Collision of Li+ and Na+ with C60: Insertion, fragmentation, and thermionic emission

Sr-centered monocyclic carbon ring Sr@C14: A new stable cluster

The study of stable neutral metal endohedral cyclo[n]carbon is helpful for discovering single-molecule devices. Extensive structural search and density functional theory calculations performed here indicate that the perfect planar alkaline metal-doped complexes Sr@C14 possess the well-defined global minima of the system with the metal atom located exactly at the center of the carbon ring. The configuration and bonding properties of C14 are different from those of pristine cyclo [14]carbon. The significant stabilization when forming Sr@C14 predominantly originates from the electrostatic interaction between Sr2+ and C142-. The detailed molecular orbital, nucleus-independent chemical shift (NICS), and ring current analyses indicate that Sr@C14 is aromatic in nature. The NICS values of Sr@C14 are considerably larger than those of benzene. Ab initio molecular dynamics simulations at different temperatures reveal that this system exhibits certain stability at low or moderate temperatures. The findings of this study effectively enrich the chemical structures and bonding patterns of metal-doped cyclo[n]carbon and provide the knowledge required to obtain novel structures of Sr@C14 in future experiments.

Model of Charge Transfer Collisions between C60 and Slow Ions

A semi-classical model describing the charge transfer collisions of C60 fullerene with different slow ions has been developed to explain available experimental data. This data reveals multiple Breit-Wigner like peaks in the cross sections, with subsequent peaks of reactive cross sections decreasing in magnitude. Calculations of charge transfer probabilities, quasi-resonant cross sections and cross sections for reactive collisions have been performed using semi-empirical interaction potentials between fullerenes and ion projectiles. All computations have been carried out with realistic wave functions for C60's valence electrons derived from the simplified jellium model. The quality of these electron wave functions have been successfully verified by comparing theoretical calculations and experimental data on the small angle cross sections of resonant C60+C60+ collisions. Using the semi-empirical potentials to describe resonant scattering phenomena in C60 collisions with ions and Landau-Zener charge transfer theory, we calculated theoretical cross sections for various C60 charge transfer and fragmentation reactions which agree with experiments.

Theoretical prediction of chiral actinide endohedral borospherenes

Actinide encapsulation can form chiral borospherenes, and the covalent character of An–B bonds dominates the formation of these actinoborospherenes.

Theoretical investigation of perfect fullerene-like borospherene Ih-B20 protected by alkaline earth metal: multi-layered spherical electride molecules as electric field manipulated second-order nonlinear optical switches

A perfect fullerene-like borospherene B₂₀ with 12 B₅ rings stabilized in the electride molecule (Mg²⁺)₁₂&B₂₀¹⁸⁻ + 6e⁻.

Stability of Coinage Metals Interacting with C60

Buckminsterfullerene (C₆₀) has been advocated as a perfect candidate material for the encapsulation and adsorption of a variety of metals and the resultant metallofullerenes have been considered for the use in different scientific, technological and medical areas. Using spin-polarized density functional theory together with dispersion correction, we examine the stability and electronic structures of endohedral and exohedral complexes formed between coinage metals (Cu, Ag and Au) and both non-defective and defective C₆₀. Encapsulation is exoergic in both forms of C₆₀ and their encapsulation energies are almost the same. Exohedral adsorption of all three metals is stronger than that of endohedral encapsulation in the non-defective C₆₀. Structures and the stability of atoms interacting with an outer surface of a defective C₆₀ are also discussed. As the atoms are stable both inside and outside the C₆₀, the resultant complexes can be of interest in different scientific and medical fields. Furthermore, all complexes exhibit magnetic moments, inferring that they can be used as spintronic materials.

Penetrating probability and cross section of the Li+-C60 encapsulation process through an ab initio molecular dynamics investigation

The endohedral complex system of Li⁺-C₆₀ has been shown to possess interesting applications in photovoltaics, supramolecular chemistry, and functionalized materials. In this study, we perform a theoretical investigation of Li⁺ encapsulation within a C₆₀ cage by employing an ab initio molecular dynamics approach. The Li⁺ cation is positioned 9 Å away from the C₆₀ center of mass, and fired towards a randomized spot in a six-membered ring with a certain level of inletting energy, which is 7.5 eV, 9 eV, 12 eV, or 15 eV. In total, 2000 samples of MD trajectories are investigated. Our statistical results yielded a penetrating probability in the range of 0.8% to 15.6% with respect to the above inletting energy, while the cross section ranges from 0.006 Ų to 0.123 Ų. Moreover, we observed that the penetrating probability exhibited direct proportionality to the inletting energy. Hence, we can determine that the minimum required inletting energy for reaction occurrence is 6.6 eV. Overall, it seems difficult for Li⁺ to penetrate through the sp²-carbon wall, because a very high inletting energy is required to open the entrance. At the same time, Li⁺ must approach closely to the center of a six-membered ring to enhance the penetration probability.

Endohedral Ca@B38: stabilization of a B382−borospherene dianion by metal encapsulation

We present the viability of an endohedralC_sCa@B₃₈which contains aC_sB₃₈²⁻borospherene cage composed of interwoven boron double chains with a σ + π double delocalization bonding pattern, extending the B_n^q(q=n− 40) borospherene family fromn= 39–42 ton= 38.

DFT study on endohedral and exohedral B38fullerenes: M@B38(M = Sc, Y, Ti) and M&B38(M = Nb, Fe, Co, Ni)

Viability of endohedral and exohedral B₃₈fullerenes with transition metal atoms was investigated using density functional theory. M@B₃₈(M = Sc, Y, Ti) possess endohedral structures as their lowest energy structures.

Endohedral C3Ca@B39+and C2Ca@B39+: axially chiral metalloborospherenes based on B39−

Axially chiral endohedral metalloborospherenesC₃/C₂Ca@B₃₉⁺predictedviafirst-principles calculations are charge-transfer complexes in nature with the universal bonding pattern of σ plus π double delocalization.

A comparative study of homolytic reactions of fullerenes with aldehydes in a mass spectrometer under electron impact and in solution under UV irradiation

C(60) was reacted in the ionization chamber of a mass spectrometer under electron impact (EI) with aldehydes, RCHO (R = Ph, p-FC(6)H(4), F(5)C(6), p-MeOC(6)H(4), α-thienyl, o-HOC(6)H(4), o-BrC(6)H(4), m-BrC(6)H(4) and t-Bu), with the transfer of R• radicals and with Me•-transfer from i-PrCHO and t-BuCHO. Paramagnetic fullerene derivatives were stabilized by the addition of the next R• radical or a hydrogen atom, or hydrogen or bromine atom loss. A detailed study showed that the reaction between C(60) and PhCHO occurred via a homolytic mechanism that matches one reported earlier for the reaction with acetone. This suggests the generality of the mechanism for the reactions of fullerenes with other species in ionization chambers under EI at ca 300°C. All aldehydes, except one, had radicals at the carbonyl group which were different from those in the ketones examined earlier in the reactions. This expanded the variety of radicals which can be transferred to fullerenes during reactions in ionization chambers under EI. Due to this and the hydrogen atom at the CO group of aldehydes, some reactions occurred that were not found for the ketones: the formation of cyclic products C(60)COC(6)H(4) and C(60)OC(6)H(4) for PhCHO, o-BrC(6)H(4)CHO and o-HOC(6)H(4)CHO, respectively, and HC(60)Ph for o- and m-BrC(6)H(4)CHO. The reaction with α- formylthiophen gives the first example of transferring an aromatic heterocyclic radical to C(60) in an ionization chamber under EI. C(70) reacted with PhCHO, p-FC(6)H(4)CHO and i- PrCHO similarly to C(60). The results for the reactions of C(60) with PhCHO and with i- PrCHO were compared with those in solution under UV irradiation. Incomplete but reasonable coincidence was found; in both modes, the addition of Ph•, PhCO• and Me• radicals to C(60) occurred, whereas some other products were formed in solution, and the explanation is given as to why this occurred. This conformity supports the hypothesis based on the results of kindred reactions with ketones and organomercurials: the results of EI-initiated homolytic reactions between fullerenes and other compounds in an ionization chamber can predict the reactivity of the fullerenes toward them in solution.

Stability of silicon-doped C60 dimers

A theoretical investigation on the structure, stability, and thermal behaviors of the smallest polymeric units, the dimers, formed from substitutionally Si-doped fullerenes is presented. A density functional based nonorthogonal tight-binding model has been employed for describing the interatomic interactions. The study focuses on those polymeric structures which involve Si-Si or Si-C interfullerene bonds. The binding energy of the dimers increases with their Si content from about 0.25 eV in C(60)-C(60) to about 4.5 eV in C(58)Si(2)-C(58)Si(2). Moreover, the C(59)SiC(59) dimer, linked through the sharing of the Si atom between the two fullerenes, has been also considered. Upon heating, the dimers eventually fragment into their constituent fullerene units. The fragmentation temperature correlates with the strength of the interfullerene bonds. C(58)Si(2)-C(58)Si(2) exhibits a higher thermal stability (fragmentation temperature of approximately 500 K) than the pure carbon C(60)-C(60) dimer (with a fragmentation temperature of approximately 325 K). Given the higher structural and thermal stabilities of the Si-doped fullerene dimers, the authors propose the use of substitutionally Si-doped fullerenes as the basic units for constructing new fullerene-based polymers.

First-principles study of electron-conduction properties of C60 bridges

The electron-conduction properties of fullerene-based nanostructures suspended between electrodes are examined by first-principles calculations based on the density functional theory. The electron conductivity of the C60-dimer bridge is low owing to the constraint of the junction of the molecules. When the fullerenes are doped electrons by being inserted Li atoms into the cages, the unoccupied state around the junction is filled and the conductivity can be significantly improved.

Simulating the thermal behavior and fragmentation mechanisms of exohedral and substitutional silicon-doped C60

Structures, thermal behavior, and fragmentation mechanisms of exohedral and substitutional silicon-doped C(60) containing 1-12 Si atoms are investigated by extensive molecular-dynamics simulations. A nonorthogonal tight-binding model is used to mimic the interatomic interactions in the doped fullerenes. Beginning from the minimum-energy structures, the temperature of the doped fullerenes is slowly increased until fragmentation takes place. A correlation can be established between the exohedral and substitutional structures and the corresponding fragmentation mechanisms and fragmentation temperatures. Exohedral C(60)Si(m) fullerenes fragment into two homonuclear pieces, the Si(m) cluster and the C(60) fullerene that remains intact. In contrast, the substitutional C(60-m)Si(m) heterofullerenes undergo structural transformations, including the partial unraveling of the cage, prior to fragmentation. Then, ejection of atoms or small molecules takes place from the distorted structures. The slow heating rate used, combined with long simulation runs, allows us to determine the fragmentation temperature of exohedral and substitutional Si-doped fullerenes as a function of the number of silicon atoms. Substitutional Si-doped fullerenes exhibit much higher fragmentation temperatures (1000-1500 K higher) than the exohedral fullerenes. This can be understood from the different bonding of the Si atoms in both structures.

Optimization and Application of Lithium Parameters for the Reactive Force Field, ReaxFF

To make a practical molecular dynamics (MD) simulation of the large-scale reactive chemical systems of Li-H and Li-C, we have optimized parameters of the reactive force field (ReaxFF) for these systems. The parameters for this force field were obtained from fitting to the results of density functional theory (DFT) calculations on the structures and energy barriers for a number of Li-H and Li-C molecules, including Li(2), LiH, Li(2)H(2), H(3)C-Li, H(3)C-H(2)C-Li, H(2)C=C-LiH, HCCLi, H(6)C(5)-Li, and Li(2)C(2), and to the equations of state and lattice parameters for condensed phases of Li. The accuracy of the developed ReaxFF was also tested by comparison to the dissociation energies of lithium-benzene sandwich compounds and the collision behavior of lithium atoms with a C(60) buckyball.

The dynamics of endohedral complex formation in surface pick-up scattering as probed by kinetic energy distributions: Experiment and model calculation for Cs@C60+

Endohedral Cs@C60 molecules were formed by implanting low energy (E0 = 30-220 eV) Cs+ ions into C60 molecules adsorbed on gold. Both growth and etching experiments of the surface deposited C(60) layer provide clear evidence for a submonolayer coverage. The Cs+ penetration and Cs@C60 ejection stages are shown to be a combined, single collision event. Thermal desorption measurements did not reveal any Cs@C60 left on the surface following the Cs+ impact. The Cs@C60 formation/ejection event therefore constitutes a unique example of a pick-up scattering by endocomplex formation. Kinetic energy distributions (KEDs) of the outgoing Cs@C60+ were measured for two different Cs+ impact energies under field-free conditions. The most striking observation is the near independence of the KEDs on the Cs+ impact energy. Both KEDs peak around 1.2 eV with similar line shapes. A simple model for the formation/ejection/fragmentation dynamics of the endohedral complex is proposed. The model leads to a strong correlation between the vibrational and kinetic energy of the outgoing Cs@C60. The KEDs are calculated taking into account the competition between the various decay processes: fragmentation and delayed ionization of the neutral Cs@C60 emitted from the surface, fragmentation of the Cs@C60+ ion, and radiative cooling. It is concluded that the measured KEDs are heavily biased by the experimental breakdown function. Good agreement between experimental and calculated KEDs is obtained.

Direct detection and quantitation of He@C60 by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry

In this paper, we report negative ion microelectrospray Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry of C60 samples containing approximately 1% 3He@C60 or 4He@C60. Resolving He@C60- and 4He@C60- from C60 containing 3 or 4 13C instead of 12C atoms is technically challenging, because the target species are present in low relative abundance and are very close in mass. Nevertheless, we achieve baseline resolution of 3He@C60- from 13C3(12C57-) and 4He@C60- from 13C4(12C56-) in single-scan mass spectra obtained in broadband mode without preisolation of the ions of interest. The results constitute the first direct mass spectrometric observation of endohedral helium in a fullerene sample at this (low) level of incorporation. The results also demonstrate the feasibility of determining the extent of He incorporation from the FT-ICR mass spectral peak heights. The present measurements are in agreement with those obtained by the pyrolysis method [1-3]. Although limited in sensitivity, the mass spectral method is faster and easier than pyrolysis.

Delayed ionization and fragmentation en route to thermionic emission: statistics and dynamics

Thermionic emission is discussed as a long time (microseconds) decay mode of energy-rich large molecules, metallic and metcar clusters, and fullerenes. We review what is known and consider the many experiments, systems, and theoretical and computational studies that still need to be done. We conclude with a wish list for future work. Particular attention is given to the experimental signatures, such as the dependence on the mode of energy acquisition, and theoretical indications of a not-quite-statistical delayed ionization and to the competition of electron emission with other decay modes, such as fragmentation or radiative cooling. Coupling of the electronic and nuclear modes can be a bottleneck and quite long time-delayed ionization can be observed, as in the decay of high Rydberg states probed by ZEKE spectroscopy, before the onset of complete energy partitioning.