Maurizio Muniz-Miranda

Light-induced variations in realgar and beta -As 4 S 4 ; X-ray diffraction and Raman studies

Abstract Transformations induced by light in realgar and β-As4S4 have been studied by means of X-ray diffraction (powder and single-crystal) and Raman laser spectroscopy. Both polychromatic and monochromatic light alter realgar and the β phase into pararealgar. By lowering the reaction rate (long-wavelength pass filter, low laser power) an intermediate product, corresponding to the so-called “χ phase,” can be observed during the alteration process from β-As4S4 to pararealgar (polychromatic filtered light and laser light) and that from realgar to pararealgar (only with polychromatic filtered light). The powder pattern of the χ phase is fairly similar to that of β-As4S4 but differs from realgar, and it can be indexed on the basis of an expanded β-phase unit cell. Lattice parameters are as follows: a = 9.758(5), b = 9.522(5), c = 9.074(5) Å, 0 = 100.84(5)°, and a = 9.757(4), b = 9.512(7), c = 9.089(4) Å, 0 = 100.97(3)° for the χ phase obtained from alteration of realgar and β-As4S4, respectively. The formation of the χ phase is preceded by a strong anisotropic increase of the unit-cell volume of realgar and the β phase. The effects of light exposure on lattice parameters were studied using single-crystal X-ray diffraction: In realgar a and c sin β increase linearly with increasing exposure times, whereas b remains substantially unchanged. In the β-As4S4 crystals, b and c sin β increase while a decreases so that, after 1560 min, the unit-cell dimensions match those of the χ phase. Raman spectroscopy, which allows monitoring of the alteration process in real time, indicates the presence of the same As4S4 cage molecule in the χ phase as in realgar and β -As4S4 and confirms the similarity between the structure of χ and that of β. However, a less-ordered crystal structure and a lower molecular symmetry is suggested from analysis of the spectroscopic results.

Spectroscopic investigation and normal mode analysis of As4S4 polymorphs

Abstract Tetra-arsenic tetrasulfide polymorphs have been investigated by IR, Raman and UV-visible spectroscopies. The mineral realgar ( α -As 4 S 4 ) and the high-temperature phase β -As 4 S 4 transform to pararealgar by exposure to monochromatic laser light with a reaction rate depending on the wavelength of the exciting radiation. The photoinduced alteration process has been monitored by Raman spectroscopy, following in real time the spectral changes of the samples undergoing irradiation. A satisfactory assignment of realgar, β -As 4 S 4 and pararealgar was obtained by employing a simplified valence force field which provided a complete set of normal coordinates suitable for describing the vibrational spectra of these As 4 S 4 polymorphs.

24 h stability of thick multilayer silicene in air

Thick epitaxial multilayer silicene films with a root 3 x root 3R(30 degrees) surface structure show only mild surface oxidation after 24 h in air, as measured by Auger electron spectroscopy. X-ray diffraction and Raman spectroscopy measurements performed in air without any protective capping, as well as, for comparison, with a thin Al2O3 cap, showed the (002) reflection and the G, D and 2D Raman structures, which are unique fingerprints of thick multilayer silicene.

SERS, XPS, and DFT Study of Adenine Adsorption on Silver and Gold Surfaces

The adsorption of adenine on silver and gold surfaces has been investigated combining density functional theory calculations with surface-enhanced Raman scattering and angle-resolved X-ray photoelectron spectroscopy measurements, obtaining useful insight into the orientation and interaction of the nucleobase with the metal surfaces.

Spectroscopic and potentiometric study of the SOD mimic system copper(II)/acetyl-L-histidylglycyl-L-histidylglycine.

Stoichiometry, stability constants and solution structures of the copper(II) complexes of the N-acetylated tetrapeptide HisGlyHisGly were determined in aqueous solution in the pH range 2-11. The potentiometric and spectroscopic data (UV-Vis, CD, EPR and Raman scattering) show that acetylation of the amino terminal group induces drastic changes in the coordination properties of AcHGHG compared to HGHG. The N3 atoms of the histidine side chains are the first anchoring sites of the copper(II) ion. At pH 4.7 and 5.6 both the imidazole rings cooperate in the formation of a 2N equatorial set, while, at higher pH values, 3N and 4N complexes are formed through the coordination of peptide N- atoms. The logbeta values of the copper complexes of AcHGHG are by far lower than those of the corresponding species in the parent CuII-HGHG system.

Normal mode analysis of 2,2′-bipyridine—II. Crystal vibrations

Abstract A normal mode analysis of the crystal spectrum of 2,2′-bipyridine was carried out using a previously determined intramolecular force field and an intermolecular potential described in terms of interactions among non-bonded atoms. New i.r. and Raman data in the region of lattice vibrations are presented, which can be satisfactorily accounted for by frequency calculations in the rigid-body approximation. Frequency splittings and shifts of internal modes, mixing with external vibrations and relative intensity of out-of-plane modes are analyzed through calculations at k = 0 which include all vibrational degrees of freedom.

Adsorption mechanism of 2-amino,5-nitropyrimidine on silver substrates, as detected by surface-enhanced Raman scattering

The adsorption of 2-amino,5-nitropyrimidine (ANPM) on silver has been investigated by surface-enhanced Raman scattering (SERS). The ligand adsorbs as neutral molecule, but the interaction changes with the pH of the Ag hydrosol. In alkaline medium the binding occurs through a nitrogen atom of the aromatic ring. When coadsorbed hydroxide ions are removed from the metal surface, the molecule interacts with the substrate via nitro group. This occurs in Ag hydrosol at neutral pH and on filters coated with colloidal silver. The use of Ag-coated filters in the SERS measurements has contributed to the understanding of the adsorption mechanism.

Normal mode analysis of 2,2′-bipyridine—I. Internal modes

Abstract The vibrational assignment of the internal modes of 2,2′-bipyridine is discussed using i.r. and Raman spectra in polarized light for a single crystal and for two different orientations of polycrystalline films. Predictions based on zero-order frequency calculations provide a useful guideline for the identification of the in plane fundamentals, while no satisfactory force field was available for the out-of-plane vibrations. A refined set of valence force constants was then obtained which, under the assumption of a planar stable conformation both in solution and in solid, can be used for further calculations of the whole crystal spectrum.

Raman and computational study of solvation and chemisorption of thiazole in silver hydrosol

A SERS investigation combined with ab initio computational analysis involving Car-Parrinello molecular dynamics simulations and Density Functional Theory approach allows fundamental information to be obtained on the behaviour of thiazole in silver aqueous suspension where solvation and chemisorption processes competitively occur.

On the occurrence of the central line (∼1025 cm−1) in the SERS spectra of pyridine adsorbed on silver hydrosols

Abstract The occurrence of a central line at ∼1025 cm−1 between the surface-enhanced Raman scattering (SERS) bands of pyridine at 1008 and 1036 cm−1 has been first detected in silver hydrosols. This band, which has no counterpart in the Raman spectrum of the free ligand, is observed in acidic aqueous suspension as corresponding to that observed in an electrochemical cell and attributed to adsorption of pyridinium cation. When pyridine is adsorbed on an aged colloid in an alkaline medium two different species are detected. A central band at about the same wave number occurs, attributable to pyridine bound to silver ion cluster on the metal surface, oxidised by ambient air.