Cyclazines‐Structure and Aromaticity or Antiaromaticity on the Magnetic Criterion

Abstract

Structure and spatial magnetic properties, through‐space NMR shieldings (TSNMRSs), of all ten cycl[2.2.2]azine to cycl[4.4.4]azine, hetero‐analogues and the corresponding hydrocarbons have been calculated at the B3LYP/6‐311G(d,p) theory level using the GIAO perturbation method and employing the nucleus independent chemical shift (NICS) concept. The TSNMRS values (actually, the ring current effect as measurable in 1H NMR spectroscopy) are visualized as iso‐chemical‐shielding surfaces (ICSS) of various size and direction, and employed to readily qualify and quantify the degree of (anti)aromaticity. Results are confirmed by NMR [δ(1H)/ppm, δ(15N)/ppm] and geometry (planar, twisted, bow‐shaped) data. The cyclazines N[2.2.2]− up to N[2.4.4]− are planar or at most slightly bowl‐shaped and, due to coherent peripheral ring currents (except in N[2.3.3]−, N[2.3.4], N[3.3.4]+ and N[2.4.4]+), develop aromaticity or anti‐aromaticity of the whole molecules dependent on the number of peripheral conjugated π electrons. The cyclazines N[2.3.3]−, N[2.3.4], N[3.3.4]+ and N[2.4.4]+ develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (in N[3.3.4]+ diatropic) including the nodal N pz lone pair into the conjugation. The residual cyclazines N[3.4.4], N[4.4.4]− and N[4.4.4]+ are heavily twisted and, therefore, are not developing peripheral or diverse ring currents. The TSNMRS information about cyclazines and the parent tricyclic annulene analogues is congruent subject to structure and number of peripheral or internal conjugated π electrons, the corresponding (anti)aromaticity is in unequivocal accordance with Hückel's rule.

Excited-state Hund’s Rule Violations in Bridged [10]- and [14]Annulene Perimeters

“…These are formally antiaromatic and likely to distort, especially through bond length alternation. 47,48,69,70 Consequently, we have not succeeded in finding species with negative singlet-triplet gaps.…”

“…47,[63][64][65][66] These have similar aromatic structures as their [10]annulene perimeters also contain 4n+2 πelectrons. 47,48 We find several bridged naphthalene analogs where the first excited state is a Hund's rule violation (Figure 8). However, all azulene analogs we have tested obey Hund's rule; for example, the azulene analog in which the bond shared by the 5-and 7membered rings is replaced by a methylene moiety is shown in Figure 8.…”

“…21,22 This has been described in terms of the [14]annulenic perimeter being stabilized by aromaticity due to its having 4n+2 π-electrons. 47,48 The structural relation between isopyrene and the [14]annulenes is clear: the simplest bridged [14]annulene-dihydroisopyrene-consists of the same π-conjugated perimeter as isopyrene Figure 7: Excited-state gaps of azupyrene, pyrene and their bridged [14]annulene perimeter analogs.…”

Experimental and computational studies of the optical properties of 2,5,8-tris(phenylthiolato)heptazine with an inverted singlet–triplet gap

“…Previous spectroscopic studies suggested that pristine heptazine is aromatic because it has the d ( 1 H) signal at 8.3 ppm 44 and that cyclazine (9b-azaphenalene) is antiaromatic, with d ( 1 H) signals at 2.1-3.7 ppm. [45][46][47] In the latter, the central nitrogen atom does not participate in the p-conjugation, leading to a rim consisting of a 12p-electron annulene.…”