ChemInform Abstract: The Concept of Ring Currents

Gomes, J.A.N.F.; Mallion, R. B.

doi:10.1002/chin.199732284

Cited by 20 publications

(117 citation statements)

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“…[1][2][3][4][5][6] that is, with the assumptions of (a) a (geometrically) planar carbon-atom skeleton, and (b) the areas of five-membered and six-membered rings being taken to be in the ratio [1][2][3][4][5][6]46]             5cot : 6cot 5 6 π π .…”

Section: Calculationsmentioning

confidence: 99%

“…Furthermore, [4,5] for a conjugated system comprising r rings, if π -London (structure) χ ⊥ stands for the 'London' [20,21,46,47] π-electron contribution to the structure's magnetic susceptibility, perpendicular to an assumed molecular plane, and π -London (benzene) χ ⊥ stands for the similarly calculated 'London' contribution [20,21,47,48] to the (diamagnetic) susceptibility of benzene, perpendicular to its own molecular plane, then, for a conjugated system comprising r rings, we define the 'susceptibility ratio': [4,5] …”

Section: Calculationsmentioning

confidence: 99%

“…Exhaustive details of the history and concept of topological ring-currents are available in two recent reviews, [3,4] and in some older ones. [5,6] The predictions of topological ring-current calculations have generally been compared with two particular ab initio approaches:…”

Section: Introductionmentioning

confidence: 99%

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Ring-Current Properties of Bispentalenes and Related Structures — Comparison of Ab Initio and Hückel-London-Pople-McWeeny (HLPM) ‘Topological’ Calculations

Dickens¹,

Mallion²

2017

Croat. Chem. Acta

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Two different groups (Cao et al. (2015) and ) have recently reported (a) experimental Nuclear Magnetic Resonance data and ab initio Nucleus Independent Chemical Shift (NICS) calculations, and (b) GIMIC (Gauge Including Magnetically Induced Currents) ab initio computations, on the magnetic properties of some derivatives of [4n+2]-π-electron conjugated systems called 'bispentalenes'. These are formed by annellating two pentalene groups with a benzene or a naphthalene moiety. The same structures are here subjected to calculations based on the simple, pseudo graph-theoretical, Hückel-London-Pople-McWeeny (HLPM) 'topological' ring-current approach. In addition, HLPM calculations are presented on some structures with [4n]-perimeters that were also studied by Cao et al., as well as some other related, specially 'designed', [4n]-and [4n+2]-perimeter structures of our own choosing. The aim is to see whether there is qualitative, or even semi-quantitative, agreement between a (presumably) numerically accurate but necessarily complex ab initio calculation and a conceptually simple, quasi graph-theoretical one -the HLPM approach -whose predictions depend only on the carbon-carbon connectivity of the structure being investigated, and on the (geometrical) areas of its individual constituent rings. The HLPM calculations agree with the more-sophisticated studies that all the structures examined, whether they be [4n]-or [4n+2]-perimeter systems, bear paramagnetic π-electron currents around their perimeters. Furthermore, all the many pentalene moieties in these conjugated systems appear to undergo incorporation into these bispentalene structures with -to greater or lesser extents -their characteristic perimeter circulations in the paramagnetic sense surviving intact. Quantitative regression comparisons between GIMIC ab initio integrated bond-current susceptibilities and HLPM bond-current intensities in a small sample of bonds in the structures studied by Sundholm et al. are found to have a correlation coefficient of 0.94 -not as high as that obtained (0.98) when similar data for a larger sample of bonds in several alternant, condensed, benzenoid were previously compared. It is again emphasised that Pople and Untch's rule about [4n+2]-annulenes being diamagnetic and [4n]-ones being paramagnetic -like the famous Hückel Rule itself -rigorously applies only to monocycles.

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Section: Calculationsmentioning

confidence: 99%

Section: Calculationsmentioning

confidence: 99%

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Ring-Current Properties of Bispentalenes and Related Structures — Comparison of Ab Initio and Hückel-London-Pople-McWeeny (HLPM) ‘Topological’ Calculations

Dickens¹,

Mallion²

2017

Croat. Chem. Acta

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“…Of the characteristic properties associated with conjugated systems, diamagnetic susceptibilities arising from π-electron ringcurrents [1][2][3][4] have been considered as one of the main diagnostics of aromaticity [5][6][7] -albeit a controversial one. [8][9][10][11] However, because of the so-called multidimensional character of aromaticity, [12][13][14][15][16][17][18][19] a quantitative measure requires the presence of several attributes, and this complicates quantitative assessments of the concept.…”

Section: Introductionmentioning

confidence: 99%

“…One should note, however, that this multidimensional character has been discussed mostly for conjugated heterocycles, [20][21][22] whilst the present paper is about benzenoid hydrocarbons. Furthermore, in addition to the global diamagnetic anisotropy, local π-electron ring-current intensities can be determined for each ring [1][2][3][4]8 of a conjugated network -in our case, a benzenoid system. In addition to the long-established criteria for aromaticity of continuous conjugationstabilising resonance-energy, and a tendency towards bond-length equalisation in planar molecules -threedimensional aromaticity has also been established.…”

Section: Introductionmentioning

confidence: 99%

Correlations between Topological Ring-Currents, π-Electron Partitions, and Six-Centre Delocalisation Indices in Benzenoid Hydrocarbons

Balaban¹,

Mallion²

2012

Croat. Chem. Acta

119

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Abstract. Comparison is made between three different indices that characterise the individual rings of a wide range of condensed benzenoid hydrocarbons. Two of these ("π-electron partitions" and the six-centre delocalisation-indices that have been called "Δ 6 -values") have been introduced only recently as potential indicators of what might be called "local aromaticity", whilst the third ("topological π-electron ringcurrents") was suggested as a possible discriminator in this regard nearly fifty years ago. Whilst linear correlations between ring currents and π-electron partitions within certain restricted classes of ring types are good (with correlation coefficients of up to 0.998), agreement between the two indices over all classes of ring types is poor. Predictions arising from a consideration of π-electron partitions and Δ 6 -values seem, on the other hand, to be in somewhat better accord. It is therefore concluded that, despite its superficially intuitive appeal, the ring-current index is out of step both with π-electron partitions and Δ 6 -values as a general indicator of so-called "local aromaticity". (doi: 10.5562/cca1846)

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Ab initio calculation of through-space magnetic shielding of linear polycyclic aromatic hydrocarbons (acenes): Extent of aromaticity

Martin

Caldwell

Carlson

et al. 2009

Journal of Molecular Graphics and Modelling

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ChemInform Abstract: The Concept of Ring Currents

Cited by 20 publications

References 0 publications

Ring-Current Properties of Bispentalenes and Related Structures — Comparison of Ab Initio and Hückel-London-Pople-McWeeny (HLPM) ‘Topological’ Calculations

Ring-Current Properties of Bispentalenes and Related Structures — Comparison of Ab Initio and Hückel-London-Pople-McWeeny (HLPM) ‘Topological’ Calculations

Correlations between Topological Ring-Currents, π-Electron Partitions, and Six-Centre Delocalisation Indices in Benzenoid Hydrocarbons

Ab initio calculation of through-space magnetic shielding of linear polycyclic aromatic hydrocarbons (acenes): Extent of aromaticity

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