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101.
Ibrahim H. A. Badr 《Mikrochimica acta》2005,149(1-2):87-94
Membrane electrodes based on 2,2-dithiobis(benzothiazole), DTBBT, as a neutral carrier for silver ions are described. Silver-selective membrane electrodes formulated with 2wt% DTBBT ionophore and 50mol% TFPB in an FPNPE plasticized poly (vinyl chloride) exhibited near-Nernstian responses towards silver ions (60.3±0.5mVdecade–1) over a wide silver ion activity range of 0.83µM to 94mM. Increasing the amount of anionic sites, TFPB, to 100 or 150mol% (relative to the DTBBT weight) resulted in super-Nernstian responses toward silver ions. Membrane electrodes prepared using a low dielectric constant plasticizer, however, exhibited sub-Nernstian responses. Polymer membrane electrodes with optimal composition (i.e., 2wt% DTBBT, 50mol% TFPB in FPNPE plasticized poly(vinyl chloride)) exhibited high potentiometric selectivity towards silver ions over alkali, alkaline earth, transition metal ions, as well as heavy metal ions such as Hg2+ and Pb2+. A good correlation was found between the potentiometric selectivity coefficients and the change in the UV-visible spectra of the ionophore upon exposure to different metal ions. The overall performance of the silver-selective membrane electrodes based on DTBBT ionophore, which is available at low cost, was found to be comparable to the performance of silver electrodes prepared with Fluka silver ionophore-IV. A DTBBT-based silver electrode was used as an indicator electrode for titrations of silver ions using standard sodium chloride solutions. Sharp inflections occur at the end point, and the data obtained showed 99.4% recovery with a standard deviation of 0.7% (n=3). In addition, the applicability of the DTBBT-based silver-selective electrode is illustrated by measuring the silver concentrations in natural water spiked with silver nitrate and by analyzing the silver in electroplating wastewater samples. The results obtained utilizing a DTBBT-based silver electrode showed very good agreement with the standard methods of analysis. 相似文献
102.
6-(Perfluoroalkyl)salicylates were prepared by [3+3] cyclization of 1,3-bis(silyl enol ethers) with 3-ethoxy-1-(perfluoroalkyl)prop-2-en-1-ones. 相似文献
103.
The synthesis of 3-methylpseudouridine (m(3)Psi) phosphoramidite, 5'-O-[benzhydryloxybis(trimethylsilyloxy)silyl]-2'-O-[bis(2-acetoxyethoxy)methyl]-3-methylpseudouridine-3'-(methyl-N,N-diisopropyl)phosphoramidite, is reported. Selective pivaloyloxymethyl protection of the Psi N1 followed by methylation at N3 was used to generate the naturally occurring pseudouridine analogue. The m(3)Psi phosphoramidite was used in combination with pseudouridine (Psi) and standard base phosphoramidites to synthesize a 19-nucleotide RNA representing helix 69 of Escherichia coli 23S ribosomal RNA (rRNA) (residues 1906-1924), containing a single m(3)Psi at position 1915 and two Psi's at positions 1911 and 1917. Our synthesis of the fully modified helix 69 RNA demonstrates the ability to make milligram quantities of RNA that can be used for further high-resolution structure studies. Site-selective introduction of the methyl group at the N3 position of pseudouridine at position 1915 causes a slight increase in the thermodynamic stability of the RNA hairpin relative to pseudouridine; RNAs containing either uridine or 3-methyluridine at position 1915 have similar stability. One-dimensional imino proton NMR and circular dichroism spectra of the modified RNAs reveal that the methyl group does not cause any substantial changes in the RNA hairpin structure. 相似文献
104.
Ibrahim MS 《Fresenius' Journal of Analytical Chemistry》2000,367(2):189-194
The electrochemical behavior of lumazine (LMZ), an important antibacterial agent, has been studied at the hanging mercury drop electrode (HMDE). The nature of the process taking place at the HMDE was clarified. Its adsorption behavior at HMDE has been studied by using a.c and cyclic voltammetry (CV). Both the molecule and its reduced product appeared to be adsorbed at the surface of the electrode. Controlled adsorptive accumulation of LMZ on the HMDE provides the basis for the direct stripping measurement of that compound in the subnanomolar concentration level. Experimental and instrumental parameters for the quantitative determination were optimized. Phase-selective a.c voltammetry provided the best signal and gave a detection limit of 0.15 microg L(-1) (9.0 x 10(-10) mol/L) LMZ in aqueous solution. Molecules or ions which may interfere were studied. 相似文献
105.
The helix-coil transition of poly[d(I-C)] and poly[d(A-T)] was studied as a function of hydrostatic pressure, temperature, and sodium ion concentration. These studies were undertaken in light of a recently published phase diagram for double stranded nucleic acids [Dubins et al. J. Am. Chem. Soc. 2001, 123, 9254-9259]. The sign and magnitude of the volume change for the heat-induced helix-coil transition, DeltaV(T), of poly[d(I-C)] and poly[d(A-T)] were dependent on the helix-coil transition temperature, T(M), at atmospheric pressure. The sign of DeltaV(T) changed from negative to positive as T(M) was increased by increasing the sodium ion concentration. For poly[d(I-C)], DeltaV(T) = 0 cm(3) mol(-1), when the sodium ion concentration is such that the spectroscopically monitored T(M) = 55 degrees C at atmospheric pressure. For poly[d(A-T)], the value of DeltaV(T) = 0 under conditions such that T(M) = 47 degrees C at atmospheric pressure. Negative values of DeltaV(T) imply that the helical form is destabilized at high pressure. Under experimental conditions where the DeltaV(T) for the transition is negative, the transition could be caused by increasing the pressure under isothermal conditions. At temperatures below T(M) measured at atmospheric pressure the midpoint of the pressure-induced helix-coil transition, P(M), decreases with increasing temperature. The volume change of the pressure-induced transitions helix-coil transition, DeltaV(P), was calculated assuming a two-state model. The magnitude of DeltaV(P) (per cooperative length) was much larger than the volume change (per base pair) measured for the heat-induced transition, DeltaV(T), calculated using the Clapeyron equation. The ratio of these two volume changes was used to calculate the cooperative length for the pressure-induced transition. This parameter depends strongly on temperature, becoming greater closer to T(M) measured at atmospheric pressure. At temperatures approaching T(M) the magnitude of the cooperative length of the pressure-induced transition is approximately twice that observed for the heat-induced transition (N(T)). On the basis of the temperature dependence of the DeltaV(T) for the two polymers the coefficient of thermal expansion of the two polymers was found to be 0.17 and 0.16 cm(3) K(-1) mol(-1) for poly[d(I-C)] and poly[d(A-T)], respectively. 相似文献
106.
Christine GozeDenis V. Kozlov Felix N. CastellanoRaymond Ziessel 《Tetrahedron letters》2003,44(48):8713-8716
The synthesis of novel ruthenium(II) bipyridine or terpyridine complexes bearing an increasing number of pyrene or toluyl moieties is described. The ruthenium complexes are constructed in a first step with ligands bearing the required bromine functions, followed in a second step by stepwise grafting of 1-ethynylpyrene or 4-ethynyltoluene promoted by Pd(0). A complex bearing a protected triethylsilylacetylene function was also prepared. In situ deprotection of this function with K2CO3 and cross-coupling with 1-bromopyrene afforded a soluble complex in which two pyrene moieties are linearly linked via ethynyl spacers to one of the bipyridine ligands. These highly coloured complexes exhibit well defined absorption and emission properties in solution at both rt and 77 K. 相似文献
107.
The microwave spectra of six isotopomers of HCl-N(2)O have been obtained in the 7-19 GHz region with a pulsed molecular beam, Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure due to all quadrupolar nuclei is resolved and the spectra are analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions. The spectroscopic constants determined include rotational constants, quartic and sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for each quadrupolar nucleus. Due to correlations of the structural parameters, the effective structure of the complex cannot be obtained by fitting to the spectroscopic constants of the six isotopomers. Instead, the parameters for each isotopomer are calculated from the A and C rotational constants and the chlorine nuclear quadrupole coupling constant along the a-axis, chi(aa). There are two possible structures; the one in which hydrogen of HCl interacts with the more electronegative oxygen of N(2)O is taken to represent the complex. The two subunits are approximately slipped parallel. For H (35)Cl-(14)N(2)O, the distance between the central nitrogen and chlorine is 3.5153 A and the N(2)O and HCl subunits form angles of 72.30 degrees and 119.44 degrees with this N-Cl axis, respectively. The chlorine and oxygen atoms occupy the opposite, obtuse vertices of the quadrilateral formed by O, central N, Cl, and H. Nuclear quadrupole coupling constants show that while the electric field gradient of the HCl subunit remains essentially unchanged upon complexation, there is electronic rearrangement about the two nitrogen nuclei in N(2)O. 相似文献
108.
Transmetalation reactions of metallic ytterbium with Hg[Co(CO)(4)](2) in the coordinating solvents pyridine and THF yield the solvent-separated ion pairs [Yb(L)(6)] [Co(CO)(4)](2) (1a, L = Pyr; 2a, L = THF). The IR spectrum of 1a in pyridine indicates that the tetracarbonylcobaltate anion is not directly bonded to the divalent Yb cation owing to the strong coordinating ability of pyridine. On the other hand, IR spectra of 2a in THF are concentration dependent. In dilute solutions there is an equilibrium between the solvent-separated ion pair and a weak contact ion pair. Higher concentrations of 2a facilitate the formation of a tight ion pair that has a low-frequency isocarbonyl absorption. Remarkably, complexes 1a and 2a are easily transformed in toluene into the two-dimensional sheetlike arrays [(Pyr)(4)Yb[(mu-CO)(2)Co(CO)(2)](2)](infinity) (1b) and [(THF)(2)Yb[(mu-CO)(3)Co(CO)](2).Tol](infinity) (2b). The two-dimensional frameworks are supported by isocarbonyl linkages. Infrared spectra of toluene solutions substantiate the existence of the isocarbonyl bridges with low-frequency absorptions at 1780 cm(-1). Compounds 1b and 2b belong to a rare class of lanthanide-transition-metal carbonyl extended arrays, only three others of which have been structurally established. Dissolving 1b in pyridine regenerates 1a, but the complete conversion of 2b into 2a cannot be achieved. Crystal data: 1a.Pyr is monoclinic, P2(1)/c, a = 11.171(1) A, b = 11.925(1) A, c = 33.978(1) A, beta = 95.10(1) degrees, Z = 4; 2a is monoclinic, C2/c, a = 17.724(1) A, b = 12.468(1) A, c = 18.413(1) A, beta = 100.34(1) degrees, Z = 4; 1b is monoclinic, C2/c, a = 11.047(1) A, b = 13.423(1) A, c = 21.933(1) A, beta = 103.49(1) degrees, Z = 4; 2b is monoclinic, C2/c, a = 28.589(1) A, b = 7.223(1) A, c = 14.983(1) A, beta = 118.90(1) degrees, Z = 4. 相似文献
109.
Six X-ray crystal structures are reported, all containing substituted triphenylmethanol derivative 4 either alone or as its mono or bis(chromium tricarbonyl) complexes. All four chromium complexes studied crystallize with two independent molecules in the crystallographic asymmetric unit. It is demonstrated that from the X-ray crystal structure of the acentric racemic (+/-)-(1pR,1' 'R)(1pS,1' 'S)-[Cr(CO)(3)(eta(6)-t-BuC(6)H(3)(CMeOMe)CPh(2)OH)], (+/-)-3, it is possible to deduce the 4-fold helical structure of the chiral (-)-(1pR,1' 'R) isomer, (-)-3. The bimetallic derivatives demonstrate the ability to control intermolecular interactions by the positioning of relative stereochemistry. 相似文献
110.
Mustafa R. Ibrahim Zacharia A. Fataftah Paul von Ragu Schleyer Peter D. Stout 《Journal of computational chemistry》1987,8(8):1131-1138
Sets of hydrogen molecule equivalents have been developed which permit the calculation of hydrogenation of different types of carbon-carbon bonds from ab initio total energies (3-21G and 6-31G* basis sets, and, to a more limited extent, for MP2/6-31G* data) of reactants and products. The calculated enthalpies of hydrogenation are in good agreement with experiment for unstrained molecules, with average errors on the order of 2 kcal/mol. The 6-31G* equivalents allow the enthalpies for strained molecules to be calculated accurately, but the 3-21G equivalents do not. The equivalents for both basis sets have been tested by calculating the enthalpies of hydrogenation of carbon-carbon bonds in nitrogen- and oxygen-containing organic molecules, free radicals, and classical carbocations. The results are in good agreement with experiment in most cases. 相似文献