Analysis of the Nucleus-Independent Chemical Shifts of [10]Cyclophenacene: Is It an Aromatic or Antiaromatic Molecule? Guglielmo Monaco* and Riccardo Zanasi* Department of Chemistry and Biology “A. Zambelli”, University of Salerno, via Giovanni Paolo II 132, Fisciano (SA) 84084, Italy *S Supporting Information ABSTRACT: [10]Cyclophenacene is an important synthetic target that shows a pair of nucleus-independent chemical shift (NICS) values for the center of mass and six-membered rings typical of an aromatic species. This is found in contrast with the global paratropic current density induced by a magnetic field parallel to the main symmetry axis. This apparent contradiction has been analyzed by studying the tensor character of the magnetic response. It turns out that the molecule displays two characters, one paratropic (antiaromatic) and another one diatropic (aromatic), depending on the orientation of the inducing magnetic field. The paratropic response, which cannot be recognized from the NICS values, is associated with a well-defined destabilization of the belt closure, as witnessed by homodesmotic reactions. A scalar measure of magnetic aromaticity, the field-independent current strength, has been introduced, which allows us to reach the conclusion that [10]cyclophenacene is indeed an aromatic molecule, although it is significantly affected by the paratropic response. T he 40 π-electron cyclic benzenoid [10]cyclophenacene 1 can be thought of as a rolled polyphenantherene-type graphite ribbon, 1 comprising ten fused benzene rings; see Figure 1 (left). According to a common definition, 1 is an “aromatic belt”, 2 or carbon nanobelt (CNB), 3 a very short armchair (5,5) carbon nanotube (CNT). 4 Owing to its bent π-conjugation, 1 is a challenging and not yet accomplished synthetic target, which is expected to exhibit a number of special properties, for example, like those of the [n]cycloparaphenilenes, 5,6 also reported as carbon nanorings (CNRs), 3 which have a similar π-distorted system. Particularly appealing is the possibility of using both CNRs 7−9 and CNBs, 10−12 as well as short CNT end-caps, 13−15 as templates for growing single-walled, single-chirality carbon nano- tubes. 16,17 The first bottom-up synthesis of a pure benzenoid CNB, a (6,6)CNT portion that is an isomer of [12]- cyclophenacene, has been reported only very recently by Povie et al. 18 The experiment that came closest to the isolation of 1 was reported by the Iyoda group in their observation of the mass peak of 1 upon laser irradiation of a stable precursor. 19,20 On the contrary, carbon-capped derivatives of 1 have been obtained by rationally designed chemical modi fication of [60]- fullerene. 21−23 Actually, the spherical π-conjugated [60]- fullerene contains 1, which can be isolated by targeted addition to double bonds at both poles of the cage, as in the case of the model compound 2; see Figure 1 (right) and refs 21−23 for details. Getting insight into the aromaticity of systems like 1 and 2 is particularly problematic because their unusual geometry hampers the identification of good reference systems. In similar cases, reference-free aromaticity indices, like the nucleus- independent chemical shift (NICS), 24−26 are often chosen. NICS values for the six-membered rings of 1 and model compound 2 indicate that the [10]cyclophenacene belt is aromatic; moreover, its center of mass 27 (CM) is predicted to be subject to an aromatic shielding effect. 21−23 These results, rather expectable for benzenoid ribbons, can be surprising instead if one considers 1 as an antiaromatic [20]annulene distorted by 10 C 2 -insertions. Prompted by this observation, we have studied the (first-order) current density (J B ) induced by a magnetic field in the electronic cloud of 1. Figure 2 shows maps of J B induced in 1 by a magnetic field parallel to the main symmetry axis. Maps have been calculated using the continuous transformation of the origin of the current Received: July 26, 2017 Accepted: September 11, 2017 Published: September 11, 2017 Figure 1. Left: structure of the D 5d (C 40 H 20 ) [10]cyclophenacene (1); symmetry distinct C−C bonds are shown. Right: structure of the C 2h (C 60 H 12 ) carbon-capped derivative of 1 (2), where the contained 1 belt has been evidenced; symmetry distinct carbon rings are enumerated. In both cases the main symmetry axis coincides with the z Cartesian direction shown in blue, while x and y axes are shown in red and green. Letter pubs.acs.org/JPCL © XXXX American Chemical Society 4673 DOI: 10.1021/acs.jpclett.7b01937 J. Phys. Chem. Lett. 2017, 8, 4673−4678