Stepwise and concerted electron-transfer/bond breaking reactions. solvent control of the existence of unstable pi ion radicals and of the activation barriers of their heterolytic cleavage

Available data from various sources seem to indicate an important role of solvation in the cleavage rates of intermediate pi ion radicals, in the passage from concerted to stepwise electron-transfer/bond breaking reaction pathways and even in the very existence of pi ion radicals. After preliminary computations treating the solvent as dielectric continuum, these expectations are examined with the help of a simple model system involving the anion radical of ONCH(2)Cl and two molecules of water, which allows the application of advanced computational techniques and a treatment of these solvent effects that emphasizes the role of solvent molecules that sit close to the charge centers of the molecule. A pi ion radical minimum indeed appears upon introduction of the two water molecules, and cleavage is accompanied by their displacement toward the leaving anion, thus offering a qualitative mimicry of the experimental observations.     

Thermodynamic forces in highly curved fluid interfaces

The identification of the force distribution in curved interfaces as a thermodynamic force [Baus and Lovett, J. Chem. Phys. 101, 377 (1995)] can be interpreted as a relation between the force distribution and the grand canonical free energy difference between two distinct systems. Using this interpretation, molecular expressions are developed for the force distribution in cylindrical and spherical interfaces that remain valid for very highly curved interfaces.     

Observation of electrostatically released DNA from gold electrodes with controlled threshold voltages

DNA oligo-nucleotides, localized at Au metal electrodes in aqueous solution, are found to be released when applying a negative bias voltage to the electrode. The release was confirmed by monitoring the intensity of the fluorescence of cyanine dyes (Cy3) linked to the 5' end of the DNA. The threshold voltage of the release changes depending on the kind of linker added to the DNA 3'-terminal. The amount of released DNA depends on the duration of the voltage pulse. Using this technique, we can retain DNA at Au electrodes or Au needles, and release the desired amount of DNA at a precise location in a target. The results suggest that DNA injection into living cells is possible with this method.     

Electrochemically modulated liquid chromatographic separations of inorganic anions

Inorganic anion retention on a porous graphitic carbon (PGC) stationary phase is investigated by electrochemically modulated liquid chromatography (EMLC). Through various combinations of the potential applied (Eapp) to the PGC packing and/or changes in the composition (sodium salts of tetrafluoroborate, sulfate, and fluoride) and concentration (10, 25, and 50 mM) of an aqueous mobile phase, conditions for the separation of two different inorganic anion mixtures (iodate, bromide, nitrite, and nitrate or iodate, bromate, and chlorate) are developed. Results show that retention was affected by both variables, with the analyte retention factor, k', changing in a few cases by as much as a factor of ca. six. Moreover, plots of In k' are linearly dependent on both Eapp and In [SE], where [SE] is the supporting electrolyte concentration. Based on these findings, insights into the retention mechanism are briefly discussed by drawing on the theory for ion exchange chromatography.     

A <Emphasis Type="Italic">QR</Emphasis>–method for computing the singular values via semiseparable matrices

Summary. The standard procedure to compute the singular value decomposition of a dense matrix, first reduces it into a bidiagonal one by means of orthogonal transformations. Once the bidiagonal matrix has been computed, the QR–method is applied to reduce the latter matrix into a diagonal one. In this paper we propose a new method for computing the singular value decomposition of a real matrix. In a first phase, an algorithm for reducing the matrix A into an upper triangular semiseparable matrix by means of orthogonal transformations is described. A remarkable feature of this phase is that, depending on the distribution of the singular values, after few steps of the reduction, the largest singular values are already computed with a precision depending on the gaps between the singular values. An implicit QR–method for upper triangular semiseparable matrices is derived and applied to the latter matrix for computing its singular values. The numerical tests show that the proposed method can compete with the standard method (using an intermediate bidiagonal matrix) for computing the singular values of a matrix.Mathematics Subject Classification (2000): 65F15, 15A18The research of the first two authors was partially supported by the Research Council K.U.Leuven, project OT/00/16 (SLAP: Structured Linear Algebra Package), by the Fund for Scientific Research–Flanders (Belgium), projects G.0078.01 (SMA: Structured Matrices and their Applications), G.0176.02 (ANCILA: Asymptotic aNalysis of the Convergence behavior of Iterative methods in numerical Linear Algebra), G.0184.02 (CORFU: Constructive study of Orthogonal Functions) and G.0455.0 (RHPH: Riemann-Hilbert problems, random matrices and Padé-Hermite approximation), and by the Belgian Programme on Interuniversity Poles of Attraction, initiated by the Belgian State, Prime Ministers Office for Science, Technology and Culture, project IUAP V-22 (Dynamical Systems and Control: Computation, Identification & Modelling). The work of the third author was partially supported by MIUR, grant number 2002014121. The scientific responsibility rests with the authors.Acknowledgments.We thank the referees for their suggestions which increased the readability of the paper.