O. Dubay

Mechanism of alkane dehydrogenation catalyzed by acidic zeolites: Ab initio transition path sampling

The dehydrogenation of propane over acidic chabazite has been studied using ab initio density-functional simulations in combination with static transition-state searches and dynamic transition path sampling (TPS) methods at elevated temperatures. The acidic zeolite has been modeled both using a small cluster and a large periodic model consisting of two unit cells, the TPS simulations allow to account for the effect of temperature and entropy. In agreement with experimental observations we find propene as the dominant reaction product and that the barrier for the dehydrogenation of a methyl group is higher than that for a methylene group. However, whereas all studies based on small cluster models (including the present one) conclude that the reaction proceeds via the formation of an alkoxy intermediate, our TPS studies based on a large periodic model lead to the conclusion that propene formation occurs via the formation of various forms of propyl cations stabilized by entropy, while the formation of an alkoxy species is a relatively rare event. It was observed only in 15% of the reactive trajectories for methyl dehydrogenation and even in only 8% of the methylene dehydrogenation reactions. Our studies demonstrate the importance of entropic effects and the need to account for the structure and flexibility of the zeolitic framework by using large periodic models.

Raman spectroscopy of template grown single wall carbon nanotubes in zeolite crystals

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts wi...

Raman Scattering And Electronic Properties Of The Cyclic Anthracene Tetramer (Picotube)

We present a study on the electronic and vibrational properties of the cyclic anthracene tetramer. FTRaman and Raman spectroscopy in the visible were used to investigate the vibrational properties of the molecule. Gaussian’98 and Gaussian’03 were employed to calculate the Raman response; the electronic properties were modeled by VASP, resulting in a gap energy of Eg = 2.24 eV. A strong correlation of the calculated modes to the measured Raman frequencies provides evidence for the D2d symmetry.

Quantized Rotational States in Endohedral Fullerenes

Results of Raman measurements on endohedral fullerene Sc2C2@C84 will be presented. We concentrated preferentially on the low‐energy part of the spectrum where signatures of the rotational state of the C2 molecule are expected. The positions of the Raman lines are consistent with those for the plane rotor. However, deviations appear in the very low‐energy part of the spectrum. The intensity of Raman lines anticipates both the plane and free rotor, depending on the temperature. This puzzling behaviour is resolved by introducing a barrier the molecule encounters during rotation. Consequently, the low‐lying rotational lines split and the line intensities redistribute in agreement with the experiment. The broadening of the line widths seen at higher temperatures likely points to thermally activated processes in Sc2C2@C84.

Raman Spectroscopy of SWNTs in Zeolite Crystals

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4‐5 crystals were studied by Raman spectroscopy and ab initio density functional calculation. It was found that only two types of nanotubes with different chiralities, (5,0) and (4,2), were responsible for the observed spectra. The frequencies of the radial breathing modes were reliably assigned. A strong response was observed for frequencies around 1250 cm−1. The laser excitation energy of 2.2 eV separates two regions with different line shapes for the G band.

Phonon softening in metallic nanotubes by a Peierls’ like mechanism

Density functional theory in the local approximation is used to study the radial dependency of the vibrational frequencies of single‐wall carbon nanotubes in the G band (1500–1600 cm−1) . In metallic nanotubes, the longitudinal A1 mode is found to be significantly softer than in insulating tubes or graphite. The mechanism that leads to the mode softening is reminiscent of the driving force inducing Peierls distortions, but the energy gained by opening the gap is not sufficient for a static lattice distortion. Instead, the corresponding vibrational frequency is lowered.

Density functional study of SWCN in channels of AlPO4 — 5 single crystals

Density functional theory (DFT) is used to study systems composed of the AlPO4‐5 zeolite (AFI) and carbon nanotubes monodispersed in the zeolite channel. The energetic stability of small freestanding isolated zigzag nanotubes as well as their stability in the AFI channel is investigated. The electronic and vibrational properties of the free (6,0) nanotube are compared with those of the tube in the AFI channel. The radial breathing mode of nanotubes in the AFI channel is analyzed in detail.

Accurate density functional calculations for the phonon dispersion relations of carbon nanotubes

Accurate calculations for the phonon dispersion relations of single wall armchair and zigzag nanotubes are presented. The calculations are performed using a plane wave basis set and density functional theory (DFT). The dependency of Raman active and infrared active modes on the radius is investigated in detail. To access the accuracy of the present calculations, the phonon dispersion relations of an isolated graphene sheet is calculated and the results are compared to experiment. As expected errors are small, but some notable discrepancies between experiment and theory are observed and discussed.