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51.
A new class of nonstoichiometric layered ternary rare-earth tin antimonides, RESn(x)()Sb(2) (RE = La, Ce, Pr, Nd, Sm), has been synthesized through reaction of the elements at 950 degrees C. In the lanthanum series LaSn(x)()Sb(2), tin can be incorporated from a maximum content of x approximately 0.7 or 0.8 to as low as x approximately 0.10. The structure of lanthanum tin diantimonide with the maximum tin content, LaSn(0.75)Sb(2), has been determined by single-crystal X-ray diffraction methods. It crystallizes in the orthorhombic space group -Cmcm with a = 4.2425(5) ?, b = 23.121(2) ?, c = 4.5053(6) ?, and Z = 4. The isostructural rare-earth analogues were characterized by powder X-ray diffraction. The structure of LaSn(0.75)Sb(2) comprises layers of composition "LaSb(2)" in which La atoms are coordinated by Sb atoms in a square-antiprismatic geometry. Between these layers reside chains of Sn atoms distributed over three crystallographically independent sites, each partially occupied at about 20%. The structure of LaSn(0.75)Sb(2) can be regarded as resulting from the excision of RE-Sb and Sb-Sb bonds in the related structures of binary rare-earth diantimonides, RESb(2), and then intercalation of Sn atoms between layers. 相似文献
52.
4-Hydroxythiazoles and N-acylimino 4-thiazolidinones show similar tautomeric behaviour to azlactones. However, the mechanism of their reaction with dimethylacetylenedicarboxylate or with ethyl vinyl ether differs fundamentally. In the thiazole series, the first step is the quaternization of the nitrogen atom. The mesoionic thiazole intermediate is then trapped by a second molecule of the dipolarophile. 相似文献
53.
Protonation constants of arsenic acid were determined at different ionic strengths in NaClO(4) (0.1, 0.5, 1.0, 3.0 mol dm(-3)), NaCl (0.5 and 1.0 mol dm(-3)) and KCl (0.5, 1.0 and 3.0 mol dm(-3)) ionic media by means of a potentiometric study. The distribution of arsenate species was defined depending on two important variables in natural environments: pH and composition. All the experimentation was performed at 25 degrees C. The differences found in the protonation constants for different medium compositions, were explained by the different behaviour of the interaction parameters of the species considered in the different media and ionic strengths. These parameters were reported for all hydrolitic As(V) species and were calculated using the Modified Bromley's Methodology (MBM). The corresponding thermodynamic stepwise formation constants were also determined (log degrees K(1)=11.58+/-0.01, log degrees K(2)=7.06+/-0.01, log degrees K(3)=2.25+/-0.01). All the results obtained showed not only the importance of the ionic strength but also of the composition of the ionic medium on the distribution of the acid-base species of As(V) as a function of pH in natural waters. 相似文献
54.
García-Raso A Fiol JJ Tasada A Albertí FM Molins E Basallote MG Máñez MA Fernández-Trujillo MJ Sánchez D 《Dalton transactions (Cambridge, England : 2003)》2005,(23):3763-3772
Alkylidene-bis(2-aminopyrimidines) (pyr2Cx, x = 2-5) are useful ligands to interact with Ag(I) yielding discrete metallocycles. Crystal structures of the [(pyr2C2)Ag(NO3)]2 and [(H-pyr2C4)Ag(NO3)2]2 have been isolated where each macrocyclic moiety interacts with their surroundings through weak interactions, yielding 3D discrete structures, On the other hand, the solution study shows that the equilibrium constants for the formation of Ag(pyr2Cx)+ complexes are higher than the literature values for Ag(I) complexes with single pyrimidines, although the differences could be explained by invoking the solid-state structures of the Ag(I)-pyr2Cx complexes. 相似文献
55.
Jose L. Zurita Maria L. G. de Soria Miguel A. Postigo Miguel Katz 《Journal of solution chemistry》1987,16(2):163-170
Densities, viscosities, enthalpies, vapor-liquid equilibria, and surface tensions were determined at 25°C for the 2-propanol+dichloromethane system. From the experimental results excess volumes, viscosities, enthalpies, Gibbs energies, and excess surface tensions were calculated. An attempt has been made to explain the observed deviations from ideal behavior on the basis of intermolecular interactions. 相似文献
56.
Bayó N Jiménez JC Rivas L Nicolás E Albericio F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1096-1103
An optimized solid-phase strategy for the preparation of the cyclic lipononadepsipeptide [N-Mst(L-Ser1), D-Ser4, L-Thr6, L-Asp8, L-Thr9]syringotoxin is reported. The strategy is based on the use of a mild orthogonal protection scheme and the incorporation of the nonproteinogenic amino acid (Z)-Dhb into the peptide chain as the dipeptide Fmoc-Thr(tBu)-(Z)-Dhb-OH. The didehydrodipeptide was synthesized by a water-soluble carbodiimide-induced beta-elimination of a protected dipeptide containing a residue of Thr with its free hydroxy side chain unprotected. 相似文献
57.
Maréchal X Mordon S Devoisselle JM Bégu S Guery B Nevière R Buys B Dhelin G Lesage JC Mathieu D Chopin C 《Photochemistry and photobiology》1999,70(5):813-819
Measurement of gastrointestinal intramucosal pH (pHim) has been recognized as an important factor in the detection of hypoxia-induced dysfunctions. However, current pH measurement techniques are limited in terms of time and spatial resolutions. A major advance in accurate pH measurement was the development of the ratiometric fluorescent indicator dye, 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). This study aimed to set up and validate a fluorescence imaging technique to measure in vivo the intramucosal pH (pHim) of the intestine. The intestine was inserted into an optical chamber placed under a microscope. Animals were injected intravenously with the pH-sensitive fluorescent dye BCECF. Fluorescence was visualized by illuminating the intestine alternately at 490 and 470 nm. The emitted fluorescence was directed to an intensified camera. The ratio of emitted fluorescence at excitation wavelengths of 490 and 470 nm was measured, corrected and converted to pHim by constructing a calibration curve. The pHim controls were performed with a pH microelectrode and were correlated with venous blood gas sampling. Results show that pHim is determined with an accuracy of +/- 0.07 pH units and a response time of 1 min. In conclusion pHim mapping of rat intestine can be obtained by fluorescence imaging using BCECF. This technology could be easily adapted for endoscopic pH measurements. 相似文献
58.
Ejnik JW Todorov TI Mullick FG Squibb K McDiarmid MA Centeno JA 《Analytical and bioanalytical chemistry》2005,382(1):73-79
Urine uranium concentrations are the best biological indicator for identifying exposure to depleted uranium (DU). Internal exposure to DU causes an increased amount of urine uranium and a decreased ratio of 235U/238U in urine samples, resulting in measurements that vary between 0.00725 and 0.002 (i.e., natural and depleted uraniums 235U/238U ratios, respectively). A method based on inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS) was utilized to identify DU in urine by measuring the quantity of total U and the 235U/238U ratio. The quantitative analysis was achieved using 233U as an internal standard. The analysis was performed both with and without the reaction gas oxygen. The reaction gas converted ionized 235U+ and 238U+ into 235UO2+ (m/z=267) and 238UO2+ (m/z=270). This conversion was determined to be over 90% efficient. A polyatomic interference at m/z 234.8 was successfully removed from the 235U signal under either DRC operating conditions (with or without oxygen as a reaction gas). The method was validated with 15 urine samples of known uranium compositions. The method detection limit for quantification was determined to be 0.1 pg U mL–1 urine and an average coefficient of variation (CV) of 1–2% within the sample measurements. The method detection limit for determining 235U/238U ratio was 3.0 pg U mL–1 urine. An additional 21 patient samples were analyzed with no information about medical history. The measured 235U/238U ratio within the urine samples correctly identified the presence or absence of internal DU exposure in all 21 patients.The opinions and assertions expressed herein are those of the authors and are not to be construed as official or as representing the views of the Armed Forces Institute of Pathology, the Department of the Army, or the Department of Defense 相似文献
59.
Kui Wu Nathan A. Yee Sangeetha Srinivasan Amir Mahmoodi Michael Zakharian Jose M. Mejia Oneto Maksim Royzen 《Chemical science》2021,12(21):7583
Correction for ‘Click activated protodrugs against cancer increase the therapeutic potential of chemotherapy through local capture and activation’ by Kui Wu et al., Chem. Sci., 2021, 12, 1259–1271, DOI: 10.1039/D0SC06099B.The authors regret that the reference to the bond-breaking bioorthogonal chemistry, termed ‘click-to-release’ was omitted from the original article. In addition, we would like to include a reference describing the synthesis of compound 1, which is an intermediate to the protodrugs described in the original article. These references are listed below as ref. 1 and 2.The Royal Society of Chemistry apologizes for these errors and any consequent inconvenience to authors and readers. 相似文献
60.
Dinglasan JA Bailey M Park JB Dhirani AA 《Journal of the American Chemical Society》2004,126(20):6491-6497
Planar tunnel junctions were fabricated by self-assembling 1,1'- ferrocenedicarboxylic acid (FDCA) onto native oxides of thermally deposited aluminum films and subsequently depositing a second aluminum film. Junctions were characterized using Reflection-Absorption Fourier Transform Infrared Spectroscopy (RAIRS) and current-voltage (I-V) spectroscopy. Before deposition of the second aluminum film, RAIRS of FDCA and ferrocenecarboxylic acid (FCA) films revealed COO(-), C=O, and Fc ring stretching modes, indicating that both types of molecules can interact strongly with the oxide and remain intact. After deposition, systems exhibited prominent COO(-) modes and weakened C=O modes, indicating further reaction with aluminum/aluminum oxide. Fc ring modes persisted in FDCA systems but disappeared in FCA systems, suggesting that the second COOH group in the FDCA molecule can act as a protecting group for the ferrocene moiety. Cyclic I-V measurements of FDCA tunnel junction systems revealed very strong ( approximately 10-fold) hysteretic differential conductance switching that was both reversible and stable. Control measurements using as prepared junctions, as well as junctions containing 1,6-hexanedioic acid, 1,9-nonanedioic acid, 1,4-dibenzoic acid, or FCA revealed only very weak ( approximately 10%) differential conductance changes. We attribute FDCA junction switching to barrier profile modifications induced by oxidation/reduction of the functionally protected ferrocene moieties. 相似文献