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31.
Bicyclic diazenium salts have been prepared from α-chloroazo species via a Lewis acid-mediated intramolecular cycloaddition. An alternative, more direct, route to these salts by the reaction of hydrazones with dimethylsulfonium ditriflate is also described. Terminal olefins provided mixtures of fused and bridged bicyclic diazenium salts. The ratio of the fused and bridged species was observed to depend on the electronics of the N-aryl substituent, which is explained by considering a concerted asynchronous cycloaddition mechanism. 相似文献
32.
Lifetime of levels up to 22+, have been measured in 78Kr and an oblate shape is assigned to the ground state using the CSM and the configuration dependent shell correction calculations.
Calculations further show that 78Kr is highly γ-soft nucleus. The experimental Q
t values coupled with theoretical calculations indicate an oblate shape for 78Kr at low spins and triaxial shape at higher spins 相似文献
33.
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35.
Literature data were collected and analyzed to guide selection of conditions for pretreatment by dilute acid and water-only
hemicellulose hydrolysis, and the severity parameter was used to relate performance of different studies on a consistent basis
and define attractive operating conditions. Experiments were then run to confirm performance with corn stover. Although substantially
better hemicellulose sugar yields are observed when acid is added, costs would be reduced and processing operations simplified
if less acid could be used while maintaining good yields, and understanding the relationship between operating conditions
and yields would be invaluable to realizing this goal. However, existing models seldom include the oligomeric intermediates
prevalent at lower acid levels, and the few studies that include such species do not account for the distribution of chain
lengths during reaction. Therefore, the polymeric nature of hemicellulose was integrated into a kinetic model often used to
describe the decomposition of synthetic polymers with the assumption that hemicellulose linkages are randomly broken during
hydrolysis. Predictions of monomer yields were generally consistent with our pretreatment data, data reported in the literature,
and predictions of other models, but the model tended to overpredict oligomer yields. These differences need to be resolved
by gathering additional data and improving the model. 相似文献
36.
A study of the dissociation pressure of crystalline K2CoCl4·2H2O. The reactions can be summed up as K2CoCl4·nH2O(c) = K2CoCl4·mH2O(c)+(nm)H2O (v). Below 50°C, n = 2 and 1, m = 1 and 0, above 50°C, n = 2 and m = 0. Below 50°C, the dihydrate is octahedral, the monohydrate and anhydrous compounds are tetrahedral. ΔH° and ΔS° are respectively 7.0 kcal and 14.2 e.u. for the loss of the first mole of water and 12.7 kcal and 32.0 e.u. for the loss of the last mole of water. Above 50°C, ΔH° and ΔS° are respectively 29.8 kcal and 77.6 e.u. for the loss of both waters. The changes in structure are discussed using the spectral and magnetic properties as indications for structural changes. 相似文献
37.
Esther Martinborough Tiziana Mordasini Denti Peter P. Castro Tara B. Wyman Carolyn B. Knobler Franois Diederich 《Helvetica chimica acta》1995,78(5):1037-1066
The complexation of N-benzyloxycarbonyl (Cbz) derivatives of the excitatory amino acids L -aspartic acid (Asp; 1 ), L -glutamic acid (Glu; 3 ), and, for the first time, L -kainic acid ((2S,3S,3S)-2-carboxy-4-(1-methylethenyl)pyrrolidine-3-acetic acid; Kai; 5 ) was studied in CDCl3 with a diversity of chiral receptors consisting of a 1,1′-binaphthyl spacer with (carboxamido)pyridine (CONH(py)) functionality attached to the 6,6′-positions in the major groove. Receptors of type A possess two N-(pyridin-2-yl)carboxamide H-bonding sites (e.g. 7 ), whereas type B-receptors have two N-(pyridine-6,2-diyl)acetamide residues attached (e.g. 8 and 9 ). Complexes of excitatory amino-acid derivatives and other, achiral α,β-dicarboxylic acids with these receptors are primarily stabilized by two sets of C?O···H? N and O? H ··· N H-bonds. Optically active type-A receptors such as (R)- and (S)- 7 showed a preference for the larger Glu derivative, whereas type- B receptors such as (R)- and (S)- 8 and (R)- and (S)- 9 formed more stable complexes with the smaller Cbz-Asp. To improve the poor enantioselectivity shown by 7–9 , additional functionality was introduced at the 7,7′-positions of the 1,1′-binaphthyl spacer, and the nature of the H-bonding sites in the 6,6′-positions was varied. Screening the diversity of new racemic receptors for binding affinity, which had been shown in many examples by Cram to correlate with enantioselectivity, demonstrated that (+)- 10 and (+)- 11 formed the most stable complexes with dicarboxylic acids, and these receptors were synthesized in enantiomerically pure form. Both are type- B binders and contain additional PhCH2O ( 10 ) and MeO ( 11 ) groups in the 7,7′-positions. By 1H-NMR binding titrations, the complexation of (R)- and (S)- 10 and (R)- and (S)- 11 with the excitatory amino-acid derivatives was studied in CDCl3, and association constants Ka between 103 and 2 · 105 l mol?1 were measured for the 1:1 host-guest complexes formed. Whereas both 10 and 11 formed stable complexes, enantioselective binding was limited to the PhCH2O-substituted receptor 10 , with the (R)-enantiomer complexing Cbz-Asp by 0.7 kcal mol?1 more tightly than the (S)-enantiomer. The structures of the diastereoisomeric complexes were analyzed in detail by experimental methods (complexation-induced changes in 1H-NMR chemical shifts, 1H{1H} nuclear Overhauser effect (NOE) difference spectroscopy) and computer modeling. These studies established that an unusual variety of interesting aromatic interactions and secondary electrostatic interactions are responsible for both the high binding affinity (? ΔG° up to 7.2 kcal mol?1) and the enantioselection observed with (R)- and (S)- 10 . In an approach to enhance the enantioselectivity by reducing the conformational flexibility of the 1,1′-binaphthyl spacer, an additional crown-ether binding site was attached to the 2,2′-positions in the minor groove of the type- B receptors (R)- and (S)- 48 . Both the binding affinity and the enantioselectivity (Δ(ΔG°) up to 0.7 kcal mol?1) in the complexation of the excitatory amino-acid derivatives by (R)- and (S)- 48 were not altered upon complexation of Hg(CN)2 at the crown-ether binding site, demonstrating lack of cooperativity between the minor- and major-groove recognition sites. 相似文献
38.
Acids catalyze the hydrolysis of cellulose and hemicellulose to produce sugars that organisms can ferment to ethanol and other
products. However, advanced low- and no-acid technologies are critical if we are to reduce bioethanol costs to be competitive
as a pure fuel. We believe carbohy drate oligomers play a key role in explaining the performance of such hydrolysis processes
and that kinetic models would help us understand their role. Various investigations have developed reaction rate expressions
based on an Arrhenius temperature dependence that is first order in substrate concentration and close to first order in acid
concentration. In this article, we evaluate these existing hydrolysis models with the goal of providing a foundation for a
unified model that can predict performance of both current and novel pretreatment process configurations. 相似文献
39.
Stable isotope methods are potentially quite useful for validating natural or enhanced mineral degradation of contaminants. For this reason, a continuous flow gas chromatograph (GC), isotope ratio mass spectrometer (IRMS) has been coupled with a quadrupole mass selective detector (MSD) to allow simultaneous mass spectral and stable carbon isotope ratio data to be obtained from a single chromatographic analysis. This allows the target contaminant and any extra-cellular degradation intermediates to be both qualified and quantified. Previously acceptable limits of precision (0.3 parts per mil) are undesirable given the small fractionation observed during aerobic degradation. To further understand the fate of organic contaminants and to gain information about the metabolic degradative pathway employed by a microorganism, routine isotopic analyses on a range of analytes have been performed. Quantities of sample producing mass-44 ion beam signal (I(44)) of 2 x 10(-10) to 1 x 10(-8) A were analysed. When the IRMS was tuned for high sensitivity, ion source nonlinearities were overcome by peak height correction from an algorithm that was produced using known isotopic standards of varying concentrations. This led to sample accuracy of <0.01 per thousand and sample precision of 0.1 per thousand. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
40.
KM Clauwaert Van Bocxlaer JF HJ Major JA Claereboudt WE Lambert Van den Eeckhout EM Van Peteghem CH De Leenheer AP 《Rapid communications in mass spectrometry : RCM》1999,13(14):1540-1545
This paper describes the investigation of the potential of a quadrupole orthogonal acceleration time-of-flight mass spectrometer (Q-TOF) equipped with an atmospheric pressure ionisation interface for quantitative measurements of small molecules separated by reversed phase liquid chromatography. To this end, the detection limits and linear dynamic range in particular were studied in an LC/MS/MS experiment using 3,4-methylenedioxymethamphetamine standards and 3,4-methylenedioxyethylamphetamine for internal standardisation. In a second phase, the experiment was repeated with real biological extracts (whole blood, serum, and vitreous humour). A calibration for 3,4-methylenedioxymethamphetamine and its metabolite 3,4-methylenedioxyamphetamine was prepared in each of these matrices again using 3,4-methylenedioxyethylamphetamine as internal standard. The resulting quantitative data were compared with those obtained by liquid chromatography with fluorescence detection for the same extracts. The Q-TOF results revealed excellent sensitivity and a linear dynamic range of nearly four decades (2-10 000 pg on-column, r(2) = 0.9998, 1/x weighting). Furthermore, all the calibration curves prepared in biological material were superimposable, LC/MS/MS and LC-fluorescence, and the quantitative results for actual samples compared very favourably. It was concluded that the Q-TOF achieves a linear dynamic range for quantitative LC/MS/MS work exceeding that of fluorescence detection and at much better absolute sensitivity. Copyright 1999 John Wiley & Sons, Ltd. 相似文献