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51.
Kickham JE Guérin F Stewart JC Stephan DW 《Angewandte Chemie (International ed. in English)》2000,39(18):3263-3266
52.
The occurrence of electrochemical processes during the operation of an electrospray ionization (ESI) source is well established. In the positive ion mode, electrons are drawn from the ESI metal capillary to a high voltage power supply. These electrons are the product of charge-balancing oxidation reactions taking place at the liquid/metal interface of the ion source. In a recent study, (Anal. Chem.2001, 73, 4836-4844), our group has shown that the introduction of a ground loop can dramatically enhance the rate of these oxidation processes. Such a ground loop can be introduced by connecting the sample infusion syringe (or the liquid chromatography column, in the case of LC-MS studies) to ground. The magnitude of the ground loop current can be controlled by the electrolyte concentration in the analyte solution, and by the dimensions of the capillary connecting the syringe needle and the ESI source. Using ferrocene as a model system, it is demonstrated that the introduction of such a ground loop can significantly enhance the signal intensity of analytes that form electrochemically ionized species during ESI. However, analytes that form protonated molecular ions, such as reserpine, also show higher signal intensities when a ground loop is introduced into the system. This latter observation is attributed to the occurrence of electrolytic solvent (acetonitrile and/or water) oxidation processes. These reactions generate protons within the ion source, and thus facilitate the formation of [M + nH](n+) ions. Overall, this work provides an example of how the careful control of electrochemical parameters can be exploited to optimize signal intensities in ESI-MS. 相似文献
53.
《Journal of organometallic chemistry》2006,691(24-25):5517-5523
Masked hexayne 18 was prepared in 11 steps from commercially available reagents. The four butenyl substituents contained within the two arylsilane residues in 18 have been used in a double ring-closing-metathesis operation in an attempt to encapsulate the π-conjugated framework. When 18 was treated with Grubbs’ 1st generation metathesis catalyst however, double ring-closing metathesis provided macrocycle 19 as the major product in good yield. Reasons why the macrocycle in 19 crowns, rather than encapsulates, the π-conjugated framework are discussed. 相似文献
54.
Dubé T Conoci S Gambarotta S Yap GP Vasapollo G 《Angewandte Chemie (International ed. in English)》1999,38(24):3657-3659
The cooperative attack of four (dipyrromethanyl)Sm(II) units on dinitrogen resulted in a novel tetranuclear samarium dinitrogen complex (shown schematically). The presence of halogen atoms inhibited reactivity with dinitrogen through the assembly of divalent samarium clusters. dipyrr=diphenylmethyldipyrrolide dianion. 相似文献
55.
Reactions of the ionized enol tautomer of acetanilide: elimination of HNCO via a novel rearrangement
Heydorn LN Carter LM Bowen RD Terlouw JK 《European journal of mass spectrometry (Chichester, England)》2003,9(4):343-350
The reactions of ionised acetanilide, C(6)H(5)NH(=O)CH(3)(.+), and its enol, C(6)H(5)NH(OH)=CH(2)(.+), have been studied by a combination of tandem mass spectrometric and computational methods. These two isomeric radical cations have distinct chemistries at low internal energies. The keto tautomer eliminates exclusively CH(2)=C=O to give ionised aniline. In contrast, the enol tautomer loses H-N=C=O, via an unusual skeletal rearrangement, to form predominantly ionised methylene cyclohexadiene. Hydrogen atom loss also occurs from the enol tautomer, with the formation of protonated oxindole. The mechanisms for H-N=C=O and hydrogen atom loss both involve cyclisation; the former proceeds via a spiro transition state formed by attachment of the methylene group to the ipso position, whereas the latter entails the formation of a five-membered ring by attachment to the ortho position. The behaviour of labelled analogues reveals that these two processes have different site selectivities. Hydrogen atom loss involves a reverse critical energy and is subject to an isotope effect. Surprisingly, attempts to promote the enolisation of ionised acetanilide by proton-transport catalysis were unsuccessful. In a reversal of the usual situation for ionised carbonyl compounds, ionised acetanilide is actually more stable than its enol tautomer. The enol tautomer was resistant to proton-transport catalysed ketonisation to ionised acetanilide, possibly because the favoured geometry of the encounter complex with the base molecule is inappropriate for facilitating tautomerisation. 相似文献
56.
The small gap distance separating a spherical colloidal particle in electrophoretic motion from a planar nonconducting surface is a required parameter for calculating its electrophoretic mobility. In the presence of an externally applied electric field, this gap distance is determined by balancing the van der Waals, electrical double layer interaction, and gravitational forces with a dielectrophoretic (DEP) force. Here, the DEP force was determined analytically by integration of the Maxwell stress over the surface of the particle. The account of this force showed that its previous omission from the analysis always resulted in underpredicted gap distances. Furthermore, the DEP force dominated under conditions of low particle density or high electric field strength and led to much higher gap distances on the order of a few microns. In one particular case, a combination of low particle density and small particle size produced two possible equilibrium gap distances for the particle. However, the particle was unstable in the second equilibrium position when subjected to small perturbations. In general, larger particles had smaller gap sizes. The effects of four other parameters on gap distance were studied, and gap distances were found to increase with lower particle density, higher electric field strength, higher particle and wall zeta potentials, and lower Hamaker constants. Retardation effects on van der Waals attraction were considered. 相似文献
57.
Summary The various techniques and methodologies of thermal conductivity measurement have been conventionally based on the determination of the rate of directional heat flow through a material having a unit temperature differential between its opposing faces. The constancy of this rate depends on the material density, its thermal resistance and the heat flow path itself. The last of these variables contributes most significantly to the true value of steady-state axial and radial heat dissipation depending on the magnitude of transient thermal diffusivity along these directions. The purpose of this paper is to exemplify the above features by defined parameters of heat flow measurement by existing methodologies. No new method is proposed here. Importantly, the relationship between the rate of heat transfer, total heat transferred and thermal conductivity at a given temperature under steady-state conditions for a fixed heat flow path will be illustrated. 相似文献
58.
The reaction of 4, 7-phenanthroline (1) with aqueous transitionmetal complexes [Mn(H2O)6][NO3]2, [Co(H2O)6][NO3]2, [Ni(H2O)6[NO3]2, [Mn(H2O)6][ClO4]2, and [Co(H2O)6][ClO4]2 does not produce coordination complexes between these metal cations and the N-donor ligand as expected. Instead, supramolecular hydrogenbonded networks are formed between the nitrogen donor atoms of 4, 7-phenanthroline and the OH groups of coordinated water molecules: M-O-H...N interactions. This motif of second-sphere coordination for 1 can be exploited as a tool for crystal engineering. As a demonstration of the generality of this new interaction as a supramolecular building block, five X-ray crystal structures are reported that utilise this hydrogen bonding scheme; [Co(H2O)4(NO3)2].(1)2 (2a), [Co(MeCN)2(H2O)4][ClO4]2.(1)2 (2b), [Ni(H2O)4(NO3)2].(1)2 (3a), [Mn(H2O)4(NO3)2].(1)2 (4a), and [Mn(H2O)6][ClO4]2.(1)(4).4H2O (4b). Each network involves complete saturation of the hydrogen-bond donor sets between the aqua complex and 1 using primarily M-O-H...N(1) and M-O-H...O(anion), interactions. Thermogravimteric analysis shows these materials to have stabililities similar to coordination polymers involving metal-ligand bonds; this demonstrates that second-sphere hydrogen bonding has potential for the construction of polymeric metal-containing materials. 相似文献
59.
《Chemical physics letters》2006,417(1-3):246-250
Ionic fragmentation of core-excited α-alanine in gas phase was observed. The most dominant ionic species is COOH+ for all core-ionizations at C 1s, N 1s, and O 1s. An increase in COO+ and a decrease in COOH+, which were observed as core-hole atom selectivity for the O 1s ionization, are explained by the enhancement of O–H bond scission. Further state-selective O–H bond scission, observed at the O 1s second peak, is attributed to the OOH1s → 3s/σ* transition. 相似文献
60.
《Electrochemistry communications》2007,9(1):173-179
The reaction between picric acid and an excess of diisopropylcarbodiimide yields a stable red–orange fluorescent zwitterionic spiro Meisenheimer complex of 1,3,5-trinitrobenzene (ZW1). The acidic character of the iminium/guanidium moiety of the triazine ring in ZW1 permits to obtain the corresponding non-fluorescent spiro Meisenheimer Complex (MC2). The redox properties of both ZW1 and MC2 have been studied using electrochemical oxidation–reduction mechanism (established by cyclic voltammetry; classical and with ultramicroelectrodes) and controlled-potential electrolysis. A potential fluorescence switching system has been established, since fluorescent properties can be reversibly modulated by conversion of both states (ON, ZW1) and (OFF, MC2) upon reduction of ZW1 or oxidation of MC2. 相似文献