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71.
72.
V. Renugopalakrishnan G. Madrid G. Cuevas A. T. Hagler 《Journal of Chemical Sciences》2000,112(1):35-42
Density functional theory was applied to the calculation of molecular structures of N-methyl formamide (NMF), N,N-dimethyl
formamide (DMF), and N,N-dimethyl acetamide (DMA). DFT calculations on NMF, DMF, and DMA were performed using a combination
of the local functional of Vosko, Wilk, and Nusair (VWN) with the nonlocal exchange functional of Becke and the nonlocal correlational
functional of Lee, Yang, and Parr (BLYP). The adiabatic connection method (ACM) of Becke has also been used, for the first
time, for the calculation of molecular structures of NMF, DMF, and DMA. The calculated molecular structures are in excellent
agreement with the experimental geometries of NMF and DMA derived from gas-phase electron-diffraction studies. Sparse experimental
data on the gas-phase geometry of DMF reported in the literature compares well with the DFT results on DMF. DFT emerges as
a powerful method to calculate molecular structures. 相似文献
73.
Patricia Gabriela Molina María Alicia Zn Hctor Fernndez 《Journal of Electroanalytical Chemistry》2002,520(1-2)
The electrooxidation of altenuene (ALT), one of the mycotoxins of the Alternaria alternata genus, on a glassy carbon disk electrode is studied for the first time by using cyclic and square wave voltammetry. From the electrochemical responses, a complex reaction mechanism could be inferred. Values of 1.06×10−5 cm2 s−1, 1.116 V and 2 were determined for the diffusion coefficient, the apparent formal potential and the electron number, respectively, for the overall electrode process by convolution analysis of linear scan voltammograms. Square wave voltammetry was used to generate Ip versus cALT* calibration curves for this fungal metabolite. A detection limit of 4.0×10−7 M was determined for a 2:1 signal to noise ratio. The acid dissociation constant for ALT was determined from conventional UV–vis spectrophotometric measurements. Experimental variations of absorbance as a function of pH at a given wavelength were fitted by using the exact equation that describes the system. Good agreement between the experimental absorbance versus pH plots and the curves generated by the fitting process was found. 相似文献
74.
Summary A new method is presented for the rapid and consecutive determination of theYoung's and bulk moduli of polymeric solids in compression loading. Usually these properties are determined by separate measurements. However, by using an undersized specimen in a standard bulk compressibility tester, it is possible to determine both moduli on the same specimen in a single test procedure. Initial loading of the undersized specimen results in a decrease in length and an increase in diameter. From these changes,Young's modulus may be calculated. Once the bore of the tester is filled, the application of additional pressure results in a decrease in the volume of the polymer from which the bulk modulus may be calculated. Both determinations may be made within minutes. The moduli values are in general agreement with published values and the calculated values for Poisson's ratio fall into the expected range.
Zusammenfassung Es wird eine neue Methode vorgestellt für die schnelle und fortlaufende Bestimmung des Elastizitäts- und Kompressions-Modul us polymerer Festkörper bei einer Druckbelastung. Normalerweise werden diese Eigenschaften durch unterschiedliche Methoden bestimmt. Durch Benutzung einer unterdimensionierten Probe in einem Standard-Kompressionsprüfgerät ist es möglich, beide Moduln an der gleichen Probe in einem einzigen Versuch zu ermitteln. Die Anfangsbelastung einer unterdimensionierten Probe führt zu einer Längenabnahme und einer Zunahme des Durchmessers. Aus diesen Änderungen läßt sich der Elastizitätsmodul berechnen. Sobald die Bohrung des Gerätes gefüllt ist, führt die Erhöhung des Druckes zu einer Volumen-Abnahme des Polymers, aus dem sich der Kompressionsmodul berechnen läßt. Beide Bestimmungen können in wenigen Minuten durchgeführt werden. Die Modulwerte stimmen im allgemeinen mit veröffentlichten Werten überein, und die für dasPoisson-Verhältnis ermittelten Werte liegen in dem erwarteten Bereich.相似文献
75.
Alicia L. Gui Guozhen Liu Muthukumar Chockalingam Guillaume Le Saux Erwann Luais Jason B. Harper J. Justin Gooding 《Electroanalysis》2010,22(16):1824-1830
4‐Nitrophenyl layers were grafted on gold and glassy carbon surfaces by electrochemical reductive adsorption of the corresponding diazonium salt. Electrochemical conversion efficiencies of 4‐nitrophenyl moieties to 4‐aminophenyl moieties on gold versus on glassy carbon in a protic medium were investigated using X‐ray photoelectron spectroscopy (XPS). In total contrast to all previous comparative studies showing greater electrochemical reactivity of aryl diazonium salt‐derived layers on gold than on glassy carbon, a much lower rate of conversion to 4‐aminophenyl was observed on gold than on glassy carbon by both cyclic voltammetry (CV) and chronoamperometry (CA) methods. The lower electron transfer rate during conversion observed on gold versus glassy carbon was proposed to be due to a mechanism related to the molecular structure rearrangement of 4‐nitrophenyl during the process on glassy carbon. However, whilst complete conversion of 4‐nitrophenyl to 4‐aminophenyl on gold by chronoamperometry was achieved, on glassy carbon complete reduction could not be achieved under the same conditions. 相似文献
76.
A selective two-step synthesis of either mono- or distannylated pyridines from commercially available pyridinols, involving its conversion to the corresponding diethyl pyridyl phosphates (pyDEP) followed by the reaction with Me3SnNa in liquid ammonia, is described.The results obtained clearly indicate that the reactions proceed through an unimolecular radical nucleophilic substitution mechanism (SRN1) with intermediacy of a monosubstitution product. 相似文献
77.
Casitas A Poater A Solà M Stahl SS Costas M Ribas X 《Dalton transactions (Cambridge, England : 2003)》2010,39(43):10458-10463
Well-defined aryl-Cu(III)-halide species undergo reductive elimination upon acid addition resulting in the formation of strong aryl-halide bonds. The computationally studied mechanism points towards ligand protonation as the rate-determining step, in agreement with previous experimental data. 相似文献
78.
Wei Xu Alicia M. Luikart Christopher E. Sims Nancy L. Allbritton 《Analytical and bioanalytical chemistry》2010,397(8):3377-3385
A novel contact printing method utilizing a sacrificial layer of polyacrylic acid (PAA) was developed to selectively modify
the upper surfaces of arrayed microstructures. The method was characterized by printing polystyrene onto SU-8 microstructures
to create an improved substrate for a cell-based microarray platform. Experiments measuring cell growth on SU-8 arrays modified
with polystyrene and fibronectin demonstrated improved growth of NIH 3T3 (93% vs. 38%), HeLa (97% vs. 77%), and HT1080 (76%
vs. 20%) cells relative to that for the previously used coating method. In addition, use of the PAA sacrificial layer permitted
the printing of functionalized polystyrene, carboxylate polystyrene nanospheres, and silica nanospheres onto the arrays in
a facile manner. Finally, a high concentration of extracellular matrix materials (ECM), such as collagen (5 mg/mL) and gelatin
(0.1%), was contact-printed onto the array structures using as little as 5 μL of the ECM reagent and without the formation
of a continuous film bridge across the microstructures. Murine embryonic stem cells cultured on arrays printed with this gelatin
hydrogel remained in an undifferentiated state indicating an adequate surface gelatin layer to maintain these cells over time. 相似文献
79.
Laurent Bultel Malena Landoni Eric Grand Alicia S. Couto José Kovensky 《Journal of the American Society for Mass Spectrometry》2010,21(1):178-190
Nitrous acid degradation of heparin followed by high-performance anion-exchange chromatography (HPAEC) separation and ultraviolet
matrix assisted laser desorption/ionization time-of-flight (UV-MALDI-TOF) analysis led to the structural determination of
six sulfated oligosaccharides. Three different matrices (α-cyano-4-hydroxycinnamic acid (CHCA), nor-harmane, and dihydroxybenzoic
acid (DHB)) have been used, and the complementary results obtained allowed in most cases to assign the position of sulfate
groups. Based on the different cleavages produced on the purified oligosaccharides in source during the MS analysis by the
use of the different matrices, this approach provides a new tool for structural analysis. 相似文献
80.