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Electronic structure, vibrational stability, and predicted infrared-Raman spectra of the As 20 , As @ Ni 12 , and As @ Ni 12 @ As 20 clusters Structure and stability of endohedral fullerene Sc 3 N@C 80 were studied by temperature-dependent Raman and infrared spectroscopy as well as by quantum-chemical ͓density-functional-based tight-binding͔ calculations. The material showed a remarkable thermal stability up to 650 K. By both theory and experiment, translational and rotational Sc 3 N modes were found. These modes give a direct evidence for the formation of a Sc 3 N-C 80 bond which induces a significant reduction of the ideal I h -C 80 symmetry. From their splitting pattern a crystal structure with more than one molecule in the unit cell is proposed. According to our results: ͑i͒ a significant charge transfer from the Sc 3 N cluster to the C 80 cage; ͑ii͒ the strength of three Sc-N bonds; ͑iii͒ the chemical bond between triscandium nitride cluster and C 80 cage; and ͑iv͒ a large HOMO-LUMO gap are responsible for the high stability and abundance of Sc 3 N@C 80 .

The radial dependency of the vibrational frequencies of single-wall carbon nanotubes in the G band (1500-1600 cm(-1)) is studied by density functional theory. In metallic nanotubes, a mode with A1 symmetry is found to be significantly softer than the corresponding mode in insulating tubes or graphite. The mechanism that leads to the mode softening is explored. It is reminiscent of the driving force inducing Peierls distortions. At ambient temperature, the energy gained by opening the gap is, however, not sufficient for a static lattice distortion. Instead the corresponding vibrational frequency is lowered.

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Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with C60. Strong evidence is provided for a highly defect free and unperturbed environment in the interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to 0.35 cm(-1) which is almost 10 times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is 484 cm(-1) corresponding to a tube diameter of only 0.50 nm. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.

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The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly 13C enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the D line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.