Designing Paramagnetic Circulenes

Abstract

In an axial magnetic field, the annular molecules coronene, corannulene, kekulene, and nonplanar [7]circulene support disjoint, counterrotating, diatropic-rim/paratropic-hub ring currents. [1] This remarkable feature represents a failure of the very popular "annulene within an annulene" (AWA) model. [2] On the other hand, a circulene comprising 10 fused pentagons around a central decagon, namely [10,5]coronene (1), is predicted to have inverted counterrotating paratropic-rim/ diatropic-hub ring currents. [3] In this case, the outer and inner cycles, which contain respectively 20 (4n) and 10 (4n + 2) carbon atoms, are essentially decoupled and here the AWA picture is compatible with the ab initio result. For all these systems, the ipsocentric approach [4] provides a unified account of the opposed currents in terms of simultaneous translational and rotational p-p* virtual excitations, and so provides a basis for further prediction and possible control of magnetic response properties in potential materials applications. For example, extensive paratropic (antiaromatic) perimeter circulation is an unusual property that is reflected in calculated magnetizability and nuclear magnetic shieldings, even when partially cancelled by the effects of a central diatropic current, as in 1.[3] Can we eliminate the cancellation and achieve greater paratropicity by reversing the central current? Here we predict that two closed-shell neutral circulenes, 2 and 3, support disjoint conrotating paratropic ring currents on both rim and hub. Indeed, the calculations indicate that 2 and 3 have a net paramagnetic response in one direction, and that in 3, remarkably, this outweighs the diamagnetic contributions to magnetizability to give a closed-shell paramagnetic molecule. Retention of paratropic current at equilibrium geometry is rare, and the received wisdom is that antiaromatic molecules will exhibit fluxionality [5] or distort to "escape" their antiaromaticity. [6] The aim is to find p systems based on a circulene-like template, but with conrotating paratropic currents on both outer and inner cycles of carbon atoms. For annular belts of hexagonal rings, it has been shown [1,7] that coupling of inner and outer circuits leads to counterrotation of ring currents. A belt of 2m fused pentagons around a central 2m-gon, as in the case of 1, has four KekulØ structures corresponding to the pairings of the two conjugated structures on rim and hub cycles. In all four, the radial 5/5 graph edges are formal single bonds and hence have zero Pauling p bond order. Thus, inner and outer circuits are decoupled, and this suggests that the AWA picture is applicable to this electronic structure. When 2m = 4n, the inner cycle should be antiaromatic; the outer 4m cycle should always be antiaromatic, independent of m. Two series of molecules can be postulated: one with 2m = 4n + 2, for which the AWA picture predicts counterrotating (paratropic-rim/diatropic-hub) ring currents, as in the case of 1, and a second with 2m = 4n, for which AWA would predict the des...

Analysis of Local and Global Aromaticity in Si3C5 and Si4C8 Clusters. Aromatic Species Containing Planar Tetracoordinate Carbon

“…According to the magnetic criteria, in the presence of an external magnetic field, aromatic (antiaromatic) molecules sustain diatropic (paratropic) currents. In contrast, in nonaromatic molecules, the currents in one direction or the other cancel out, giving a resultant current strength close to zero [53][54][55]76,81,90,91]. In this work, we define "local ring current" as the current circuits distributed in a local molecular ring.…”

“…Aromaticity is not an observable property, yet it is generally assessed in terms of structural, energetic, and magnetic criteria [3,[43][44][45][46][47][48]. Aromatic and antiaromatic species sustain diatropic and paratropic ring currents when exposed to a uniform magnetic field perpendicular to the molecular plane [49][50][51][52][53][54]. This magnetic response allows the interpretation of experimental NMR spectroscopy and magnetic anisotropy measurements on (anti) aromatic molecules [45,55].…”

Analysis of Local and Global Aromaticity in Si₃C₅ and Si₄C₈ Clusters. Aromatic Species Containing Planar Tetracoordinate Carbon

“…According to the magnetic criteria, in the presence of an external magnetic field, aromatic (antiaromatic) molecules sustain diatropic (paratropic) currents. In contrast, in nonaromatic molecules, the currents in one direction or the other cancel out, giving a resultant current strength close to zero [54][55][56]81,85,89,90]. In this work, we define as "local ring current" the current circuits distributed in a local molecular ring.…”

“…Aromaticity is not an observable property, yet it is generally assessed in terms of structural, energetic, and magnetic criteria [3,[44][45][46][47][48][49]. Aromatic and antiaromatic species sustain diatropic and paratropic ring current when exposed to a uniform magnetic field perpendicular to the molecular plane [50][51][52][53][54][55]. This magnetic response allows to interpret experimental NMR spectroscopy and magnetic anisotropy measurements on (anti) aromatic molecules [46,56].…”

On the Nullity of Altans and Iterated Altans

“…One strategy for design of carbon nanostructures that should support concentric currents is based on the venerable annulene-within-an-annulene model [12][13][14][15], which predicts combinations of inner and outer currents on concentric cycles connected by 'spoke' bonds. Depending on the lengths of inner and outer circuits, and the strength of the spoke coupling between them, any of the four combinations of diatropic and paratropic current may appear [13,[16][17][18]. As with many chemical models, this approach gives a rule of thumb rather than a rigorous prediction [19], but it can serve as a starting point for more detailed explanations.…”