Original scientific paper

https://doi.org/10.5562/cca3219

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

Timothy K. Dickens ; University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England, United Kingdom
Roger B. Mallion ; School of Physical Sciences, University of Kent, Canterbury CT2 7NH, England, United Kingdom

Abstract

Two different groups (Cao et al. (2015) and Sundholm et al. (2016)) 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 sur-viving 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.

This work is licensed under a Creative Commons Attribution 4.0 International License.

Keywords

topological ring-currents; bispentalenes; [4n]- and [4n+2]- π-electron systems; comparisons with ab initio calculations

Hrčak ID:

191410

URI

https://hrcak.srce.hr/191410

Publication date:

18.12.2017.

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