13C CP/MAS NMR studies of vitamin E model compounds

13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2.     

High-pressure [2 + 4]-cycloaddition of dimethylmaleic anhydride to 3,4-dimethoxyfuran; synthesis of dimethoxycantharidin preliminary communication

As an active diene (more active than furan itself), 3,4-dimethoxyfuran ( 1 ) affords with many dienophiles the respective cycloadducts in a high yield [2]. It has recently been found that under thermal conditions 1 easily reacts with maleic anhydride and its monomethyl derivative, but not with dimethylmaleic anhydride ( 2 ) [3]. This is probably due to steric hindrance resulting from the location of two methyl groups on the double bond of the dienophile. Since all Diels-Alder reactions in particular those with steric hindrance are pressure-sensitive [4]. we resolved to perform the title reaction under conditions of static high pressure.     

Synthesis of γγγ-trifluorocarbonyl compounds

γγγ-Trifluorocarbonyl compounds are easily obtained in a good yield by introduction of the 1,1,1-trifluoroethyl moiety (CF₃-CH₂-) on the -methylene group of a ketone.     

The Photophysics of β-Tyrosine and Its Simple Derivatives

Synthesis and photophysical studies of (O-methyl)--tyrosine (-tyrosine; an analogue of tyrosine, in which the amino group is moved from the - to the -carbon, closer to the phenol ring) and its derivatives with a blocked amino and/or carboxyl group were performed to explain the nature of the fluorescence of tyrosine derived analogues. All -tyrosine derivatives, except Ac-Tyr(Me), displayed the monoexponential fluorescence decay. The biexponential fluorescence decay observed for Ac-Tyr(Me) is assumed to be the result of the presence of two low-energy conformations (extended and with an intramolecular hydrogen bond). Higher quenching of the fluorescence of -tyrosine derivatives by the N-acetyl group than by the N-methylamide group moved farther was found, contrary to the data found for the respective derivatives of natural tyrosine. The obtained photophysical data are discussed with theoretical calculations (AMBER, AM1) on the basis of the rotamer model.     

Negative ion source for chlorine isotope ratio measurements

A negative chlorine ion source has been designed and constructed. The source utilizes direct surface ionization of chloromethane gas on a hot metal filament. Four different alloys for the filament material were tested: W99Th1, W75Re25, Hf97.5Zr2.5 and Mo52.5Re47.5. We conclude that the best filament material is the MoRe alloy, for which the signal‐to‐noise ratio is optimal. The ion source is used for chlorine isotope ratio measurements with higher precision and sensitivity than the positive ionization source used previously. Inasmuch as only negative ions of the two isotopes of interest are observed, no corrections to the measured isotope ratio are necessary, and less rigously purified samples may be analyzed. The negative ion currents are considerably larger than positive ion currents obtained with an electron ionization source. This implies higher analytical precision (typically 0.005 permil) and sensitivity. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrical conductivity and phase transformations in the composite ionic conductors AgI : α-Al2O3 prepared via a high-pressure route

Silver ion conducting composites of the general formula xAgI : (1 − x)α-Al₂O₃, where 0.2 ≤ x ≤ 0.8 (vol fraction) were prepared using a high-pressure synthesis route (T = 400 °C, p = 7.5 GPa). The microstructure of as-received rigid, non-brittle and dense samples was investigated by SEM. Other studies — DSC, XRD and impedance spectroscopy, were done as a function of temperature in the 20-200 °C temperature range. Close correlations were observed between the temperature dependences of the XRD patterns, the electrical conductivity and events seen on DSC traces. It was found out that the electrical conductivity at room temperature of all as-received composite materials was higher (by a factor between 7 and 100, depending on composition) than that measured after the heating-cooling cycle. This effect and other observed correlations were explained referring to the residual stress concept.     

Identification of the structure parameters applying a moving load

An approach to the flexural stiffness identification of a linear structure is proposed. The idea of the presented approach is to transform the dynamical problem into a static one by integrating the input and output signals. The output signal is the structure displacement due to different kinds of loads such as a pulse acting at a given point, moving a load of deterministic or random type. The obtained solution for the one-point force can be easily generalized to a set of point forces, which can be a model of the pressure of vehicle axes. The presented method can be applied to the identification of structure parameters of bridges. It allows also to take into account some stochastic disturbances following the movement of vehicles through the pavement roughness.     

L,L‐Lactide and ε‐Caprolactone Block Copolymers by a ‘Poly(L,L‐lactide) Block First’ Route

L,L ‐lactide (LA) and ε‐caprolactone (CL) block copolymers have been prepared by initiating the poly(ε‐caprolactone) (PCL) block growth with living poly(L,L ‐lactide) (PLA*). In the previous attempts to prepare block copolymers this way only random copolyesters were obtained because the PLA* + CL cross‐propagation rate was lower than that of the PLA–CL* + PLA transesterification. The present paper shows that application of Al‐alkoxide active centers that bear bulky diphenolate ligands results in efficient suppression of the transesterification. Thus, the corresponding well‐defined di‐ and triblock copolymers could be prepared.