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61.
62.
Timms PL Norman NC Pardoe JA Mackie ID Hinchley SL Parsons S Rankin DW 《Dalton transactions (Cambridge, England : 2003)》2005,(3):607-616
The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods. 相似文献
63.
A fully validated and clinically relevant assay was developed for the assessment of nevirapine concentrations in neonate blood plasma samples. Solid-phase extraction with an acid-base wash series was used to prepare subject samples for analysis. Samples were separated by high performance liquid chromatography and detected at 280 nm on a C8 reverse-phase column in an isocratic mobile phase. The retention times of nevirapine and its internal standard were 5.0 and 6.9 min, respectively. The method was validated by assessment of accuracy and precision (statistical values <15%), specificity, and stability. The assay was linear in the range 25-10,000 ng/mL (r2 > 0.996) and the average recovery was 93% (n = 18). The lower limit of quantification (relative standard deviation <20%) was determined to be 25 ng/mL for 50 microL of plasma, allowing detection of as little as 1.25 ng of nevirapine in a sample. This value represents an increase in sensitivity of up to 30-fold over previously published methods. 相似文献
64.
J. L. Gardea-Torresdey K. J. Tiemann J. R. Peralta-Videa J. G. Parsons M. Delgado 《Microchemical Journal》2004,76(1-2):65
Traditional treatment methods used to clean-up heavy metal contamination of soils and waters are cost intensive whereas more cost effective methods need to be developed. The use of plant materials to remediate heavy contamination has been studied for the past two decades. This technique has shown much promise for many of the common heavy metal contaminants, but few studies have focused on the lanthanide series elements. By investigating the binding and interactions of the lanthanide elements to alfalfa biomass, a more complete understanding of the binding mechanisms and the interactions of heavy metals with biomaterials can be obtained. Different chemical functional groups on the alfalfa biomass, carboxyl, amino, sulfur, and ester groups, were modified to investigate the binding mechanisms of erbium(III) and holmium(III). Batch experiments were performed with native and chemically modified alfalfa biomass suggesting that the carboxyl groups play a major role in the binding of erbium(III) and holmium(III) to the alfalfa biomass. In addition, X-ray absorption spectroscopy (XAS) studies corroborated the data obtained from the batch experiments. 相似文献
65.
Alexander Alex Sophie Harvey Teresa Parsons Frank S. Pullen Patricia Wright Jo‐Anne Riley 《Rapid communications in mass spectrometry : RCM》2009,23(17):2619-2627
Prediction of tandem mass spectrometric (MS/MS) fragmentation for non‐peptidic molecules based on structure is of immense interest to the mass spectrometrist. If a reliable approach to MS/MS prediction could be achieved its impact within the pharmaceutical industry could be immense. Many publications have stressed that the fragmentation of a molecular ion or protonated molecule is a complex process that depends on many parameters, making prediction difficult. Commercial prediction software relies on a collection of general heuristic rules of fragmentation, which involve cleaving every bond in the structure to produce a list of ‘expected’ masses which can be compared with the experimental data. These approaches do not take into account the thermodynamic or molecular orbital effects that impact on the molecule at the point of protonation which could influence the potential sites of bond cleavage based on the structural motif. A series of compounds have been studied by examining the experimentally derived high‐resolution MS/MS data and comparing it with the in silico modelling of the neutral and protonated structures. The effect that protonation at specific sites can have on the bond lengths has also been determined. We have calculated the thermodynamically most stable protonated species and have observed how that information can help predict the cleavage site for that ion. The data have shown that this use of in silico techniques could be a possible way to predict MS/MS spectra. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
66.
Michael G. Minnich Derek C. Miller Patrick J. Parsons 《Spectrochimica Acta Part B: Atomic Spectroscopy》2008
The application of the large-bore direct injection high efficiency nebulizer (LB-DIHEN) for the determination of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in urine by inductively coupled plasma mass spectrometry (ICP-MS) is described. The LB-DIHEN is compared with the standard method using a concentric pneumatic nebulizer and cyclonic spray chamber. In addition to the toxicological significance of As, Cd, Pb, and Hg, these elements represent a cross-section of analytical issues including spectral interferences (e.g., 40Ar35Cl+ on 75As+ and 98Mo16O+ on 114Cd+) and memory effects (Hg). In this study, the low sample consumption of the LB-DIHEN is used to reduce the volume of urine needed for analysis, and to reduce the volume of final diluted sample required for analysis. Eliminating the spray chamber and reducing the dead volume of the nebulizer reduces memory effects, especially for analytes such as Hg. The Dynamic Reaction Cell (DRC) is used in this study to attenuate the background level of ArCl+ in spite of the increase in the solvent load and, in turn, the urine matrix (chloride) delivered to the plasma by the LB-DIHEN. This is the first report on coupling the LB-DIHEN to a standard autosampler for unattended sample analysis. The robustness of direct injection nebulization for routine analysis and the issues associated with automation of the sample introduction process are discussed. Although the figures of merit (sensitivity, limit of detection, and precision) determined for both nebulizers are slightly poorer for the LB-DIHEN than for the concentric pneumatic nebulizer, there is not a clinically significant difference between the results for both sample introduction systems. The accuracy of results is assessed using archived urine materials that are circulated by several different proficiency testing (PT) programs and external quality assessment schemes (EQAS). Results obtained using the LB-DIHEN were within the acceptable range established by a consensus pool generated using different methods, none of which are likely to be using direct injection nebulization. Internal quality control sample results obtained using the LB-DIHEN were compared to those obtained using the conventional nebulizer. Reported results were similar for both nebulizers. Thus, these results show that the LB-DIHEN is certainly feasible for the analysis of urine specimens. 相似文献
67.
68.
Terence Parsons 《Logica Universalis》2008,2(1):3-11
The truth conditions that Aristotle attributes to the propositions making up the traditional square of opposition have as
a consequence that a particular affirmative proposition such as ‘Some A is not B’ is true if there are no Bs. Although a different
convention than the modern one, this assumption remained part of centuries of work in logic that was coherent and logically
fruitful.
相似文献
69.
To explore the effect of intermolecular interactions on the photophysics of 2-aminopurine (2AP) in a well-defined environment, we have investigated the fluorescence properties of single 2AP crystals, having determined their X-ray structure. In the crystal, 2AP is subject to base-stacking and hydrogen-bonding interactions similar to those found in DNA. The crystal shows dual fluorescence: pi-stacked molecules in the bulk of the lattice have redshifted excitation and emission spectra, while molecules at defect sites have spectra similar to those of 2AP in solution or in DNA. Heterogeneous intermolecular interactions in the crystal give rise to multiexponential fluorescence decay characteristics similar to those observed for 2AP-labelled DNA. The presence of about 13 % of the 7H tautomer in the crystal confirms that 9H-7H tautomerisation of 2AP occurs in the ground state. Long-wavelength excitation of a 2AP-labelled oligonucleotide duplex produced redshifted emission similar to that observed in the crystal, indicating that pi-stacking interaction of 2AP with nucleobases gives rise to a low energy excited state. 相似文献
70.
Sarkar U Glaser R Parsons ZD Barnes CL Gates KS 《Journal of chemical crystallography》2010,40(7):624-629