers, which disrupts the overall conjugated circuit. These results highlight the importance of choosing an adequate computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule that is responsible for the loss of aromaticity. We believe the strategy here employed will be helpful in designing new large aromatic molecular nanorings. </p> </div> </div> </div>

How Many Electrons Does a Molecular Electride Hold?

“…B3LYP usually gives the lowest average electron numbers in the NNA basins, the largest zero-electron probabilities, and the lowest oxidation states. This result is in line with the large delocalization errors documented in conjugated and aromatic molecules. − , CAM-B3LYP and M06-2X do not suffer from large delocalization errors and often provide similar NNA electron numbers, although slightly lower than those given by MP2. On the other hand, HF, which tends to overestimate electron localization, provides the largest electron numbers in the NNA basin.…”

“…All structures were optimized at the CAM-B3LYP/ma-TZVP , level of theory. It has been recently documented that this functional avoids large delocalization errors, − which could give rise to spurious critical points in the potential energy surface. , For these optimized geometries, single-point calculations with various basis sets and density functional approximations (CAM-B3LYP, B3LYP, , M06-2X, and MN15) as well as Hartree–Fock (HF) and MP2 methods have been performed. Although HF completely neglects electron correlation and, in most cases, density functional approximations (DFAs) provide more accurate energies and geometries, DFAs struggle to reproduce the electron density of some electrides with unpaired electrons.…”

“…At the same time, the occupations of the EffAOs are used to calculate a measure of the uncertainty of the oxidation state assignment ( vide infra ). The EOS analysis has been recently used to assign oxidation states in many nontrivial molecules. ,− In this work, we have employed APOST3D (dev. ver.…”

COMPAS-3: a Data Set of peri-Condensed Polybenzenoid Hydrocarbons

“…In this regard, we note that we deliberately chose the CAM-B3LYP functional, which has been shown not to suffer from overdelocalization errors; 90,91 such errors could lead to spurious results, including exaggerated planarity and over-estimation of aromatic stabilization. Nevertheless, to try to pinpoint the source of the discrepancy, we studied the relationship between the size of the molecule and the difference in relative energy, ∆E rel = E rel (DFT) − E rel (xTB).…”

Quantifying the electronic dimension of global aromaticity in macrocycles

“…Aromaticity is one of the most longstanding concepts in physical organic chemistry, yet in many cases the assignment of a molecule as "aromatic" or "antiaromatic" elicits heated debate, [1][2][3] especially as a molecule's structure strays further from that of the archetype aromatic, benzene.…”