Isomers of the C84 fullerene: A theoretical consideration within energetic,structural, and topological approaches

Currently, there is no common conception on the stability and achievability of fullerene isomers under the arc-discharge synthesis conditions. Different approaches operate with energetic, structural, and topological parameters of the fullerene molecules to explain why some fullerene isomers are more preferable than the others. In the present work, we have selected the most appropriate approaches based on topological roundness, information entropy, nuclear volumes, and sphericities and compared their predictions with the relevant experimental data on the C₈₄ fullerene isomers (obeying the isolated pentagon rule) and density functional theory estimates of their stability. We have found that the molecules of most stable (and most abundant) C₈₄ isomers have the minimal extremal roundness, maximal sphericity, and largest volume and vice versa. In the case of the information entropy, no correlation is observed. Interestingly, the found correlation between the volume and the stability of the C₈₄ isomers is unexpectedly inverse, i.e. more stable C₈₄ isomers have larger inner cavities inside, though traditionally large volume is associated with instability of hollow framework molecules. We assume that the large volumes allows enhancing the sphericity of the fullerene cages as we found the last one favoring the stability and the unstable C₈₄ fullerene cages having low volumes are far from the spherical shape. We think that the results obtained may be extrapolated to other fullerene isomeric sets and contribute to the understanding the grounds of the interconnection in the triad “topology – structure – energy” underlying structural chemistry.     

Depth profiling of fullerene-containing structures by time-of-flight secondary ion mass spectrometry

A new variant of depth profiling for thin-film fullerene-containing organic structures by the method of time-of-flight (TOF) secondary ion mass spectrometry (SIMS) on a TOF.SIMS-5 setup is described. The dependence of the yield of C₆₀ molecular ions on the energy of sputtering ions has been revealed and studied. At an energy of sputtering Cs⁺ ions below 1 keV, the intensity of C₆₀ molecular ions is sufficiently high to make possible both elemental and molecular depth profiling of multicomponent (multilayer) thin-film structures. Promising applications of TOF-SIMS depth profiling for obtaining more detailed information on the real molecular composition of functional organic materials are shown.     

Study on structural phase transitions and relaxation processes of Cs2Co(SO4)2·6H2O and Cs2Zn(SO4)2·6H2O crystals

The structural phase transitions and relaxation processes of Cs₂Co(SO₄)₂·6H₂O and Cs₂Zn(SO₄)₂·6H₂O single crystals were investigated, with the phase transitions of both crystals being determined from NMR data. The spin–lattice relaxation time, T₁, of the ¹³³Cs nucleus in two crystals undergoes a significant change near the phase transition temperature, T_C, and these changes coincide with the changes in the splitting of the ¹³³Cs resonance lines. The variations in the temperature dependence for the splitting of the ¹³³Cs resonance lines and T₁ near T_C are related to changes in the symmetry of surrounding Cs⁺. In addition, the ¹³³Cs T₁ of Cs₂Co(SO₄)₂·6H₂O, which contains paramagnetic ions, was found to be shorter than that of Cs₂Zn(SO₄)₂·6H₂O. This relaxation time is inversely proportional to the square of the magnetic moment of the paramagnetic ions. The differences between the ¹³³Cs T₁ of these compounds are probably due to the differences between the electronic structures of their metal ions.     

Anharmonicity in the V<Subscript>2</Subscript>O<Subscript>3</Subscript> molecule and thermodynamic properties of V<Subscript>2</Subscript>O<Subscript>3</Subscript> in the gas phase

A quantum-mechanical calculation of the relative stability, structural parameters, and vibrational frequencies of V₂O₃ molecule isomers for different spin states was carried out using the BPW91/6-311+G(d, p) method. It was shown that the isomer with the C _s structure (nonplanar VOVO rectangle with an O atom attached to it) in the X ⁵ A″ electronic state possesses the maximum stability. The energy of the C _2v symmetry structure was higher than the lowest energy by just 23 cm⁻¹. It definitely indicated the impossibility of usage of the harmonic model in order to calculate the thermodynamic functions of V2O3 (g). A model is proposed based on which the energy levels and vibrational sums of states for this type of motion were calculated for the C _s → C _2v → C _s transition coordinate. These data, as well as results obtained from quantum-mechanical calculations, were used to calculate the thermodynamic functions of V₂O₃ (g) in the temperature range of T = 100–6000 K. The calculations were performed with the five excited electronic states with energies from 1000 to 9000 cm⁻¹ taken into account. A comparison with the data calculated in the “rigid rotator-harmonic oscillator” approximation was performed.     

High-temperature thermal conductivity of NdCoO3 and GdCoO3

Thermal conductivity of NdCoO₃ and GdCoO₃ has been studied from 300 to 1200 K. The conductivity is mainly excitonic, arising from the interactions of the t_2g–e_g states of cobalt ions in the oxides and shows characteristic maxima at 1030 and 975 K, respectively, corresponding with their semiconductor-metal transition temperatures. Contributions to thermal conductivity by phononic, electronic, ambipolar, and excitonic modes of conduction are calculated from measured thermal and electrical conductivity data in this temperature range. The energy of exciton formation in both the compounds is found to be 0.11 eV, same as in the case of LaCoO₃.     

The relative stability in and thermodynamic functions of the system of Si2H4(g) isomers: Comparison of results from two different evaluations of the system energetics

Thermodynamic consequences of the interplay of four recently found isomers of Si₂H₄(g) are studied. The partition functions and energetics of the isomers are constructed on the basis of available quantum-chemical data, and the results from two different approximations of potential energy are compared. Substantial values of isomerism contributions to the overall thermodynamics of the system are pointed out, as well as their sensitivity to interisomer energetics.     

Generalized topological efficiency – case study with C84 fullerene

Abstract

A pure topological mechanism able to explain fullerenes stability is presented here. The non-trivial case of the C₈₄ fullerene isomers with isolated Pentagons Is in fact analyzed. This original computational approach is based on the generalization of the topological efficiency potentials which may be used to study all types of carbon allotropes.     

Kinetic energy release and mean life time of metastable ions produced from C3H3N

Metastable ions C₃H_nN⁺ (n = 1, 2 or 3) and C₂H₂⁺ produced by electron impact on the neutral C₃H₃N (acrylonitrile) undergo metastable decay reactions resulting in the fragment ions C₃H_mN⁺ (m = 1, 2) or C₂H₂⁺. We have monitored these reactions in a double focusing mass spectrometer of reversed B-E geometry. To identify the fragment ions and determine their kinetic energy release distribution (KERD), the mass-analyzed ion kinetic energy (MIKE) scan technique was utilized. For the above mentioned ions we obtained average KER values ranging from 9 up to 23 meV. Moreover, the fractions of decaying ions were measured. We have found the existence of two states of the metastable C₂H₂⁺ ion, one with a short life time of about 0.27 μs and the other one which is most likely a completely stable ion state.     

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

Kinetic Instability of Boron Heterofullerenes

Abstract

Although boron heterofullerenes are produced abundantly in the gas phase, they cannot be isolated in macroscopic amounts. Those with two to six boron atoms were found to have minimum bond resonance energies (min BRE) of less than -0.100 |β|. All carbon-boron bonds have negative BREs, suggesting that boron atoms markedly destabilize nearby π bonds. on this theoretical basis, typical boron heterofullerenes are predicted to be kinetically very unstable and chemically very reactive. In contrast, the molecular anions with 60 π electrons of boron heterofullerenes are predicted to be kinetically much more stable. These anions do not have bonds wirh large negative BREs. Some of them may be isolable as salts or endohedral complexes. This study constitutes the first attempt to apply the BRE method to heteroconjugated systems.     

Acenes adducts with C70 fullerene: Anthracene,tetracene and pentacene

The mono-adducts of C₇₀ fullerene with pentacene, tetracene and antracene were synthesized. The C₇₀/acene adducts were studied with FT-IR and the electronic absorption spectroscopy. Thermogravimetric analysis (TGA) in combination with derivative thermogravimetry (DTG) and differential thermal analysis (DTA) were employed to determine the quantitative composition of the adducts and their relative thermal stability. Differential scanning calorimetry (DSC) was employed as well for the determination of the stability of the adducts. It was found that the C₇₀/acene adducts decompose with endothermal reactions. The enthalpy of the decomposition reaction and the decomposition temperatures were determined and found in agreement with the DTG and DTA results.     

Energy levels and crystal-field parameters for Pr and Nd ions in rare earth (RE) tellurium oxides RE2Te4O11 revisited - Ascent/descent in symmetry method applied for triclinic site symmetry

Triclinic site symmetry presents considerable challenges in the studies of energy levels and crystal-field parameters (CFPs) for rare earth (RE) ions in crystals. Assignment of the spectral features to the irreducible representations of low symmetry point groups is difficult and may be unreliable. Fittings of large number of CFPs, even if the number of the available experimental energy levels is sufficient, often yield multiple solutions with relatively low and close rms deviations. These challenges call for better methods of fitting and analysis of CFPs. In this paper, we employ the ascent/descent in symmetry (ADS) method and three computational approaches: (i) a crystallographic data analysis program to identify approximated higher symmetry, (ii) the superposition model (SPM) analysis, and (iii) the pseudosymmetry axes method (PAM) for the combined coordination factors obtained using SPM. As a case study, the experimental CF splittings of Pr³⁺ and Nd³⁺ ions at triclinic C₁ sites in tellurium oxides RE₂Te₄O₁₁ are re-analyzed. Two alternative ADS chains C₁ ↔ C₂ ↔ D₂ and C₁ ↔ C_s ↔ C_2v were independently obtained. For each approximation of the actual C₁ symmetry of the REO₈ polyhedron in RE₂Te₄O₁₁, the ligands’ positions are determined and used in SPM to calculate CFP sets, which are used as starting for additional fittings. Comparative analysis of the fitted and calculated CFP sets enables assignment of appropriate axis system to the fitted CFP sets, thus avoiding the ambiguities occurring in previous ADS applications. The very good compatibility and consistency of the newly determined CFP sets indicate high reliability of both approaches. These CFP sets were used to simulate magnetic susceptibility data for polycrystalline samples of Pr₂Te₄O₁₁ and Nd₂Te₄O₁₁. Wider applications of the proposed procedures in optical spectroscopy studies of low symmetry systems may improve reliability of the CFP sets reported in literature.     

Molecular structures of the open-shell IPR isomers of fullerene C90

Fifteen open-shell isomers off the forty six IPR isomers of fullerene C₉₀ were found and investigated: 5 (C_s), 7 (C₁), 9 (C₁), 10 (C_s), 11 (C₁), 20 (C₁), 21 (C₁), 22 (C₁), 23 (C₂), 24 (C₁), 25 (C_2v), 41 (C₂), 42 (C₂), 43 (C₂), and 44 (C₂). According to developed by us approach the positions of single, double, and delocalized π-bonds in the molecules of these isomers are shown for the first time. The obtained results of electronic and geometrical structures are fully supported by DFT method with the B3LYP functional at the 6-31G and 6-31G* levels. Molecules of these open-shell isomers contain different radical substructures (mainly the phenalenyl-radical substructures), they should be unstable and could not be obtained as empty molecules. Nevertheless, there is a possibility of obtaining them in polymeric forms or as endohedral or exohedral derivatives.     

Maximal Non-Adjacent Addition to Fullerene-70: Computation of All the Closed Shell Isomers of C70X26

Abstract

Computation of the patterns for saturation non-adjacent addition to D_5h-C₇₀ to yield closed-shell products shows that maximal addition is achieved with C₇₀X₂₆. There are ten possible addition patterns and the relative energies of these isomers is assessed by simple Hückel and molecular modelling calculations (for X = H and CH₃). the identity of the lowest energy isomer is found to depend upon the size of the addend.     

The role of sputter etching and annealing processes on the formation of β-FeSi2 thin films

For formation of β-FeSi₂ using ion beam sputter deposition (IBSD) method, sputter etching (SE) followed by thermal annealing is effective substrate treatment to obtain highly (100)-oriented β-FeSi₂ on Si(100). However, the best condition of these treatments are not yet known. In this work, the effect of SE together with annealing process on the orientation of the film is investigated. Prior to the deposition of Fe, the substrate is irradiated by Ne⁺ ions with various energy and fluence followed by thermal annealing at 1073 K for 60 min. The overall results show the most suitable SE condition using Ne⁺ ion on IBSD method is the energy of 1 keV with the fluence of 3.0×10¹⁹ ions /m².     

Eu@C86 isomers: Calculated relative populations

Abstract

Relative populations of four energy-lowest IPR (isolated-pentagon-rule) isomers of Eu@C₈₆ are computed using the Gibbs energy based on characteristics from density functional theory calculations (M06-2X/3-21G?～?SDD entropy term, M06-2X/6-31G*～SDD or B2PLYP(D)/6-31G*～SDD energetics). The calculations confirm that the recently isolated Eu@C₁(7)-C₈₆ species is a major isomer in a relevant temperature region. Relationship to the empty C₈₆ cages is discussed, too.     

Fullerane,the Hydrogenated C60 Fullerene: Properties and Astrochemical Considerations

Abstract

Hydrogenated C₆₀ fullerene, C₆₀H₃₆ was prepared in different solvents using Zn/HCl as reducing agents. The structure of C₆₀H₃₆ was confirmed both by electronic and FT‐IR spectroscopy and the purity of the reaction product was checked by HPLC analysis. It has been confirmed that C₆₀H₃₆ is not stable in air, especially in presence of light which enhances the oxidation. The oxidation of C₆₀H₃₆ was studied by FT‐IR spectroscopy and by differential scanning calorimetry (DSC) in air; the formation of hydroxyl groups on the fullerene cage and ketonic groups (involving cage breakdown) have been detected. Furthermore, the action of O₃ on C₆₀H₃₆ was investigated and it has been found that O₃ exerts practically the same effect of air but causing an enhanced cage breakdown. The thermal stability of C₆₀H₃₆ was checked by a thermogravimetric analysis (TGA) coupled with a differential thermal analysis (DTA) under N₂ flow. The vaporization of C₆₀H₃₆ occurs at very high temperature: the DTA trace has shown an endothermic peak at 540°C (at a heating rate of 20°C/min). C₆₀H₃₆ shows an electronic absorption spectrum with a maximum at about 217 nm and it is able to match both in position and in half width the peak at 217.5 nm observed in the spectrum of the interstellar extinction of light which was attributed to hydrogenated, radiation processed and thermally annealed carbon dust. Similarly, the absorption spectrum of C₆₀H₃₆ is able to match several infrared emission bands (called UIBs) detected from certain astrophysical objects like the protoplanetary nebulae (PPNe). It is proposed that hydrogenated fullerenes can be used as model compounds in the laboratory simulation studies of interstellar carbon dust.     

Electronic structure and electrical transport characteristics of C60, 2C60 and 4C60 fullerene molecules

The extended Hückel method and the Green’s function method were used to calculate the electronic structure and electrical transport of Au electrode-C₆₀, 2C₆₀ or 4C₆₀ fullerene-Au electrode systems. Furthermore, their electronic structure and electrical transport characteristics were compared and analyzed. The results show that (i) owing to the contact with the Au electrodes, the C₆₀, 2C₆₀ and 4C₆₀ molecules change in their electronic structures significantly, and their energy gaps between LUMO and HOMO are narrow; (ii) the bonding between C₆₀, 2C₆₀ or 4C₆₀ fullerene and Au electrodes is partially covalent and partially electrovalent; and (iii) the conductance of the three fullerenes conforms to the order of C₆₀ > 2C₆₀ > 4C₆₀. Translated from Journal of Materials Science and Engineering, 2006, 24(1): 53–56 [译自: 材料科学与工程学报]     

Inelastic neutron scattering results on pure and doped fullerenes

Via inelastic neutron scattering (INS) generalized phonon densities of states are obtained for C₆₀ and superconducting A₃C₆₀ (A=K, Rb). Changes in the intramolecular modes agree with and augment recent Raman (RA), infrared (IR), and INS measurements, i.e., in the vicinity of the high-frequency modes of C₆₀ [H _g (8),A _g (2),H _g (7)] we find a pronounced shift of about 10 meV toward smaller energy transfers upon intercalating with potassium. Separated from these vibrations by a gap are the intermolecular spectra which for the doped samples display well-defined optic modes at 11 and 14 meV due to the vibrations of Rb and K ions. Indications of a weak anomalous temperature dependence of low-energy phonons ( <3 meV) nearT _c are observed for K₃C₆₀.