The intriguing class of altan‐molecules

Abstract

-molecules are formed from parent molecules with outgoing C-H bonds by substituting the C-H bonds with C-C bonds to alternating carbon atoms of an annulene. If the outgoing C-H bonds are even, which happens for totally fused aromatic hydrocarbons, the altan-molecule has a 4n perimeter expected to hold a paratropic circulation, in consideration of its poor coupling with the inner part of the molecule. This qualitative expectation is borne out in many systems by ab initio and tight-binding calculations, but it breaks down when the perimeters of the parent molecule and its altan derivative are of close dimensions. The theoretical design of these molecules is reviewed. The possible importance of their anions and their openshell singlet ground states is also briefly discussed.

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

“…It should be noted that the HLPM bond-currents rigorously obey Kirchhoff's law of current conservation at junctions [4,5,27,52] -an attribute not, in general, shared by the corresponding quantities (such as integrated current-strengths (susceptibilities) [17][18][19] ) arising from the ab initio approaches, [7][8][9][10][11][12][13][14][15][16][17][18][19] except in the limit of an infinite basis set. [14,18,23] The bond currents are thus the microscopic analogy of the currents in the wires in a macroscopic electrical network and the ring currents are analogous to loop currents flowing around the several irreducible [4,5,27] cycles of the network.…”

“…Now that ab initio methods (e.g., Refs. [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.…”

“…All of these have been specially 'designed' for the present study. Each of (10)- (15) contains two pentalene moieties (outlined in blue, for emphasis, in Figure 4), whilst (16)-(19)) each contain four pentalene units (as shown in Figure 5). The third column of Table 3 shows the average perimeter current in all cases to be in the paramagnetic sense, with values ranging from [5,47] are expressed as a ratio to the 'London' contribution to the diamagnetic susceptibility of benzene; '+' indicates an overall net diamagnetic susceptibility: '-' indicates an overall net paramagnetic susceptibility.…”

“…Please see (a) Ref. [29] for comparisons of HLPM bond-currents [30] with integrated current-strengths calculated for some altan structures by Monaco and Zanasi [13] (by use of the 'ipsocentric' [7][8][9][10][11][12][13][14][15] ab initio formalism), and (b) pp. 340-342 of Ref.…”

“…[7][8][9][10][11][12][13][14][15][16][17][18][19]) to compute not just pictorial, semiquantitative current-density maps, as heretofore, but also to calculate numerical quantities (variously called bondcurrent susceptibilities or bond-current strengths [13,14] or integrated current-strengths [18,39] or current-strength susceptibilities [39] ) which are the equivalent of bond currents [4,24,27] in the HLPM approach. This gives the opportunity directly to compare the quantitative predictions of ab initio methods with those of the HLPM formalism when quantities calculated from both approaches are expressed as a (dimensionless) ratio to the corresponding quantity computed, by the same respective method, for benzene.…”