Evidence for Extensive Single‐Electron‐Transfer Chemistry in Boryl Anions: Isolation and Reactivity of a Neutral Borole Radical

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron‐centered nucleophilicity are still rare and sought‐after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single‐electron‐transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole‐based radical. The radical was characterized by elemental analysis, single‐crystal X‐ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron‐based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B? Sn and B? Pb bonds, the latter of which was the first isolated and structurally characterized compound with a “noncluster” B? Pb bond.     

Boron Radical Cations from the Facile Oxidation of Electron‐Rich Diborenes

The realization of a phosphine‐stabilized diborene, Et₃P?(Mes)B?B(Mes)?PEt₃ ( 4 ), by KC₈ reduction of Et₃P?B₂Mes₂Br₂ in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC‐stabilized diborene IMe?(Dur)B?B(Dur)?IMe ( 1 ). Importantly, both species feature unusual electron‐rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the B? B double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making 1 a stronger neutral reductant as 4 ( 1 : E_1/2=?1.55 V; 4 : ?1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to 4 . Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron‐centered radical cations ( 5 and 6 ).     

Very Strong Organosuperbases Formed by Combining Imidazole and Guanidine Bases: Synthesis,Structure, and Basicity

New structural motives for organosuperbases, that are easy to prepare and highly basic are urgently required in many areas of chemistry. The synthesis of N,N′‐bis(imidazolyl)guanidine bases (BIG bases) is reported. Their pK_α values are determined as 26.1–29.3 in THF. They are thus probably the strongest known phosphorous‐free organic bases both in solution and in the gas phase. Calculations help to determine the structural and electronic factors giving rise to the high basicity.     

Hard-coupled model of local direct resistance heating of thin sheets

A monolithic model of local direct resistance heating of thin sheets is presented. The model respects all material nonlinearities of the system such as the temperature dependences of physical properties of individual parts of the system. Numerical computations are performed by a finite element method of higher order of accuracy and realized by our own codes Hermes2D and Agros2D. The methodology is illustrated by a typical example, whose results are discussed.     

Literatur

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Elektrometrische Titrationsmethoden

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Comparison of Metallothionein Detection by Using Brdicka Reaction and Enzyme‐Linked Immunosorbent Assay Employing Chicken Yolk Antibodies

In this study, two methods for metallothioneins (MT) determination in a biological sample were compared. Particularly, twenty five human and nine pig blood serum samples and liver and kidney samples from thirty five carps (Cyprinus carpio) were analyzed by enzyme‐linked immunosorbent assay and differential pulse voltammetry Brdicka's reaction. The results obtained by these two methods were in good agreement. For commercially available MT standard the correlation coefficient between the single concentrations signal height was 0.99. In biological samples the correlation coefficients were 0.90 for fish liver and kidney samples, 0.91 for pig blood serum and 0.93 for human blood serum.     

Physical and perceptual estimation of differences between loudspeakers

Differentiating the restitution of timbre by several loudspeakers may result from standard measurements, or from listening tests. This work proposes a protocol keeping a close relationship between the objective and perceptual evaluations: the stimuli are musical excerpts, and the measuring environment is a standard listening room. The protocol involves recordings made at a listener position, and objective dissimilarities are computed using an auditory model simulating masking effects. The resulting data correlate very well with listening tests using the same recordings, and show similar dependencies on the major parameters identified from the dissimilarity matrices. To cite this article: M. Lavandier et al., C. R. Mecanique 334 (2006).     

Superconvergence in the generalized finite element method

In this paper, we address the problem of the existence of superconvergence points of approximate solutions, obtained from the Generalized Finite Element Method (GFEM), of a Neumann elliptic boundary value problem. GFEM is a Galerkin method that uses non-polynomial shape functions, and was developed in (Babuška et al. in SIAM J Numer Anal 31, 945–981, 1994; Babuška et al. in Int J Numer Meth Eng 40, 727–758, 1997; Melenk and Babuška in Comput Methods Appl Mech Eng 139, 289–314, 1996). In particular, we show that the superconvergence points for the gradient of the approximate solution are the zeros of a system of non-linear equations; this system does not depend on the solution of the boundary value problem. For approximate solutions with second derivatives, we have also characterized the superconvergence points of the second derivatives of the approximate solution as the roots of a system of non-linear equations. We note that smooth generalized finite element approximation is easy to construct. I. Babuška’s research was partially supported by NSF Grant # DMS-0341982 and ONR Grant # N00014-99-1-0724. U. Banerjee’s research was partially supported by NSF Grant # DMS-0341899. J. E. Osborn’s research was supported by NSF Grant # DMS-0341982.     

Anwendung der Röntgenspektrographie für die Untersuchung kolloider Systeme

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