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981.
Selective determination of dithiocarbamate (DTC) fungicides is mainly performed by hydrophilic interaction liquid chromatography (HILIC). According to Crnogorac and Schwack, DTC analyses by HILIC only lead to meaningful results with a zwitterionic polymer-based hydrophilic interaction liquid chromatography (ZIC-pHILIC) column. Considering the limited availability of this special type of column and the importance of DTC residue analysis, several new HILIC columns were evaluated as alternatives to the ZIC-pHILIC column. Detection was carried out by ultraviolet light and by mass spectrometry (MS) on a single quadrupole mass spectrometer coupled to an electrospray ionization interface operating in negative mode. On nearly all tested columns, separation of dimethyldithiocarbamates, ethylenebis(dithiocarbamates), and propylenebis(dithiocarbamates) was achieved with ammonium acetate eluents (pH 6.8). However, due to ion suppression by the buffer and the limited alkaline pH stability, the tested silica-based columns were not suitable for the sensitive analysis of DTCs. The polymer-based iHILIC-Fusion was the only alternative that offered high MS sensitivity, when a buffer containing 15?mM aqueous ammonium hydroxide and 7.5?mM ammonium hydrogen carbonate (pH 9.8) was used, but the separation of the three DTC subclasses was poor. Thus, considering both selectivity and sensitivity, the originally proposed polymer-based ZIC-pHILIC column still outperformed all the tested newly available alternative HILIC columns. 相似文献
982.
We present a simple, robust, and highly efficient method for optimizing all parameters of many-body wave functions in quantum Monte Carlo calculations, applicable to continuum systems and lattice models. Based on a strong zero-variance principle, diagonalization of the Hamiltonian matrix in the space spanned by the wave function and its derivatives determines the optimal parameters. It systematically reduces the fixed-node error, as demonstrated by the calculation of the binding energy of the small but challenging C(2) molecule to the experimental accuracy of 0.02 eV. 相似文献
983.
Matassa R Sadun C D'Ilario L Martinelli A Caminiti R 《The journal of physical chemistry. B》2007,111(8):1994-1999
Toluidine blue (TB) dye molecules are intensively utilized for large-area photophysics applications such as carcinoma detection, photoinactivation of bacteria, biosensors, and photovoltaic cells. Understanding the nature of the TB aggregation state becomes an essential point of the research process in order to know the structure-function relationship and to foresee technological applications of this class of metachromatic-dye molecules. However, no structural information on toluidine blue is available in the literature, maybe because of the poor crystalline character of the aggregate. Here, we present the first structural determination of TB organic molecules using the energy dispersive X-ray diffraction technique. The investigation highlights dimeric arrangements of stacked molecules in antiparallel fashion, forming a superstructure of two dimers in a transverse arrangement. The behavior of the TB higher aggregates indicates that these dye molecules, in spite of repulsion due to similar charge (cationic dyes), undergo self-aggregation to form helical conformations. 相似文献
984.
Höbartner C Pradeepkumar PI Silverman SK 《Chemical communications (Cambridge, England)》2007,(22):2255-2257
A deoxyribozyme is identified that mediates the site-selective depurination of its 5'-terminal guanosine nucleotide using periodate (IO(4)-) as an obligatory cofactor. 相似文献
985.
986.
Bron M Teschner D Knop-Gericke A Jentoft FC Kröhnert J Hohmeyer J Volckmar C Steinhauer B Schlögl R Claus P 《Physical chemistry chemical physics : PCCP》2007,9(27):3559-3569
The hydrogenation of acrolein over pure and supported silver has been investigated with a focus on the influence of catalyst structure and reaction pressure (mbar to 20 bar range) on activity and selectivity. An onset of formation of allyl alcohol beyond 100 mbar reaction pressure (at 250 degrees C) is ascribed to a change in adsorption geometry upon increasing coverage. Smaller silver particles (in the nanometer range), the proximity of a reducible oxide component as well as high pressure lead to enhanced allyl alcohol formation; the selectivity to the other main product propionaldehyde is reduced. The silver dispersion changed depending on the reaction pressure. Moreover, the presence of oxygen, most likely as subsurface oxygen, and the presence of defects are of paramount importance for the catalytic behaviour. The considerable changes of the silver catalysts under reaction conditions and the pressure dependence call for in situ measurements to establish true structure-activity/selectivity relationships for this system. 相似文献
987.
Krämer PM Martens D Forster S Ipolyi I Brunori C Morabito R 《Analytical and bioanalytical chemistry》2007,387(4):1435-1448
Immunochemical methods (in particular immunoassays) have been applied to spring and surface water samples, respectively, which
were set-up as reference materials (RM) within two proficiency testing campaigns. For the first set of proficiency tests (PTs)
described here (which were actually the second round of PTs organized, spring 2005), three ELISAs (enzyme-linked immunosorbent
assays) were employed in the enzyme tracer format for isoproturon, diuron, and atrazine, respectively. Results were evaluated
in comparison with conventional reference methods (LC, GC). Based on their Z-score laboratory performances, the results for isoproturon and diuron were satisfactory, both for fortified spring water
and for the blind solution. The results for atrazine were strongly influenced by other triazines present and needed detailed
interpretation. For the second set of PTs described here (which were actually the third round of PTs organized, spring 2006),
two ELISAs in the coating antigen format were used for isoproturon and diuron, and the result was included with the results
obtained by conventional methods during the PTs. The results (the Z-scores) for isoproturon were again classified as satisfactory, in both fortified surface water and blind solution. The results
for diuron in ELISA showed an influence of the water matrix, while the analysis of the blind solution was satisfactory. In
addition, an ELISA in the enzyme tracer format was applied to analyze isoproturon, diuron, and atrazine in surface water samples,
which had been set-up and spiked during a field trial (tank experiment) at the Maas River at Eijsden, The Netherlands. The
immunoassay results were compared with those from an in-house on-line SPE LC/MS–MS used as reference. Although the immunochemical
results were sometimes higher than those determined in the reference analysis, the general concentration trends in the samples
were similar. The contribution of immunochemical methods to the implementation of the European Water Framework Directive is
also discussed. 相似文献
988.
Fritz C Cross I Moore BC Woodhouse J 《The Journal of the Acoustical Society of America》2007,122(6):3640-3650
This study is the first step in the psychoacoustic exploration of perceptual differences between the sounds of different violins. A method was used which enabled the same performance to be replayed on different "virtual violins," so that the relationships between acoustical characteristics of violins and perceived qualities could be explored. Recordings of real performances were made using a bridge-mounted force transducer, giving an accurate representation of the signal from the violin string. These were then played through filters corresponding to the admittance curves of different violins. Initially, limits of listener performance in detecting changes in acoustical characteristics were characterized. These consisted of shifts in frequency or increases in amplitude of single modes or frequency bands that have been proposed previously to be significant in the perception of violin sound quality. Thresholds were significantly lower for musically trained than for nontrained subjects but were not significantly affected by the violin used as a baseline. Thresholds for the musicians typically ranged from 3 to 6 dB for amplitude changes and 1.5%-20% for frequency changes. Interpretation of the results using excitation patterns showed that thresholds for the best subjects were quite well predicted by a multichannel model based on optimal processing. 相似文献
989.
Claudia Loerbroks Andreas Heimermann Walter Thiel 《Journal of computational chemistry》2015,36(15):1114-1123
This article reports a combined quantum mechanics/molecular mechanics (QM/MM) investigation on the acid hydrolysis of cellulose in water using two different models, cellobiose and a 40‐unit cellulose chain. The explicitly treated solvent molecules strongly influence the conformations, intramolecular hydrogen bonds, and exoanomeric effects in these models. As these features are largely responsible for the barrier to cellulose hydrolysis, the present QM/MM results for the pathways and reaction intermediates in water are expected to be more realistic than those from a former density functional theory (DFT) study with implicit solvent (CPCM). However, in a qualitative sense, there is reasonable agreement between the DFT/CPCM and QM/MM predictions for the reaction mechanism. Differences arise mainly from specific solute–solvent hydrogen bonds that are only captured by QM/MM and not by DFT/CPCM. © 2015 Wiley Periodicals, Inc. 相似文献
990.
Walking Down the Chalcogenic Group of the Periodic Table: From Singlet to Triplet Organic Emitters
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Dr. Adrian Kremer Dr. Claudia Aurisicchio Dr. Federica De Leo Dr. Barbara Ventura Prof. Johan Wouters Dr. Nicola Armaroli Dr. Andrea Barbieri Prof. Davide Bonifazi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(43):15377-15387
The synthesis, X‐ray crystal structures, ground‐ and excited‐state UV/Vis absorption spectra, and luminescence properties of chalcogen‐doped organic emitters equipped on both extremities with benzoxa‐, benzothia‐, benzoselena‐ and benzotellurazole ( 1X and 2X ) moieties have been reported for the first time. The insertion of the four different chalcogen atoms within the same molecular skeleton enables the investigation of only the chalcogenic effect on the organisation and photophysical properties of the material. Detailed crystal‐structure analyses provide evidence of similar packing for 2O – 2Se , in which the benzoazoles are engaged in π–π stacking and, for the heavier atoms, in secondary X???X and X???N bonding interactions. Detailed computational analysis shows that the arrangement is essentially governed by the interplay of van der Waals and secondary bonding interactions. Progressive quenching of the fluorescence and concomitant onset of phosphorescence features with gradually shorter lifetimes are detected as the atomic weight of the chalcogen heteroatom increases, with the tellurium‐doped derivatives exhibiting only emission from the lowest triplet excited state. Notably, the phosphorescence spectra of the selenium and tellurium derivatives can be recorded even at room temperature; this is a very rare finding for fully organic emitters. 相似文献