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

Abstract

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.

Analyzing Magnetically Induced Currents in Molecular Systems Using Current-Density-Functional Theory

“…For the BPAN molecule the range of different bond lengths makes the comparison more complex, but again the ratios are in a similar range to those reported in Ref. 84 and the relative magnitudes on the central naphthalene moiety are weaker than those on the pentalene moieties by a similar amount. The agreement between the HLPM method, the GIMIC approach and the present work suggests that for planar systems ring currents are consistently described and well interpreted by simple approaches such as HLPM.…”

“…The pentalene, naphthalene, and BPAN molecules were recently studied using the Hückel–London–Pople–McWeeny (HLPM) method by Dickens and Mallion . In ref bond currents are quoted as a dimensionless value relative to the benzene value.…”

“…The pentalene, naphthalene, and BPAN molecules were recently studied using the Hückel–London–Pople–McWeeny (HLPM) method by Dickens and Mallion . In ref bond currents are quoted as a dimensionless value relative to the benzene value. As discussed earlier, the planar quadrature approach yields ring currents for naphthalene and benzene whose ratio is very close to the HLPM value of 1.093.…”

Topological Ring Currents and Bond Currents in Some Neutral and Anionic Altans and Iterated Altans of Corannulene and Coronene

“…The numerical values of for structures 1 – 6 and their dianions are given in the right-hand columns of Tables , , and . Of these, only one ( 1 ) has a negative value for , which (by a naïvebut, in practice, unwarrantedextension from monocyclic to polycyclic systems) might intuitively − be thought of as indicating anti-aromaticitybut please see ref for further critical comment on this kind of reasoning. All of the others have positive values for this index, and those with the greatest values (of ca.…”

Calculated Magnetic Properties of a Recently Synthesized Corannulene–Dibenzobistetracene Hybrid

“…Our attention was attracted to these NICS calculations reported by Xu et al on this extensive 24-ring unsaturated structure because we have for some time been taking opportunities, when they arise, for example, in refs − , to compare ab initio calculations of the magnetic properties of various conjugated systems with predictions of the rudimentary, quasi graph-theoretical Hückel–London–Pople–McWeeny (HLPM) method of calculating π-electron ring currents. This approach is reviewed in detail in ref Such comparisons have been made previously in, for example, refs − .…”