The induced magnetic field of C
2
N
(
N
= 3-14) carbon rings was dissected to contributions from out-of-plane and in-plane π orbitals revealing two concurrent long range shielding or deshielding ...cones as a manifestation of the dual aromatic and antiaromatic character of C
4
n
+2
and of C
4
n
rings respectively. Aromaticity based on the magnetic criterion was evaluated with regard to the bonding pattern and geometrical characteristics that elucidate the influence of bond length and bond angle alteration on out-of-plane and in-plane magnetic responses. Ground state polyynic geometries of C
4
n
+2
rings exhibit comparable shielding cones to annulenes, decreasing the magnetic response with regard to the ring size and similar π
out
and π
in
diatropicity. Transition state cumulenic rings display increased aromaticity expressed by a very strong constant magnetic response and augmented π
out
diatropicity with regard to π
in
. The variations of the induced magnetic field are explained on the basis of frontier orbital interactions through rotational excitations, which enable further rationalization of the aromatic/antiaromatic behavior.
Two orthogonal sets of π orbitals induce concurrent (de)shielding cones demonstrating the double (anti)aromaticity of sp-hybridized carbon rings.
Cycloparaphenylenes (CPPs) exhibit selective host capabilities, featuring the ability to incorporate smaller CPPs to form double-walled host-guest complexes. Moreover, CPPs can also be stabilized by ...global aromaticity under twofold oxidation or reduction, involving electronic conjugation along with the overall structural backbone. Herein we explore the structural modifications, bonding, electron delocalization and magnetic properties of doubly reduced double-walled CPP complexes with DFT methods, in the isolated and aggregate
n
+ 5CPP⊃
n
CPP
2−
(
n
= 5-8) species. Our results show that the hosts undergo structural, bonding and delocalization deformations towards quinoidal configurations and exhibit global long-ranged shielding cones similar to global aromatic free dianionic CPPs, accounting for charge delocalization on the outer nanohoops, whereas the guests preserve local aromatic benzenoid configurations, resulting in global and local aromatic circuits within the host-guest aggregate. This observation suggests that in multi-layered related species electronic delocalization will be retained at the outer structural surface. The aromaticity of the hosts is manifested in the strong upfield shifts of the guests
1
H-NMR signals. Hence, CPP complexes can be extended to doubly reduced species stabilized by global host aromaticity expanding our understanding of doubled-walled nanotubes at the nanoscale regime.
The charge is delocalized on the outer nanohoop of double-walled doubly reduced
n
+ 5CPP⊃
n
CPP
2−
complexes enabling global host aromaticity which strongly shields the guests delivering upfield
1
H-NMR chemical shifts.
Nanoscaled 2D and 3D carbon structures with closed curved π-surfaces are of relevance in the development of desirable building units for materials science. Such species are able to sustain local and ...global aromatic circuits involving isolated regions or the overall structural backbone, respectively. Here we account for local and global aromaticity under rotation of representative two- and three-dimensional species involving
para
-connected and fused edge-sharing phenyl rings (8CPP, 10CPP, CNB), and C
60
fullerene at different charge states. Our results denote that nanoscaled 2D global aromatics mimic the behaviour of the most prototypical aromatic 6π-circuit, given by benzene, where the shielding cone properties vary along the rotation motion. In contrast, 3D spherical aromatics remain almost invariant under rotation, given the distinctive characteristics of such species, differing from 2D global aromatics. Dissection of orbital contributions reveals that π-orbitals are determinants for shifting from non-aromatic to spherical aromatic species. Under rotation, the variation of the anisotropic effect inherent to such nanoscaled structures is accounted for, which is relevant to rationalize variation in NMR signal shifts upon the formation of host-guest aggregates.
Nanoscaled 2D and 3D carbon structures with closed curved π-surfaces are of relevance in the development of desirable building units for materials science, where the connection between small and larger molecular systems is revealed.
The induced magnetic field of B40 was dissected into contributions from π, σ and core electrons revealing the origins for the formation of the strong long range shielding response characterizing the ...spherical aromatic nature of the cavernous D2d structure. Our analysis showed the complementary role of π and σ orbitals for the formation of the global shielding cone, with weak π contributions at a long range and strong σ contributions inside the cage, supporting the molecule as double aromatic with weak π and strong σ delocalization. Similar local variations of both π and σ magnetic responses were identified portraying peripheral diatropic and local paratropic currents. The weak π aromaticity is explained on the basis of symmetry rules pertaining to its electronic structure which forbid small gap paratropic rotational excitations.
Orbital contributions to the magnetic response depend on the method used to compute them. Here, we show that dissecting nuclear magnetic shielding tensors using natural localized molecular orbitals ...(NLMOs) leads to anomalous core contributions. The arbitrariness of the assignment might significantly affect the interpretation of the magnetic response of nonplanar molecules such as C
60
or 14helicene and the assessment of their aromatic character. We solve this problem by computing the core- and σ-components of the induced magnetic field (and NICS) and the magnetically induced current density by removing the valence electrons (RVE). We estimate the core contributions to the magnetic response by performing calculations on the corresponding highly charged molecules, such as C
6
H
6
30+
for benzene, using gauge-including atomic orbitals and canonical molecular orbitals (CMOs). The orbital contributions to nuclear magnetic shielding tensors are usually estimated by employing a natural chemical shielding (NCS) analysis in NLMO or CMO bases. The RVE approach shows that the core contribution to the magnetic response is small and localized at the nuclei, contrary to what NCS calculations suggest. This may lead to a completely incorrect interpretation of the magnetic σ-orbital response of nonplanar structures, which may play a major role in the overall magnetic shielding of the system. The RVE approach is thus a simple and inexpensive way to determine the magnetic response of the core- and σ-electrons.
Dissecting the magnetic shielding tensor using natural localized molecular orbitals leads to an anomalous contribution from the core electrons. We solve this problem by computing the core- and σ-contributions to the magnetic response by removing the valence electrons (RVE).
The induced magnetic field of B40 was dissected into contributions from π, σ and core electrons revealing the origins for the formation of the strong long range shielding response characterizing the ...spherical aromatic nature of the cavernous D2d structure. Our analysis showed the complementary role of π and σ orbitals for the formation of the global shielding cone, with weak π contributions at a long range and strong σ contributions inside the cage, supporting the molecule as double aromatic with weak π and strong σ delocalization. Similar local variations of both π and σ magnetic responses were identified portraying peripheral diatropic and local paratropic currents. The weak π aromaticity is explained on the basis of symmetry rules pertaining to its electronic structure which forbid small gap paratropic rotational excitations.
Materials combining amplified nonlinear optical responses and wide optical gaps that ensure high laser-induced damage thresholds and broadband transparency are highly required for photonic ...applications. In this article, we use well-established quantum chemical methods to demonstrate that a special class of polyaromatic hydrocarbons, in which multiple borazine (B3N3) units substitute aromatic carbon sextets, are suitable for the design and synthesis of systems meeting both above requirements. Computations conducted in a wide assortment of purposely designed nanographene model systems exposed robust doping/property relations which point out that multiple B3N3 doping can be considered as an efficient strategy to enhance the quadratic nonlinear optical responses of a given polyaromatic hydrocarbon maintaining at the same time wide optical gaps. A detailed analysis of the underlying charge transfer mechanism revealed that the observed features stem from local in character electronic excitations occurring between the incorporated B3N3 units and neighboring aromatic sextets.
The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in ...two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization.
Dications of cycloparaphenyles (nCPPs) are known to exhibit in‐plane global aromaticity, contained in a nanobelt structure. Recently synthesized ortho and meta isomers of nCPPs break the radial ...symmetry of π structure incorporating perpendicular oriented π orbitals. Herein we set to explore the aromaticity of neutral and dicationic ortho and meta isomers of 8CPP by dissecting the induced magnetic field to contributions of the twofold radial/perpendicular π system using delocalized canonical molecular orbitals (CMO), and introducing the natural localized molecular orbitals (NLMO) analysis with DFT methods. The dications sustain a reduced global aromatic character of the radial π system under a perpendicular orientation of the external field which declines from ortho to meta isomer and reinforces local aromaticity of ortho ring while it destroys aromaticity of meta ring. Aromaticity variations are determined by symmetry governed rotational excitations of frontier π orbitals. The parallel orientation reveals a substantial reduction of local aromaticity verified with NICSπ analysis and electron delocalization indices.
How aromatic? Ortho and meta isomers of the 8‐clycloparaphenylene dication retain a decreasing global aromatic character of the twofold radial‐perpendicular π system, while local aromaticity is augmented in the ortho ring and becomes antiaromatic in meta ring.
The induced magnetic fields originating from the π system of planar conjugated polycyclic hydrocarbons and spherical fullerenes are accurately reproduced by their corresponding hydrogen skeletal ...models (HSMs). Moreover, the individual contribution per molecular orbital is also reproduced unraveling simple symmetry rules related to canonical molecular orbitals. Hence, fast, handy and accurate 3D visualization of shielding and deshielding cones is realized, enabling the interpretation of global and local π aromaticity and antiaromaticity of PAHs and spherical species in a simple and concise manner to facilitate further interpretations of large sized hydrocarbon systems.
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