Ring-Current Assessment of the Annulene-Within-an-Annulene Model for some Large Coupled Super-Ring Conjugated-Systems

Abstract

Abstract. The Annulene-Within-an-Annulene (AWA) model for conjugated super-ring systems, proposed by Barth and Lawton nearly fifty years ago, is further tested on six newly considered 'coupled' structures by means of π-electron ring-currents and bond-currents calculated via the Hückel-London-PopleMcWeeny (HLPM) formalism. Super-ring systems are said to be decoupled when the bonds connecting the central ring to the outer perimeter never appear as anything other than single bonds in any Kekulé structure that can be devised for the system as a whole. The preliminary conclusions of other recent investigations -by the same HLPM method and by ipso-centric ab-initio approaches -are verified. Phenylene-[5]-circulene, a (coupled) alternant isomer of the much-studied (decoupled) non-alternant system [10,5]-coronene, is also considered. It is advised that the AWA model should in future either be used with considerable caution in very specific circumstances, or it should be abandoned altogether.

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

“…[7][8][9][10][11][12][13][14][15][16][17][18][19]) are routinely available, however, this consideration is, of course, no longer so relevant. That is why, in the current work, and earlier but recent ones, [27][28][29][30][31][32][33] the above reservation about non-iterative calculations expressed in Refs. [47,[61][62][63][64][65] have been temporarily set aside in order to test how well the predictions of the ab initio methods compare with even the most basic, non-iterative pseudograph-theoretical version of the Hückel-London-PopleMcWeeny approach; [1][2][3][4][5][6] in this formalism, the calculated quantities (ring currents and bond currents) are effectively regarded -despite the cautions and caveats considered by one of us (RBM) in Ref.…”

“…This is perhaps hardly surprising when it is recalled that the structures being dealt with here are not monocyclic annulenes and thus the rules of Pople and Untch [57] do not necessarily apply; (the situation is somewhat reminiscent of the analogous failure generally observed in the context of the so-called the 'annulene-within-an-annulene model' -see, for example, Refs. [3,29,32].) (b) The HLPM calculations reported also reflect the view [38,39] that the current flow around the perimeters of the several pentalene moieties in (3)- (5) shows that the pattern observed in pentalene ( (2)) itselfnamely, that of a strong current in the clockwise (paramagnetic) sense -essentially survives intact, to a greater-or-lesser extent, when pentalene becomes incorporated into structures of this sort, as one or more pentalene moieties.…”

“…This is especially the case when, as here, some rings in a given structure bear diamagnetic ring-currents and other rings are calculated to support paramagnetic ring-currents -a difficulty that was pointed out almost fifty years ago in a much-neglected paper by Jung. [51] We have exploited this concept of 'susceptibility ratio' for that very purpose on several previous occasions [3,4,28,32,47] and, as we wish to do so again here, the susceptibility ratios calculated from equation (1) are listed in the right-hand columns of Tables 1-4 (for structures (1)- (21)). …”

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

“…It is perhaps material to point out at this stage that what we have for some time been calling [6,13,14,17,21,22,26,[28][29][30][31] 'topological HLPM calculations' are precisely what Fowler et al have very recently [34][35][36][37] given the appellation 'graphtheoretical Current-Density Hückel-London (CD-HL) calculations'. Given the same geometrical assumptions, the two 1 2 Figure 1.…”

“…(The only circumstances in recent work where a different assumption about ring areas needs to be made are in the case of structures with 'holes', such as kekulene [6,22,[30][31][32][33] and the p-coronenes. [13,14] ) Lillington et al [1] have pointed out that 1 is indeed planar (with C4h symmetry) and so also is 2, in a transitional form (with D4h symmetry); in reality, however, 2 is bowlshaped in its equilibrium conformation, with symmetry C4v.…”