Ipsocentric ring current

The criterion of ipsocentric ring current has been used to assess aromaticity in S-N heterocycles (and related inorganic ring systems). Current density maps indicate that the ten r-electron systems [SsNs], [S4N3] " and [S4N4] ", and the fourteen r-electron system [S5N5] " support diatropic k currents, reinforced by a circulations. 

It has been shown that there is no direct relation between electron delocalization and the presence of a ring current, other than that a delocalized system is a necessary but not sufficient condition to lead to a ring current. In the case of Al " derived compounds, there is clearly both o and n electron delocalization although only the a system gives rise to a ring current. The analysis of CMO-NICS data is shown to lead to a different conclusion although this method also contains occupied-occupied terms and is not so easily interpretable as ipsocentric ring current maps. 

The few-electron character of ring currents uncovered by the ipsocentric approach prompts many questions about the links between aromaticity/strobilism and distortivity. Just two examples from recent work [14] will be discussed briefly here. 

ReUance on simple classical electron coimting rules corresponds to an approximate description of the clamped cycle in terms of its annulene perimeter, and this is clearly insufficient. We can propose three perturbed perimeter models, whereby the simple perimeter annulene analogy is considered as the zeroth-order solution (Model 0) in a perturbative treatment. Model I includes the non-perimeter bonds perturbatively. Model II includes the perimeter heteroatoms perturbatively. Model III includes both (see Scheme 5). By means of these pictorial perturbative models (I, II and III) it is shown below that, when first-order corrections to the angular momentum character and orbital energies of the perimeter annulene (Model 0) are taken into account, it is possible, within the ipsocentric model, to give a unified rationalisation for the survival of the original ring current in XHXH clamped monocycles, and its extinction in HC = CH-clamped monocycles, even at the simple Hiickel level of theory. 

Clearly, a different picture arises with CMO-NICS. However, CMO-NICS do not have the nice conceptual interpretability as orbital resolved ring currents. The latter, in the ipsocentric formulation, arise purely from occupied-virtual transitions whereas in CMO-NICS many other terms are involved too, including occupied-occupied interactions [88], This may lead to the, according to us wrong conclusion, that the strong diatropic currents in the compounds studied here are also in part due to the n system. 

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