Induced magnetic field in sp-hybridized carbon rings: analysis of double aromaticity and antiaromaticity in cyclo[2N]carbon allotropes

Abstract

Two orthogonal sets of π orbitals induce concurrent (de)shielding cones demonstrating the double (anti)aromaticity of sp-hybridized carbon rings.

Aromaticity of ortho and meta 8‐Cycloparaphenylene and Their Dications: Induced Magnetic Field Analysis with Localized and Delocalized Orbitals in Strained Nanohoops

“…In CPPs the π orbitals are oriented towards the interior of the macrocycle forming a radial π system of sp 2 carbon atoms, in contrast to the classical planar aromatics where the π orbitals are oriented perpendicular to the molecular plane. A similar radial π system is observed in double aromatic sp hybridized cyclo[ n ]carbon allotropes [8,9] . Although [ n ]CPPs are comprised of n aromatic rings, their radial π system does not support a global ring current and the CPPs are considered as non‐aromatic benzenoid structures [10–12] .…”

“…[10][11][12]17,18] According to the magnetic criterion of aromaticity and molecular orbital analysis, [n]CPP 2 � support a global diatropic current and a consequent global diatropic magnetic response, in analogy to the respective all-cis-annulenes with 4n + 2 and 4(n-1) + 2 radial π electrons for the dianions and the dications respectively. [10,12,18] Theoretical findings of global aromaticity were experimentally supported by near-infrared fluorescence of [6][7][8][9]]CPP 2 + . [17] The extra stabilization of charged aromatic CPPs is a crucial feature for the design of novel organic electronic devices [7] and hence a deeper understanding of in-plane aromaticity can provide valuable insight towards the design of molecules that can act as charge transport materials.…”

“…The π magnetic response can be further dissected to CMO contributions allowing a detailed interpretation based on electronic configuration via occupied to virtual orbitals rotational excitations. [9,35] These excitations obey to selection rules and are allowed when the triple direct product of orbitals' irreducible representations with the rotational operator along the direction of the external field contains the totally symmetric representation. [38] A π!π* rotational excitation induces a paratropic response depending on the energy gap and the overlap of rotated CMOs.…”

“…A similar radial π system is observed in double aromatic sp hybridized cyclo[n]carbon allotropes. [8,9] Although [n]CPPs are comprised of n aromatic rings, their radial π system does not support a global ring current and the CPPs are considered as non-aromatic benzenoid structures. [10][11][12] However, the electronic structure of CPPs can be modified by reduction or oxidation resulting to anionic [13,14] and cationic [15][16][17] species with increased bond length alteration and shortening of connecting C ipso À C ipso bonds, adopting a quinoidal character which supports global π delocalization.…”

“…In planar systems the π/σ partitioning is typically achieved with canonical molecular orbital analysis (CMO) of the shielding tensor. [9,27,35,36] In curved and highly strained π conjugated systems this dissection scheme is not applicable, because lower energy CMOs have mixed π/σ character. Here we introduce the natural localized molecular orbitals [37] (NLMOs) dissection scheme for the induced magnetic field, allowing the accurate extraction of the π magnetic response of curved molecules.…”