Present State of Molecular Structure Calculations

An Analysis of Topological Ring-Currents and their Use in Assessing the Annulene-Within-an-Annulene Model for Super-Ring Conjugated Systems

“…Those authors observed 8 that, although the ab-initio formulations are of course potentially of considerable value in making accurate predictions of experimentally verifiable quantities, the problem with them is that '...the more complex they are the more difficult they become to interpret and visualise.' More-simplistic and intuitive approaches, by contrast, frequently encapsulate what Coulson 85 once memorably called 'primitive patterns of understanding'.…”

“…That said, it should, however, be borne in mind that, in this discussion, we have deliberately eschewed versions [85][86][87][88][89][90] of the Hückel-London-Pople-McWeeny approach [1][2][3][4] in which resonance integrals between adjacent carbon-atoms are made iteratively self-consistent with respect to the corresponding calculated bondorders. We have recently re-emphasised 13 that if realistic paramagnetic ring-currents are needed in systems like these -for example, for the purposes of predicting 1 H-NMR chemical-shifts [5][6][7][8] -then such an iterative calculation 'is vital'.…”

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

“…As will be seen there, the process of calculation is by no means simple: it is a multi-step one, with specified procedures and parametrisations being adopted at every stage. This therefore gives us the opportunity to consider further what Coulson called 'primitive patterns of understanding'; [40] this itself is very much in the spirit of Dirac's remark, [41] some thirty years earlier, that it would be '... desirable that approximate methods of applying quantum mechanics should be developed that can lead to an explanation of the main features of complex atomic systems without too much computation'. We do this by subjecting the same structures as were studied in Refs.…”

Comparisons of π-Electron Ring-Current and Bond-Current Patterns Calculated by Topological (‘HLPM’) and Ab Initio (‘Ipso-Centric’) Formalisms for Two Isomeric Conjugated Hydrocarbons, Corazulene and Cornaphthalene

“…[1] Once again, therefore, the simple topological HLPM approach [2][3][4][5][6]28,29] -which depends on knowledge only of the molecular graph [12] of the conjugated system in question (in the form of a vertex-adjacency matrix describing it), [46] and the areas of its constituent rings -has been seen to demonstrate a possibly unexpected ability to reproduce complex patterns of current in large polycyclic hydrocarbons. According to common belief, such predictive success should ostensibly be the preserve only of sophisticated, and much less intuitive, [47,48] ab initio calculations. This conclusion is entirely consistent with our own previous findings [21,22] and with the view expressed in the recent, independent and simultaneous work of Gershoni-Poranne et al on phenylenes, [34] that the relative currents calculated by the (equivalent of the) HLPM topological approach '...are remarkably similar to those extracted from the pseudo-π maps, [8] which themselves mirror the full ab initio maps [7,11,15,19,20,49] '…and that, in general, the HLPM topological method '...has a remarkable ability…to capture essential features of delocalised systems including patterns of current…' There is therefore now a growing body of evidence [17,21,22,32−37] that this is the case.…”