Synthesis,Structure, and Fluxionality of Strained Hypercoordinate Silicon‐Bridged [1]Ferrocenophanes

The first hypercoordinate sila[1]ferrocenophanes [fcSiMe(2‐C₆H₄CH₂NMe₂)] ( 5 a ) and [fcSi(CH₂Cl)(2‐C₆H₄CH₂NMe₂)] ( 5 b ) (fc=(η⁵‐C₅H₄)Fe(η⁵‐C₅H₄)) were synthesized by low‐temperature (?78 °C) reactions of Li[2‐C₆H₄CH₂NMe₂] with the appropriate chlorinated sila[1]ferrocenophanes ([fcSiMeCl] ( 1 a ) and [fcSi(CH₂Cl)Cl] ( 1 d ), respectively). Single‐crystal Xray diffraction studies revealed pseudo‐trigonal bipyramidal structures for both 5 a and 5 b , with one of the shortest reported Si???N distances for an sp³‐hybridized nitrogen atom interacting with a tetraorganosilane detected for 5 a (2.776(2) Å). Elongated Si? C_ipso bonds trans to the donating NMe₂ arms (1.919(2) and 1.909(2) Å for 5 a and 5 b , respectively) were observed relative to both the non‐trans bonds ( 5 a : 1.891(2); 5 b : 1.879(2) Å) and the Si? C_ipso bonds of the non‐hypercoordinate analogues ([fcSiMePh] ( 1 b ): 1.879(4), 1.880(4) Å; [fcSi(CH₂Cl)Ph] ( 1 e ): 1.881(2), 1.884(2)). Solution‐state fluxionality of 5 a and 5 b , suggestive of reversible coordination of the NMe₂ group to silicon, was demonstrated by means of variable‐temperature NMR studies. The ΔG^≠ of the fluxional processes for 5 a and 5 b in CD₂Cl₂ were estimated to be 35.0 and 37.6 kJ mol^?1, respectively (35.8 and 38.3 kJ mol^?1 in [D₈]toluene). The quaternization of 5 a and 5 b by MeOTf, to give [fcSiMe(2‐C₆H₄CH₂NMe₃)][OTf] ( 7 a‐ OTf) and [fcSi(CH₂Cl)(2‐C₆H₄CH₂NMe₃)][OTf] ( 7 b‐ OTf), respectively, supported the reversibility of NMe₂ coordination at the silicon center as the source of fluxionality for 5 a and 5 b . Surprisingly, low room‐temperature stability was detected for 5 b due to its tendency to intramolecularly cyclize and form the spirocyclic [fcSi(cyclo‐CH₂NMe₂CH₂C₆H₄)]Cl ( 9 ‐Cl). This process was observed in both solution and the solid state, and isolation and Xray characterization of 9 ‐Cl was achieved. The model compound, [Fc₂Si(2‐C₆H₄CH₂NMe₂)₂] ( 8 ), synthesized through reaction of [Fc₂SiCl₂] with two equivalents of Li[2‐C₆H₄CH₂NMe₂] at ?78 °C, showed a lack of hypercoordination in both the solid state and in solution (down to ?80 °C). This suggests that either the reduced steric hindrance around Si or the unique electronics of the strained sila[1]ferrocenophanes is necessary for hypercoordination to occur.     

Link between the kinetic‐ and exchange‐energy functionals in the generalized gradient approximation

An approximate kinetic‐energy functional of the generalized gradient approximation form was derived following the “conjointness conjecture” of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange‐energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed‐shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic‐energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Detection of proline,arginine, and lysine using iodine‐azide reaction in TLC and HPTLC

The application of a modified iodine‐azide procedure for the detection of proline, arginine, and lysine is described. Phenyl isothiocyanate was used to transform amino acids into phenyl thiocarbamyl derivatives (derivatization in situ). The developed plates were sprayed with a mixture of sodium azide and starch solution, adjusted to pH 5.5, and exposed to iodine vapour. Due to the catalytic effect of the C? S bond, the spots appeared white on a violet‐grey background and were stable for 20 minutes. The detection limits were found to lie in the pmole range. The iodine‐azide test is compared with other procedures (iodine, UV, ninhydrin).     

Solubility Measurements of Surfactants and Dissolution of Cytochrome c in 1,1,1,2‐Tetrafluoroethane Modified with Organic Solvents

The solubilities of four surfactants, sodium bis‐2‐ethylhexyl sulfosuccinate (AOT), cetyltrimethylammonium chloride, didodecyldimethylammonium bromide, and n‐dodecyltrimethylammonium chloride in liquid refrigerant 1,1,1,2‐tetrafluoroethane (R134a) modified with methanol, ethanol, and 2‐propanol were measured. The measurements were performed under 25 and 50 bar at 25 °C. The solubility of AOT was as high as 92 mM in R134a with addition of 2% ethanol. Cytochrome c molecules were then verified to be soluble in the AOT‐containing and ethanol‐modified R134a solution. Accordingly, liquid R134a is suggested as a potential extraction solvent for hydrophilic compounds.     

Voltammetric Determination of Antibacterial Nitro‐Compounds at Activated Carbon Fibre Microelectrodes

The electrochemical behavior of the nitrofuran antibiotics nitrofurantoin (NFA) and nitrofurazone (NFZ), as well as the related nitroimidazole drug, metronidazole (MNZ) at electrochemically activated carbon fibre microelectrodes (CFMEs) is reported. These nitrofurans have been widely used in the form of feed additives for the treatment of gastrointestinal infections in cattle, pigs and poultry. Electrochemical activation of CFMEs by repetitive square‐wave (SW) voltammetric scans between 0.0 and +2.6 V, produced an unexpected high cathodic response from these compounds. This is attributed to the increase of the carbon fibre surface area due to its fracture and the appearance of deep fissures along the main fibre axis. Penetration of the nitroderivative compounds into the micro‐channels of the activated fibres was profited to develop a sensitive and selective method for NFA based on stripping SW voltammetry. The metabolite of NFA, 1‐aminohydantoin (AHD), did not exhibit cathodic response in the potential range between ?0.9 and +0.1 V. However, AHD gave a SW oxidation signal at activated CFMEs, with a potential of +0.95 V, which did not appear at non‐activated microelectrodes. Based on this finding, the simultaneous determination of NFA and AHD was performed by amperometry using two activated CFMEs in a parallel configuration connected to a multichannel detector. This methodology was also used to carry out degradation studies of NFA in acidic and basic media under photodegradation or thermal degradation conditions.     

Electrochemical Determination of Ascorbic Acid in Room Temperature Ionic Liquid BPPF6 Modified Carbon Paste Electrode

A room temperature ionic liquid N‐butylpyridinium hexafluorophosphate (BPPF₆) was used as a binder to make an ionic liquid modified carbon paste electrode (IL‐CPE), which showed good characteristics such as simple preparation procedure, fast electrochemical response and good conductivity. The electrochemical oxidation of ascorbic acid (AA) on the new IL‐CPE was carefully studied. The oxidation peak potential of AA on the IL‐CPE appeared at 109 mV (vs. SCE), which was about 338 mV decrease of the overpotential compared to that obtained on the traditional carbon paste electrode (CPE) and the oxidation peak current was increased for about four times. The electrochemical parameters of AA on the IL‐CPE were calculated with the charge transfer coefficient (α) and the electrode reaction rate constant (k_s) as 0.87 and 0.800 s^?1, respectively. Based on the relationship of the oxidation peak current and the concentration of AA a sensitive analytical method was established with cyclic voltammetry. The linear range for AA determination was in the range from 1.0×10^?5 to 3.0×10^?3 mol/L with the linear regression equation as I_p (μA)=?2.52–0.064C (μmol/L) (n=13, γ=0.9942) and the detection limit was calculated as 8.0×10^?6 mol/L (3σ). The proposed method was free of the interferences of coexisting substances such as dopamine (DA) and amino acids etc., and successfully applied to the vitamin C tablets determination.     

Stereochemical analysis of N-cyclohexylidene-N-(1-phenylethyl)amine derivatives

The configurational properties of a series of cyclohexylidene imines are discussed on the basis of their 1H, 13C and 15N NMR spectral data.     

A two-stage approach to automatic determination of 1H NMR coupling constants

(1)H NMR scalar coupling constants are a rich source of information on molecular structure, but their extraction from spectra can be less than straightforward. Previous approaches to J extraction include methods proposed by Hoye, Golotvin, and the 'modified J-doubling' method. Here we describe the ACCA method, currently implemented in the NMR package MestReC, which allows a high degree of automation in the extraction of coupling patterns even in the case of complex multiplets with sublinewidth splitting. The new approach is illustrated by application to strychnine, for which it has detected previously unreported couplings.     

The Reactivity of Cr‐Te Superlattice Reactants and of Co‐deposited Cr‐Te Films: Studies with in‐situ X‐ray Diffractometry

The reactivity of a Cr‐Te multilayer film consisting of 326 Cr/Te double‐layers with about 88(3) at% Te was investigated with in‐situ X‐ray diffraction. The occurrence of a superstructure reflection caused by the multilayer system is observed. During the annealing procedure between 80 and 90 °C the layer‐by‐layer arrangement is destroyed and elemental Te crystallizes. After 6 h isothermal annealing at 140 °C CrTe₃ starts to crystallize. The amount and the crystal size of CrTe₃ increase with raising temperature and Te is consumed during the growth process. At 220 °C the formation of crystalline Cr₂Te₃ is observed and between 220 and 260 °C CrTe₃ is completely decomposed into Cr₂Te₃. The two Cr tellurides grow as highly textured materials. The size of CrTe₃ crystals is significantly larger than that of Cr₂Te₃. For a co‐deposited Cr‐Te film with composition 77(3) at% Te the tri‐telluride and elemental Te crystallize simultaneously. The texture of CrTe₃ is less pronounced and different reflections of the tri‐telluride occur besides the (h00)‐reflections. Up to 160 °C a pronounced increase of the crystallite size is observed and the crystals are significantly larger than those obtained with the multifilm sample. At 130 °C the crystallization kinetics seem to be controlled by nucleation.     

Predicting the shape and structure of face-centered cubic gold nanocrystals smaller than 3 nm.

Although a number of computational studies have examined the relative stability of icosahedral and decahedral gold clusters from 1 to 3 nm in size, few studies have focussed on the variety of face-centered cubic (fcc) nanoparticles in this size regime. In most cases small fcc gold particles are assumed to adopt the truncated octahedral shape, but in light of the fact that the shape and structure of gold nanoparticles is known to vary, the relative stability of fcc polyhedra may change with size. Presented here are results of first-principles calculations investigating the preferred shape of gold particles less than 3 nm in size. Our results indicate that the equilibrium shape of fcc gold nanoparticles less than 1 nm is the cuboctahedron, but this shape rapidly becomes energetically unstable with respect to the truncated octahedron, octahedron and truncated cube shapes as the size increases.