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SUMMARY:Topological Ring-Currents in Conjugated Systems - Dr Roger Mallion
 \, School of Physical Sciences\, University of Kent
DTSTART:20120502T131500Z
DTEND:20120502T141500Z
UID:TALK35230@talks.cam.ac.uk
CONTACT:Robert Best
DESCRIPTION:(in collaboration with Dr Tim Dickens\, University Chemical La
 boratory)\n\nIn 1958\, when working at the University Chemical Laboratory\
 , Nobel Laureate the late Sir John Pople performed the first quantum-mecha
 nical calculation of a π-electron ring-current intensity — a feat indep
 endently and almost simultaneously achieved by McWeeny\, who published onl
 y136 pages later in Volume 1 of Molecular Physics\, a newly founded journa
 l edited by the Cambridge Professor of Theoretical Chemistry at the time\,
  H. C. Longuet-Higgins\, who himself later contributed to this field. The 
 superficially different\, but entirely equivalent\, methods that Pople and
  McWeeny used were extensions of the approach introduced twenty years earl
 ier by London\, whose formalism was itself based on Hückel molecular-orbi
 tals. During the 1960s and 1970s\, the Hückel method was gradually supers
 eded by more-sophisticated semi-empirical approaches and\, ultimately\, by
  ab-initio ones. Over the course of the last forty years\, however\, the H
 ückel model has\, to some extent\, been rehabilitated because it is now m
 ore generally appreciated that\, when certain assumptions about ring areas
  are made\, ring-current intensities calculated by the Hückel–London–
 Pople–McWeeny (HLPM) approach are effectively graph-theoretical in natur
 e\, their signs and relative sizes being dependent on (and\, de facto\, la
 tent in) the carbon-carbon connectivity of the polycyclic\, conjugated hyd
 rocarbon under study\, and the ring currents are thus completely pre-deter
 mined once the structural formula of the system in question has been writt
 en down. \n\nThe authors have recently revived this so-called ‘topologic
 al’ (HLPM) formalism and have applied it\, with some success\, to severa
 l problems such as the ‘emptiness’ (or otherwise) of the central ring 
 in the family of structures related to perylene\, and the so-called ‘ann
 ulene-within-an-annulene’ model of certain ‘super-ring’ conjugated s
 ystems. Attention has also been directed to the conceptual advantages and 
 disadvantages of considering bond currents in conjugated structures\, inst
 ead of the more-traditional ring currents — the two sets of quantities b
 eing connected by the analogue\, in the context of microscopic molecular s
 ystems\, of Kirchhoff’s First Law (as conventionally applied to macrosco
 pic electrical networks). The ring currents for a given structure constitu
 te a collection of independent quantities\, whereas the family of bond cur
 rents in all the bonds of a given molecule are not themselves independent\
 , by virtue of Kirchhoff’s First Law. The talk will also review four alt
 ernative topological methods that rely on the concept of ‘circuits of co
 njugation’ and which have recently been proposed by independent teams le
 d by Mandadao\, Fowler\, Randić\, and Ciesielski\, in order to explore bo
 nd currents and ring currents.\n
LOCATION:Department of Chemistry\, Cambridge\, Pfizer lecture theatre
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