Ab-initio study of initial atmospheric oxidation reactions of C3 and C4 alkanes

Second-order, Møller–Plesset (MP2)-unrestricted Hartree–Fock calculations with full geometry optimization in the 6-31G(d, p) basis set were carried out to study the initial atmospheric oxidation reactions of alkanes. All structures in the initial hydrogen abstraction reaction by an OH radical and the subsequent addition of molecular oxygen to the alkyl radical were characterized for alkanes with three and four carbon atoms. The reaction paths for the formation of the peroxyl radicals were obtained and discussed in the light of similarities along series involving primary, secondary, and tertiary hydrogens. A 0.999 correlation was found between the height of our barriers for the OH abstraction of a primary hydrogen atom from alkanes containing one to four carbon atoms and the optimally estimated activation energies for this reaction recently presented. From the slope and the intersection at zero activation energy an equation was obtained that yields scaled values of the activation energies to account for the tunnel effect and for the error due to the basis set and the method employed. We present new results for the abstraction of the less favored primary hydrogens in propane, butane, and isobutane, which should be important at high temperatures. Negative net activation energies were obtained for the addition of molecular oxygen to all the alkyl radicals formed in the first reaction. The structure of the peroxyl radicals is discussed; and very good correlations are observed for similar compounds, regardless of the length of the carbon chain. A revision of some experimental values is suggested. Single point density functional calculations at the MP2 geometries were also performed with the B3LYP functional for comparison. The observed trends are exactly the same for the two methods. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 845–856, 1999     

Ab initio compact group model potentials for describing environment effects in cluster calculations

A method for the determination of ab initio group model potentials within the Hartree–Fock framework is reported. Following the theory of separability of many electron systems, a new way to incorporate the effect of complete chemical entities by means of polycenter compact model potentials is presented. The interaction between active and frozen electrons is partitioned as a sum of long- and short-range terms. The long-range term is described as the effect of −2e charges placed in the center of the charge of the frozen group molecular orbitals; the short-range one, the exchange and Pauli repulsion, is developed as a spectral representation in a nonorthogonal basis set. An algorithm to solve the problem associated with the rotation of the polycenter model potential is presented and implemented in an all-purpose quantum chemical program. In order to check the method, a group model potential for H₂O was obtained and tested. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1145–1152, 1999     

2,2′‐Methyl­enebis(3‐hydroxy‐5,5‐di­methyl­cyclo­hex‐2‐en‐1‐one)

The title compound, C₁₇H₂₄O₄, crystallizes with two independent mol­ecules, both lying across twofold rotation axes in space group Pccn, in a unit cell whose dimensions closely mimic those of a tetragonal cell. Each mol­ecule contains paired O—H⋯O hydrogen bonds [H⋯O = 1.81 and 1.83 Å, O⋯O = 2.640 (2) and 2.642 (2) Å, and O—H⋯O = 168 and 162°].     

Effect of Cold Atmospheric Plasma Jet Associated to Polyene Antifungals on Candida albicans Biofilms

The increasing incidence of antifungal resistance represents a great challenge in the medical area and, for this reason, new therapeutic alternatives for the treatment of fungal infections are urgently required. Cold atmospheric plasma (CAP) has been proposed as a promising alternative technique for the treatment of superficial candidiasis, with inhibitory effect both in vitro and in vivo. However, little is known on the association of CAP with conventional antifungals. The aim of this study was to evaluate the effects of the association between CAP and conventional polyene antifungals on Candida albicans biofilms. C. albicans SC 5314 and a clinical isolate were used to grow 24 or 48 h biofilms, under standardized conditions. After that, the biofilms were exposed to nystatin, amphotericin B and CAP, separately or in combination. Different concentrations of the antifungals and sequences of treatment were evaluated to establish the most effective protocol. Biofilms viability after the treatments was compared to negative control. Data were compared by One-way ANOVA and post hoc Tukey (5%). The results demonstrate that 5 min exposure to CAP showed more effective antifungal effect on biofilms when compared to nystatin and amphotericin B. Additionally, it was detected that CAP showed similar (but smaller in magnitude) effects when applied in association with nystatin and amphotericin B at 40 µg/mL and 60 µg/mL. Therefore, it can be concluded that the application of CAP alone was more effective against C. albicans biofilms than in combination with conventional polyene antifungal agents.     

DFT and MP2 molecular orbital determination of OH–toluene–O2 isomeric structures in the atmospheric oxidation of toluene

We have performed an exhaustive theoretical study, using a density functional theory (DFT) and ab initio techniques, of the possible isomers of the OH–toluene–O₂ radical. DFT calculations of the all electron type using the hybrid B3LYP approach and 6‐31G* orbital basis set were employed. In addition to the well‐established ortho position, addition of OH at C₁ on the benzene ring of toluene was also considered for the initial methylhydroxycyclohexadienyl adduct. In all, 28 different intermediate structures of the OH–toluene–O₂ system, consisting of peroxyl radicals, bicyclic structures, and epoxides, have been explored through fully optimized electronic structure calculations. Starting from the 1,3‐O₂‐methylorthohydroxycyclohexadienyl radical, or ortho‐OH adduct, several peroxyl radicals are found to have low‐lying structures contained within a small energy range (about 1 kcal/mol). Only two bicyclic structures are stable with respect to the methylhydroxycyclohexadienyl radical plus O₂, one of them being clearly favored. The four possible epoxy structures are all found to lie more than 15 kcal/mol lower than any of their peroxyl and bicyclic isomers. The preference, first noted by Bartolotti and Edney, for structures in which the OH group lies on the same side of the ring as the O₂ group, is obeyed in all cases. If the 1‐CH₃, 1‐OH cyclohexadienyl radical (or C₁–OH adduct) is used as the initial adduct, three peroxyl radicals are expected to be formed, while two bicyclic structure and three epoxides need to be considered. These structures are found to be, in general, less stable than the ones arising from the ortho adduct. However, the 4‐O, 2,3‐epoxy, 1,1‐methylhydroxycyclohexadienyl radical is found to be the most stable of all the isomers considered, and this, by more than 3 kcal/mol. In this work, most structures were also calculated with the MP2 method with a 6‐31G* basis set. The geometries obtained with the two methods are similar. Contrary to the B3LYP method, MP2 always yields an extra stability to structures in which the C₁ carbon atom has sp³ hybridization. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 716–730, 2000     

Ex‐situ Antimony Screen‐printed Carbon Electrode for Voltammetric Determination of Ni(II)‐ions in Wastewater

An Ex‐situ antimony film screen‐printed carbon electrode (Ex‐situ SbSPCE) was successfully applied for the determination of Ni(II), by means of adsorptive stripping voltammetry using dimethylglyoxime as complexing agent, in a certified reference wastewater sample. This electrode is proposed as an alternative to more conventional antimony film electrodes. Ex‐situ SbSPCE was analytically characterized and the obtained parameters suggest that Ex‐situ SbSPCE behaves much better than both Ex‐situ BiSPCE and Bi_spSPE for Ni(II) determination. The results confirm the applicability of Ex‐situ SbSPCE for the determination of low concentration levels of Ni(II) in natural samples with a very high reproducibility and good trueness.     

First-order phase transition from the vortex liquid to an amorphous solid

We present a systematic study of the topology of the vortex solid phase in superconducting Bi2Sr2CaCu2O8 samples with low doses of columnar defects. A new state of vortex matter imposed by the presence of geometrical contours associated with the random distribution of columns is found. The results show that the first-order liquid-solid transition in this vortex matter does not require a structural symmetry change.     

Films and nanorods of transparent conducting oxide ITO by a citric acid sol route

We report the synthesis of nanorods of Sn-doped In₂O₃ (ITO) through sol electrophoresis with a template. A citric acid-based sol was developed for the formation. This technique enables the synthesis of ITO nanorods with controlled size and Sn-doping concentration. The nanorods synthesized have diameters of 75–145 nm and show a bulk resistivity of 5 cm. PACS 61.46.+w; 73.63.Bd     

Usefulness of parallel factor analysis to handle the matrix effect in the fluorescence determination of tetracycline in whey milk

The determination of tetracycline by fluorescence spectrophotometry in complex matrices has some difficulties, because the presence of other compounds in the matrix affects the analytical signal. In this work, the effect of some inorganic species that are present in whey milk on the fluorescence signal of tetracycline is studied using a D-optimal experimental design. Next, an experimental strategy is proposed in conjunction with Parallel Factor Analysis, PARAFAC, modeling that leads to suitably modeling the severe matrix effect in the determination of tetracycline in whey milk. A specific design is performed in such a way that the lack of trilinearity due to the effect of the presence of interferents on the signal is obviated. Then, ten test samples from three brands of milk, spiked with different quantities of tetracycline and measured in 2 days were analysed using this methodology (mean of the absolute value of the relative errors: 5.1%). The developed analytical method fulfils the property of trueness, the relative errors being, both in calibration and prediction, inside the interval set by Commission Decision 2002/657/EC at these concentration levels. Decision limits (CCα) at x₀ = 0 μg L⁻¹ and at x₀ = 100 μg L⁻¹ were 13.2 and 112.4 μg L⁻¹ respectively, for α = 0.05; whereas detection capabilities (CCβ) were 25.9 μg L⁻¹ and 124.4 μg L⁻¹ respectively for α = β = 0.05.     

NMR characterization of the host-guest inclusion complex between beta-cyclodextrin and doxepin

The interaction between doxepin, a member of the tricyclic antidepressant (TCA) class of drugs, with beta-cyclodextrin (beta-CD) was investigated using NMR. Several TCAs have been reported to form a complex with beta-CD having 1:1 stoichiometry. Previous results from UV-visible spectroscopy, fluorescence measurements, and molecular modeling indicated that for imipramine, desipramine, and amitriptyline, the TCA aliphatic tail is included in the cyclodextrin cavity with apparently no interaction of the tricyclic ring. An alternative view of the doxepin-beta-CD complex is presented in this work using analysis of complexation-induced chemical shifts (CICSs), the method of continuous variation (Job's analysis), and analysis of ROESY spectra. The Job's plot derived from the NMR spectral data confirms that the complex formed has 1:1 stoichiometry. The largest changes in the CICS data were observed for the aromatic protons of one of the doxepin rings, with much smaller chemical shift changes observed for the protons of the other aromatic ring and the doxepin tail. Perhaps the most significant evidence for inclusion of the doxepin tricyclic ring is the strong ROESY cross peaks between the doxepin aromatic resonances and the protons located inside the beta-CD cavity. Changes in the doxepin (1)H NMR spectrum and the behavior of ROESY exchange cross peaks suggest that inclusion complex formation decreases the rate of internal motions of doxepin.