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排序方式: 共有197条查询结果,搜索用时 264 毫秒
51.
Venkataramanan NS Premsingh S Rajagopal S Pitchumani K 《The Journal of organic chemistry》2003,68(19):7460-7470
The kinetics of oxygenation of several para-substituted phenyl methyl sulfides and sulfoxides with a series of 5-substituted and sterically hindered oxo(salen)chromium(V) complexes have been studied by a spectrophotometric technique. Though the reaction of sulfides follows simple second-order kinetics, sulfoxides bind strongly with the metal center of the oxidant and the oxygen atom is transferred from the oxidant-sulfoxide adduct to the substrate. The reduction potentials, E(red), of eight Cr(V) complexes correlate well with the Hammett sigma constants, and the reactivity of the metal complexes is in accordance with the E(red) values. The metal complexes carrying bulky tert-butyl groups entail steric effects. Organic sulfides follow a simple electrophilic oxidation mechanism, and the nonligated sulfoxides undergo electrophilic oxidation to sulfones using the oxidant-sulfoxide adduct as the oxidant. Sulfoxides catalyze the Cr(V)-salen complexes' oxygenation of organic sulfides, and the catalytic activity of sulfoxides is comparable to pyridine N-oxide and triphenylphosphine oxide. The rate constants obtained for the oxidation of sulfides and sulfoxides clearly indicate the operation of a pronounced electronic and steric effect in the oxygenation reaction with oxo(salen)chromium(V) complexes. 相似文献
52.
53.
Vyacheslav N. Fishman Bruno Linclau Dennis P. Curran Kasi V. Somayajula 《Journal of the American Society for Mass Spectrometry》2001,12(9):1050-1054
A new fluorinated compound mixture has been developed for the calibration of the double focusing mass spectrometer in the mass range of 100–3000 Da in the positive electron ionization (EI) mode. Current calibration standards for EI have either limited mass range [perfluorotributylamine (PFTBA), perfluorokerosene (PFK), s-triazines (TRIS)] or poor peak intensities with significant chemical background in the instrument for several days (perfluoroalkyl phosphazine). The newly synthesized fluorinated silyl alkyl amines mixture is proposed as a reference/calibration standard for EI-MS. This standard produced abundant parent and fragment ions across the entire mass range without any memory effect. 相似文献
54.
Viswanath RN Kramer D Weissmüller J 《Langmuir : the ACS journal of surfaces and colloids》2005,21(10):4604-4609
We report an experimental study of the variation of surface stress with surface charge density for nanoporous Pt immersed in aqueous solutions of NaF of various concentration. As the concentration is reduced, we find initially an increase in the magnitude of the surface stress-charge coefficient, followed by saturation at a value of -1.9 V. Since specific adsorption is expected to be reduced as the solution becomes more dilute, the results support the notion that changes in the bonding at the metal surface play a decisive role in determining the change in the surface stress during double-layer charging. 相似文献
55.
56.
Rampur S. Viswanath Philip J. Miller Charles A. Cody 《Journal of Physics and Chemistry of Solids》1979,40(3):223-229
IR transmission, near-normal IR reflection and polarized Raman spectra of single crystals of selenious acid (H2SeO3) have been obtained at temperatures between 77°K and 335°K. The observed bands in the O-H stretching region have been assigned to two types of O-H….O bonds in the crystal structure. The spectral results appear to rule out correlation field splittings, intramolecular coupling, or Fermi resonance as explanations for the observed splitting. A proposed model based on intermolecular coupling of O-H….O vibrations to explain the splitting of the O-H stretching region is given. 相似文献
57.
The adsorption of oxygen on W(100) single crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED) and retarding field work function measurements with the aim of obtaining a better understanding of the adsorption kinetics and of the structures of the adsorbed layer. The AES results reveal step-wise changes of the sticking coefficients in the coverage range 0 to 1, and activated adsorption at higher coverages. Upon room temperature adsorption a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the well-known p(2 × 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 K and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room temperature adsorbates changes their structure irreversibly. At temperatures below 750 K some new structures are observed. Combining the results obtained in this study with other published work leads to a considerable revision of the previously accepted model of the adsorption of oxygen on W(100). 相似文献
58.
The co-adsorption of Cu on O2 and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases sO, the sticking coefficient of O2. For longer oxygen exposures and for higher adsorption temperatures, θO reaches values larger than those on the clean surface for the same O2 exposure. Except at the highest θO values and temperatures, the sticcking coefficient for copper, sCu, is unity and is independent of the oxygen coverage θO in the range studied (0 ? θO ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation . The work function is a complicated function of θO and θCu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior. 相似文献
59.
Sevagapandian Sankarapandian Rajagopal Gurusamy Nehru Kasi Athappan Periakaruppan 《Transition Metal Chemistry》2000,25(4):388-393
Complexes of the general formula, ML2 [M = CuII, NiII, CoII and OVIV; L = 1,2,3,5,6,7,8,8a-octahydro-3-hydroxyimino-N-(4-X-phenyl)-l-phenyl-5-(phenylmethylene)-2-naphthalenecarboxamide (X =
H, Me, OMe, Cl)] have been prepared and characterized on the basis of elemental analysis, magnetic moments and i.r., e.p.r.
and electronic spectra. These metal complexes contain the N4 chromophore with the ligand coordinating through nitrogens of the azomethine and deprotonated anilide functions. C.v. measurements
indicate that the copper(II) complexes are quasi-reversible in acetonitrile solution. Square planar and square pyramidal structures
are assigned respectively to the copper(II) and oxovanadium(IV) complexes, whereas tetrahedral geometry is assigned to the
nickel(II) and cobalt(II) complexes. Deprotonated anilide nitrogen is involved in coordination and the presence of an electron-donating
group para to the anilide function decreases the ΔE values of the d–d transitions while the value is found to increase when electron-withdrawing groups are substituted. Line
spacing in the e.p.r. spectra of the copper(II) and oxovanadium(IV) complexes increases when methyl group is para to the anilide group, and decreases when this group is replaced by methoxy or chloro. The ν(C–N) of the anilide group and
the ν(C-N) of the azomethine function of the oxime metal complexes are metal-sensitive and the blue shift for the above stretching
frequencies follows the order: copper(II) > oxovanadium(IV) > nickel(II) ≈ cobalt(II).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
60.
Anusree Viswanath Kuttatheyil Marcel Handke Jens Bergmann Dr. Daniel Lässig Dr. Jörg Lincke Prof. Dr. Jürgen Haase Dr. Marko Bertmer Prof. Dr. Harald Krautscheid 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(3):1118-1124
Spectroscopic techniques are a powerful tool for structure determination, especially if single‐crystal material is unavailable. 113Cd solid‐state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical‐shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by 113Cd solid‐state NMR is reported. The results obtained demonstrate that 113Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non‐single‐crystal materials. Furthermore, this method allows the observation of guest‐induced phase transitions supporting understanding of the structural flexibility of coordination frameworks. 相似文献