Aromaticity of the doubly charged [8]circulenes

Abstract

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes. The gauge including magnetically induced current (GIMIC) method has been employed for calculating the current density susceptibilities. The aromatic character and current pathways are deduced from the calculated current density susceptibilities showing that the neutral [8]circulenes have two concentric pathways with aromatic and antiaromatic character, respectively. The inner octatetraene core (the hub) is found to sustain a paratropic (antiaromatic) ring current, whereas the ring current along the outer part of the macrocycle (the rim) is diatropic (aromatic). The neutral [8]circulenes can be considered nonaromatic, because the sum of the ring-current strengths of the hub and the rim almost vanishes. The aromatic character of the doubly charged [8]circulenes is completely different: the dianionic [8]circulenes and the OC-, CH-, CH2-, SiH-, GeH-, SiH2-, and GeH2-containing dicationic species sustain net diatropic ring currents i.e., they are aromatic, whereas the O-, S-, Se-, NH-, PH- and AsH-containing dicationic [8]circulenes are strongly antiaromatic. The present study also shows that GIMIC calculations on the [8]circulenes provide more accurate information about the aromatic character than that obtained using local indices such as nuclear-independent chemical shifts (NICSs) and (1)H NMR chemical shifts.

Some Observations on Triplet Ground-States in the Context of ‘Topological’ (HLPM) Ring-Currents in Conjugated Systems

“…We have also noted this occurrence of what we are somewhat colloquially calling 'the triplet ground-state problem' in some charged species of a hypothetical conjugated system (structure 3, on the left-hand side of Figure 2) and in the case of an extant hydrocarbon of the [8]-circulene family (structure 4, on the right-hand side of Figure 2), the latter of which was recently considered by Baryshikov et al [32] These two structures will be used in this presentation, along with structures 1 and 2 (in Figure 1), in order to illustrate the observations that are being documented here about the 'triplet ground-state problem'.…”

Electronic structure, aromaticity and spectra of hetero[8]circulenes

“…Our quantum chemical calculations at the GIAO/ B3LYP/6-311+G(d,p) level of theory confirmed the antiaromatic character of the inner eight-membered ring of quasicirculenes 40 and 42 ± 46 based on the positive values of NICS(0) and NICS(1). 118 We also found that the aromaticity of dithienothiophene quasicirculenes strongly depends on the positions of the sulfur atoms in the annulated moiety. The strict alternation of single and double bonds in the series of quasicirculenes 45, 46 and 40 containing the odd number of thiophene moieties makes it impossible to predict the existence of the corresponding quasicirculenes containing an even number of thiophene rings.…”

Calculations of magnetically induced current densities: theory and applications

“…Neutral [8]circulenes have two concentric current pathways. The inner octatetraene core (the hub) sustains a paratropic ring current and along the outer edge of the macrocycle (the rim) flows a diatropic ring current of the same size . Neutral [8]circulenes are globally nonaromatic.…”