Theoretical studies on the BN substituted fullerenes C70−2x(BN)x (x=1–3)—isoelectronic equivalents of C70

The equilibrium structures and relative stabilities of BN-doped fullerenes C_70−2x(BN)_x (x=1–3) have been studied at the AM1 and MNDO level. The most stable isomers of C_70−2x(BN)_x have been found out and their electronic properties have been predicted. The calculation results show that the BN substituted fullerenes C_70−2x(BN)_x have considerable stabilities, though they are less stable than their all carbon analog. For C₆₈BN, the isomers whose BN is located in the most chemically active bonds of C₇₀ (namely B and A) are among the most stable species, of which B is predicted to be the ground state. The stabilities of C₆₈BN decrease and the dipole moments increase with increasing the distance between the heteroatoms. For C₆₆(BN)₂, the lowest energy species is the isomer in which the B–N–B–N bond is formed; For C₆₄(BN)₃, the most stable species should have three BN units located in the same hexagon to form B–N–B–N–B–N ring. The ionization potentials and the affinity energies of the most stable species of BN-doped C₇₀ are almost the same as those of C₇₀ because of the isoelectronic relationship. The ionization potentials and affinity energies depend on the relative position of the heteroatoms in C₆₈BN, the chemical reactivities of the isomers whose heteroatoms are well separated should differ significantly from their all carbon analog.     

Semiempirical molecular dynamics studies of C60/C70 fullerene oxides: C60O, C60O2 and C70O

The spectral analysis indicates that all isomers of C₆₀O, C₇₀O and C₆₀O₂ have an epoxide-like structure (an oxygen atom bridging across a C–C bond). According to the geometrical structure analysis, there are two isomers of fullerene monoxide C₆₀O (the 5,6 bond and the 6,6 bond), eight isomers of fullerene monoxide C₇₀O and eight isomers of fullerene dioxide C₆₀O₂. In order to simulate the real reaction conditions at 300 K, the calculation of the different isomers of C₆₀O, C₆₀O₂ and C₇₀O fullerene oxides was carried out using the semiempirical molecular dynamics method with two different approaches: (a) consideration of the geometries and thermodynamic stabilities, and (b) consideration of the ozonolysis mechanism. According to the semiempirical molecular dynamic calculation analysis, the probable product of this ozonolysis reaction is C₆₀O with oxygen bridging over the 6–6 bond (C_2v). The most probable product in this reaction contains oxygen bridging across in the upper part of C₇₀ (6–6 bond in C₇₀O-2 or C₇₀O-4) an epoxide-like structure. C₆₀O₂-1, C₆₀O₂-3 and C₆₀O₂-5 are the most probable products for the fullerene dioxides. All of these reaction products are consistent with the experimental results. It is confirmed that the calculation results with the semiempirical molecular dynamics method are close to the experimental work. The semiempirical molecular dynamics method can offer both the reaction temperature effect by molecular dynamics and electronic structure, dipole moment by quantum chemistry calculation.     

Surface-enhanced vibrational Raman spectroscopy of C60 and C70 on rough silver surfaces

The observation of the surface-enhanced vibrational Raman spectra of vapor-deposited C₆₀ and C₇₀ on rough silver films is reported. Both near-monolayer and multilayer films of pure C₆₀ and of C₆₀/C₇₀ mixtures are studied. The films are obtained by evaporating fullerene samples at temperatures of 683–875 K in ultra-high vacuum. Mixed fullerene samples were greatly enriched in C₇₀ by making use of the slightly different vapor pressures of the two major components at the low end of this temperature range. The spectra contain all the lines of the normal Raman spectra as well as several additional lines caused by a reduction in the stringency of the normal Raman selection rules. These results demonstrate the potential of this technique for detecting small quantities of fullerenes and obtaining their vibrational spectra.     

Co-aggregation of fullerene C60 and thiophene in the non-aromatic solvent cyclohexene

Co-aggregation of fullerene C₆₀ and thiophene has been studied calorimetrically in cyclohexene at 25 °C. The total aggregation heat is found to depend on initial concentration of thiophene and fall between −1.9 and −5.8 kJ mol⁻¹. The corresponding thiophene/fullerene molar ratio (“co-aggregation number”) ranges from 7 to 12. The data are rationalized by formation of heteromolecular nanoaggregates with intermolecular contacts of both fullerene–thiophene and fullerene–fullerene types. A physical model describing interaction between fullerenes and π-donors in solution is substantiated and used to explain heterogeneity of composites containing fullerenes.     

Temperature dependence of Kováts indices in gas chromatography revisited

The performance of Fourier transform infrared spectroscopy (FT-IR) detection coupled to high-performance liquid chromatography for the analysis of C₆₀ and C₇₀ fullerenes was investigated. The isocratic separation method involved an octadecylsilane (ODS) column and an acetonitrile–toluene (1:1) mobile phase. The hyphenated system was designed with a split valve to control eluent volume leading to the FT-IR detector; this allowed for additional coupling of the liquid chromatograph to ultraviolet–visible detection. On-line FT-IR spectra of C₆₀ and C₇₀ were matched with standard off-line FT-IR spectra from the literature. In addition, with band chromatograms individual fullerenes can be identified using FT-IR active modes known specifically for each fullerene. Few changes to a pre-existing HPLC–UV method were necessary for the HPLC–FT-IR method, and there was no need for fraction collection to identify the fullerenes C₆₀ and C₇₀.     

Effect of solvent polarity on the aggregation of fullerenes: a comparison between C60 and C70

Effect of solvent polarity on the aggregation behaviour of C₇₀ has been investigated in several mixed solvents using optical absorption, fluorescence, dynamic light scattering and scanning electron microscopic measurements and compared with those observed for the other fullerene analogue, C₆₀. It is seen that similar to C₆₀, aggregation of C₇₀ also requires the solvent polarity to exceed some critical value. In terms of solvent dielectric constant the critical solvent polarity, required for C₇₀ aggregation is found to be in the range of 27–31, which is much higher than that required for C₆₀ aggregation (12–14). The large difference in the critical solvent polarity required for C₆₀ and C₇₀ aggregation has been rationalized on the basis of the molecular shapes and the polarizabilities of two fullerene molecules.     

Yield of fullerenes generated by contact arc method under He and Ar: dependence on gas pressure

The yields of toluene-soluble material from carbon soot depend on the buffer gas as well as the pressure. Helium was more effective for yielding fullerenes than argon, and the optimum pressure was 20 Torr, under which a maximum yield of about 13 wt% was obtained. C₆₀ showed a maximum abundance at 20 Torr, while C₇₀ and higher fullerenes (C₇₆, C₇₈ and C₈₄) at a slightly higher pressure between 20 and 50 Torr. Raw soot was also studied by electron microscopy. The pressure dependence of the fullerene yield is discussed in terms of the cooling rate and diffusion of carbon vapor around the evaporation source.     

Electronic structures of dihedral (D5h, D5d, D6h, D6d) fullerenes

By means of our proposed method for Hückel theory calculation, we have calculated the electronic structures of dihedral (D_5h, D_5d, D_6h, D_6d) fullerences, which are generated from icosahedral C₂₀, C₆₀, C₈₀, C₁₈₀, C₂₄₀ and C₅₄₀, respectively. From the calculated results of 1224 fullerence molecules, certain rules on the stability and chemical reactivity have been drawn for such types of dihedral fullerenes.     

Donor–acceptor interaction of fullerene C60 with triptycene in molecular complex TPC·C60

A molecular complex of fullerene C₆₀ with triptycene, TPC·C₆₀ is obtained. The complex has a three-dimensional packing of C₆₀ molecules. According to the IR spectra, the freezing of free rotation of C₆₀ molecules in the complex is maintained up to 360 K. The XP-spectra of TPC·C₆₀ show the suppression of π–π^* transitions of TPC phenylene rings. The separation of C₆₀ molecules by TPC ones in TPC·C₆₀ results in low intensity of the C₆₀ transitions in the 420–500 nm range in an optical spectrum. This absorption is assumed as that attributed to intermolecular transitions between adjacent C₆₀ molecules.     

基于曲率和电子结构的掺杂C50和C70富勒烯的稳定性研究

使用密度泛函理论(DFT)-B3LYP/6-31G^*方法研究了B、N、Si、P和Co在C₅₀和C₇₀中的掺杂能和电子结构, 并基于曲率理论和电子结构探讨了掺杂富勒烯的结构稳定性. 计算结果表明, 掺杂能随着原子曲率的增大而减小, 随着掺杂物种原子半径的增大而增大, B、N、P和Co的掺杂有利于C₅₀结构的稳定, 而B和N的掺杂不利于C₇₀结构的稳定; 除了用于反映原子活性的曲率主要决定掺杂反应性, 各不等价碳原子在C₅₀和C₇₀的最高占据分子轨道(HOMO)中所占成分对掺杂能的影响也很大, 且其成分越大越有利于掺杂. 此外, 掺杂原子得失电子情况与其电负性有关. 本工作将为富勒烯结构稳定性的研究提供理论依据.     

Giant motion of La atom inside C82 cage

An anomalous charge density distribution of La atom encapsulated in a C₈₂ cage has been revealed for La@C₈₂ by the maximum entropy method (MEM)/Rietveld analysis using synchrotron powder diffraction data. The obtained La atom charge density shows a feature almost like a bowl or a hemisphere, suggesting that the La atom has a giant motion (large amplitude motion) inside the C₈₂ cage at room temperature. From the obtained MEM charge density, the main results are (1) the cage structure of La@C₈₂ (I) has C_2V symmetry; (2) La atom locates at an off-centered position adjacent to a six-membered ring of the carbon cage; (3) the nearest La–C distance is 2.55(8) and (4) the amount of charge transfer from the La atom to the carbon cage is about 3.2 e, which corresponds to the nominal electronic structure, La³⁺@C₈₂³⁻.     

Peptide anchored Langmuir–Blodgett films of a fullerene amphiphile

A new amphiphilic derivative of fullerene C₆₀ bearing an oligoglycyl tail (C₆₀CHCOgly₂OEt, 2) formed stable Langmuir floating films at the air–water interface. This occurred when the molecular assembly was stabilized by anchoring the amphiphilic C₆₀'s to the aqueous subphase, via hydrogen bonding interactions between a dipeptide (Gly–L–Leu) dissolved in the water subphase, and the oligoglycyl chain. The compression (π−A) isotherm of the Langmuir floating film constructed in such a way showed no hysteresis, was steep, and evidenced that the monolayer collapsed at a surface pressure π65 mN m⁻¹, thus confirming that the film was tightly packed, extremely stable, and rigid. A limiting area per molecule of 89.1 Ų was extrapolated, in agreement with the calculated cross-section area of the C₆₀ fullerene. On the contrary, when the dipeptide was absent and pure water was used as the subphase, the π−A isotherm yielded a limiting area <55 Ų which indicated the formation of multiple layers; moreover it showed significant hysteresis, the film was fragile, and it collapsed at π≈50 mN m⁻¹. Once anchored by the dipeptide, the floating monolayer of 2 could be transferred onto hydrophobic quartz, glass and silicon substrates, by successive vertical dipping cycles, each cycle made up of two down-strokes and two up-strokes, to yield the Langmuir–Blodgett film. Up to 200 down- and up-strokes could be repeated reproducibly, a noteworthy result for non-covalently assembled LB films of fullerenes. The transfer ratio was 1.0, except for the second down-stroke of each cycle that gave a transfer ratio of zero, making the sequence of successful transfers: D, U, U, (cleaning and spreading), D, U, U, (cleaning and spreading), and so on (D=down-stroke, U=up-stroke). The total number of deposited layers was therefore 150. X-ray diffraction spectra were registered and exhibited a peak, which was fitted by a Montecarlo method of simulation to obtain the distribution of the repeat unit responsible for scattering; such distribution, with thickness between 20 and 60 Å, was consistent with the size of the amphiphile and the transfer sequence. The UV–Vis spectra of the LB film exhibited the characteristic C₆₀ bands, and the absorption peaks in the 200–400 nm range were proportional to the number of layers, indicating that the deposition was reproducible and that the molecular environment of C₆₀ in each layer remained constant.     

Water clusters in a confined nonpolar environment

The stability and structure of water clusters in a confined nonpolar environment is investigated theoretically by examining the encapsulation of water molecules inside a fullerene (C₆₀) cage. While the Hartree–Fock (6-31G) calculations suggest H₂O@C₆₀ to be marginally more stable (−0.5 kcal/mol) than the isolated water and C₆₀ molecules, second order Møller–Plesset perturbation theory suggests it to be much more stable (−9.9 kcal/mol). It is shown that encapsulation results in the breaking of hydrogen bonds and rearrangement of water clusters. The tetramer inside the cage, for example, is tetrahedral in arrangement, in contrast to a square planar geometry observed in the gas phase.     

Molecular structure of 3-methylthiophene studied by gas electron diffraction combined with microwave spectroscopic data

The molecular structure of 3-methylthiophene

has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r_g(S---C₂) = 1.719(2) Å, r_g(C₂=C₃) = 1.370(3) Å, r_g(C₃---C₆) = 1.497(6) Å, r_g(C₂---H) = 1.101(5) Å, CSC = 91.6(2)°, SC₂C₃ = 113.3(5)°, SC₅C₄ = 111.3(3)°, C₂C₃C₆ = 123.2(11)° and C₃C₆H = 112(2)°. The values of r(S---C₂) − r(S---C₅) and r(C₂=C₃) − r(C₄=C₅) were fixed at the 3–21G* value of 0.002Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Use of cerium oxide (CeO(2)) as a packing material for the chromatographic separation of C(60) and C(70) fullerenes

Cerium oxide (CeO₂) was tested as a packing material in liquid chromatography for the separation of C₆₀ and C₇₀ fullerenes. The separation of C₆₀ and C₇₀ fullerenes could be achieved within 20 min by using pure n-hexane as the mobile phase. Furthermore, some higher fullerenes could also be separated in less than 40 min. The peak area was reproducible to a large extent. The separation of fullerenes by liquid chromatography on CeO₂ is shown to be an effective method for their isolation in large amounts. The column efficiency of the CeO₂ column was compared with commercial silica gel and ODS columns. The main advantage of the CeO₂ column is its ability to separate large amounts of fullerenes (C₆₀ and C₇₀) in toluene.     

Theoretical Studies on Structures,Stabilities, NMR Spectra and Designing Methods of Dihedral Fullerenes of C3 Series

Using bowl shaped carbon intermediates to construct dihedral fullerenes is an advisable method. Assu- ming that cap shaped C21 extends the size through building pentagons and hexagons at the U and V clefts of the brims, a series of homologous carbon intermediates was generated, in which most of the members have been unknown up to now. The joins between these homologous intermediates gave the C3 dihedral series under the restriction of C3 sym- metrical axis. The investigations point out that the stabilities of these fullerenes not only relate to the shapes of cages and the co-planarities of polygons, but also associate with the equalizations of bond lengths and the pentagonal dis- tributions. The stabilities reveal that the pentagonal distribution in cages is not negligible to the Jr delocalization, be- sides the co-planarities of hexagons and pentagons. Analyzing the possible Stone-Wales（SW） rearrangements in those fullerenes with dehydrogenated pyracyclene units（DPUs） can help us to find out the highly stable isomers. Based on the geometrical optimizations, the calculations provided the theoretical chemical shifts of unknown fullerenes and the data reconfirmed the existence of members C78 and C84. The symmetry adaptation analyses for the frontier orbitals support the formative mechanism of consecutive pentagonal and hexagonal fusions, but the simulated routes are more complicated than the pentagon road（PR） mechanism, which include not only C2 but also C3 additive reactions.     

内嵌金属碳氮化物团簇富勒烯的稳定性与生成机理

Fullerene molecules have nano-scale cavities in which various metal or metal clusters of different sizes can be embedded to form metallofullerenes with unique core-shell structures. The physical and chemical properties of metallofullerenes can be modified through the interaction between the encapsulated metals and the fullerene cages. As such, the investigation of metallofullerenes with novel structures has been a principal research focus in the field of fullerenes. In this study, we investigated the size matching effect between encapsulated clusters and fullerene cages for the endohedral metal carbonitride clusterfullerenes in order to discover new metallofullerenes. The stability and electronic structure of the metallofullerenes formed by encapsulating M₃NC clusters (M = Y, La, Gd) into D₂(186)-C₉₆ and D₂(35)-C₈₈ fullerenes were studied using quantum chemical calculations. It was found that the fullerene cages formed stable structures by accepting six electrons transferred from the encapsulated clusters. The change in configuration of the encapsulated clusters was clarified by a comparison with the corresponding M₃N@C_2n metal nitride clusterfullerenes; the size matching effect between M₃NC cluster and fullerene cage was elucidated on the basis of the calculated results and previous studies on Sc₃NC@I_h(7)-C₈₀. For the D₂(186)-C₉₆ fullerene, the Gd₃NC cluster was found to have smaller changes in the configuration as compared with the La₃NC cluster, proving that Gd₃NC is more suitable than La₃NC for encapsulation in the D₂(186)-C₉₆ fullerene cage. In addition, it was determined that the La₃NC cluster requires a large structural change to maintain its planar configuration. For the D₂(35)-C₈₈ fullerene cage, the Y₃NC cluster is more suitable than Gd₃NC for encapsulation owing to the smaller size of the Y₃NC cluster. The spatial distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of Gd₃NC@D₂(186)-C₉₆ were found to be similar to those of Gd₃N@D₂(186)-C₉₆. However, a unique endohedral cluster-based occupied molecular orbital was found for Gd₃NC@D₂(186)-C₉₆. This orbital is derived from the interaction between the NC unit and the Gd atoms. The spatial distribution of the HOMO of Y₃NC@D₂(35)-C₈₈ is similar to that of Y₃N@D₂(35)-C₈₈, while the LUMO of Y₃NC@D₂(35)-C₈₈ has a much larger contribution from the endohedral cluster as compared to Y₃N@D₂(35)-C₈₈. Thus, the addition of a carbon atom in the cluster has a remarkable impact on the electronic structure of the metallofullerenes. With respect to structural characteristics, we found that the three fullerene cages, D₂(186)-C₉₆, D₂(35)-C₈₈, and I_h(7)-C₈₀, have a uniform distribution of five-membered carbon atom rings; these fullerenes can be greatly stabilized in the form of C_2n^6- anions. However, the formation mechanism of fullerenes and metallofullerenes, at present, is poorly understood. Based on the structural analysis, we propose a direct mechanism for the formation of fullerenes without the Stone-Wales isomerization, i.e., the rearrangement of five-membered rings through the addition of carbon atoms and the transformation into larger carbon cages while maintaining stable structural units.     

Formation of [C60C5H5] adduct cations and theoretical study of the isomers

We study here the reactions between C₆₀ and planar C₅H₅⁺ cations that lead to the formation of [C₆₀C₅H₅]⁺ adduct cations in the chemical ionization source of the mass spectrometer. The structures, stabilities and charge locations of some possible isomers of [C₆₀C₅H₅]⁺: σ-adduct, π-complex, [1,4]- and [l,2]-addition cations, are studied by AM1 semiempirical molecular orbital calculations. We find that the most stable is the σ-addition cation. Another interesting and stable structure is the π-complex cation which is bonded by the electrostatic interaction at the inter-ring distance of 1.589 Å with the C_5v symmetry. The C₅H₅⁺ cyclopentadienium cation seems to be an “inverted umbrella” sitting on a five-membered ring of the C₆₀ cage.     

Dramatic persistence (minutes) of the triplet excited state and efficient singlet oxygen generation for C60 incorporated in Y zeolite and MCM-41 silicate

The triplet excited state of C₆₀ (λ_max=780 nm) lives minutes and can be monitored by conventional spectrophotometers when this fullerene is incorporated inside LiY, as opposed to C₆₀@HY and C₆₀@MCM-41 wherein C₆₀ triplet lives in the submillisecond time scale. C₆₀ adsorbed in LiY or MCM-41 efficiently generates ¹O₂ that was detected by its characteristic NIR emission (λ_em=1270 nm).     

The synthesis, structural features and thermal behaviour of thiophene-containing dithiocarboxylate complexes of zinc(II)

The complexes [Zn₂(S₂CTR)₄] (T = 2,5-disubstituted thiophene, R = C₄H₉ (1), C₆H₁₃ (2), C₈H₁₇ (3), C₁₂H₂₅ (4) and C₁₆H₃₃ (5)) have been synthesized and their structural features investigated. Compared to the analogous dithiobenzoate complexes, the crystal structure determination of 2 revealed that the thiophene induces a “step-rod” chain pattern instead of the linear, rodlike structure found for the corresponding dithiobenzoates. Complexes 1–5 did not display mesophases under thermal conditions, but an irregular melting pattern was observed for 3 and 4.