Preparation and redox properties of N,N,N-1,3,5-trialkylated flavin derivatives and their activity as redox catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.     

Surprising Homolytic Gas Phase Co−C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B₁₂ designed as potential ‘antivitamins B₁₂’. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co−C )-BDEs of 38.4 and 40.6 kcal mol⁻¹, respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol⁻¹ for the corresponding α-isomers. Surprisingly, the observed (Co−C )-BDEs of the Co_β-aryl-cobinamides were smaller than the (Co−C )-BDE of Co_β-methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co−C )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.     

Transition metal allyls : IV. The (η3-allyl)2M complexes of nickel,palladium and platinum: reaction with tertiary phosphines

A series of 1 : 1 adducts have been prepared by treating the bis-η³-allyl complexes of nickel, palladium and platinum with tertiary phosphines. Investigations of their structure in solution as well as in the crystal have shown that both 18-electron (η³-allyl)₂ML complexes as well as 16-electron (η¹-allyl)-(η³-allyl)ML complexes may be formed.     

Scanning force microscopy based rapid force curve acquisition on supported lipid bilayers: experiments and simulations using pulsed force mode.

In situ pulsed force mode scanning force microscopy (PFM-SFM) images of phase separated solid-supported lipid bilayers are discussed with the help of computer simulations. Simultaneous imaging of material properties and topography in a liquid environment by means of PFM-SFM is severely hampered by hydrodynamic damping of the cantilever. Stiffness and adhesion images of solid-supported membranes consisting of cholesterol, sphingomyelin, and 1,2-dioleyl-phosphatidylcholine obtained in aqueous solution exhibit contrast inversion of adhesion and stiff. ness images depending on parameters such as driving frequency, amplitude, and trigger setting. Simulations using a simple harmonic oscillator model explain experimental findings and give a deeper insight into the way PFM-SFM experiments have to be performed in order to obtain interpretable results and hence pave the way for reliable material contrast imaging at high speed.     

How we came to produce C60-fullerite

At the beginning two remarks. One to the nomenclature: Fullerite does not name a specific substance; it names a solid entirely consisting of fullerenes, i.e. closed-cage all-carbon molecules. When our method of synthesis is applied, a solid is obtained in which C₆₀ is the most abundant species. This material may thus be called C₆₀-fullerite. The other remark regarding the production of fullerite: in view of the ease with which this could be achieved, I have the feeling that fullerene molecules must belong to a very frequently overlooked molecular species in carbon chemistry. Since the synthesis is simple and does not require many words to be described, I thought it might be interesting to tell how we came to make fullerite.     

Deoxygenation of supporting electrolytes in stripping voltammetry by glocuse and co-immobilized glocuse oxidase and catalase in a flow system

Carrier solutions for stripping voltammetry in flow systems are deoxygenated by reaction with glucose added to the carrier. The reaction was catalyzed by glocuse oxidase and catalase co-immobilized in an enzyme reactor which was inserted before the injector. The oxygenated was removed at least as efficiently as with nitrogen purging and the voltametric behaviour of cadmium(II), lead(II) and zinc(II) was unaffected by the glucose/gluconic acid system. A particular advantage is the rapid start-up compared to the lengthy purging of carrier solution when nitrogen degassing is used. The enzyme reactor made from porous glass was effective for several months.     

Experimental and theoretical study of the hyperfine structure in the (5d+6s)96p configurations of Pt I

The hyperfine structure of the odd configurations 5d ⁹6p and 5d ⁸6s6p was studied with laser optogalvanic spectroscopy and high resolution interferometry in a platinum hollow cathode. A parametric hyperfine structure analysis for the configurations (5d+6s)⁹6p with 10 excited odd-parity levels was performed for¹⁹⁵Pt. We obtained one-electron hfs parameters,a _5d ⁰¹ =1160(320) MHz,a _6s ¹⁰ =35600(5300) MHz anda _6p ⁰¹ =1300(780) MHz for the configuration 5d ⁸6s6p. The corresponding $\left\langle {r^{ - 3} } \right\rangle _{nl}^{k_s k_l } $ values are compared with those known for other 5d-elements.     

Untersuchungen über das dielektrische Verhalten von Polypropylen

Zusammenfassung Das Verhalten von Polypropylen im elektrischen Wechselfeld wurde im Frequenzbereich von 0,15 kHz bis 300 kHz und im Temperaturbereich von –75 C bis +140 C untersucht. Es konnte sowohl ein Tieftemperaturdispersionsgebiet —-Absorption — als auch ein Hochtemperaturdispersionsgebiet —-Absorption — beobachtet werden. Die Untersuchungsergebnisse wurden, um die Breite des Dispersionsgebietes und die zugehörigen -Stufen zu bestimmen, nach demCole-Cole-Kreisbogengesetz analysiert. Aus dem Anstieg der Geraden: logf _max 1/T _K wurden sowohl für die- als auch fur die-Absorption die experimentellen Aktivierungsenergien berechnet. Eine Deutung der Ergebnisse wurde versucht.     

Copper and Silver Atoms in the β-Cage of a Zeolite: Model Calculations

The electronic structures of the 4-4 SBU, the β-cage, and the β-cage with two 4-4 SBU's attached to it have been studied by means of EH-MO calculations. No indication of the formation of a band structure has been found. The HOMO region consists of many closely spaced, localized states, 98.6% of them concentrated on the O-atoms. Reversible color changes of Cu⁺¹ and Ag⁺¹ zeolites observed upon hydration-dehydration experiments can be understood as charge-transfer transitions from the HOMO concentrated on the zeolite O-atoms to the metal cations. As soon as the Cu⁺¹ or Ag⁺¹ are partially hydrated, the ns* and np* states are shifted to higher energies. The luminescence observed with dehydrated Cu⁺¹-zeolites X is caused by a 4p*←HOMO absorption, followed by spontaneous 4s*←4p* emission. After a detailed study of a Cu⁺¹ in the 6-6 SBU, we discuss the electronic structure of a β-cage filled with 1,2,4,8, and 9 Cu⁺¹. In each case, the β-cage is found to be too small to allow the formation of a band structure. The levels caused by the added copper are distinctly quantized. Calculations on [Ag₃(H₂O)₃]³⁺ in a β-cage are reported. The direct interaction between the Ag-atoms is significant. As a consequence, the states formed by Ag 5s and 5p atomic orbitals are delocalized over the three Ag-centers. In both the Cu⁺¹ and the Ag⁺¹ zeolites, the ligand-field picture is found to be insufficient to explain the electronic structure, when the metal is coordinated to the zeolite oxygen framework.     

Deviation from Ohm's law in electric field assisted capillary liquid chromatography

Earlier studies of electric field assisted LC (EF-LC) have shown that the effect on charged analytes of the application of an electric field over a capillary LC column is relatively small. Charged analytes can only be affected by the electric field while present in the mobile phase, which makes the effective time for influence of the electric field t(0) independent of retention time. Because the charged analytes only can be affected for a short time the electric field strength ought to be high in order to increase the impact of the electric field on the separation. We have, however, found that only a relatively low electric field strength can be used in EF-LC when pressure is used as main driving force. The useful field strength was limited by a dramatic increase in the current. This increase in current was found to origin from an increased concentration of buffer ions that have an electrophoretic mobility towards the pumped flow.