Self-assembly of endohedral metallofullerenes: a decisive role of cooling gas and metal-carbon bonding

The formation mechanism of Sc-based metallofullerenes: a molecular dynamics simulation study

The practical applications of endohedral metallofullerenes (EMFs) are mainly constrained by their low yields. Understanding the formation mechanisms is therefore crucial for developing methods for high-yield and selective synthesis. To address this, a novel force-field parameter set, "CSc.ff", was created using a single-parameter search optimization method, then molecular dynamics simulations of various systems with a carbon-to-scandium atomic ratio of 12.5 were carried out. The simulations were run under a constant atomic number, volume, and energy (NVE) ensemble. The influence of the working gas, helium, as well as temperature gradients on the formation process was examined. Our simulations reveal that the cage growth patterns of Sc-based EMFs (Sc-EMFs) closely resemble those of hollow fullerenes, evolving from free carbon atoms to chains, rings, and, ultimately, to cage-shaped clusters. Importantly, the Sc-EMFs formed in the simulation frequently exhibit structural defects or under-coordinated carbon atoms. Scandium atoms, whether at the periphery or on the surface of these cages, can be incorporated into the cages, forming Sc-EMFs. Helium was found to not only promote the formation of carbon cages but also facilitate the encapsulation of scandium atoms, playing a crucial role in the formation of cluster fullerenes. Moreover, cooling effectively inhibits the uncontrollable growth of the carbon cage and is essential for forming stable, appropriate-sized cages. This study enhances our understanding of the formation of Sc-EMFs and provides valuable insights for developing more efficient synthetic methods.

Laser-induced ultrafast spin-transfer processes in non-linear zigzag carbon chain systems

We combine the high-level quantum chemistry theory CCSD and EOM-CCSD together with local and global Λ processes to investigate the details of the laser-induced ultrafast spin manipulation scenarios in non-linear zigzag carbon chain systems Ni2@C32H32 and Ni2@C36H36. The spin density distribution, which is calculated on each many-body state using a Mulliken population analysis, fulfills the requirements to accomplish the spin dynamics processes. Various spin-flip and spin-transfer scenarios are accomplished. All the spin-dynamics processes can be achieved within subpicosecond times. Under the influence of a magnetic field, we find that the spin-transfer scenarios are preserved, while the local spin-flip scenario on a Ni atom can be significantly inhibited depending on the strength of the magnetic field. The impact of the propagation direction of the laser pulse on the spin dynamics processes by varying their polar and azimuthal angles in spherical coordinates is investigated. Additionally, we find that double laser pulses successfully induce the spin-transfer processes. Our outcomes underline the significant potential of carbon chain systems as building blocks for developing future all-optical integrated logic processing units.

Ultrafast control of laser-induced spin-dynamics scenarios on two-dimensional Ni3@C63H54 magnetic system

The concept of building logically functional networks employing spintronics or magnetic heterostructures is becoming more and more popular today. Incorporating logical segments into a circuit needs physical bonds between the magnetic molecules or clusters involved. In this framework, we systematically study ultrafast laser-induced spin-manipulation scenarios on a closed system of three carbon chains to which three Ni atoms are attached. After the inclusion of spin-orbit coupling and an external magnetic field, different ultrafast spin dynamics scenarios involving spin-flip and long-distance spin-transfer processes are achieved by various appropriately well-tailored time-resolved laser pulses within subpicosecond timescales. We additionally study the various effects of an external magnetic field on spin-flip and spin-transfer processes. Moreover, we obtain spin-dynamics processes induced by a double laser pulse, rather than a single one. We suggest enhancing the spatial addressability of spin-flip and spin-transfer processes. The findings presented in this article will improve our knowledge of the magnetic properties of carbon-based magnetic molecular structures. They also support the relevant experimental realization of spin dynamics and their potential applications in future molecular spintronics devices.

Discovery of a Superatom inside the Fullerene Cage

The stability of endohedral clusterfullerenes is generally understood in terms of the metal cluster shape, cage structure, and metal-cage interactions, with the electronic state of the internal cluster mostly neglected. Herein, theoretical calculations reveal that the (Ti₃C₃)⁶⁺ unit of recently synthesized Ti₃C₃@I_h(7)-C₈₀ exhibits a superatomic state with a perfect closed-shell 1S²1P⁶1D¹⁰ electronic configuration in accordance with the famous jellium model. This "trapped superatom" features considerable aromaticity and hyperconjugation interactions never reported for other clusterfullerenes. Besides the localized two-center two-electron (2c-2e) Ti-C/C-C bonds, it also has two 3c-2e Ti-C-Ti bonds. Furthermore, the ring strain of the cyclopropane-like C₃ core is effectively released upon the metal coordination. All these factors greatly stabilize the (Ti₃C₃)⁶⁺ cluster, showing the critical role of metal-to-cage charge transfer and cage encapsulation in enhancing the stability of this exotic metal cluster.

Density-functional tight-binding: basic concepts and applications to molecules and clusters

The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation, treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, non-adiabatic dynamics. A number of applications are reviewed, focusing on -(i)- the variety of systems that have been studied such as small molecules, large molecules and biomolecules, bare orfunctionalized clusters, supported or embedded systems, and -(ii)- properties and processes, such as vibrational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given.

Trapping Metallic Oxide Clusters inside Fullerene Cages

The sub-nanometer sized void inside a fullerene cage permits the accommodation of a single atom, atomic cluster, or even small molecule, resulting in the formation of endohedral fullerenes. Particularly, clusterfullerenes can be formed by encapsulating multiple metallic ions in most cases along with nonmetal ions (i.e., N^3-, C₂^2-, S^2-, O^2-) inside the fullerene cage. Such an association makes clusterfullerene more functional than empty fullerenes and conventional mono-metallofullerenes. To date, a variety of clusterfullerenes have been reported, including metal nitrides, carbides, oxides, sulfides, cyanides, and so on. Among them, oxide clusterfullerenes (OCFs) can contain variable oxide clusters (i.e., M₄O₂, M₄O₃, M₃O, and M₂O; M = Sc or other metal), yielding one of the most versatile families. Thus, OCFs may provide a more convenient platform for developing new functional molecules and for studying previously less-explored topics such as formation mechanisms of clusterfullerenes. In this Account, we review recent progress in the field of OCFs, including their synthesis, isolation, and structural and electrochemical studies as well as the preliminary exploration into their potential functions and applications. Thanks to the concrete crystallographic results of an OCF series, we can track the transition of endohedral cluster and fullerene cage. It is suggested that the configuration and internal dynamics of the oxide cluster are highly dependent on not only the cage size but also cage structure. On the other hand, based on the experimental observations, two competitive transformation pathways are established for the majority of OCFs, verifying the bottom-up or top-down formation mechanism. It is also found that the redox behaviors of OCFs are more or less comparable to their isoelectronic species with common cage structure and similar cluster geometry but varied greatly with the cluster variety (i.e., Sc₂O vs Sc₄O_2-3). The mechanism behind such phenomena has been discussed. In addition, the potential of Dy-based OCFs as single molecular magnets (SMMs) is presented theoretically. Nevertheless, experimental advance remains to be achieved.

Evaluation of the effect of nickel clusters on the formation of incipient soot particles from polycyclic aromatic hydrocarbons via ReaxFF molecular dynamics simulations

In the present study, the ReaxFF reactive molecular dynamics simulation method was applied to investigate the effect of a small nickel cluster (Ni13) on the formation of nascent soot from polycyclic aromatic hydrocarbon (PAH) precursors. A series of NVT simulations was performed for systems of a Ni13 cluster and various PAH monomers, namely, naphthalene, anthracene, pyrene, coronene, ovalene, and circumcoronene, at temperatures from 400 to 2500 K. At low temperatures, the PAHs form soot particles via binding and stacking around nickel clusters. Larger soot particles are formed due to the early initiation of clustering provided by nickel compared to those observed in homogenous PAH systems. At 1200-1600 K, the PAH monomers show a chemisorption tendency onto the nickel surface, which results in incipient soot particles. Chemical nucleation was observed at 2000 K where nickel-assisted dehydrogenation and chemisorption of PAH led to the growth of stable soot particles, which did not occur in the absence of Ni-clusters. At a high temperature (2500 K), nickel significantly accelerates the ring-opening and graphitization of PAH molecules and increases the size of the fullerene-type soot as compared to that of homogenous systems.

Thermodynamics of hydration of fullerols [C60(OH)n] and hydrogen bond dynamics in their hydration shells

Molecular dynamics simulations of fullerene and fullerols [C₆₀(OH)_n, where n = 2-30] in aqueous solutions have been performed for the purpose of obtaining a detailed understanding of the structural and dynamic properties of these nanoparticles in water. The structures, dynamics and hydration free energies of the solute molecules in water have been analysed. Radial distribution functions, spatial density distribution functions and hydrogen bond analyses are employed to characterize the solvation shells of water around the central solute molecules. We have found that water molecules form two solvation shells around the central solute molecule. Hydrogen bonding in the bulk solvent is unaffected by increasing n. The large decrease in solvation enthalpies of these solute molecules for n > 14 enhances solubilisation. The diffusion constants of solute molecules decrease with increasing n. The solvation free energy of C₆₀ in water is positive (52.8 kJ/mol), whereas its value for C₆₀(OH)₃₀ is highly negative (-427.1 kJ/mol). The effects of surface hydroxylation become more dominant once the fullerols become soluble.

Formation of Nickel Clusters Wrapped in Carbon Cages: Toward New Endohedral Metallofullerene Synthesis

Despite the high potential of endohedral metallofullerenes (EMFs) for application in biology, medicine and molecular electronics, and recent efforts in EMF synthesis, the variety of EMFs accessible by conventional synthetic methods remains limited and does not include, for example, EMFs of late transition metals. We propose a method in which EMF formation is initiated by electron irradiation in aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) of a metal cluster surrounded by amorphous carbon inside a carbon nanotube serving as a nanoreactor and apply this method for synthesis of nickel EMFs. The use of AC-HRTEM makes it possible not only to synthesize new, previously unattainable nanoobjects but also to study in situ the mechanism of structural transformations. Molecular dynamics simulations using the state-of-the-art approach for modeling the effect of electron irradiation are performed to rationalize the experimental observations and to link the observed processes with conditions of bulk EMF synthesis.

Reversible ultrafast spin switching on Ni@B80endohedral fullerene

We demonstrate ultrafast (∼100 fs) and reversible spin switching on the endohedral fullerene Ni@B₈₀viaΛ processes.

An atlas of endohedral Sc2S cluster fullerenes

Low-energy Sc₂S@C_n isomers are connected by an intricate web of Stone–Wales isomerization and Endo–Kroto C₂ insertions, giving clues to their formation.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc-Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH4 and Ti/CH4 systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH4 system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH4 system are Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster-cage strain in the endohedral fullerenes through electrochemical measurements.

The importance of dynamics studies on the design of sandwich structures: a CrB24 case

A series of BO-MD simulations showed that the sandwich structure of CrB₂₄ has an extremely poor dynamic stability, and a highly symmetric endohedral structure was obtained during the MD simulations.