Remarkable oxidation stability of glycals: excellent substrates for cerium(IV)-mediated radical reactions

The remarkable stability of glycals under oxidative conditions becomes apparent by their redox data in solution, computed HOMO energies, and behavior on the addition of electrophilic radicals generated in the presence of cerium(IV) ammonium nitrate. Oxidation potentials up to 2.03 V vs ferrocene were obtained, which are exceptionally high for cyclic enol ethers but correlate nicely with the reaction times of the radical reactions. Protecting groups have a strong influence on the oxidation stability and HOMO energies of glycals as E(ox) is shifted from O-silyl over O-benzyl to O-acetyl by more than 500 mV. Interestingly, this effect must be transmitted through sigma-bonds, even up to the para-position of a benzoate group, as verified by a wide variation of remote substituents in the carbohydrate. Favorable interactions of the sigma*-orbital of the adjacent C-O bond with the HOMO of the double bond are proposed as a mechanistic rationale, which might be important for the redox behavior of other allylic systems. Finally, donors and acceptors in the 1-position exert the strongest influence on the oxidation stability, shifting the potentials by almost 1 V and resulting in different follow-up reactions of the cerium(IV)-mediated additions of malonates. It is the remarkable oxidation stability of glycals that makes them valuable building blocks in carbohydrate chemistry.     

Why triple bonds protect acenes from oxidation and decomposition

An experimental and computational study on the impact of functional groups on the oxidation stability of higher acenes is presented. We synthesized anthracenes, tetracenes, and pentacenes with various substituents at the periphery, identified their photooxygenation products, and measured the kinetics. Furthermore, the products obtained from thermolysis and the kinetics of the thermolysis are investigated. Density functional theory is applied in order to predict reaction energies, frontier molecular orbital interactions, and radical stabilization energies. The combined results allow us to describe the mechanisms of the oxidations and the subsequent thermolysis. We found that the alkynyl group not only enhances the oxidation stability of acenes but also protects the resulting endoperoxides from thermal decomposition. Additionally, such substituents increase the regioselectivity of the photooxygenation of tetracenes and pentacenes. For the first time, we oxidized alkynylpentacenes by using chemically generated singlet oxygen ((1)O(2)) without irradiation and identified a 6,13-endoperoxide as the sole regioisomer. The bimolecular rate constant of this oxidation amounts to only 1 × 10(5) s(-1) M(-1). This unexpectedly slow reaction is a result of a physical deactivation of (1)O(2). In contrast to unsubstituted or aryl-substituted acenes, photooxygenation of alkynyl-substituted acenes proceeds most likely by a concerted mechanism, while the thermolysis is well explained by the formation of radical intermediates. Our results should be important for the future design of oxidation stable acene-based semiconductors.     

Impact of the non-measured infrared spectral range of the imaginary refractive index on the derivation of the real refractive index using the Kramers-Kronig transform

A quantitative analysis to asses the influence of a non-measured imaginary index spectrum on the extracted real refractive index is presented. The investigation was done on the Mid-IR spectral range, where the ‘‘measured’’ imaginary spectrum is defined between 800 and 4500 cm⁻¹. The influence of bands of various locations and shapes in the non-measured IR spectral region (0-800 cm⁻¹) on the n values obtained by the computational procedure of the Kramers-Kronig transform was investigated. Additional analysis was conducted to estimate the relevance of different assumptions that are commonly made with regard to the non-measured range (e.g. linear extrapolation or the effect of uncertainty in the precise band location). The results show that the contribution of an unmeasured band at any wavenumber is well described by a simple function of the band location and , regardless of the band shape. Furthermore, the error caused by incorrect band location can also be described by a simple function of the band location, the band location error and . The simple functions can be used to estimate the impact that ignoring or misplacing a band will have on the extracted n spectrum, without performing the whole KK integration. These relationships were validated on two data sets of optical constants of crystalline ammonium sulfate and water in the Mid-IR range.     

Formation of superconductive supersaturated TaZr solid solutions by pulsed electron beam treatment

Amorphous TaZr films were prepared by simultaneous evaporation onto sapphire substrates. Pulsed electron beam treatment with deposited energy densities above a threshold of about 2 J cm^?2 led to a crystallization of the films and to the formation of supersaturated TaZr solid solutions with a bcc structure up to Zr concentrations of 54 at %. The crystalline alloys were superconducting with a maximum transition temperature T_c of 7.7 K. The films decomposed totally at a temperature of 1100°C during furnace annealing.     

New Singlet Oxygen Donors Based on Naphthalenes: Synthesis,Physical Chemical Data,and Improved Stability

Singlet oxygen donors are of current interest for medical applications, but suffer from a short half‐life leading to low singlet oxygen yields and problems with storage. We have synthesized more than 25 new singlet oxygen donors based on differently substituted naphthalenes in only a few steps. The influence of functional groups on the reaction rate of the photooxygenations, thermolysis, half‐life, and singlet oxygen yield has been thoroughly studied. We determined various thermodynamic data and compared them with density functional calculations. Interestingly, remarkable stabilities of functional groups during the photooxygenations and stabilizing effects for some endoperoxides during the thermolysis have been found. Furthermore, we give evidence for a partly concerted and partly stepwise thermolysis mechanism leading to singlet and triplet oxygen, respectively. Our results might be interesting for “dark oxygenations” and future applications in medicine.     

Convenient syntheses and transformations of 2-C-malonyl carbohydrates

2-C-malonyl carbohydrates were synthesized in only few steps and high yields by radical additions of malonates to glycals. For the first time, the undesired formation of nitrates was completely suppressed with anhydrous cerium ammonium nitrate (CAN) as oxidizing agent. A coherent explanation for the high stereoselectivities of the additions to gluco-configured glycals was provided by variation of the substituents in the 3-position. We established steric effects for the face selectivity, and electronic effects strongly influence the reactivity of the double bonds. The scope and limitation of transition-metal-mediated radical reactions in the synthesis of 2-C-branched carbohydrates was thoroughly investigated. Thus, unsaturated disaccharides and benzyl-protected glycals were used as substrates for the first time. Finally, the 2-C-malonyl carbohydrates were transformed into various products by decarboxylation, saponification and reduction, which afforded interesting precursors for C-disaccharides. In this paper we describe the syntheses of more than 40 new 2-C-analogues of carbohydrates, which were isolated in high yields in analytically pure form. Therefore, the transition-metal-mediated radical addition of malonates to glycals offers a simple and convenient entry to such important carbohydrate derivatives.     

Superconducting and structural properties of YBaCuO thin films deposited by inverted cylindrical magnetron sputtering

Thin films of YBaCuO have been deposited onto Al₂O₃, MgO, SrTiO₃ and ZrO₂ substrates by inverted cylindrical magnetron sputtering. The main advantage of this preparation technique is the high reproducibility of the results allowing systematic studies of the film properties as a function of deposition parameters. Optimum preparation conditions were a low discharge voltage (100 V), a high oxygen partial pressure (2×10^–1 Torr) in an oxygen argon mixture, and substrate temperatures around 800°C. The films grow highly textured on all substrates yielding single crystalline growth on SrTiO₃ and MgO as proved by X-ray diffraction and channeling. The zero resistance values of the sharp transtions usually are around 90 K. highest critical current densities were determined in films grown on MgO and SrTiO₃ with values up to 5.5×10⁶ A/cm² at 77 K.     

Integrated explosive preconcentrator and electrochemical detection system for 2,4,6-trinitrotoluene (TNT) vapor

This article reports on an integrated explosive-preconcentration/electrochemical detection system for 2,4,6-trinitrotoluene (TNT) vapor. The challenges involved in such system integration are discussed. A hydrogel-coated screen-printed electrode is used for the detection of the thermally desorbed TNT from a preconcentration device using rapid square wave voltammetry. Optimization of the preconcentration system for desorption of TNT and subsequent electrochemical detection was conducted yielding a desorption temperature of 120 °C under a flow rate of 500 mL min⁻¹. Such conditions resulted in a characteristic electrochemical signal for TNT representing the multi-step reduction process. Quantitative measurements produced a linear signal dependence on TNT quantity exposed to the preconcentrator from 0.25 to 10 μg. Finally, the integrated device was successfully demonstrated using a sample of solid TNT located upstream of the preconcentrator.     

Extraction of optical constants from mid-IR spectra of small aerosol particles

A direct method for extracting optical constants in the mid-infrared (IR), using small particle's spectra is presented. The method is based on the direct extraction of the optical constants from the measured spectra using the Rayleigh approach for absorbance cross section of small particles. This was achieved by using an experimental system combining a scanning mobility particle sizing system attached to a long-path IR cell, allowing simultaneous measurements of aerosol size distribution and their IR spectra.The inversion procedure was tested on crystalline ammonium sulfate aerosols, for which high resolution set of optical constants was obtained and were found to be in good agreement with recently published data. Since the extraction of the k and n spectra is deduced from the refractive index dependent complex function, the exact band features can be obtained, unlike the commonly used iterative methods that modify simultaneously both band features and scale of k and n during the calculation procedure. The suggested procedure is simple to apply; nevertheless, it is sensitive to scaling errors of the final constants resulting from uncertainties in total particle volume measurements.     

NMR spectroscopic conformational analysis of 4‐methylene‐cyclohexyl pivalate—The effect of sp2 hybridization

The conformational equilibrium of the axial/equatorial conformers of 4‐methylene‐cyclohexyl pivalate is studied by dynamic NMR spectroscopy in a methylene chloride/freon mixture. At 153 K, the ring interconversion gets slow on the nuclear magnetic resonance timescale, the conformational equilibrium (?ΔG°) can be examined, and the barrier to ring interconversion (ΔG^#) can be determined. The structural influence of sp² hybridization on both ΔG° and ΔG^# of the cyclohexyl moiety can be quantified.