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The decrease in D-->A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND(3)BF(3) by powder neutron diffraction. The B-N internuclear distance is 1.554(3) A, 0.118 A shorter than the most recent gas-phase microwave value and 0.121 A shorter than the single molecule geometry optimized (1.672 A, CISD/6-311++G(d,p)) bond length. The crystal structure also shows N-D.F-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen ((15)N and (14)N) and boron ((11)B and (10)B) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the (11)B and (14)N QCCs show large changes because of both effects. An analysis of the (10)B[(19)F] line shape in solid ND(3)(10)BF(3) yields a (11)B QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH(3)BF(3)](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T(1) measurements yield an activation energy for rotation of the ND(3) group in solid ND(3)BF(3) of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T(1) anisotropy gave an E(a) of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 x 10(12) s(-)(1). Our calculated value for a [NH(3)BF(3)](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T(1) measurements. 相似文献
5.
Ceramides are important intracellular second messengers that play a role in the regulation of cell growth, differentiation and programmed cell death. Qualitative and quantitative analysis of these second messengers requires sensitive and specific analytical methods to detect endogenous levels of individual ceramide species and to differentiate between them. Nine synthetic ceramides were separated by liquid chromatography coupled to tandem mass spectrometry on a C18 bonded silica column. The lipids were eluted in gradient elution mode using a mixture of water, acetonitrile and 2-propanol as mobile phase. They were detected by reaction monitoring performed on positive ion electrospray generated ions. Collision-induced fragmentations conducted on ceramides produced a well characteristic product ion at m/z 264, making multiple reaction monitoring (MRM) well suited for various ceramides quantitative measurements. After optimization of the extraction step, the proposed methodology was able to identify and quantify different ceramide species issued from human cancer cells. The method could be validated for C16, C18 and C20 ceramides, quantified at the nanogram level. The validation exhibits good results with respect to linearity, accuracy and precision. 相似文献
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John J. Fenske Andrew Hashimoto Michael H. Penner 《Applied biochemistry and biotechnology》1998,73(2-3):145-157
The relative toxicity of the combined nonxylose components in prehydrolysates derived from three different lignocellulosic
biomass feedstocks was determined. One woody (poplar) and two herbaceous (corn stover and switchgrass) feedstocks were dilute-acid
pretreated under temperature and acid conditions chosen to optimize xylose recovery in the liquid prehydrolysate; xylose yields
averaged 96,89,and 87% of theoretical for switchgrass,corn stover,and poplar,respectively. Prehydrolysates from each feedstock
were neutralized,adjusted to equivalent xylose concentrations,and bioassayed for toxicity,using a standardized fermentation
protocol withPichia stipitis NRRL 11545. Full time-courses for ethanol production (30-60 h) clearly illustrate the distinct inhibitory effects of prehydrolysates
from different feedstocks. The relative toxicity of the prehydrolysates,ranked in order of decreasing toxicity,is poplar-derived
prehydrolysates > switchgrass-derived prehydrolysates > corn stover-derived prehydrolysates. The inhibition of ethanol production
appeared to be the result of a general slowdown of yeast metabolism,rather than the result of the production of alternative,
nonethanol end products. Ethanol yields averaged 74,83,and 88% of control values for poplar,switchgrass,and corn stover prehydrolysates,
respectively. Volumetric ethanol productivities (g ethanol L/h) averaged 32,70,and 102% of control values for poplar,switchgrass,and
corn stover prehydrolysates,respectively. Ethanol productivities correlated closely with acetate concentrations in the prehydrolysates;
however, regression lines correlating acetate concentrations and ethanol productivities were found to be feedstock-dependent.
Oregon State University Agricultural Experiment Station Technical Publication Number 11114 相似文献
7.
Werner Unterberger Bernd Jenewein Bernhard Kl?tzer Simon Penner Wolfgang Reichl Günther Rupprechter Di Wang Di Wang Robert Schl?gl Konrad Hayek 《Reaction Kinetics and Catalysis Letters》2006,87(2):215-234
Summary The effect of hydrogen reduction on the structure and catalytic properties of “thin film”and “inverse”model systems for supported
metal catalysts is discussed. Thin film model catalysts were obtained by epitaxial growth of Pt and Rh nanoparticles on NaCl(001),
which were coated with amorphous or crystalline supports of alumina, silica, titania, ceria and vanadia. Structural and morphological
changes upon hydrogen reduction between 473 and 973 K were examined by high resolution electron microscopy. Metal-oxide interaction
sets in at a specific reduction temperature and is characterized by an initial “wetting”stage, followed by alloy formation
at increasing temperature, in the order VOx< TiOx< SiO2< CeOx< Al2O3. “Inverse”model systems were prepared by deposition of oxides on a metal substrate, e.g. VOx/Rh and VOx/Pd. Reduction of inverse systems at elevated temperature induces subsurface alloy formation. In contrast to common bimetallic
surfaces, the stable subsurface alloys of V/Rh and V/Pd have a purely noble metal-terminated surface, with V positioned in
near-surface layers. The uniform composition of the metallic surface layer excludes catalytic ensemble effects in favor of
ligand effects. Activity and selectivity, e.g. for CO and CO2methanation and for partial oxidation of ethene, are mainly controlled by the temperature of annealing or reduction. Reduction
above 573 K turned out to be beneficial for the catalytic activity of the subsurface alloys, but not for the corresponding
thin film systems which tend to deactivate viaparticle encapsulation.</o:p> 相似文献
8.
Low-energy ion scattering with monolayer sensitivity was applied to investigate ultrathin films of zinc on Pd(1?1?1). Uptake curves taken at 150?K indicate the simultaneous growth of multilayers with negligible interlayer transport. Annealing experiments for two-monolayer films reveal a rapid decrease in the zinc content on the surface layer at temperatures above 300?K, forming a metastable state with a Pd:Zn surface ratio of approx. 1:1 in the temperature region between 400 and 550?K. This state is most easily explained as a slightly buckled p(2?×?1)-PdZn surface alloy, with Zn atoms located approx. 0.25?? above their Pd counterparts. 相似文献
9.
Methyl 6‐O‐p‐nitrobenzoyl‐β‐d‐glucoside was synthesized by reacting methyl 4,6‐O‐p‐nitrobenzylidine‐β‐d‐glucoside with N‐bromosuccinimide (NBS). First, methyl β‐d‐glucoside was converted into methyl 4,6‐O‐p‐nitrobenzylidine‐β‐d‐glucoside with p‐nitrobenzaldehyde. Later, methyl 4,6‐O‐p‐nitrobenzylidine‐β‐d‐glucoside was opened oxidatively with NBS to give methyl 6‐O‐p‐nitrobenzoyl‐β‐d‐glucoside. 相似文献
10.
Calculations of the nonequilibrium rate constant for the model system H2O2 + M → 2 OH + M over the temperature range of 300–1900°K, assuming that only vibrational, or that both vibrational and rotational, energy is transferred in a collision, show that (1) inefficient energy transfer leads to a distinctly non-Arrhenius temperature dependence, the nonlinearity being in principle different for different M, and (2) despite different activation energies for different M, the order of M efficiencies is preserved throughout the temperature range. A reversal of M efficiencies can occur only if there is a change of mechanism of the reaction over the temperature range investigated. 相似文献