The complexation of anions by chloro- and cyanoacetanilides; IR, 1H-NMR and computation study

Complexation ability of simple chloro- and cyanoacetanilides has been determined using IR as well as ¹H-NMR titration experiments. Only 1:1 anion–amide complex stoichiometry has been found. The structures of several free anilides and their complexes with bromide anion have been studied by DFT calculation.     

Upper limit on the diffuse flux of ultrahigh energy tau neutrinos from the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth's crust. Tau leptons from nu(tau) charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of nu(tau) at EeV energies. Assuming an E(nu)(-2) differential energy spectrum the limit set at 90% C.L. is E(nu)(2)dN(nu)(tau)/dE(nu)<1.3 x 10(-7) GeV cm(-2) s(-1) sr(-1) in the energy range 2 x 10(17) eV< E(nu)< 2 x 10(19) eV.     

Observation of the suppression of the flux of cosmic rays above 4 x 10 (19) eV

The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional, variantE;{-gamma}, at energies between 4 x 10;{18} eV and 4 x 10;{19} eV is 2.69+/-0.02(stat)+/-0.06(syst), steepening to 4.2+/-0.4(stat)+/-0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min.     

Electron energy distribution function in ionization waves of different amplitudes

The changes of the body and of the tail of the electron energy distribution function are studied in dependence on amplitudes of ionization waves in neon.     

Poly(3,4-ethylenedioxythiophene)-modified nafion membrane for direct methanol fuel cells

Membranes Nafion 117 are modified with poly(3,4-ethylenedioxythiophene) (PEDT) by chemical polymerization of EDT with H₂O₂ or FeCl₃ as the oxidants in a two-compartment cell. Depending on the oxidant and polymerization conditions, PEDT is deposited either as a thin film on the membrane surface or inside the Nafion membrane depending on whether FeCl₃ or H₂O₂ is used as the oxidant. The decrease in the ionic conductivity and methanol permeability is studied as a function of the polymerization time. A linear dependence is found with H₂O₂ and a t ^−1/2 dependence, with FeCl₃. The contributions of PEDT and Nafion to the overall conductivity of the composite membranes are separated by impedance measurements. The modified membranes (FeCl₃) are also tested in direct methanol fuel cells (DMFC). The methanol permeation through the membranes is measured by operating the fuel cell in an electrolysis mode. The influence of the modified membranes on the DMFC current-voltage characteristics is studied with 2 M CH₃OH and O₂ at 1.2 bar_abs and 80°C. Membrane electrode assemblies (MEAs) are prepared by hot pressing the modified membrane between two commercial gas diffusion electrodes with 1 mg cm⁻² of Pt loading. A decrease of the methanol permeation of 25% is observed at MEA with the modified membrane (1 h polymerization time), compared with that of MEA with a Nafion membrane. However, the overall DMFC performance decreases in the same relation: a maximal power density of 36 W cm⁻² is measured at MEA with a PEDT-modified membrane compared with 45 W cm⁻² for MEA with a Nafion membrane. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1330–1339. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Calculation of structural behavior of indirect NMR spin-spin couplings in the backbone of nucleic acids

Calculated indirect NMR spin-spin coupling constants (J-couplings) between (31)P, (13)C, and (1)H nuclei were related to the backbone torsion angles of nucleic acids (NAs), and it was shown that J-couplings can facilitate accurate and reliable structural interpretation of NMR measurements and help to discriminate between their distinct conformational classes. A proposed stepwise procedure suggests assignment of the J-couplings to torsion angles from the sugar part to the phosphodiester link. Some J-couplings show multidimensional dependence on torsion angles, the most prominent of which is the effect of the sugar pucker. J-couplings were calculated in 16 distinct nucleic acid conformations, two principal double-helical DNAs, B- and A-, the main RNA form, A-RNA, as well as in 13 other RNA conformations. High-level quantum mechanics calculations used a baseless dinucleoside phosphate as a molecular model, and the effect of solvent was included. The predicted J-couplings correlate reliably with available experimental data from the literature.     

Computational study on spectral properties of the selected pigments from various photosystems: structure-transition energy relationship

In this study, the most important kinds of pigments (chlorophylls, bacteriochlorophylls, phycobilins, and carotenoids) from various photosystems were explored. For the most stable conformations, electronic transitions were determined at the TDDFT/6-31+G(d) level with the B3PW91 functional and compared to measured spectra. The group of carotenoids was also investigated at the TDA/TDDFT level with the BLYP functional. The energies of Qy transitions are systematically blue-shifted by about 50-100 nm in the case of (bacterio)chlorophyll and pheophytin molecules. Nevertheless, the correct relative order of the Q lines among various chlorophyll types was obtained through comparison with experimental results. Much better agreement was obtained for the Soret band, for which the differences between calculated and measured transitions were at most 35 nm. In the case of phycobilins, the first transition line was estimated to be at lower frequencies (around 500 nm) with a very similar blue shift of about 100 nm from experimental values. The influence of anchoring cysteine side chain(s) was found to be marginal. A dominant effect of the linear polyene chain on the determined spectral lines was found in the case of carotenoids. Nevertheless, the impact of beta-cycles and epoxy and keto groups is clearly visible as well. The high intensity of the first allowed transition matches different characters of the HOMO and LUMO. In the case of fucoxanthin, the TDA method also predicts the Bu- state to lie below the 1Bu+ state. Because the shift of electron transitions is approximately proportional to the size of the pi-conjugated system, the shift of the calculated transitions compared to experimental values is practically constant for the same excitations of (bacterio)chlorophyll and phycobilin molecules. However, this is not true for carotenoids, for which both the transition energy and the shift of the transition vary with the number of conjugated double bonds.     

Pre-treatment of polyethylene terephthalate by Grignard reagents for high quality polypyrrole coatings and for altering the hydrophobicity

Polypyrrole-coated polyethylene terephthalate is very attractive for various applications as electrically conductive fiber. This paper investigates the importance of polyester pre-treatment on the quality of the polypyrrole coating and its conductivity efficiency. The results showed nucleophilic pre-treatments by Grignard reagents resulting in the significantly higher surface quality and strongest adhesion of the polypyrrole coating to the polyester fibers compared to the other published methods. Moreover, several Grignard reagents were used for altering the hydrophobicity and morphology of the surface.