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31.
Solutions of selenium(IV) standards with different acid matrices were stored in containers constructed of boosilicate glass, conventional polyethylene, and fluorinated ethylenepropylene (teflon FEP). After 50 days of storage in FEP, there were highly significant losses of Se(IV) from standards in either 5% HCl/5% H2SO4, or 5% H2SO4. Increasing the hydrochloric acid concentration, e.g., 15% HCl/5% H2SO4, greatly reduced this loss. Addition of selenium-75 (selenate-free) indicated that the losses did not result from physical adsorption onto container surfaces. It is shown that the losses were caused by oxidation of Se(IV) to Se(VI). 相似文献
32.
Bhatt AI May I Volkovich VA Collison D Helliwell M Polovov IB Lewin RG 《Inorganic chemistry》2005,44(14):4934-4940
The reduction of selected lanthanide cations to the zerovalent state in the room-temperature ionic liquid [Me3N(n)Bu][TFSI] is reported (where TFSI = bistriflimide, [N(SO2CF3)2]-). The lanthanide cations were introduced to the melt as the TFSI hydrate complexes [Ln(TFSI)3(H2O)3] (where Ln = La(III), Sm(III) or Eu(III)). The lanthanum compound [La(TFSI)3(H2O)3] has been crystallographically characterized, revealing the first structurally characterized f-element TFSI complex. The lanthanide in all three complexes was shown to be reducible to the metallic state in [Me3N(n)Bu][TFSI]. For both the Eu and Sm complexes, reduction to the metallic state was achieved via divalent species, and there was an additional observation of the electrodeposition of Eu metal. 相似文献
33.
A summary is given about hitherto studied and applied versions of sample pretreatment and separation techniques of89Sr and90Sr from fission and activation products in plate-out experiments for security studies of graphite moderated gas cooled high temperature pebble bed reactors. The latest and most succesfully applied version with a combined double-column system is described and its performance and results are discussed in detail. 相似文献
34.
Subpicosecond pulse generation has been examined in synchronously pumped mode-locked ring dye laser systems. These include hybrid and composite absorber/gain media arrangements as well as a simple synchronous cavity. The shortest pulses recorded were 0.3 ps for the hybrid system, and this has been shown to be critically dependent on the positioning of the absorber jet in the centre of the cavity to better than 50 m. Stable operation for subpicosecond pulse generation has been achieved in the ring configuration with greater wavelength tunability and higher average power conversion efficiency than with conventional cavity arrangements. 相似文献
35.
A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects.
This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode
potentiometry at 25.0‡C and ionic strengths (I) from 0.1 to 5.0M in NaCI media and atI = 1.0M in KC1 and Me4NCl media. These measurements provided evidence of weak but significant ionpairing between SO2/3 -and Na+ with a formation constant of logK
Na = 0.431 in 1.0M Me4NCl. This was in very good agreement with the value logK
Na = 0.410 measured directly by Na+ ion-selective electrode potentiometry. Evidence is also presented for an extremely weak association of K+ and SO
2
3
-with logK
k
= 0.22 in 1.0M Me4NCl. 相似文献
36.
May JP Fournier P Pellicelli J Patrick BO Perrin DM 《The Journal of organic chemistry》2005,70(21):8424-8430
This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl esters to the corresponding 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) amino acids with generally facile separation of syn-cis and anti-cis diastereomers. Structural X-ray diffraction data are presented for both diastereomers of Tr-Hpi-Gly-OMe, which allow for a putative assignment of the other 13 pairs of diastereomers reported herein, based on correlations with 1H NMR chemical shifts. Selective and high yielding deprotection at either the N or C terminus is described, allowing the Hpi motif to be introduced efficiently into potential targets with minimal protecting group manipulation. Two tripeptides containing Hpi and cysteine were prepared and treated with acid in the Savige-Fontana reaction to produce a cyclic tryptathionine linkage, characteristic of both amatoxins and phallotoxins. 相似文献
37.
Z. Y. Chen B. D. May A. W. Castleman Jr. 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,25(3):239-246
The potential energy surface (PES) of linear Ar 3 + is calculated at the MP4/6-31G* level including all single, double, triple and quadruple excitations. The results show that the PES of the linear Ar 3 + has a very flat valley along the asymmetric stretching vibration normal mode, ν3. A higher level quadratic configuration interaction calculation including single, double and triple substitutions QCISD (T) along this flat valley suggests that an asymmetric geometry energy minimum reported earlier based on MP2 [1] is due to symmetry breaking in UHF. The global minimum of the PES is found to be for the symmetric geometry atR ab =R bc =2.66±0.01 Å, which is in good agreement with the MRD-CI calculation [2] and expectations from our earlier photodissociation experiments [3]. The calculational results are compared with other theoretical calculations, and are discussed in the context of the photodissociation and dynamics of dissociation experiments conducted on Ar 3 + . 相似文献
38.
Ralf Wischnat Joachim RudolphRoman Hanke Roger KaeseAchim May Heidi TheisUndine Zuther 《Tetrahedron letters》2003,44(23):4393-4394
An efficient scalable synthesis of 2-halothiazolium-type peptide coupling reagents has been developed. The key step is the formation of the 2-bromothiazole scaffold through cyclization of α-thiocyanato ketones with hydrogen bromide. Using this method, the new coupling reagent 2-bromo-N-methylthiazolium bromide (BMTB) was synthesized. BMTB was tested in a difficult model coupling reaction of two sterically hindered N-methylated amino acids and showed higher activity than the well-established peptide coupling reagent HATU. 相似文献
39.
H. Abramowicz F. Dydak J. G. H. de Groot J. Knobloch J. May P. Palazzi F. Ranjard W. D. Schlatter J. Steinberger H. Taureg W. von Rüden H. Wahl J. Wotschack P. Buchholz F. Eisele H. P. Klasen K. Kleinknecht B. Pszola B. Renk H. J. Willutzki T. Flottmann C. Geweniger J. Krolikowski K. Tittel C. Guyot J. P. Merlo A. Para B. Peyaud J. Rander J. Rothberg J. P. Schuller R. Turlay J. T. He T. Z. Ruan W. M. Wu 《Zeitschrift fur Physik C Particles and Fields》1982,12(3):225-227
Right handed weak quark currents coupled to the usual left handed weak lepton current would be seen in inclusive antineutrino scattering on nuclei as a contribution at largey with the quark (not antiquark) structure function. We do not see such a term, and can therefore put an upper limit on the relative strengths of such right handed currents: \(\varrho ^2 = \frac{{\sigma _R }}{{\sigma _L }}< 0.009\) , 90% confidence. This measurement puts limits on the mixing angle of left-right symmetric models. In distinction to similar limits derived from muon decay or β decay, our limits are also valid if the right handed neutrino is heavy. 相似文献
40.
The 25% niobium substituted crystalline titanosilicate with the composition Na1.5Nb0.5Ti1.5O3SiO4·2H2O (Nb-TS) was synthesized under hydrothermal conditions. Its selectivity for radioactive 137Cs and 89Sr was compared with the TS, Na2Ti2O3SiO4·2H2O, having sitinakite topology. The Nb-TS shows significantly higher uptake value for 137Cs but lower for 89Sr than the TS. To investigate the origin of selectivity, the ion exchanged Cs+ and Sr2+ forms with the composition, CsxNaHyNb0.5Ti1.5O3SiO4·zH2O (x=0.1, 0.2 and 0.3, x+y=0.5 and z=1-2) and Sr0.2Na0.6H0.5Nb0.5Ti1.5O3SiO4·H2O, respectively, were structurally characterized from the X-ray powder diffraction data using the Rietveld refinement technique. Simultaneously the kinetics of 137Cs and 89Sr uptake was investigated for the NbV free and doped samples. While the Cs+ and Sr2+ exchanged form of Nb-TS and the Cs+ exchanged form of TS retain the symmetry of the parent compound, the Sr2+ exchanged form of TS undergoes a symmetry change. The differences in the uptake of Cs+ and Sr2+ result from the different coordination environments of cesium and strontium in the eight-ring channel, that result from various hydration sites in the tunnel. The origin of selectivity appears to arise from the higher coordination number of cesium or strontium. Other effects due to NbV substitution are reflected in the increase of both, the a- and c-dimensions and thus the unit cell volume, and the population of water vs. Na+ in the channel to charge-balance the Nb5+↔Ti4+ substitution. 相似文献