Further studies of the formation of mercury barbiturate films at a mercury anode

Potential sweep and step experiments have been used to reinvestigate the electrochemistry of 5,5′-disubstituted barbituric acids at a mercury electrode and it is confirmed that while all such compounds form a monolayer of an insoluble mercury barbiturate, the mechanism and kinetics of these oxidations depend on the detailed structure of the barbiturates. The experiments have been extended to barbituric acid and 5-ethylbarbituric acid and these would suggest that when the barbiturate is not fully substituted in the 5-position, more than a monolayer of the mercury salt is formed.     

Bad numbers: coping with flawed decision support

Evidence is accumulating that many spreadsheet-based decision support systems contain errors. These errors can result in bad numbers, which in turn could lead to bad decisions. We review the literature on the origins and consequences of bad (erroneous) numbers produced by models and/or decision support systems built around them. Then, we present a case study in which an experiment in visual support for a sequential decision-making task was tainted by bad numbers. Both the literature review and the experiment indicate a robust human ability to overcome flawed decision support. We conclude with questions that need to be addressed in order to better understand the capabilities of humans to deal with erroneous results from decision support systems.     

Extracting nucleation rates from current–time transients: further comments

Nucleation rate constants, over a range of deposition potentials, are obtained for the electrocrystallisation of nickel onto a vitreous carbon electrode using the pre-pulse method. This method is based on the analysis of the early stages of two transients, one obtained as a result of the application of a single potential step, and the other resulting from the application of two potential steps (the first as a pre-pulse potential of large amplitude and short duration followed by another at which the nucleation rate is to be determined). A′-values thus obtained are compared with those estimated from the fit of the transients (each recorded at a single potential step at which A′ is to be determined) to the generalised equation derived for the model of growth of right-circular cones. The values obtained by the two methods are shown to differ from one another by about a factor of 20. Though it has been possible to obtain, with the application of two potential steps, transients which display, in the early stages of deposition, a rising current proportional to the square of the deposition time, it is shown to be impossible to describe adequately the behaviour of these transients in terms of instantaneous formation and subsequent growth of centres for the case of the deposition of nickel onto a vitreous carbon electrode. In the light of the above findings the suitability of the pre-pulse method as a means of estimating A′ has therefore been questioned in this example of electrocrystallisation. It has been shown that, even for electrocrystallisation processes where the pre-pulse method is a valid method of estimating A′, this method is, strictly speaking, applicable only to the deposition processes obtained at the lower range of potentials (at which A′ is 20 times less than the product of the radial growth rate and the square root of the density of nucleation sites). The inability of classical and atomistic theories of nucleation to arrive at plausible values for critical nucleation size has once again been demonstrated.     

Zur umsetzung von trimethylsilyl-methylen-dimethylsulfuran mit carbonylverbindungen : Reaktionen mit organosiliziumverbindungen, 1. mitt.

The first step in the reaction of trimethylsilylmethylene-dimethylsulfurane 1 (prepared from (CH₃)₃SiCH₂S?(CH₃)₂ī with 1 equivalent (CH₃)₃COK) with carbonyl compounds (R¹)(R²)CHCOCH₂R³2 leads Peterson-like way to the non-isolable vinylsulfoniumdimethylsilanolate-intermediate B or its recombined product A. Depending on the reaction conditions and the nature of the substituents R¹, R² and R³ there exist five different pathways to the end products 3–7. Frequently one can find an elimination of trimethylsilanole followed by a 2,3-sigmatropic rearrangement of an allyl-substituted methylene - sulfurane - intermediate to the 2,3,3 - trialkyl - 5 -thiahex -1 - enes 3 [CH₃SCH₂C(R¹)(R²)C(CH₂R³)=CH₂]. Sometimes this formation of 3 is suppressed by means of two equivalents of (CH₃)₃COK. In this case demethylation of the assumed precursor B arises yielding the E,Z-vinylsulfides 4 [CH₃SCH=C(CH₂R³)(CHR¹R²)], whereas the formation of the methylthiomethyl-dialkylcar-binoles 5 [CH₃SCH₂C(OH)(CH₂R³)(CHR¹R²)] can be explained by the demethylation of the precursor A. Treatment with five equivalents of (CH₃)₃COK yields in one case the 2,2,5,7 - tetramethyl - 5 - hydroxy - 3 - oxaoctane (7a) as

a consequence of a extrusion of dimethylsulfide. Frequently also epoxides of type 6 [CH₂-C(CH₂R³)(CHR¹R²)] can be isolated. Their formation is compared with the well-known preparation of epoxides by the dimethylsulfonium-methylide on the one and dimethylsulfoxoniummethylide on the other side.     

Biochemical and spectroscopic characterization of the putative photoreceptor for phototaxis in amoebae of the cellular slime mould Dictyostelium discoideum

Using sucrose gradient centrifugation, anion exchange chromatography on a SMART system, isoelectric focusing (IEF) and sodium dodecylsulphate polycrylamide gel electrophoresis (SDS PAGE), a 45.5. kDa membrane protein was isolated from amoebae of the cellular slime mould Dictyostelium discoideum. The absorption spectrum of the isolated protein corresponds well with the action spectrum for the photoaccumulation of the amoebae of Dictyostelium discoideum showing a major peak between 405 and 412 nm and some minor peaks between 500 and 600 nm. The remarkable similarity of the two spectra leads to the hypothesis that the isolated protein-pigment complex may serve as a photoreceptor for amoebal phototaxis.     

The visible absorption spectrum of OBrO, investigated by Fourier transform spectroscopy

By the utilization of a new laboratory method to synthesize OBrO employing an electric discharge, the visible absorption spectrum of gaseous OBrO has been investigated. Absorption spectra of OBrO have been recorded at 298 K, using a continuous-scan Fourier transform spectrometer at a spectral resolution of 0.8 cm(-1). A detailed vibrational and rotational analysis of the observed transitions has been carried out. The FTS measurements provide experimental evidence that the visible absorption spectrum of OBrO results from the electronic transition C(2A2)-X(2B1). Vibrational constants have been determined for the C(2A2) state (omega(1) = 648.3 +/- 1.9 cm(-1) and omega 2 = 212.8 +/- 1.2 cm(-1)) and for the X(2B1) state (omega 1 = 804.1 +/- 0.8 cm(-1) and omega 2 = 312.2 +/- 0.5 cm(-1)). The vibrational bands (1,0,0), (2,0,0), and (1,1,0) show rotational structure, whereas the other observed bands are unstructured because of strong predissociation. Rotational constants have been determined experimentally for the upper electronic state C(2A2). By modeling the band contours, predissociation lifetimes have been estimated. Further, an estimate for the absorption cross-section of OBrO has been made by assessing the bromine budget within the gas mixture, and atmospheric lifetimes of OBrO have been calculated using a photochemical model.