Die Anwendbarkeit potentiometrische Titrierungen in der technischen Analyse

Ohne Zusammenfassung     

A numerical model for the 3-D non-linear vibrations of an N-string

We present a non-linear numerical model describing the 3-D vibrations of a planar network of N sections of string which are connected together at one common mobile extremity. We call such a network N-string. For small-amplitude vibrations perpendicular to the N-string equilibrium plane, the numerical results coincide with the already known analytical solutions of the linear model. This non-linear model makes it possible to describe small- or large-amplitude 3-D vibrations of any kind of N-string subjected to an initial plucking. The equations of motion are also presented in a dimensionless form and a vast dimensionless physical parameter space is identified. The numerical model can be extended to more complex networks of strings.     

Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]Helicenes Featuring an Overcrowded Alkene

The straightforward, multigram-scale synthesis of the partially saturated H₆-fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H₆-fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.     

Trapping of Terminal Platinapolyynes by Copper(I) Catalyzed Click Cycloadditions; Probes of Labile Intermediates in Syntheses of Complexes with Extended sp Carbon Chains,and Crystallographic Studies

The silylated hexatriynyl complex trans-(C₆F₅)(p-tol₃P)₂Pt(C≡C)₃SiEt₃ ( PtC₆TES ) is converted in situ to PtC₆H (wet n-Bu₄N⁺ F⁻, THF) and cross coupled with the diyne H(C≡C)₂SiEt₃ ( HC₄TES ; CuCl/TMEDA, O₂) to give PtC₁₀TES (71 %). This sequence is repeated twice to afford PtC₁₄TES (65 %) and then PtC₁₈TES (27 %). An analogous series of reactions starting with PtC₈TES gives PtC₁₂TES (60 %), then PtC₁₆TES (43 %), and then PtC₂₀TES (17 %). Similar cross couplings with H(C≡C)₂Si(i-Pr)₃ ( HC₄TIPS ) give PtC₁₂TIPS (68 %), PtC₁₄TIPS (68 %), and PtC₁₆TIPS (34 %). The trialkylsilyl species (up to PtC₁₈TES ) are converted to 3+2 “click” cycloadducts or 1,4-disubstituted 1,2,3-triazoles trans-(C₆F₅)(p-tol₃P)₂Pt(C≡C)_n-1C=CHN(CH₂C₆H₅)N=N (29–92 % after workups). The most general procedure involves generating the terminal polyynes PtC _x H (wet n-Bu₄N⁺ F⁻, THF) in the presence of benzyl azide in DMF and aqueous CuSO₄/ascorbic acid. All of the preceding complexes are crystallographically characterized and the structural and spectroscopic properties analyzed as a function of chain length. Two pseudopolymorphs of PtC₂₀TES are obtained, both of which feature molecules with parallel sp carbon chains in a pairwise head/tail packing motif with extensive sp/sp van der Waals contacts.     

Weinreb Amide,Ketone and Amine as Potential and Competitive Secondary Molecular Stations for Dibenzo-[24]Crown-8 in [2]Rotaxane Molecular Shuttles

This paper reports the synthesis and study of new pH-sensitive DB24C8-based [2]rotaxane molecular shuttles that contain within their axle four potential sites of interaction for the DB24C8: ammonium, amine, Weinreb amide, and ketone. In the protonated state, the DB24C8 lay around the best ammonium site. After either deprotonation or deprotonation-then-carbamoylation of the ammonium, different localizations of the DB24C8 were seen, depending on both the number and nature of the secondary stations and steric restriction. Unexpectedly, the results indicated that the Weinreb amide was not a proper secondary molecular station for the DB24C8. Nevertheless, through its methoxy side chain, it slowed down the shuttling of the macrocycle along the threaded axle, thereby partitioning the [2]rotaxane into two translational isomers on the NMR timescale. The ketone was successfully used as a secondary molecular station, and its weak affinity for the DB24C8 was similar to that of a secondary amine.     

Fulvenes, fulvalenes, and azulene: are they aromatic chameleons?

On the basis of the theory of Baird on reversal of Hückel's rule for aromaticity and antiaromaticity of annulenes when going from the electronic ground state (S0) to the lowest pipi* triplet state (T1) (J. Am. Chem. Soc. 1972, 94, 4941), we argue that fulvenes, fulvalenes, and azulene are "aromatic chameleons". The dipole moments of fulvenes in T1 should be of comparable magnitude to those of S0, but due to the reversal of Hückel's aromaticity rule in T1, their dipole should be in the opposite direction. Thereby, they are capable of adopting some aromaticity in both the T1 and S0 states as they adapt their dipolar resonance structures. The same applies to fulvalenes and azulene in their lowest quintet states (Q1) when compared to S0. Our hypothesis on chameleon behavior is supported by quantum chemical OLYP, CASSCF, and CASPT2 calculations of dipole moments, pi-orbital populations, and energies.     

Hopf bifurcations in time-delay systems with band-limited feedback

We investigate the steady-state solution and its bifurcations in time-delay systems with band-limited feedback. This is a first step in a rigorous study concerning the effects of AC-coupled components in nonlinear devices with time-delayed feedback. We show that the steady state is globally stable for small feedback gain and that local stability is lost, generically, through a Hopf bifurcation for larger feedback gain. We provide simple criteria that determine whether the Hopf bifurcation is supercritical or subcritical based on the knowledge of the first three terms in the Taylor-expansion of the nonlinearity. Furthermore, the presence of double-Hopf bifurcations of the steady state is shown, which indicates possible quasiperiodic and chaotic dynamics in these systems. As a result of this investigation, we find that AC-coupling introduces fundamental differences to systems of Ikeda-type [K. Ikeda, K. Matsumoto, High-dimensional chaotic behavior in systems with time-delayed feedback, Physica D 29 (1987) 223–235] already at the level of steady-state bifurcations, e.g. bifurcations exist in which limit cycles are created with periods other than the fundamental “period-2” mode found in Ikeda-type systems.     

Comparison of microwave-assisted digestion procedures for total trace element content determination in calcareous soils

The aim of the study was to determine total trace (Cd, Co, Cr, Cu, Mn, Pb and Zn) and major (Al and Fe) element concentrations in calcareous soils using microwave-assisted digestion procedures. The literature showing lack of consensus regarding digestion procedures and unsatisfying recoveries for calcareous materials, four procedures using various acid combinations (HCl, HNO₃, H₂O₂, HF) and volumes were tested using a certified reference material (CRM 141R) and natural calcareous soil samples. Digests were analysed by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Repeatability (R.S.D. <5%) and recoveries (82-116%) showed that the procedures were precise and accurate for most elements. Five calcareous soil samples from a Champagne vineyard plot were, then, subjected to these procedures. In calcareous materials, the presence of HF resulted in Al being severely underestimated (recovery <5%) and Co overestimated (recovery >124%) due to complex formation or spectrochemical interferences, respectively. As digestion was not significantly influenced by the addition of H₂O₂, the procedure corresponding to Aqua regia (HCl-HNO₃) appeared as the best compromise and was selected for further multielemental environmental studies on calcareous materials, even if the absence of HF could lead to incomplete digestion of accessory silicate minerals. Results for a vineyard plot showed that the soils were contaminated (3.65 mg kg⁻¹ Cd, 67 mg kg⁻¹ Cr, 278 mg kg⁻¹ Cu, 143 mg kg⁻¹ Pb and 400 mg kg⁻¹ Zn) as a consequence of urban waste and copper-treatment applications.     

Grafting characterization of natural rubber latex particles: wet-STEM imaging contributions

Natural rubber latexes investigated in this study have been chemically modified by seeded emulsion polymerization. Depending on the water affinity of the monomer involved (MMA or DMAEMA), the expected result was the grafting of the corresponding polymer inside or on the surface of the latex particles. The present article focuses on the grafting characterization of these modified natural rubber latexes. In this purpose, non-imaging classical experimental methods such as dynamic light scattering and nuclear magnetic resonance have been completed by microscopy techniques, including transmission imaging in scanning electron microscopy (SEM) and a recent imaging mode called wet-STEM. It consists in transmission imaging in an Environmental SEM operating in the wet-mode, allowing transmission observations of particles suspended in a liquid layer with good resolution and contrast. In the present study, we have adopted a comparative characterization approach between a nongrafted natural rubber latex and two grafted ones. Such an approach indeed contributes to highlight the particles morphology resulting from chemical modification using either MMA or DMAEMA. Transmission images in SEM of thin foils are relatively well interpreted and are completed with wet-STEM images of latexes in their native state, bringing important contributions for grafting characterization.     

An Enantiopure Cyclometallated Iridium Complex Displaying Long‐Lived Phosphorescence both in Solution and in the Solid State

A new enantiopure cyclometallated iridium complex bearing a [4]helicenic ‐coordinating and two ‐coordinating dfppy (2‐(2,4‐difluorophenyl)‐pyridyl) ligands was prepared. This complex displayed long‐lived phosphorescence both in solution and in the solid state. Its chiroptical properties, namely electronic circular dichroism and circularly polarized luminescence, were also examined. Comparison with former chiral complexes enabled assignment of the Δ_Ir‐(?) and Λ_Ir‐(+) absolute configurations.