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71.
Our struggles and ultimate success in achieving a total synthesis of phomactin A are described. Our strategy features an intramolecular oxa-[3 + 3] annulation to construct its unique ABD-tricyclic manifold. Although the synthesis would constitute a distinctly new approach with the 12-membered D-ring of phomactin A being assembled simultaneously with the 1-oxadecalin at an early stage, the ABD-tricycle represents a unique structural topology that would pose a number of unprecedented challenges. One challenge concerned elaborating this tricycle to have oxygenation at the proper carbon atoms. To overcome this, we would utilize a Kornblum-DeLaMare ring-opening of a peroxide bridge as well as a challenging late-stage 1,3-allylic alcohol transposition. Further, the structural intricacies of the ABD-tricycle were uncovered by a conformational analysis that would be critical for the C5a-homologation. 相似文献
72.
Guingab-Cagmat JD Stevens SM Ratliff MV Zhang Z Gold MS Anagli J Wang KK Kobeissy FH 《Electrophoresis》2011,32(13):1692-1705
Protein tyrosine nitration is a post-translational modification commonly used as a marker of cellular oxidative stress associated with numerous pathophysiological conditions. We focused on ubiquitin carboxyl terminal hydrolase-L1 (UCH-L1) and glyceraldehyde-3-phosphate (GAPDH) which are high-abundant brain proteins that have been identified to be highly susceptible to oxidative modification. Both UCH-L1 and GAPDH have been linked to the pathogenesis of Alzheimer's and Parkinson's disease, however specific nitration sites have not been elucidated. Identification of specific nitration sites and quantitation of endogenous nitrated proteins are important in correlating this modification to disease pathology. In this study, purified UCH-L1 and GAPDH were nitrated in vitro with peroxynitrite and the presence of nitrated proteins was confirmed by anti-3-nitrotyrosine Western blots. Data-dependent LC-MS/MS analysis identified several distinct tyrosine nitration sites in UCH-L1 (Tyr-80) and GAPDH (Tyr-47, Tyr-92, and Tyr-312). Subsequent validation with synthetic peptides was conducted for selected nitropeptides. An LC-MS/MS method was developed for semi-quantitative determination of the synthetic nitropeptides: KGQEVSPKVY(*) (UCH-L1) and mFQY(*) DSTHGKF (GAPDH). The nitropeptides were detectable in the mid-attomole range and the peak area response was linear over three orders of magnitude. Targeted analysis of endogenous UCH-L1 and GAPDH nitration was then conducted in an in vivo second-hand smoke rat model to evaluate the utility of this approach. 相似文献
73.
Leach ES Hopkinson A Franklin K van Duijneveldt JS 《Langmuir : the ACS journal of surfaces and colloids》2005,21(9):3821-3830
It is shown how stable, nonaqueous suspensions of fully exfoliated smectite clays such as Laponite and montmorillonite can be obtained. Suspensions in toluene and in a branched aliphatic solvent (polydecene) were characterized using elemental analysis, rheometry, and small-angle X-ray scattering. For Laponite, stable suspensions were obtained by adsorbing a dichain poly(isobutylene) based stabilizer to the particles. This approach did not result in full exfoliation for montmorillonite particles, possibly because the stabilizer was able to connect individual clay sheets at the edges during the treatment process. Instead, a quaternary ammonium surfactant, dimethyl dioctadecylammonium bromide (DODAB), was first adsorbed to the clay. Subsequently, adsorption of the poly(isobutylene) based stabilizer to this pretreated clay resulted in fully exfoliated suspensions of montmorillonite. Suspensions of clay particles treated only with DODAB have been studied by several authors before and are included in this study for comparison. A detailed discussion of the suspension behavior of the different clay samples is given. The suspension routes presented here may enable further studies of the structure and flow behavior of suspensions of thin, flexible clay platelets as a function of aspect ratio. 相似文献
74.
Ziauddin Ahmed Carsten Müller Jaya J. Johnston Kevin Nguyen Christopher P. J. Schubert Karina Abitaev 《Liquid crystals》2019,46(6):896-904
We investigate the origins of ‘de Vries-like’ liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S2 in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and ‘de Vries-like’ properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit ‘de Vries-like’ properties. The S2(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with ‘de Vries-like’ behaviour. However, the Leff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S2 in compensating for the molecular tilt. 相似文献
75.
76.
A liquid chromatography–electrospray-tandem mass spectrometry (ES-MS/MS) method was developed for the speciation analysis of four organic selenium species of relevance to human urinary metabolism, namely trimethylselenomium ion (TMSe+), selenomethionine (SeMet) and the two selenosugars, methyl 2-acetamido-2-deoxy-1-seleno-β-d-galactos/-glucos-amine (SeGalNAc and SeGluNAc, respectively). Their chromatographic separation was achieved by using a cation exchange pre-column coupled in-series with a reversed-phase high-performance liquid chromatography column, along with an isocratic mobile phase. Online detection was performed using ES-MS/MS in selective reaction monitoring mode. SeGalNAc was detected as the major human urinary metabolite of selenium in the samples analysed, whereas TMSe+ was detected in the urine of one volunteer before and after receiving a selenium supplement. SeMet was not detected as a urine excretory metabolite in this study. Spiking experiments performed with the urine samples revealed significant signal suppression caused by coeluting matrix constituents. To overcome such interferences, isotopically labelled 13CD382SeGalNAc was used as an internal standard, whereas in the absence of an isotopically labelled internal standard for TMSe+, the standard addition method was applied. Quality control for the accurate quantitation of TMSe+ and SeGalNAc was carried out by analysing spiked human urine samples with appropriate selenium standards over a concentration range of 10–50 μg Se L−1. The method has achieved a limit of detection in the presence of urine matrix comparable to that of HPLC-inductively coupled plasma-mass spectrometry for the four selenium species: 1.0 μg Se L−1 for TMSe+, 5.6 μg Se L−1 for SeMet, and 0.1 μg Se L−1 for both SeGalNAc and SeGluNAc. 相似文献
77.
Synthesis of (−)‐Hebelophyllene E: An Entry to Geminal Dimethyl‐Cyclobutanes by [2+2] Cycloaddition of Alkenes and Allenoates 下载免费PDF全文
Dr. Johannes M. Wiest Dr. Michael L. Conner Prof. M. Kevin Brown 《Angewandte Chemie (International ed. in English)》2018,57(17):4647-4651
The first synthesis of hebelophyllene E is presented, along with assignment of its previously unknown relative configuration through synthesis of epi‐ent‐hebelophyllene E. Development of a catalytic enantioselective [2+2] cycloaddition of alkenes and allenoates provides access to the required chiral geminal dimethylcyclobutanes. Key to its success is the identification of a novel oxazaborolidine catalyst which promotes the cycloaddition in high enantioselectivities with good functional‐group tolerance (9 examples, up to 97:3 e.r.). Thus, a late‐stage cycloaddition using a fully functionalized alkene, followed by a diastereoselective reduction allows a concise entry to this class of natural products. 相似文献
78.
Copper‐Catalyzed Heteroarylboration of 1,3‐Dienes with 3‐Bromopyridines: A cine Substitution 下载免费PDF全文
Kevin B. Smith Dr. Yuan Huang Prof. M. Kevin Brown 《Angewandte Chemie (International ed. in English)》2018,57(21):6146-6149
A method for the heteroarylboration of 1,3‐dienes is presented. The process involves an unusual cine substitution of 3‐bromopyridine derivatives to deliver highly functionalized heterocyclic products. Mechanistic studies are included that clarify the details of this unusual process. 相似文献
79.
Metabolome analysis of human cerebrospinal fluid (CSF) is challenging because of low abundance of metabolites present in a
small volume of sample. We describe and apply a sensitive isotope labeling LC-MS technique for qualitative analysis of the
CSF metabolome. After a CSF sample is divided into two aliquots, they are labeled by 13C-dansyl and 12C-dansyl chloride, respectively. The differentially labeled aliquots are then mixed and subjected to LC-MS using Fourier-transform
ion cyclotron resonance mass spectrometry (FTICR MS). Dansylation offers significant improvement in the performance of chromatography
separation and detection sensitivity. Moreover, peaks detected in the mass spectra can be readily analyzed for ion pair recognition
and database search based on accurate mass and/or retention time information. It is shown that about 14,000 features can be
detected in a 25-min LC-FTICR MS run of a dansyl-labeled CSF sample, from which about 500 metabolites can be profiled. Results
from four CSF samples are compared to gauge the detectability of metabolites by this method. About 261 metabolites are commonly
detected in replicate runs of four samples. In total, 1132 unique metabolite ion pairs are detected and 347 pairs (31%) matched
with at least one metabolite in the Human Metabolome Database. We also report a dansylation library of 220 standard compounds
and, using this library, about 85 metabolites can be positively identified. Among them, 21 metabolites have never been reported
to be associated with CSF. These results illustrate that the dansylation LC-FTICR MS method can be used to analyze the CSF
metabolome in a more comprehensive manner. 相似文献
80.