1,721,053 research outputs found
GLYCOSYLATION USING A ONE-ELECTRON-TRANSFER HOMOGENEOUS REAGENT - A NOVEL AND EFFICIENT SYNTHESIS OF BETA-LINKED DISACCHARIDES
No full textVariously substituted S-glycosides react in acetonitrile with the primary or secondary hydroxy group of O-glycosides in the presence of tris (4-bromophenyl)ammoniumyl hexachloroantimonate, a stable and commerically available radical cation, to give the corresponding β-O-linked disaccharides in high yield
Formation reversible d'un hydrure de palladium (II) cationique HPd(PPh3) 2+ par addition oxydante de l'acide acetique ou formique avec le palladium(0)
No full textRates of the oxidative addition of benzyl halides to a metallacyclic palladium(II) complex and of the reductive elimination from a benzyl-palladium(IV) complex.
No full textPd(II) metallacyclic complexes are key intermediates in sequential reactions in which three new
C−C bonds are formed in an aromatic ring. The Pd(II) metallacyclic complex 10 with
phenanthroline as ligand undergoes oxidative addition to benzyl bromide or chloride in DMF to
generate benzyl-Pd(IV) complexes. The rate constant of the oxidative addition is determined by
means of electrochemical techniques, with the expected reactivity order PhCH2Br (k1Br = 3.6 M−1 s−1)
> PhCH2Cl (k1Cl = 6 × 10−3 M−1 s−1) at 29 °C. When the benzyl-Pd(IV) complex is generated from
PhCH2Br, the oxidative addition is followed by a slow C−C reductive elimination, which gives a
Pd(II) complex. The rate constant of the reductive elimination has been determined in DMF (k2Br = 6
× 10−4 s−1) at 29 °C
Electrochemical Reduction of Dioxygen in the Presence of 4,6-Dimethyl-2-Thiopyrimidine in DMF
No full textStructural effects in the reductive activation of (Indenyl)RhL2 complexes: the reduction of the anti and syn isomers of Cr(CO)3-Indenyl-Rh(cod).
No full textThe electrochemical reduction of anti- and syn-[Cr(CO)(3)(indenyl)Rh(cod)] complexes (cod = cyclooctadiene) has been investigated, to evaluate, in each stereochemical situation, the effects of the contemporary presence of two metals coordinated to the indenyl ligand. In contrast to the reduction of monometallic [(indenyl)Rh(cod)], which gives rise to two well separated monoelectronic waves, the reduction of both bimetallic complexes occurs at more positive potentials in a single bielectronic and chemically reversible wave. These results indicate that an ECrevE mechanism is active, in which the chemical step following the first electron transfer is the reversible structural rearrangement of the 35-electron radical anion, which allows the second electron transfer to occur at a more positive potential than the first one. This structural rearrangement is more marked when the two metals are syn, which suggests the presence of an interaction between the two metals in this configuration. Side reactions correspond to the dissociation of the mono- and dianions; yet the dianions are surprisingly less frangible than the monoanions
Performance of a Numerical Method Based on the Hopscotch Algorithm and on an Oblate Spheroidal Space Coordinate-Expanding Time Grid for Simulation of Voltammetric Curves at an Inlaid Disk Microelectrode
No full textThe application of a simulation procedure which combines the Hopscotch algorithm with a hyperbolic space grid in solving the initial value problems at an inlaid disk electrode for electrochemical processes of any reversibility degree is discussed. The importance of the resolution of the space and time grid in connection with the use of the Hopscotch algorithm and of the Butler-Volmer equation to gain good accuracy, is stressed. The improvements introduced to define analytically the diffusion layer at different mass transfer conditions and to save computing time with the use of an exponentially expanding time grid and a cubic-spline interpolation make the simulation algorithm suitable to regression procedures for parameter estimation
Digital Simulation Via the Hopscotch Algorithm of a Microelectrode-Based Channel Flow-Through Amperometric Detector.
No full textThe current flowing at a band ultramicroelectrode in a rectangular channel under mass transfer-limited conditions is computed numerically. The procedure is based on the "hopscotch" algorithm and on an appropriate space transformation, delta = tanh(x/alpha). The edge effect is found to influence the response depending on the flow rate. The polarization times necessary to reach the steady-state current are also computed under different operating conditions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Time-dependent diffusion-migration at cylindrical and spherical microelectrodes: steady- and quasi-steady-state analytical solution can be used under transient conditions.
No full textMass transport at cylindrical and spherical microelectrodes involving diffusion and migration is analyzed by means of numerical simulation under transient conditions. The origin of the intrinsic difficulties encountered during the numerical solution of the diffusion-migration equations using implicit finite differences are outlined, especially for the particular case when the number of electrons transferred equals the charge number of the electroactive species. The numerical results for transient conditions have been compared to the general analytical solutions for the current enhancement or diminishment due to migration under steady- and quasi-steady-state conditions at 1D geometry microelectrodes (Amatore, C.; Fosset, B.; Bartelt, J.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1988, 256, 255-268). This yields that the analytical limiting currents are applicable, within experimental error, to the analysis of transient diffusion-migration current responses at microelectrodes of cylindrical and spherical geometries except extremely short times after the application of the potential step, i.e., when current measurements are anyway already corrupted by ohmic drop when the supporting electrolyte concentration is low. Also, this confirms that the current enhancements or diminishments due to migration are identical for both electrode geometries when steady- or quasi-steady states are approached and do not drastically differ even under transient regimes
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