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排序方式: 共有205条查询结果,搜索用时 15 毫秒
41.
Chloe C. Ence Whitney K. Walker Ryjul W. Stokes Erin E. Martinez Spencer M. Sarager Stacey J. Smith David J. Michaelis 《Tetrahedron》2019,75(24):3341-3347
The synthesis of six chiral titanium-containing phosphinoamide ligands is discussed. These ligands assemble chiral heterobimetallic Pd–Ti complexes, enable enantioselective intramolecular allylic aminations with hindered amine nucleophiles and achieve selectivity up to 53% ee. Mechanistic studies demonstrate the reversibility of the enantio-determining C–N bond forming step, which leads to a gradual increase in the % ee of the reaction over time. These results represent a rare example of enantioselective heterobimetallic catalysis and suggest that these new ligands could find broad application in enantioselective transition metal catalysis. 相似文献
42.
Vaishnavi Rajagopal Chris Stokes Alessandra Ferzoco 《Journal of the American Society for Mass Spectrometry》2018,29(2):260-269
We report a custom-geometry linear ion trap designed for fluorescence spectroscopy of gas-phase ions at ambient to cryogenic temperatures. Laser-induced fluorescence from trapped ions is collected from between the trapping rods, orthogonal to the excitation laser that runs along the axis of the linear ion trap. To increase optical access to the ion cloud, the diameter of the round trapping rods is 80% of the inscribed diameter, rather than the roughly 110% used to approximate purely quadrupolar electric fields. To encompass as much of the ion cloud as possible, the first collection optic has a 25.4 mm diameter and a numerical aperture of 0.6. The choice of geometry and collection optics yields 107 detected photons/s from trapped rhodamine 6G ions. The trap is coupled to a closed-cycle helium refrigerator, which in combination with two 50 Ohm heaters enables temperature control to below 25 K on the rod electrodes. The purpose of the instrument is to broaden the applicability of fluorescence spectroscopy of gas-phase ions to cases where photon emission is a minority relaxation pathway. Such studies are important to understand how the microenvironment of a chromophore influences excited state charge transfer processes. 相似文献
43.
Braddock DC Bhuva R Millan DS Pérez-Fuertes Y Roberts CA Sheppard RN Solanki S Stokes ES White AJ 《Organic letters》2007,9(3):445-448
[reaction: see text] Sharpless asymmetric dihydroxylation was regioselective for the trans olefin in an E vs Z vs terminal triene substrate. To test a biosynthetic hypothesis, the resulting diol underwent diastereoselective bromoetherification to provide the des-chloro core of marine natural products obtusallenes II and IV. Alternatively, anionic chloride ring-opening of a Z-beta,gamma-unsaturated epoxide gave separable regioisomeric halohydrins. Bromoetherification gave the fully elaborated core of obtusallenes II and IV with all of the relative stereochemistry correctly set. 相似文献
44.
Grubisic A Li X Stokes ST Cordes J Ganteför GF Bowen KH Kiran B Jena P Burgert R Schnöckel H 《Journal of the American Chemical Society》2007,129(18):5969-5975
Anion photoelectron spectroscopy and density functional theory were employed to study aluminum hydride clusters, AlnHm- (4 相似文献
45.
Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz 总被引:2,自引:0,他引:2
The potential of red thermoluminescence (RTL) emission from quartz, as a dosimeter for baked sediments and volcanic deposits, has received some attention over the past decade. While there have been some important observations relating to signal stability, saturation characteristics and emission wavebands, there has not been a systematic analysis of RTL properties of older (i.e., >1 Ma), quartz-bearing known age volcanic deposits. We have undertaken such an analysis using independently-dated silicic volcanic deposits from New Zealand, ranging in age from 300 ka through to 1.6 Ma. We observed a complex RTL emission in most volcanic quartzes, which consists of a number of discrete high temperature (i.e, >220°C) TL peaks. Isothermal analysis indicates a stable dating trap (E=2.03 eV; S=4.20×1015) which is stable at ambient (c. 20°C) temperatures for >109 a. We confirm the slow onset of saturation with dose, and the limited extent of sensitivity changes due to dosing and TL readout. As such, there is much potential for exploiting the dosimeter in dating studies and we present the results from a modified single aliquot regenerative (SAR) procedures which indicate that there is a good agreement between RTL dating and other methods over time scales 105–106 a. This paper presents a summary of the most important related results of our findings and outlines the configuration of photomultiplier and filter combinations which maximizes RTL detection for temperatures up to 500°C. 相似文献
46.
Stephen J. Valentine Sarah T. Stokes Ruwan T. Kurulugama Fabiane M. Nachtigall David E. Clemmer 《Journal of the American Society for Mass Spectrometry》2009,20(5):738-750
The transport of ions through multiple drift regions is modeled to develop an equation that is useful for an understanding
of the resolving power of an overtone mobility spectrometry (OMS) technique. It is found that resolving power is influenced
by a number of experimental variables, including those that define ion mobility spectrometry (IMS) resolving power: drift
field (E), drift region length (L), and buffer gas temperature (T). However, unlike IMS, the resolving power of OMS is also influenced by the number of drift regions (n), harmonic frequency value (m), and the phase number (Φ) of the applied drift field. The OMS resolving power dependence upon the new OMS variables (n, m, and Φ) scales differently than the square root dependence of the E, L, and T variables in IMS. The results provide insight about optimal instrumental design and operation. 相似文献
47.
Timon Geib Lekha Sleno Rabea A. Hall Caroline S. Stokes Dietrich A. Volmer 《Journal of the American Society for Mass Spectrometry》2016,27(8):1404-1410
We describe a systematic comparison of high and low resolution LC-MS/MS assays for quantification of 25-hydroxyvitamin D3 in human serum. Identical sample preparation, chromatography separations, electrospray ionization sources, precursor ion selection, and ion activation were used; the two assays differed only in the implemented final mass analyzer stage; viz. high resolution quadrupole-quadrupole-time-of-flight (QqTOF) versus low resolution triple quadrupole instruments. The results were assessed against measured concentration levels from a routine clinical chemiluminescence immunoassay. Isobaric interferences prevented the simple use of TOF-MS spectra for extraction of accurate masses and necessitated the application of collision-induced dissociation on the QqTOF platform. The two mass spectrometry assays provided very similar analytical figures of merit, reflecting the lack of relevant isobaric interferences in the MS/MS domain, and were successfully applied to determine the levels of 25-hydroxyvitamin D for patients with chronic liver disease. 相似文献
48.
Warren SA Stokes S Frampton CS White AJ Spivey AC 《Organic & biomolecular chemistry》2012,10(24):4685-4688
The first synthesis of hydroxy-iso-evoninic acid (2), a pyridyl diacid found as a macrodilactone bridging ligand in bioactive Celastraceae sesquiterpenoid-based natural products, has been achieved in 9 steps and an overall yield of 26%. The synthesis utilizes a benzilic ester rearrangement (BER) and a late stage benzylic oxidation to give access to all four stereoisomers whose absolute stereochemistry was assigned following chromatographic separation and anomalous dispersion X-ray crystallography. 相似文献
49.
A. Grubisic X. Li S. T. Stokes J. Cordes G. F. Gantefoer K. H. Bowen B. Kiran P. Jena R. Burgert H. Schnoeckel 《ChemInform》2007,38(31):no-no
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. 相似文献
50.
Z Chi B M Cullum D L Stokes J Mobley G H Miller M R Hajaligol T Vo-Dinh 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2001,57(7):1377-1384
In this work, we present the fluorescence spectra of anthracene and pyrene vapors at different elevated temperatures (from 150 to 650 degrees C) excited with the 337 nm line of a nitrogen laser. We describe the high temperature effects on the resulting spectral properties including spectral intensity, spectral bandwidth and spectral shift. We found that the PAH fluorescence spectral bandwidths become very broad as the temperature increases. The broadening is mainly due to thermal vibrational sequence congestion. We also have found that the fluorescence intensity of pyrene vapor increases with increasing temperature, which results from the increase of the pyrene vapor absorption cross section at 337 nm. 相似文献