Raman spectrum of NaAlSi2O6 jadeite. A quantum mechanical simulation

The Raman spectrum of NaAlSi₂O₆ jadeite is simulated and compared with two recent experimental data sets. In one experiment, only 17 (out of 30 symmetry allowed) peaks and a qualitative estimate of the intensities are provided. In the second case, the digitalized spectrum is available, from which we have been able to extract 20 evident peaks and an estimate of the relative intensities. The present calculation is based on an ab initio quantum mechanical treatment. Using an all‐electron Gaussian‐type basis set, together with the hybrid B3LYP density functional, the full set of 30 active modes and their (polycrystalline and polarized) intensities are obtained. The simulated intensities (not available in a previous study of the same system) permit the two experimental spectra to be reconciled and explain why the missing peaks were not seen. This ultimately leads to excellent agreement between experiment and theory. By artificially varying the mass of the Na ⁺ and Al^{3 +} cations in the simulations, which can be performed automatically and at essentially no computational cost, the vibrational modes to which these ions contribute are identified. We conclude that quantum mechanical simulation can be a very useful complementary tool for the interpretation of experimental Raman spectra. Copyright © 2014 John Wiley & Sons, Ltd.     

Reversible formation of a PdC(x) phase in Pd nanoparticles upon CO and O2 exposure

The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O(2) at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O(2) mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules.     

Layered Na(0.71)CoO(2): a powerful candidate for viable and high performance Na-batteries

The present study reports on the synthesis and the electrochemical behavior of Na(0.71)CoO(2), a promising candidate as cathode for Na-based batteries. The material was obtained in two different morphologies by a double-step route, which is cheap and easy to scale up: the hydrothermal synthesis to produce Co(3)O(4) with tailored and nanometric morphology, followed by the solid-state reaction with NaOH, or alternatively with Na(2)CO(3), to promote Na intercalation. Both products are highly crystalline and have the P2-Na(0.71)CoO(2) crystal phase, but differ in the respective morphologies. The material obtained from Na(2)CO(3) have a narrow particle length (edge to edge) distribution and 2D platelet morphology, while those from NaOH exhibit large microcrystals, irregular in shape, with broad particle length distribution and undefined exposed surfaces. Electrochemical analysis shows the good performances of these materials as a positive electrode for Na-ion half cells. In particular, Na(0.71)CoO(2) thin microplatelets exhibit the best behavior with stable discharge specific capacities of 120 and 80 mAh g(-1) at 5 and 40 mA g(-1), respectively, in the range 2.0-3.9 V vs. Na(+)/Na. These outstanding properties make this material a promising candidate to construct viable and high-performance Na-based batteries.     

Self-assembly of a catechol-based macrocycle at the liquid-solid interface: experiments and molecular dynamics simulations

This combined experimental (STM, XPS) and molecular dynamics simulation study highlights the complex and subtle interplay of solvent effects and surface interactions on the 2-D self-assembly pattern of a Schiff-base macrocycle containing catechol moieties at the liquid-solid interface. STM imaging reveals a hexagonal ordering of the macrocycles at the n-tetradecane/Au(111) interface, compatible with a desorption of the lateral chains of the macrocycle. Interestingly, all the triangular-shaped macrocycles are oriented in the same direction, avoiding a close-packed structure. XPS experiments indicate the presence of a strong macrocycle-surface interaction. Also, MD simulations reveal substantial solvent effects. In particular, we find that co-adsorption of solvent molecules with the macrocycles induces desorption of lateral chains, and the solvent molecules act as spacers stabilizing the open self-assembly pattern.     

NIR emitting ytterbium chelates for colourless luminescent solar concentrators

A new oxyiminopyrazole-based ytterbium chelate enables NIR emission upon UV excitation in colorless single layer luminescent solar concentrators for building integrated photovoltaics.     

Does adsorption at hydroxyapatite surfaces induce peptide folding? Insights from large-scale B3LYP calculations

Large-scale periodic quantum mechanical calculations (509 atoms, 7852 atomic orbitals) based on the hybrid B3LYP functional focused on the peptide folding induced by the adsorption on the (001) and (010) hydroxyapatite (HA) surfaces give interesting insights on the role of specific interactions between surface sites and the peptide, which stabilize the helix conformation over the "native" random coil ones for in silico designed model peptides. The two peptides were derived from the 12-Gly oligomer, with one (P1, C-tGGKGGGGGGEGGN-t) and two (P2, C-tGGKGGKEGGEGGN-t) glutamic acid (E) and lysine (K) residue mutations. The most stable gas-phase "native" conformation for both peptides resulted in a random coil (RC) structure, with the helix (H) conformation being ≈100 kJ mol(-1) higher in free energy. The two peptide conformations interact with the HA (001) and (010) surfaces by C═O groups via Ca(2+) ions, by hydrogen bond between NH(2) groups and the basic PO(4)(3-) groups and by a relevant fraction due to dispersion forces. Peptide adsorption was studied on the dry (001) surface, the wet one envisaging 2 H(2)O per surface Ca(2+) and, on the latter, also considering the adsorption of microsolvated peptides with 4 H(2)O molecules located at sites responsible of the interaction with the surface. The P1 mutant does prefer to be adsorbed as a random coil by ≈160 kJ/mol, whereas the reverse is computed for P2, preferring the helix conformation by ≈50 kJ/mol. Adsorption as helix of both P1 and P2 mutants brings about proton transfer toward the HA surfaces with a large charge transfer component to the interaction energy.     

Scaling dynamics for a particle in a time-dependent potential well

Some dynamical properties for a classical particle confined in an infinitely deep box of potential containing a periodically oscillating square well are studied. The dynamics of the system is described by using a two-dimensional non-linear area-preserving map for the variables energy and time. The phase space is mixed and the chaotic sea is described using scaling arguments. Scaling exponents are obtained as a function of all the control parameters, extending the previous results obtained in the literature.     

On the Finite Spectral Triple of an Almost-Commutative Geometry

In this short communication, we examine the relevance of the signature of the space-time metric in the construction of the product of a pseudo-Riemannian spectral triple with a finite triple describing the internal geometry. We obtain arguments favoring the appearance of SU(2) and U(1) as gage groups in the standard model.     

Reshaping the trajectory and spectrum of nonlinear Airy beams

We demonstrate theoretically and experimentally that a finite Airy beam changes its trajectory while maintaining its acceleration in nonlinear photorefractive media. During this process, the spatial spectrum reshapes dramatically, leading to negative (or positive) spectral defects on the initial spectral distribution under a self-focusing (or defocusing) nonlinearity.