Electrokinetics of charged spherical colloidal particles taking into account the effect of ion size constraints

The electrokinetic properties of suspended spherical particles are examined using a modified standard electrokinetic model, which takes into account the finite ion size and considers that the minimum approach distance of ions to the particle surface need not be equal to their effective radius in the bulk solution. We calculate the conductivity increment and the electrophoretic mobility and present a detailed interpretation of the obtained results, based on the analysis of the equilibrium and field-induced ion concentrations, as well as the convective fluid flow in the neighborhood of the particle surface. We show that when charge reversal takes place, the sign of the concentration polarization remains unchanged while the sign of the electrophoretic mobility only changes under favorable circumstances.     

Chemical composition and antibacterial activity of the essential oil of Espeletia nana

The essential oil of the leaves of Espeletia nana Cuatrec., obtained by hydrodistillation, was analyzed by GC-MS, which allowed the identification of 24 components, which made up 99.9% of the oil. The most abundant compounds were a-pinene (38.1%), beta-pinene (17.2%), myrcene (15.0%), spathulenol (4.2%), bicyclogermacrene (4.0%), a-zingiberene (4.0%), and gamma-himachalene (3.7%). Antibacterial activity was tested against Gram-positive and Gram-negative bacteria using the agar disk diffusion method. Activity was observed only against Gram-positive bacteria. MIC values were determined for Staphylococcus aureus ATCC 25923 (200 microg/mL) and Enterococcusfaecalis ATCC 29212 (600 microg/mL).     

(1)H relaxation dispersion in solutions of nitroxide radicals: Effects of hyperfine interactions with (14)N and (15)N nuclei

(1)H relaxation dispersion of decalin and glycerol solutions of nitroxide radicals, 4-oxo-TEMPO-d(16)-(15)N and 4-oxo-TEMPO-d(16)-(14)N was measured in the frequency range of 10 kHz-20 MHz (for (1)H) using STELAR Field Cycling spectrometer. The purpose of the studies is to reveal how the spin dynamics of the free electron of the nitroxide radical affects the proton spin relaxation of the solvent molecules, depending on dynamical properties of the solvent. Combining the results for both solvents, the range of translational diffusion coefficients, 10(-9)-10(-11) m(2)∕s, was covered (these values refer to the relative diffusion of the solvent and solute molecules). The data were analyzed in terms of relaxation formulas including the isotropic part of the electron spin - nitrogen spin hyperfine coupling (for the case of (14)N and (15)N) and therefore valid for an arbitrary magnetic field. The influence of the hyperfine coupling on (1)H relaxation of solvent molecules depending on frequency and time-scale of the translational dynamics was discussed in detail. Special attention was given to the effect of isotope substitution ((14)N∕(15)N). In parallel, the influence of rotational dynamics on the inter-molecular (radical - solvent) electron spin - proton spin dipole-dipole coupling (which is the relaxation mechanism of solvent protons) was investigated. The rotational dynamics is of importance as the interacting spins are not placed in the molecular centers. It was demonstrated that the role of the isotropic hyperfine coupling increases for slower dynamics, but it is of importance already in the fast motion range (10(-9)m(2)∕s). The isotope effects is small, however clearly visible; the (1)H relaxation rate for the case of (15)N is larger (in the range of lower frequencies) than for (14)N. It was shown that when the diffusion coefficient decreases below 5 × 10(-11) m(2)∕s electron spin relaxation becomes of importance and its role becomes progressively more significant when the dynamics slows done. As far as the influence of the rotational dynamics is concerned, it was show that this process is of importance not only in the range of higher frequencies (like for diamagnetic solutions) but also at low and intermediate frequencies.     

Synthesis and Crystal Structure of a New Zn(II) Nitronyl Nitroxide Complex [Zn(NIT4Py)(PDA)(H2O)3]

1 INTRODUCTION Nitronyl nitroxides, independently or in combina- tion with metal ions, have been one of the most wi- dely studied systems in molecular magnetism for understanding radical-radical or metal-radical inte- ractions as well as for synthesizing organic ferroma- gnets and metal-radical magnetic materials[1～4]. Up to now, lots of metal-radical complexes have been reported[5～8]. On the other hand, cooperative inter- molecular interactions, such as coordination bonds, hydrogen bon…     

Structure and bonding in monomeric iron(III) complexes with terminal oxo and hydroxo ligands

We report on the structure and bonding in the title iron(III) complexes, containing the tris[(N'-tert-butylureayl)-N-ethyl]amine ligand, with density functional theory techniques. In agreement with the experimental data, a high-spin electronic state is favored for all of the systems we considered. H bonds between the terminal oxo and hydroxo ligands and NH groups present in the organic ligand coordinated to the metal have a remarkable effect on the overall coordination geometry. In fact, the structure of model complexes without H bonds shows shorter Fe-O bond lengths. This is a consequence of the ability of the H bonds to stabilize a remarkable amount of electron density localized on the terminal oxo and hydroxo ligands. Energy analysis indicates that each H bond stabilizes the nonheme complexes by roughly 35 kJ/mol. Molecular orbital analysis indicates a reduction of two Fe-O bonding electrons on going from a complex with a terminal oxo ligand to a complex with a terminal hydroxo ligand. This reduction in the number of bonding electrons is also supported by frequency analysis.     

Role of sequence and conformation on the photochemistry and the photophysics of A-T DNA dimers: an experimental and computational approach

The role of base sequence and conformation on the photochemistry and photophysics of thymidylyl (3'-5')-2'-deoxyadenosine sodium salt (TpdA) and 2-deoxyadenylyl (3'-5')-thymidine ammonium salt (dApT) was studied. To this end, nanosecond transient absorption at 266 nm, steady-state irradiation at 254 nm, and quantum chemical calculations were used. The transient absorption spectra show the solvated electron broad band in the visible region for each dinucleotide. In addition, low-intensity absorption bands are observed in the UV region, which are attributed to the deprotonated and protonated neutral radicals of adenine and thymine bases. Photoionization (PI) occurs by one- and two-photon pathways; the latter accounting for approximately 70% of the net PI yield. A diffusion-limited rate constant of 2.0 x 10(10) M(-1) s(-1) was obtained for the reaction of the neutral molecule with the photoejected electron in both sequences. The photodestruction yield, measured from the chromophore loss at 260 nm, decreases in the presence of well-known electron scavengers. This suggests the participation of base radical anions as one of the photodegradation pathways, which is higher in TpdA than in dApT. The intermediacy of a radical ion pair (charge separated state) between the adjacent adenine and thymine bases is proposed in the formation of the [2 + 2] cycloadduct intermediate. The [2 + 2] cycloadduct intermediate is known to be the precursor of the thymine-adenine eight-member ring photoproduct (TA*). Conformational constrains in the radical ion pair are suggested to explain the absence of the TA* photoproduct in dApT. This hypothesis is supported by semiempirical calculations performed on all relevant reactive intermediates proposed to participate in the mechanism of formation of TA*. Altogether, the results show that sequence and conformation profoundly influence the photochemistry and the photophysics of these DNA model systems.     

Determination of pesticides in waters by capillary gas chromatography with atomic emission detection

Capillary gas chromatography with atomic emission detection (GC-AED) was successfully used to separate and quantify 14 pesticides (organochlorines, organophosphorus compounds and pyrethrins) in water samples after liquid-liquid extraction with ethyl acetate. Monitoring the emission lines for elements such as chlorine, bromine and sulfur ensures nearly specific chromatograms for these elements, and markedly increases selectivity. Calibration curves were obtained by plotting peak area versus concentration and the correlation coefficients relating to linearity were at least 0.999. Each chromatographic separation takes 21 min and, since two injections are necessary, it requires approximately 1 h to analyze one sample. The method shows a precision of 4.3-8.2% (RSD), depending on the compound. The application of liquid-liquid extraction with ethyl acetate led to recoveries from spiked samples ranging from 76 to 113%. The sensitivity and linearity for the elements chlorine, sulfur, nitrogen and phosphorus were checked under the optimized conditions at their customary emission wavelength. The sensitivity and linearity for these compounds decreased in the order (atom and emission wavelength) Cl (479 nm)>S (181 nm)>P (178 nm)>N (174 nm). A study of the GC-AED system's response to chlorine concentration in eight pesticide molecules was performed and a linear relationship was found with a correlation coefficient of 0.987.     

Carboxylate-bridged copper(II)-lanthanide(III) complexes [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n)(Ln = Dy,Ho, Er,Y; oda = oxydiacetate)

The hydrothermal reaction of Ln(2)O(3) (Ln = Dy and Ho), Cu(OAc)(2).2H(2)O, and oxydiacetic acid in the approximate mole ratio of 1:3:8 resulted in the formation of two new members of the isostructural series of polymers formulated as [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n), crystallizing in the hexagonal crystal system, space group P6/mcc (No. 192). Temperature-dependent magnetic susceptibilities and EPR spectra are reported for the heterometallic compounds Cu-Dy 1, Cu-Ho 2, Cu-Er 3, and Cu-Y 4. The results are discussed in terms of the structure of the compounds, the electronic properties of the lanthanide ions, and the exchange interactions between the magnetic ions.     

Ferromagnetism and chirality in two-dimensional cyanide-bridged bimetallic compounds

The combination of hexacyanoferrate(III) anions, [Fe(CN)(6)](3)(-), with nickel(II) complexes derived from the chiral ligand trans-cyclohexane-1,2-diamine (trans-chxn) affords the enantiopure layered compounds [Ni(trans-(1S,2S)-chxn)(2)](3)[Fe(CN)(6)](2).2H(2)O (1) and [Ni(trans-(1R,2R)-chxn)(2)](3)[Fe(CN)(6)](2).2H(2)O (2). These chiral systems behave as ferromagnets (T(c) = 13.8 K) with a relatively high coercive field (H(c) = 0.17 T) at 2 K. They also exhibit an unusual magnetic behavior at low temperatures that has been attributed to the dynamics of the magnetic domains in the ordered phase.     

Fluorimetric determination of diacetyl and 2,3-pentanedione with isoniazide and a zirconium salt

Diacetyl and 2,3-pentanedione react with isoniazide in acidified solutions to give the corresponding hydrazones which, by reacting with a zirconium (IV) salt, form fluorescent complexes. Both reactions have been studied separately. Variables such as pH, heating time, and excess of reactives are discussed. The method may be used for the determination of these compounds in foods.