Alkyl halide charge transfer complexes with hard Lewis bases

The unusual charge transfer complexes formed between alkyl halide acceptors and hard Lewis base donors (amines and alcohols) in low dielectric solvent were examined using ultraviolet spectroscopy. The lambda(max) of the complex decreases with increasing ionization potential of the donor. The complex formation equilibria were probed by thermodynamic analysis and concentration variation. At ambient temperatures complex formation is generally slightly exergonic with a negative complexation entropy. The complex extinction coefficients are much lower (<10 l mol(-1) cm(-1)) than for typical charge transfer complexes. These complexes are extraordinary within a classical context since the halide acceptors have a negative electron affinity. They exhibited an atypical hypsochromic shift with increasing solvent dielectric constant.     

Theoretical and experimental study on the photoluminescence in BaTiO3 amorphous thin films prepared by the chemical route

A polymeric precursor method was used to synthesize BaTiO₃ amorphous thin film processed at low temperature. The luminescence spectra of BaTiO₃ amorphous thin films at room temperature revealed an intense single-emission band in the visible region. The visible emission band was found to be dependent of the thermal treatment history. Photoluminescence (PL) properties for different annealing temperatures were investigated. It was concluded that the intensity of PL is strongly dependent on both the heat treatment of the films and the presence of an inorganic disordered phase. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We discuss the nature of visible PL at room temperature in amorphous barium titanate in the light of the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the barium titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous barium titanate.     

A Raman and dielectric study of a diffuse phase transition in (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb_1-xCa_xTiO₃ thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca²⁺ content. On the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE–PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. PACS 77.80.Bh; 77.55.+f; 78.30.-j; 77.80.-e; 68.55.-a     

Synthesis,Ferroelectric and Optical Properties of (Pb,Ca)TiO3 Thin Films by Soft Solution Processing

Calcium modified lead titanate sol was synthesized using a soft solution processing, the so-called polymeric precursor method. In soft chemistry method, soluble precursors such as lead acetate trihydrate, calcium carbonate and titanium isopropoxide, as starting materials, were mixed in aqueous solution. Pb_0.7Ca_0.3TiO₃ thin films were deposited on platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure, dielectric and optical properties of the thin films were investigated. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 299 and 0.065, respectively, for a thin film with 230 nm thickness annealed at 600°C for 2 h. The remanent polarization (2P_r) and coercive field (E _c) were 32 C/cm² and 100 kV/cm, respectively. Transmission spectra were recorded and from them, refractive index, extinction coefficient, and band gap energy were calculated. Thin films exhibited good optical transmissivity, and had optical direct transitions. The present study confirms the validity of the DiDomenico model for the interband transition, with a single electronic oscillator at 6.858 eV. The optical dispersion behavior of PCT thin film was found to fit well the Sellmeir dispersion equation. The band gap energy of the thin film, annealed at 600°C, was 3.56 eV. The results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of PCT thin films.     

Simultaneous analysis of free amino acids and biogenic amines in honey and wine samples using in loop orthophthalaldeyde derivatization procedure

This work presents a RP-HPLC method for the simultaneous quantification of free amino acids and biogenic amines in liquid food matrices and the results of the application to honey and wine samples obtained from different production processes and geographic origins. The developed methodology is based on a pre-column derivatization with o-phthaldialdehyde carried out in the sample injection loop. The compounds were separated in a Nova-Pack RP-C(18) column (150 mm x 3.9 mm, 4 microm) at 35 degrees C. The mobile phase used was a mixture of phase A: 10 mM sodium phosphate buffer (pH 7.3), methanol and tetrahydrofuran (91:8:1); and phase B: methanol and phosphate buffer (80:20), with a flow rate of 1.0 ml/min. Fluorescence detection was used at an excitation wavelength of 335 nm and an emission wavelength of 440 nm. The separation and quantification of 19 amino acids and 6 amines was carried out in a single run as their OPA/MCE derivatives elute within 80 min, ensuring a reproducible quantification. The method showed to be adequate for the purpose, with an average RSD of 2% for the different amino acids; detection limits varying between 0.71 mg/l (Asn) and 8.26 mg/l (Lys) and recovery rates between 63.0% (Cad) and 98.0% (Asp). The amino acids present at the highest concentration in honey and wine samples were phenylalanine and arginine, respectively. Only residual levels of biogenic amines were detected in the analysed samples.     

Leakage current, ferroelectric and structural properties in Pb1−xBaxTiO3 thin films prepared by chemical route

Single-phase perovskite structure Pb_1−xBa_xTiO₃ thin films (x=0.30, 0.50 and 0.70) were deposited on Pt/Ti/SiO₂/Si substrates by the spin-coating technique. The dielectric study reveals that the thin films undergo a diffuse type ferroelectric phase transition, which shows a broad peak. An increase of the diffusivity degree with the increasing Barium contents was observed, and it was associated to a grain decrease in the studied composition range. The temperature dependence of the phonon frequencies was used to characterize the phase transition temperatures. Raman modes persist above tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of breakdown of the local cubic symmetry by chemical disorder. The absence of a well-defined transition temperature and the presence of broad bands in some interval temperature above FE-PE phase transition temperature suggested a diffuse type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. The leakage current density of the PBT thin films was studied at different temperatures and the data follow the Schottky emission model. Through this analysis the Schottky barrier height values 0.75, 0.53 and 0.34 eV were obtained to the PBT70, PBT50 and PBT30 thin films, respectively.     

Characterization of a crystal grown from Ocimum Basilicum leaves and branches

Leaves and branches of Ocimum Basilicum were dried, reduced to coarse powder and extracted in aqueous metanol and aqueous hexane. This paper discusses the X‐ray diffraction and FT‐Raman spectra measured at room temperature. The data obtained have been used to propose a general assignment of the vibrational fundamentals on the base of frequencies assigned in other related single crystals. From the results we can conclude that the crystal has a high symmetry. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simultaneous spectrophotometric determination of pyrantel pamoate and febantel in pharmaceutical preparations using partial least-squares regression

The multivariate spectroscopic determination of the combination of pyrantel pamoate (PP) and febantel (FB) in pharmaceutical preparations was carried out by partial least-squares regression. The UV absorbance spectra of standard mixtures of PP and FB were measured at concentrations ranging from 5.76–11.52 and 6–12 μg/mL, respectively, in a diluent solution of methanol and acetonitrile. The best model was selected with variable selection (280–320, 380–400 nm), 2 latent variables and mean centered. External validation was performed using 13 different mixtures, which provided prediction errors lower than 1%. The method was validated in accordance with international and Brazilian guidelines. This method was successfully employed to quantify the drug content in two different pharmaceutical formulations (solid — capsules, liquid — oral suspensions) with good prediction capacity. Furthermore, its advantages include the use of simple and accessible reagents and equipment, and a low operating cost.     

Synthesis and crystal structure of the N4-oxide of an hydroxybenzo[f]quinoxalin-6(5H)-one obtained by oxidative cleavage of beta-lapachone-quinoxaline

The N4-oxide of the 5-(3-hydroxy-3-methylbutyl)-5-hydroxybenzo[f]quinoxalin-6(5H)-one was synthesized from beta-lapachone-quinoxaline by MCPBA oxidation and the crystal structure has been determined (C₁₇H₁₈N₂O₄), Mr=297.3, a=15.880(5), b=16.998(1), c=5.587(2) ?, V=1508.1(5) ?³, D _x=1.31, space group P2₁2₁2, Z=4, S=1.109 and R _obs=0.06. The central ring of the molecule is in a twisted-boat conformation and the molecular packing is done through two intermolecular hydrogen bonds O–H···O [2.072(6)] and [2.177(5) ?], forming columns along the direction parallel to the c-axis.