Cluster <Emphasis Type="Italic">ab initio</Emphasis> calculations for the C<Subscript>60</Subscript>F<Subscript>24</Subscript>, C<Subscript>60</Subscript>Cl<Subscript>24</Subscript>, and C<Subscript>60</Subscript>Br<Subscript>24</Subscript> halofullerenes

This paper reports on ab initio cluster calculations of the equilibrium geometry, electronic structure, and vibrational properties of the C₆₀, C₆₀F₂₄, C₆₀Cl₂₄, C₆₀Br₂₄ molecules.     

Synthesis,crystal structure,Hirshfeld surface analysis,and vibrational and DFT investigation of [C<Subscript>6</Subscript>H<Subscript>10</Subscript>(NH<Subscript>3</Subscript>)<Subscript>2</Subscript>]<Subscript>3</Subscript>CuBr<Subscript>4</Subscript>.3Br

Single crystal of a new organic–inorganic hybrid material [C₆H₁₀(NH₃)₂]₃CuBr₄.3Br was synthesized by the slow evaporation method at room temperature and characterized by X-ray diffraction, FTIR, Raman spectroscopy, UV–Vis, dielectric measurements, and Hirschfield surface analysis. The title compound crystallizes in trigonal system \( P\overline{3} \).The crystal packing is governed by the N-H…Br and non-classical C-H…Br hydrogen-bonding interactions between the 1, 2-diamoniumcyclohexane cations, the tetrahedral [CuBr₄]^3? anions, and the isolated ion Br^?. Theoretical calculations were performed using density functional theory (DFT) for studying the molecular structure, vibrational spectra, and optical properties of the investigated molecule in the ground state. The optimized geometrical parameters obtained by DFT calculations are in good agreement with single crystal XRD data. The optical properties were investigated by optical absorption and show two bands at 260 and 305 nm.     

Photophysics of C<Subscript>60</Subscript> and C<Subscript><Emphasis Type="Bold">60</Emphasis></Subscript><Superscript><Emphasis Type="Bold">-</Emphasis></Superscript> in faujasite zeolites

While the intercalation of C₆₀ and the formation of C ₆₀ ^- in the supercages of NaX and NaY are confirmed by using ¹²⁹Xe NMR and ESR, the photophysical properties of C₆₀ and C ₆₀ ^- are characterized by monitoring transient reflectance spectra and kinetics, fluorescence kinetics, and diffuse reflectance spectra. C ₆₀ ^- is considerably more abundant in NaY than in NaX. This difference is explained in terms of polarity difference between two zeolites. Both C₆₀ and C ₆₀ ^- have remarkably elongated excited-state lifetimes due to their collision-free environment in zeolitic nanocavities although C ₆₀ ^- has much shorter lifetimes than C₆₀. C ₆₀ ^- , in particular, shows intense absorption and emission due to its reduced symmetry in zeolites. Received 13 July 2001 and Received in final form 8 October 2001     

Photopolymerization of C<Subscript>60</Subscript> and Li@C<Subscript>60</Subscript> studied by second-harmonic generation and infrared spectroscopy

The photopolymerization of C₆₀ and Li@C₆₀ films was investigated by means of optical second-harmonic generation. The films were deposited under ultra-high-vacuum conditions and irradiated in situ with an Ar⁺ laser at 514 nm. The second harmonic generated by a Nd:YAG laser working at 1064 nm was monitored after different steps of irradiation. Photopolymerization was observed after very low irradiation doses, of the order of 10²⁰ photons/cm², and confirmed with infrared absorption spectroscopy. Similar kinetics for C₆₀ and Li@C₆₀ were observed. The measurements give evidence for photopolymerization of the endohedral fullerene Li@C₆₀. PACS 78.30.Na; 82.50.Hp; 81.05.Tp     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

X-ray emission and X-ray photoelectron spectroscopic studies of fullerene fluoride C<Subscript>60</Subscript>F<Subscript>24</Subscript>

The structure of the fullerence fluoride C₆₀F₂₄ of the T _h symmetry contains two types of chemically different carbon atoms, namely, atoms of isolated double bonds and atoms of CF groups. X-ray photoelectron and x-ray emission spectroscopic studies of C₆₀F₂₄ revealed a difference in the widths of the x-ray bands corresponding to these types of atoms. Nonempirical quantum-chemical calculations performed for C₅₉NF ₂₄ ⁺ ions with a hole in the C 1s core level of the fullerence fluoride showed that the difference in the bandwidths may be due to the fact that the vibrational states of the system are different when 1s electrons are removed from chemically nonequivalent atoms.     

Spin reorientation effects in GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> induced by applied field and temperature

Magnetic properties of GdFe₃(BO₃)₄ single crystals were investigated by ⁵⁷Fe-Mössbauer spectroscopy and static magnetic measurements. In the ground state, the GdFe₃(BO₃)₄ crystal is an easy-axis compensated antiferromagnet, but the easy axis of iron moments does not coincide with the crystal C₃ axis, deviating from it by about 20°. The spontaneous and field-induced spin reorientation effects were observed and studied in detail. The specific directions of iron magnetic moments were determined for different temperatures and applied fields. Large values of the angle between the Fe³⁺ magnetic moments and the C₃ axis in the easy-axis phase and between Fe³⁺ moments and the a₂ axis in the easy-plane phase reveal the tilted antiferromagnetic structure.     

Geometry of the internal dynamics of C<Subscript>2</Subscript>H<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> carbocation

Using the symmetry group chain methods, the internal dynamics of the simplest carbocation, C₂H ₃ ⁺ , is analyzed under the traditional assumptions that the equilibrium structures of the carbocation are planar and that the nonrigid motion between them is in-plane. This geometry of the internal dynamics is shown to agree with the data of the microwave spectroscopy on the splittings of rotational energy levels caused by the nonrigid motion. Previously, this statement was based on the model that violated the requirement of self-adjointness of operators of physical quantities.     

Excitons and energy transfer in KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> and RbPb<Subscript>2</Subscript>Br<Subscript>5</Subscript> laser crystals

Excitonic states, radiative relaxation of electronic excitations, and energy transfer to luminescence centers in both undoped and rare-earth activated (Pr, Er, Nd, Ho, Tb, Tm) KPb₂Cl₅ and RbPb₂Br₅ crystals were studied using low-temperature (8 K) time-resolved VUV spectroscopy under selective photoexcitation by synchrotron radiation.     

Probing the isomer,fluorination and bond effects in C<Subscript>3</Subscript>H<Subscript>6</Subscript>, cyclo-C<Subscript>3</Subscript>H<Subscript>6</Subscript> and C<Subscript>3</Subscript>F<Subscript>6</Subscript> molecules using electron impact

We have carried out experimental and theoretical studies on electron scattering from the C₃H₆ isomers and C₃F₆ molecules and we report on total, differential as well as theoretical integral elastic cross-sections for these molecules. Vibrational excitation functions are also presented for the typical vibrational peaks in C₃H₆ and cyclo-C₃H₆ for the angle of 90^○, impact energy range of 1–16 eV and loss energies of 0.12 eV and 0.13 eV, respectively. In the cross-sections, clear differences in peak positions and magnitudes between the C₃H₆ isomers can be viewed as the isomer effect. The same is observed between C₃H₆ and C₃F₆ in a clear manifestation of the fluorination effect. The resemblance of the π^* shape resonance in the cross-sections, observed at about 2.2 eV for C₃H₆ and 3.5 eV for C₃F₆, to those in C₂H₄ and C₂F₄ clearly points to the effect of the double bond in the molecular structures for these molecules. Theoretical analysis is performed to provide rationales for the scattering dynamics.     

II. Electron-impact dissociative excitation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact dissociative ionization of C₂ H₂⁺ and C₂ D₂⁺ to CH⁺, C⁺, C₂⁺ , H⁺, CH₂⁺ and C₂D⁺ fragments are determined for electron energies ranging from the corresponding threshold to 2.5 keV. Results obtained in a crossed beams experiment are analyzed to estimate the contribution of dissociative ionization to each fragment formation. The dissociative ionization cross sections are seen to decrease for more than an order of magnitude, from CH⁺ (5.37±0.10) × 10^-17 cm² over C⁺ (4.19± 0.16) × 10^-17 cm², C₂D⁺ (3.94±0.38) × 10^-17 cm², C₂⁺ (3.82±0.15) × 10^-17 cm² and H⁺ (3.37±0.21) × 10^-17 cm² to CH₂⁺ (2.66±0.14) × 10^-18 cm². Kinetic energy release distributions of fragment ions are also determined from the analysis of the product velocity distribution. Cross section values, threshold energies and kinetic energies are compared with the data available from the literature. Conforming to the scheme used in the study of the dissociative excitation of C₂H₂⁺ ( C₂ D₂⁺ )\left( {\rm C}_2 {\rm D}_2^+ \right), the cross-sections are presented in a format suitable for their implementation in plasma simulation codes.     

Thermodynamic properties of C<Subscript>60</Subscript> polyfullerites

This paper discusses the results of calorimetric studies of the 1D C₆₀ (orthorhombic) and 2D C₆₀ (tetragonal and rhombohedral) fullerites, as well as of the graphite-like polyfullerite, which are produced from a starting C₆₀ fullerite subjected to a pressure of 1–8 GPa at temperatures ranging from 300 to 1270 K. The analysis is made primarily of the C _p ⁰ heat capacity measurements performed in adiabatic calorimeters in the 5-to 350-K range.     

CaCuMn<Subscript>6</Subscript>O<Subscript>12</Subscript> vs. CaCu<Subscript>2</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>: A comparative study

The ferrimagnetic compounds Ca(Cu_xMn_3?x)Mn₄O₁₂ of the double distorted perovskites AC₃B₄O₁₂ family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn³⁺)_C for (Cu²⁺)_C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(Cu_xMn_3?x)Mn₄O₁₂ is driven by strong antiferromagnetic exchange interaction of (Cu²⁺)_C with (Mn³⁺/Mn⁴⁺)_B coordinated octahedra. The competing interactions of (Mn³⁺)_C with (Mn³⁺/Mn⁴⁺)_B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.     

Three types of photomagnetic effects in Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> crystals

Three different photomagnetic effects caused by ultraviolet light in paramagnetic crystals based on molecules of spiropyrans (Sp) Sp₃Cr(C₂O₄)₃ and SpI have been revealed and separated: (1) in the high-temperature range (30–300 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is determined by the charge transfer between chromium ions and spiropyran molecules; (2) in the low-temperature range (2 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is due to the photoisomerization of spiropyran molecules, the change in the crystal field, and the splitting of the levels of Cr³⁺ ions in zero field; and (3) in the temperature range 2–20 K, the generation of radiation-induced paramagnetic defects contributes to the magnetic moment of the organic sublattice Sp⁺.     

On the interaction of self-assembled C<Subscript>60</Subscript>F<Subscript>18</Subscript> polar molecules with the Ni(100) surface

The current work is dedicated to investigation of the interaction between self-assembled polar molecules of fullerene fluoride C₆₀F₁₈ with the chemically active surface Ni(100) under radiation and heat treatments. X-ray photoelectron spectroscopy is used in combination with quantum-chemical simulation. For the first time, the transformation of an as-deposited dielectric continuous 2D thin film to a 3D island-type assembly with molecular ordering within the islands is shown to take place. The degree of coverage of the Ni surface by C₆₀F₁₈ islands (0.6–0.7) and their height (~6 nm) are estimated. Quantum-chemical simulation shows that the chemisorption energy of the C₆₀F₁₈ molecule on the Ni surface equals ~6.6 eV and fluorine atoms are located at a distance of 1.9 Å above the Ni surface. The results of the investigation provide an opportunity to create nanoscale ordered structures with local changes in the work function.     

The structure of fullerite C<Subscript>60</Subscript> intercalated with molecular oxygen

A (O₂) _x C₆₀ sample with a high content of oxygen (x ≥ 0.4) and free of technological solvent impurities was obtained by precipitation from solution. For the first time, the results of the determination of the x coefficients using ¹³C NMR and elemental analysis were compared. It was shown by Raman spectroscopy, mass spectrometry, and NMR that the inclusion of oxygen into fullerite was accompanied by a decrease in the frequency of O=O stretching vibrations by no less than 12 cm⁻¹ compared with gaseous O₂. Nevertheless, oxygen exists in the molecular form in (O₂)_0.4C₆₀ and is released in the form of O₂ as the sample is heated to 373 K. The number of oxygen molecules occupying octahedral pores closets to the fullerene molecule takes on all the possible values, from 0 to 6. At room temperature, the (O₂) _x C₆₀ sample lost oxygen much more slowly than similar products prepared by diffusion saturation of pure fullerite with oxygen.     

Kinetic C<Subscript>(2)-H</Subscript> exchange study of<Emphasis Type="Italic">Cis</Emphasis>-[(en)<Subscript>2</Subscript>Co(HIm)(enH)]Br<Subscript>4</Subscript> by NMR spectroscopy: Evidence for the acidity of C<Subscript>(2)-H</Subscript> proton

C_(2)-H exchange incis-[(en)₂Co(HIm)(enH)]Br₄ (Im, imidazole; en, ethylenediamine) was studied over a pD range of 9.4 to 10.2 at 60°C where thecis-[(en)₂Co(Him)(enH)]⁴⁺ andcis-[(en)₂Co(HIm)(en)]³⁺ are the species present in significant concentrations.Cis-[(en)₂Co(HIm)(enH)]⁴⁺ has pK_a1 7.86 and pK_a2 9.82 with pK_a1 corresponding to coordinated enH ionization and pK_a2 to ImH ionization. The kinetic data may be interpreted in terms of an intramolecular H-D exchange mechanism, where the “dangling”-ND₂CH₂CH₂ND₂ group acts as a base to remove the imidazole C_(2)-H proton from the coordinated DIm moiety.     

Ion-pair states of I<Subscript>2</Subscript>, Br<Subscript>2</Subscript>, IBr,and ICl

The experimentally determined energies and rotational constants of the vibrational levels v = 0–20 of the Ion-Pair states Ω = 0⁺, Ω = 1 of the I₂, Br₂, IBr, and ICl molecules are modeled. The model used includes three diabatic states, which correlate to X⁺(3P, 1D) + Y^～(1S₀). These states are coupled by the spin-orbit interaction, which is assumed to be independent of the internuclear distance. For IBr and ICl, as well as for the ungerade states of I₂ and Br₂, satisfactory results are obtained. The model is less applicable to the gerade states of I₂ and Br₂, which is possibly results from the retainment of the asymptotic J _A J _B coupling of the angular momenta at equilibrium internuclear distances.     

Magnetic and structural anomalies of Na<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>C<Subscript>60</Subscript> (n = 2, 3)

Sodium fullerides Na _n C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure of Na₂C₆₀ at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed in this work in Na₂C₆₀ fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure of Na₃C₆₀ is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic susceptibility with increasing temperature has been observed in the Na₃C₆₀ fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the ²³Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained by a metal-insulator transition caused by a structural transition.     

I. Electron-impact ionization and dissociation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact ionization and dissociation of C₂H₂⁺ and C₂D₂⁺ have been measured for electron energies ranging from the corresponding thresholds up to 2.5 keV. The animated crossed beams experiment has been used. Light as well as heavy fragment ions that are produced from the ionization and the dissociation of the target have been detected for the first time. The maximum of the cross-section for single ionization is found to be (5.56 ± 0.03)× 10^-17 cm² around 140 eV. Cross-sections for dissociation of C₂ H₂⁺ (C₂D₂⁺) to ionic products are seen to decrease for two orders of magnitude, from C₂D⁺ (12.6 ± 0.3) × 10^-17 cm² over CH⁺(9.55 ± 0.06) × 10^-17 cm², C⁺ (6.66 ± 0.05) × 10^-17 cm², C₂⁺ (5.36 ± 0.27) × 10^-17 cm², H⁺ (4.73 ± 0.29) × 10^-17 cm² and CH₂⁺ (4.56 ± 0.27) × 10^-18 cm² to H₂⁺ (5.68 ± 0.49) × 10^-19 cm². Absolute cross-sections and threshold energies have been compared with the scarce data available in the literature.