Cyclazines‐Structure and Aromaticity or Antiaromaticity on the Magnetic Criterion

Abstract

AbstractStructure 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 in1H 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 cyclazinesN[2.2.2]−up toN[2.4.4]−are planar or at most slightly bowl‐shaped and, due to coherent peripheral ring currents (except inN[2.3.3]−,N[2.3.4],N[3.3.4]+andN[2.4.4]+), develop aromaticity or anti‐aromaticity of the whole molecules dependent on the number of peripheral conjugated π electrons. The cyclazinesN[2.3.3]−,N[2.3.4],N[3.3.4]+andN[2.4.4]+develop two ring currents of different direction within the same molecule, in which the dominating ring current proves to be paratropic (inN[3.3.4]+diatropic) including the nodalNpzlone pair into the conjugation. The residual cyclazinesN[3.4.4],N[4.4.4]−andN[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.