Assessing the Extent of π-Electron Delocalization in Naphtho-Annelated Fluoranthenes by Means of Topological Ring-Currents

Abstract

The extent of π-electron delocalization in the fivemembered rings of 14 naphtho-annelated fluoranthenes has recently been assessed by means of several approaches, including application of the following indices: the energy effect (ef), bond resonanceenergy (BRE), multicenter delocalization-energies (MCI), the index arising from the harmonic-oscillator model of aromaticity (HOMA), and nucleus-independent chemical shifts (NICS). The calculated Huckel−London−Pople−McWeeny (HLPM) ("topological") ring-current intensities (TRC) for the five-membered rings of these same structures are here compared with the above-named indices in order to assess how well TRC parallels these other criteria. The indices ef and BRE are the ones that correlate best with TRC, and it is suggested that this is because all three approaches are founded on the Huckel model. TRC does not, however, confirm the proposal that cyclic delocalization in the five-membered rings can be greater than in any of the six-membered rings in this series of conjugated systems. Certain ostensible shortcomings of the NICS approach are discussed, and some doubts are emphasized regarding the legitimacy of associating "ring currents" with "aromaticity".

Doubly zwitterionic, di-reduced, highly electron-rich, air-stable naphthalenediimides: redox-switchable islands of aromatic–antiaromatic states

“…Although, NICS is a decent tool for interpretation of the aromaticity of a variety of cyclic compounds yet it has been criticized due to its unreliability to describe the aromaticity of the polycyclic structures 28. This is because NICS calculates the induced magnetic field at single point in space of a ring also affected by the σ-electrons while aromaticity is a property of global and local ring currents induced by a single ring and the rings associated with that ring 29…”

A study on the aromaticity and magnetic properties of N-confused porphyrins

“…In general, in monocyclic aromatic compounds, magnetically induced diatropic and paratropic RC are associated with their aromaticity and antiaromaticity, respectively [55]. The Hückel-London theory was used here to derive RC patterns of the above compounds [36][37][38][39][40][41]. In order to better understand the role of the diatropic and paratropic contributions of the different circuits to the RC, we investigated the trend of the magnetic properties of the different circuits.…”

“…TRE, BRE and CRE are given in units of |β|, where β is the standard resonance integral in Hückel theory. RC strength herein has been obtained using the Hückel-London model [36][37][38][39][40][41]. Van-Catledge's set of Hückel parameters have been used for nitrogen [42].…”

A study on the aromaticity and conjugation pathways of diheterodiazuliporphyrins and their dications

“…RCs have been calculated using the Hückel-London model. [37][38][39][40][41][42][43] The standard set of Hückel heteroatom parameters proposed by Van-Catledge has been used here in order to calculate the TRE, BRE, and CRE aromaticity values. [46] In general, positive values for TRE, BRE, and CRE indicate aromaticity, whereas negative values indicate the antiaromaticity of those molecules.…”

“…[28][29][30][31][32][33][34] In the present paper, we describe the global aromaticity of diheterodiazuliporphyrins and their dications utilizing the TRE method (an energetic index) [35,36] and the ring current (RC) method (a magnetic index). [37][38][39][40][41][42][43] The effect of the azulene units on the global aromaticity of the porphyrinoid macrocycle is investigated. The origin of global and local aromaticities as well as the main conjugation pathways have been identified using the bond resonance energy (BRE) [44] and circuit resonance energy (CRE) indices.…”