Lothar Dunsch

Diameter grouping in bulk samples of single-walled carbon nanotubes from optical absorption spectroscopy

The influence of the synthesis parameters on the mean characteristics of single-wall carbon nanotubes in soot produced by the laser vaporization of graphite has been analyzed using optical absorption spectroscopy. The abundance and mean diameter of the nanotubes were found to be most influenced by the furnace temperature and the cobalt/nickel catalyst mixing ratio. Via an analysis of the fine structure in the optical spectra, the existence of preferred nanotube diameters has been established and their related fractional abundance could be determined. The results are consistent with nanotubes located mainly around the armchair axis.

Metal Sulfide in a C82 Fullerene Cage: A New Form of Endohedral Clusterfullerenes

The row of endohedral fullerenes is extended by a new type of sulfur-containing clusterfullerenes: the metal sulfide (M(2)S) has been stabilized within a fullerene cage for the first time. The new sulfur-containing clusterfullerenes M(2)S@C(82)-C(3v)(8) have been isolated for a variety of metals (M = Sc, Y, Dy, and Lu). The UV-vis-NIR, electrochemical, and FTIR spectroscopic characterization and extended DFT calculations point to a close similarity of the M(2)S@C(82) cage isomeric and electronic structure to that of the carbide clusterfullerenes M(2)C(2)@C(2n). The bonding in M(2)S@C(82) is studied in detail by molecular orbital analysis as well as with the use of quantum theory of atom-in-molecules (QTAIM) and electron localization function (ELF) approaches. The metal sulfide cluster formally transfers four electrons to the carbon cage, and metal-sulfur and metal-carbon cage bonds with a high degree of covalency are formed. Molecular dynamics simulations show that Sc(2)S cluster exhibits an almost free rotation around the C(3) axis of the carbon cage, resulting thus in a single line (45)Sc NMR spectrum.

Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules

Metal-cage and intracluster bonding was studied in detail by quantum theory of atoms in molecules (QTAIM) for the four major classes of endohedral metallofullerenes (EMFs), including monometallofullerenes Ca@C(72), La@C(72), M@C(82) (M=Ca, Sc, Y, La), dimetallofullerenes Sc(2)@C(76), Y(2)@C(82), Y(2)@C(79)N, La(2)@C(78), La(2)@C(80), metal nitride clusterfullerenes Sc(3)N@C(2n) (2n=68, 70, 78, 80), Y(3)N@C(2n) (2n=78, 80, 82, 84, 86, 88), La(3)N@C(2n) (2n=88, 92, 96), metal carbide clusterfullerenes Sc(2)C(2)@C(68), Sc(2)C(2)@C(82), Sc(2)C(2)@C(84), Ti(2)C(2)@C(78), Y(2)C(2)@C(82), Sc(3)C(2)@C(80), as well as Sc(3)CH@C(80) and Sc(4)O(x)@C(80) (x=2, 3), that is, 42 EMF molecules and ions in total. Analysis of the delocalization indices and bond critical point (BCP) indicators such as G(bcp)/rho(bcp), H(bcp)/rho(bcp), and |V(bcp)|/G(bcp), revealed that all types of bonding in EMFs exhibit a high degree of covalency, and the ionic model is reasonable only for the Ca-based EMFs. Metal-metal bonds with negative values of the electron-density Laplacian were found in Y(2)@C(82), Y(2)@C(79)N, Sc(4)O(2)@C(80), and anionic forms of La(2)@C(80). A delocalized nature of the metal-cage bonding results in a topological instability of the electron density in EMFs with an unpredictable number of metal-cage bond paths and large elipticity values.

Endohedral clusterfullerenes--playing with cluster and cage sizes.

The family of endohedral fullerenes was significantly enlarged within the past six years by the clusterfullerenes containing structures like the M(2)C(2) carbides and the M(3)N nitrides. While the carbide clusters are generated under the standard arc burning conditions according to the stabilisation energy the nitride clusterfullerene type is formed by varying the composition of the cooling gas atmosphere in the arc burning process. The special situation in nitride clusterfullerene synthesis is described in detail and the optimum conditions for the production of nitride clusterfullerenes as the main product in fullerene synthesis are discussed. A review of new nitride clusterfullerenes reported recently is given summarizing the structures, properties and the stability of metal nitride clusterfullerenes. It is shown that all cages with even carbon atoms of C(68) and beyond are available as endohedral nitride clusterstructures. Furthermore the nitride clusterfullerenes are that class of endohedral fullerenes forming the largest number of non-IPR structures. Finally the prospects of this evolving field are briefly discussed taking the superior stability of these endohedral clusterfullerenes into account.

An endohedral single-molecule magnet with long relaxation times: DySc2N@C80.

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.

Structure and stability of endohedral fullerene Sc3N@C80: A Raman, infrared, and theoretical analysis

Structure and stability of endohedral fullerene Sc3N@C80 were studied by temperature-dependent Raman and infrared spectroscopy as well as by quantum-chemical [density-functional-based tight-binding] calculations. The material showed a remarkable thermal stability up to 650 K. By both theory and experiment, translational and rotational Sc3N modes were found. These modes give a direct evidence for the formation of a Sc3N–C80 bond which induces a significant reduction of the ideal Ih–C80 symmetry. From their splitting pattern a crystal structure with more than one molecule in the unit cell is proposed. According to our results: (i) a significant charge transfer from the Sc3N cluster to the C80 cage; (ii) the strength of three Sc–N bonds; (iii) the chemical bond between triscandium nitride cluster and C80 cage; and (iv) a large HOMO–LUMO gap are responsible for the high stability and abundance of Sc3N@C80.

In situ Raman and Vis-NIR spectroelectrochemistry at single-walled carbon nanotubes

Abstract The population of valence-band electronic states of single-walled carbon nanotubes (SWCNT) was tuned electrochemically in aqueous electrolyte solution. Depending on the applied potential, reversible changes of intensity and frequency of the Raman-active tangential mode and radial breathing mode were observed. The maximum intensity occurs at ca. −0.2 V vs. Ag/AgCl independent of the electrode material used as a support (Pt, Au, Hg). SWCNT are sensitive to photoanodic breakdown upon prolonged exposure to anodic bias and laser light. Reversible bleaching of the first band-gap transition in semiconducting tubes (at 0.7 eV) occurs after anodic polarization.

Initial states in the electropolymerization of aniline and p-aminodiphenylamine as studied by in situ FT-IR and UV-Vis spectroelectrochemistry

The study on the very first stages of electrochemical polymerization of aniline and p-aminodiphenylamine (p-ADPA) on electrodes by in situ FT-IR/ATR and UV-Vis transmission techniques is presented. The in situ FT-IR/ATR measurements indicate the highest polymer growth rate during the reversed cathodic potential scan in potentiodynamic electropolymerization. In this scan direction the p-ADPA radical is formed in a less anodic potential region by symproportionation of the soluble oxidized dimer N-phenyl-quinonediimine with p-ADPA formed by re-reduction. The resulting radical formed by symproportionation causes the polymer growth. The radical cations form the tetrameric Willstatter blue and red imine by dimerization and further oxidation. Therefore, the main step in polymer deposition is the radical reaction of p-ADPA. By further reactions of the first oligomers, insoluble higher oligomers are formed and deposited on the electrode. The preferred occurrence of these reactions is the reason for the more intense polymer growth by cycling of the potential in comparison to potentiostatic methods. Comparative studies of the p-ADPA polymerization were done under the same conditions. The comparison of FT-IR vibration modes of PANI and polymerized p-ADPA shows significant structural differences of both polymers. By FT-IR and UV-Vis spectroscopy it is shown that the main structure of polymerized p-ADPA is the aniline tetramer Willstatter blue/red imine.

Hindered cluster rotation and 45Sc hyperfine splitting constant in distonoid anion radical Sc3N@C80-, and spatial spin-charge separation as a general principle for anions of endohedral fullerenes with metal-localized lowest unoccupied molecular orbitals.

DFT calculations of Sc(3)N@C(80) in the neutral and anionic states are performed which revealed that in the neutral state of the nitride clusterfullerene the lowest energy structure has C(3) symmetry, while in the anionic and dianionic states the C(3v) conformer has the lowest energy. Barriers to the cluster rotation inside the cage are also found to increase in the charge states. The (45)Sc hyperfine slitting constant, a(Sc), in Sc(3)N@C(80) anion radical is calculated by different theoretical approaches and in different conformations of Sc(3)N cluster. It is found that a(Sc) is strongly dependent on the cluster orientation with respect to the cage, covering a range form -10 to +25 Gauss at the B3LYP/6-311G*//PBE/TZ2P level of theory. A thorough analysis of the computed values as well as comparison of unrestricted and orbital-restricted calculations revealed that the polarization contribution to a(Sc) is about -10 Gauss and does not depend on the cluster orientation. Dependence of the predicted a(Sc) values on the density functional form (LSDA, BP, PBE, BLYP, OLYP, TPSS, B3LYP, and TPSSh), the basis set, as well as on the scalar-relativistic and spin-orbit corrections were investigated. The analysis of the charge distribution in the Sc(3)N@C(80)(-) anion radical revealed an interesting peculiarity of its electronic structure: while the spin density mostly resides on the cluster, only a slight decrease of its charge is found using both Bader and Mulliken definitions of atomic charges. A set of other endohedral metallofullerenes, including nitride clusterfullerenes Sc(3)N@C(2n) (2n = 68, 70, 78) and Y(3)N@C(2n) (2n = 78-88), carbide clusterfullerenes Sc(2)C(2)@C(68), Sc(2)C(2)@C(82), Sc(3)C(2)@C(80), Ti(2)C(2)@C(78), Y(2)C(2)@C(82), and dimetallofullerenes Sc(2)@C(76), Y(2)@C(82), La(2)@C(2n) (2n = 72, 78, 80), was also studied in the neutral and anionic state, and a spatial charge-spin separation is found to be a general rule for all endohedral fullerenes with high contribution of metal atoms to the LUMO.

In situ uv-vis esr spectroelectrochemistry

Simultaneous in situ measurements by both ESR and UV-vis spectroscopy have been carried out at the same electrode for the first time during a single electrochemical experiment in order to spectroscopically characterize the electrochemical reaction products. The experimental technique, including a special TE102 optical ESR cavity and an electrochemical cell for both ESR and UV-vis spectroscopy in transmission, is described. It is shown that the UV-vis absorbance measured in situ, as well as the ESR intensity characteristics of the electrochemical system under study, can be understood in terms of the faradaic current. The reliability of this system is proved by measuring the organic redox couple of methyl-substituted p-phenylenediamine and by comparing calculated and experimental curves.