Icosahedral C60: an aromatic molecule with a vanishingly small ring current magnetic susceptibility

Revisiting the quasi-aromaticity in polynuclear metal chalcogenide clusters and their derivative "cluster-assembly" crystalline structures

The concept of aromaticity is primarily invented to account for the high stability of conjugated organic compounds that possess a specific structural and chemical stability with (4n + 2) π electrons. In 1988, quasi-aromaticity was theoretically proposed for the Mo3S44+ core in the Mo3(μ3-S)(μ-S)3(χ-dtp)3(μ-dtp) L compound (χ: chelating ligand; dtp: (EtO)2PS2-) illustrated by canonical molecular orbitals. However, the origin of the quasi-aromaticity and chemical bonding remains ambiguous, lacking a thorough analysis in terms of stability and quantitative measurement of the aromatic character. Thus, in this work, we systematically reported the electronic structure and aromaticity of a series of polynuclear metal chalcogenide clusters [M3X4(H2O)9]4+ (M = Cr, Mo, W, and Sg; X = O, S, Se, and Te) to explore an efficient tool of NICS index values at specific points to measure the quasi-aromaticity and to figure out the (d-p-d) π three-center bonding as the predominant origin from the arrangement of three Mo atoms and three bridged X atoms. Interestingly, derived from the Mo3⋯S3 quasi-plane, the extended sandwich cluster model of a S3⋯Mo3⋯S3 (Mo3S6) structure can be seen as the seed unit of the popular MoS2 nanomaterials, with the resemblance between both molecular and periodic systems regarding geometries, electronic structures, and chemical bonding. Additionally, the highly symmetric Mo3S4 core in [Mo3X4(H2O)9]4+ can be arranged in a staggered and stacked manner to create the Mo6S82- building block, corresponding to the crystalline structures in BaMo6S8 Chevrel phases, albeit with slight deformations. But the neutral Mo6S8 cluster can be seen as the seed structure for the Mo3S4 periodic materials for the high resemblance in terms of geometry, electronic structures and chemical bonding. Drawing upon the observed similarities between cluster models and materials, we propose a new concept termed "cluster-assembly" materials. This concept involves the expansion from a high-symmetry and/or aromatic stable cluster seed unit to form the corresponding derivative materials, presenting an alternative paradigm for investigating crystals and enriching our comprehension of the stabilities exhibited by both gas-phase clusters and solid-state materials. The concept of "cluster-assembly" materials not only contributes to the formulation of design strategies for novel materials or stable clusters but also provides valuable insights into the extension of periodic aromaticity.

Tailoring Titanium Carbide Clusters for New Materials: from Met-Cars to Carbon-Doped Superatoms

Tailoring materials with prescribed properties and regular structures is a critical and challenging research topic. Early transition metals were found to form supermagic M8C12 metallocarbohedrenes (Met-Cars); however, stable metal carbides are not limited to this common stoichiometry. Utilizing self-developed deep-ultraviolet laser ionization mass spectrometry, here, we report a strategy to generate new titanium carbides by reacting pure Tin clusters with acetylene. Interestingly, two products corresponding to Ti17C2 and Ti19C10 exhibit superior abundances in addition to the Ti8C12 Met-Cars. Using global-minimum search, the structures of Ti17C2 and Ti19C10 are determined to be an ellipsoidal D4d and a rod-shaped D5h geometry, respectively, both with carbon-capped Ti4C moieties and superatomic features. We illustrate the electronic structures and bonding nature in these carbon-doped superatoms concerning their enhanced stability and local aromaticity, shedding light on a new class of metal-carbide nanomaterials with atomic precision.

Observation of High Magnetic Bistability in Lanthanide (Ln = Gd, Tb and Dy)-Grafted Carbon Nanotube Hybrid Molecular System

There is an immense research interest in molecular hybrid materials posing novel magnetic properties for usage in spintronic devices and quantum technological applications. Although grafting magnetic molecules onto carbon nanotubes (CNTs) is nontrivial, there is a need to explore their single molecule magnetic (SMM) properties post-grafting to a greater degree. Here, we report a one-step chemical approach for lanthanide-EDTA (Ln = GdIII, 1; TbIII, 2 and DyIII, 3) chelate synthesis and their effective grafting onto MWCNT surfaces with high magnetic bistability retention. The magnetic anisotropy of an Ln-CNT hybrid molecular system by replacing the central ions in the hybrid complex was studied and it was found that system 1 exhibited a magnetization reversal from positive to negative values at 70 K with quasi-anti-ferromagnetic ordering, 2 showed diamagnetism to quasi-ferromagnetism and 3 displayed anti-ferromagnetic ordering as the temperature was lowered at an applied field of 200 Oe. A further analysis of magnetization (M) vs. field (H) revealed 1 displaying superparamagnetic behavior, and 2 and 3 displaying smooth hysteresis loops with zero-field slow magnetic relaxation. The present work highlights the importance of the selection of lanthanide ions in designing SMM-CNT hybrid molecular systems with multi-functionalities for building spin valves, molecular transistors, switches, etc.

Electronic Structure Calculations on Endohedral Complexes of Fullerenes: Reminiscences and Prospects

The history of electronic structure calculations on the endohedral complexes of fullerenes is reviewed. First, the long road to the isolation of new allotropes of carbon that commenced with the seminal organic syntheses involving simple inorganic substrates is discussed. Next, the focus is switched to author's involvement with fullerene research that has led to the in silico discovery of endohedral complexes. The predictions of these pioneering theoretical studies are juxtaposed against the data afforded by subsequent experimental developments. The successes and failures of the old and modern quantum-chemical calculations on endohedral complexes are summarized and their remaining deficiencies requiring further attention are identified.

Reversible Diels–Alder Addition to Fullerenes: A Study of Dimethylanthracene with H2@C60

The study of isolated atoms or molecules inside a fullerene cavity provides a unique environment. It is likely to control the outer carbon cage and study the isolated species when molecules or atoms are trapped inside a fullerene. We report the Diels–Alder addition reaction of 9,10-dimethyl anthracene (DMA) to H2@C60 while 1H NMR spectroscopy is utilized to characterize the Diels–Alder reaction of the DMA with the fullerene. Through 1H NMR spectroscopy, a series of isomeric adducts are identified. The obtained peaks are sharp, precise, and straightforward. Moreover, in this paper, H2@C60 and its isomers are described for the first time.

The spin–orbit effects on platinabenzene: a ring current and electron delocalization approach

The aromaticity of platinabenzene was studied using the electronic structure and magnetic descriptors with relativistic corrections. It presents aromatic character following the Hückel's rule, negative NICS values, and diatropic current densities.

Aromaticity Survival in Hydrofullerenes: The Case of C66 H4 with Its π-Aromatic Circuits

The isolated-pentagon rule (IPR) is a determining structural feature that accounts for hollow fullerene stabilization and properties related to C_n (n≥60) cages. The recent characterization of an unprecedented non-IPR hydrofullerene, C_2v C₆₆ H₄ , bearing two heptagons with adjacent fused-pentagon motifs, largely dismisses this feature. Herein, employing DFT calculations, the ¹³ C NMR spectroscopy and aromatic behavior of C_2v C₆₆ H₄ are explored. The results show the presence of three π-aromatic circuits at the bottom boat section of C₆₆ H₄ , indicating the unique features of this hydrofullerene in comparison to those of pristine C₆₀ . In addition, under specific orientations of the external field, certain π-aromatic circuits are enabled, resulting in a more aromatic fullerene than that of C₆₀ , but lower than that of the spherical aromatic C₆₀ ^6- fulleride. Notably, under a field aligned with the saturated carbon atoms, nonaromatic characteristics are exposed. This reveals that spherical-like cages can involve a complex magnetic response that heavily depends on the orientation of the applied field.

Co13O8—metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co₁₃O₈ that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co₁₃O₈ cluster that forms a new class of metal oxides that we named as ‘metalloxocubes’. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co₁₃ to the metalloxocube Co₁₃O₈ with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

On the formation of spherical aromatic endohedral buckminsterfullerene. Evaluation of M@C60 (M = Cr, Mo, W) from relativistic DFT calculations

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606- phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

Shielding cone behavior in the spherical aromatic He@C606-: origin of the record for the most shielded encapsulated 3He nucleus and comparison to He@C706

The textbook concept of shielding cone is one of the characteristic properties for planar aromatic species, which explains the nuclear shielding of neighbor molecules located above and below the molecular plane. Here, we explore its resemblance in spherical aromatic fullerenes, where particularly for C₆₀^6-, it explains the record for the highest shielding observed via ³He-NMR for the endohedral ³He@C₆₀^6- species. Our results compare the behavior for He@C₆₀ and He@C₆₀^6- denoting relevant changes in their magnetic response behavior, where an induced shielding cone was observed for the latter with its long-range shielding character. In contrast to planar aromatics which give rise to a shielding cone property only for a perpendicularly oriented field, it was found that the shielding cone in spherical aromatic fullerenes occurs to any orientation. Thus, the orientation dependence behavior of the shielding cone is accounted, unraveling a characteristic shielding cone pattern in He@C₆₀^6- which further rationalizes its record on the most shielded He nucleus, upon rotation or tumbling from the aromatic ring about the applied field. For the C₇₀ case, the opposite situation has been characterized via ³He-NMR experiments, which is explained in terms of the deshielding/shielding region inside the C₇₀ cage. Graphical abstract.

Aromaticity of unsaturated BEC4 heterocycles (E = N, P, As, Sb, O, S, Se, Te)

A compendium of pnictogen and chalcogen substituted boron heterocycles were assessed for their aromatic character by first principles density functional theory. Group-15 and Group-16 elements were placed at the ortho-, meta-, and para-positions of six-membered rings relative to boron to assess their impact on the aromaticity of the unsaturated heterocycles. Aromaticity was analyzed by a multidimensional approach using nuclear independent chemical shifts, gauge-including magnetically induced current, as well as natural bond orbital and natural resonance theory analyses. Based on these methods, we observe a general decline of aromaticity in heavier pnictaborines while the chalcogen analogues maintain relatively strong aromatic character. These general trends result from complementary π-π* natural bond order interactions that sustain resonance within the ring of each heterocycle establishing a pattern of cyclic delocalization. Consequently, natural resonance theory displays strong resonance, which is corroborated with the signed modulus of ring current, toroidal vortices of current maps, and elevated average induced current throughout the ring. The 1,3-configurations for pnictaborines and chalcogenaborines are generally more aromatic compared to the 1,2- and 1,4-isomers, which contain π-holes that limit diatropism within the heterocycles. However, an energetic trend favors the 1,2-heterocycles in both groups, with a few exceptions driven in large-part by π-donation of the lone pair on the heteroatom to the pz orbital on the adjacent boron resulting in stabilization. The importance of planarity for high aromaticity is demonstrated, especially in the pnictaborine isomers where pyramidalization at the pnictogen is favored, while bond regularity seems a less important criterion.

A Quantum Ruler for Magnetic Deflectometry

Matter-wave near-field interference can imprint a nano-scale fringe pattern onto a molecular beam, which allows observing its shifts in the presence of even very small external forces. Here we demonstrate quantum interference of the pre-vitamin 7-dehydrocholesterol and discuss the conceptual challenges of magnetic deflectometry in a near-field interferometer as a tool to explore photochemical processes within molecules whose center of mass is quantum delocalized.

High-precision measurement of the electron spin g factor of trapped atomic nitrogen in the endohedral fullerene N@C60

The electronic g factor carries highly useful information about the electronic structure of a paramagnetic species, such as spin-orbit coupling and dia- or paramagnetic (de-)shielding due to local fields of surrounding electron pairs. However, in many cases, a near "spin-only" case is observed, in particular for light elements, necessitating accurate and precise measurement of the g factors. Such measurement is typically impeded by a "chicken and egg situation": internal or external reference standards are used for relative comparison of electron paramagnetic resonance (EPR) Larmor frequencies. However, the g factor of the standard itself usually is subject to a significant uncertainty which directly limits the precision and/or accuracy of the sought after sample g factor. Here, we apply an EPR reference-free approach for determining the g factor of atomic nitrogen trapped within the endohedral fullerene C60:N@C60 in its polycrystalline state by measuring the 1H NMR resonance frequency of dispersing toluene at room temperature. We found a value of g=2.00204(4) with a finally reached relative precision of ∼20 ppm. This accurate measurement allows us to directly compare the electronic properties of N@C60 to those found in atomic nitrogen in the gas phase or trapped in other solid matrices at liquid helium temperature. We conclude that spin-orbit coupling in N@C60 at room temperature is very similar in magnitude and of same sign as found in other inert solid matrices and that interactions between the quartet spin system and the C60 molecular orbitals are thus negligible.

C50Cl10, a planar aromatic fullerene. Computational study of 13C-NMR chemical shift anisotropy patterns and aromatic properties

The structural characterization of D_5h-C₅₀Cl₁₀ as an IPR-violating fullerene provides an interesting case of an oblate structure displaying a planar-aromatic character provided by the face-to-face disposition of two IPR structural motifs, as unraveled by DFT calculations.

Formation of Spherical Aromatic Endohedral Metallic Fullerenes. Evaluation of Magnetic Properties of M@C28 (M = Ti, Zr, and Hf) from DFT calculations

The small C₂₈ cage has been shown experimentally to encapsulate titanium, zirconium, and hafnium (M), among other elements. Here, we explore computationally its magnetic response properties accounting for both global and local shielding tensors. Our results exhibit a continuous shielding region for M@C₂₈ for an orientation-averaged applied field thereby differing from that observed for the hollow C₂₈ structure. Moreover, under a specific orientation of the applied field a long-ranged shielding cone is obtained supporting the spherical aromatic behavior expected by the 2(N + 1)² Hirsch rule for M@C₂₈, standing for its particular abundance. The comparison between the hollow and endohedral C₂₈ fullerenes exhibits a characteristic long-range behavior at the outside region of the structure. The particular shape of the local chemical shift anisotropy tensor at a representative carbon atom exhibits inherent patterns as a consequence of the spherical aromatic behavior. This shows the capabilities from NMR experiments to account for the nonaromatic → aromatic variation. We envisage that the current approach will be beneficial in comparative studies of aromatic and electronic structure properties, to gain a deeper understanding of the geometrical and electronic structure situation in other endohedral species beyond that available from the information provided by routine NMR measurements.

Can the current density map topology be extracted from the nucleus independent chemical shifts?

Aromatic compounds are characterised by the presence of a ring current when in a magnetic field. As a consequence, current density maps are used to assess (the degree of) aromaticity of a compound. However, often a more discrete set of so-called Nucleus Independent Chemical Shift (NICS) values is used that is derived from the current density. It is shown here that there is no simple one-to-one relationship that allows reconstructing current density maps from only NICS-values. NICS values should therefore not be used as aromaticity indices without analysis of the ab initio computed current density map.

From C(58) to C(62) and back: Stability, structural similarity, and ring current

An increasing number of observations show that non-classical isomers may play an important role in the formation of fullerenes and their exo- and endo-derivatives. A quantum-mechanical study of all classical isomers of C₅₈ , C₆₀ , and C₆₂ , and all non-classical isomers with at most one square or heptagonal face, was carried out. Calculations at the B3LYP/6-31G* level show that the favored isomers of C₅₈ , C₆₀ , and C₆₂ have closely related structures and suggest plausible inter-conversion and growth pathways among low-energy isomers. Similarity of the favored structures is reinforced by comparison of calculated ring currents induced on faces of these polyhedral cages by radial external magnetic fields, implying patterns of magnetic response similar to those of the stable, isolated-pentagon C₆₀ molecule. © 2016 Wiley Periodicals, Inc.

The shielding cone in spherical aromatic structures: insights from models for spherical 2(N + 1)2 aromatic fullerenes

The shielding cone in spherical aromatic fullerenes is of long-range character and permits multiple orientations.

4N electron aromatic cycles in polycyclic hydrocarbons

Antiaromatic frameworks become aromatic in polycyclics through fusion, resulting in 4N electron hydrocarbons that possess aromatic properties by energetic, magnetic and geometric criteria.

The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes

The molecular structure and chemical reactivity of endohedral metallofullerenes can be greatly predicted and rationalized by their local and global aromaticity.

σ, π aromaticity and anti-aromaticity as retrieved by the linear response kernel

The chemical importance of the linear response kernel from conceptual Density Functional Theory (DFT) is investigated for some σ and π aromatic and anti-aromatic systems. The effect of the ring size is studied by looking at some well known aromatic and anti-aromatic molecules of different sizes, showing that the linear response is capable of correctly classifying and quantifying the aromaticity for five- to eight-membered aromatic and anti-aromatic molecules. The splitting of the linear response in σ and π contributions is introduced and its significance is illustrated using some σ-aromatic molecules. The linear response also correctly predicts the aromatic transition states of the Diels-Alder reaction and the acetylene trimerisation and shows the expected behavior along the reaction coordinate, proving that the method is accurate not only at the minimum of the potential energy surface, but also in non-equilibrium states. Finally, the reason for the close correlation between the linear response and the Para Delocalisation Index (PDI), found in previous and the present study, is proven mathematically. These results show the linear response to be a reliable DFT-index to probe the σ and π aromaticity or anti-aromaticity of a broad range of molecules.

Multidimensionality of delocalization indices and nucleus-independent chemical shifts in polycyclic aromatic hydrocarbons II: proof of further nonlocality

In a recent contribution, we examined the effect of 10- and 14-center circuits on the nucleus-independent chemical shifts NICSs using multicenter bond indices (MCBIs) (Fias et al., J Comput Chem 2008, 29, 358). In this study, the nonlocal contributions to the NICS are further investigated for a larger set of polycyclic aromatic hydrocarbons (PAHs). To achieve this, the NICSs are predicted using the MCBI and compared with ab initio results. The NICSs of the central ring of perylene- and benzo-[ghi]perylene-like fragments and of coronene appear to have other nonlocal contributions than the ones previously studied. It is shown that a model based on the MCBI-ring current maps and the inclusion of new circuits proves the existence and shows the nature of these new nonlocal effects on the NICS. This new model leads to a better understanding of the differences between the NICSs and delocalization indices. The results show that the NICS value is not only significantly influenced by the higher order circuits encircling the ring at which it is evaluated but also by the local aromaticity of the surrounding rings, and occasionally, like in the case of coronene, the NICSs are even influenced by currents farther away in the molecule.

Multidimensionality of delocalization indices and nucleus independent chemical shifts in polycyclic aromatic hydrocarbons

The aromaticity and local-aromaticity of a large set of polycyclic aromatic hydrocarbons (PAHs) is studied using multicenter delocalization indices from generalized population analysis and the popular nucleus independent chemical shift (NICS) index. A method for the fast computation of the NICS values is introduced, using the so-called pseudo-pi-method. A detailed examination is made of the multidimensional nature of aromaticity. The lack of a good correlation between the NICS and the multicenter delocalization indices is reported and the grounds discussed. It is shown through a thorough statistical analysis that the NICS values arise not only from local aromaticity of the benzenoid rings, but also from other circuits. It is shown that the NICS indices do not reveal the individual aromatic nature of a specific ring, contrary to the delocalization indices.