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891.
de Visser SP 《Journal of the American Chemical Society》2006,128(30):9813-9824
Density functional calculations on the oxygenation reaction of propene by a model for taurine/alpha-ketoglutarate dioxygenase (TauD) enzyme are presented. The oxo-iron active species of TauD is shown to be a powerful and aggressive oxidant, which is able to hydroxylate C-H bonds and epoxidize C=C bonds with low barriers. In the case of propene oxygenation, the hydroxylation and epoxidation mechanisms are competitive on a dominant quintet spin state surface. We have compared the mechanism and thermodynamics of TauD with oxo-iron heme catalysts, such as the cytochromes P450, and found some critical differences. The TauD model is found to be much more reactive toward oxygenation of substrates than oxo-iron complexes in a heme environment with much lower reaction barriers. We have analyzed this and assigned this to the strength of the O-H bond formed after hydrogen abstraction from a substrate, which is at least 10 kcal mol(-)(1) stronger in five-coordinated oxo-iron nonheme complexes than in six-coordinated oxo-iron heme complexes. Since, the metal in TauD enzymes is five-coordinated, whereas in heme-enzymes it is six-coordinated there are some critical differences in the valence molecular orbitals. Thus, in oxo-iron heme catalysts one of the antibonding pi orbitals is replaced by a low-lying nonbonding delta orbital resulting in a lower overall spin state. Moreover, heme-enzymes have an extra oxidation equivalent located on the heme, which is missing in non-heme oxo-iron catalysts. As a result, the oxo-iron species of TauD reacts via single-state reactivity on a dominant quintet spin state surface, whereas oxo-iron heme catalysts react via two-state reactivity on competing doublet and quartet spin states. 相似文献
892.
By means of heterodyned two-dimensional IR photon echo experiments on liquid formamide and isotopomers the vibrational frequency dynamics of the N-H stretch mode, the C-D mode, and the C=O mode were obtained. In each case the vibrational frequency correlation function is fitted to three exponentials representing ultrafast (few femtoseconds), intermediate (hundreds of femtoseconds), and slow (many picoseconds) correlation times. In the case of N-H there is a significant underdamped contribution to the correlation decay that was not seen in previous experiments and is attributed to hydrogen-bond librational modes. This underdamped motion is not seen in the C-D or C=O correlation functions. The motions probed by the C-D bond are generally faster than those seen by N-H and C=O, indicating that the environment of C-D interchanges more rapidly, consistent with a weaker C-D...O=C bond. The correlation decays of N-H and C=O are similar, consistent with both being involved in strong H bonding. 相似文献
893.
Sam Byrne Basel Bandak Jennifer Kane Jorge Yañez Lorena Cornejo 《Microchemical Journal》2010,94(1):28-162
Chinchorros, a fishermen culture, who lived about 7000 years ago in the coastal region of the Atacama Desert in the northern outpost of present-day Chile, practiced an intricate system of mummification of their dead. The drinking water in this region is rich with arsenic, and the mummies were found in these arsenic endemic areas. Well preserved mummy hair samples provided a unique opportunity to explore the ancient arsenic exposure of the Chinchorros by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a single hair strand without any elaborate sample preparation. Forty-six hair samples from mummies found in five burial sites around the Atacama Desert, Chile, were used for this study. After cleaning, hair strands were placed on mounting tape and ablated using a Nd-YAG UV laser coupled to ICP-MS. A suite of contemporary human hair from the same region with known arsenic concentrations was used for calibration of LA-ICP-MS. Satisfactory linear calibration functions were obtained for arsenic in hair. The method detection limit was 0.8 µg/g and the sample throughput for this method is ∼ 10 samples per hour. It appears that mummies from the Morro (Arica), Iquique and Camarones had the elevated concentration of arsenic in hair (AsH > 10 µg/g) in this sub-set of samples, where Morro had the broad distribution of As concentrations. 相似文献
894.
The synthesis of 4-chloro-N-(naphthalen-1-ylmethyl)-5-(3-(piperazin-1-yl)phenoxy)thiophene-2-sulfonamide (B-355252) using an MW-assisted nucleophilic aromatic substitution (SNAr) reaction will be discussed. Utilization of this method allowed for the rapid generation of B-355252 heteroaryl ether core structure in the presence of cesium carbonate in dimethylformamide or tripotassium phosphate in N-methyl-2-pyrrolidone in 94% yield. Evaluation of B-355252 enhancement of nerve growth factor’s ability to stimulate neurite outgrowths was determined using NS-1 cells. 相似文献
895.
Sam Elliott 《Journal of Functional Analysis》2009,256(12):4162-4186
Building on techniques used in the case of the disc, we use a variety of methods to develop formulae for the adjoints of composition operators on Hardy spaces of the upper half-plane. In doing so, we prove a slight extension of a known necessary condition for the boundedness of such operators, and use it to provide a complete classification of the bounded composition operators with rational symbol. We then consider some specific examples, comparing our formulae with each other, and with other easily deduced formulae for simple cases. 相似文献
896.
The program of Reverse Mathematics (Simpson 2009) has provided us with the insight that most theorems of ordinary mathematics are either equivalent to one of a select few logical principles, or provable in a weak base theory. In this paper, we study the properties of the Dirac delta function (Dirac 1927; Schwartz 1951) in two settings of Reverse Mathematics. In particular, we consider the Dirac Delta Theorem, which formalizes the well-known property ${\int_\mathbb{R}f(x)\delta(x)\,dx=f(0)}$ of the Dirac delta function. We show that the Dirac Delta Theorem is equivalent to weak K?nig’s Lemma (see Yu and Simpson in Arch Math Log 30(3):171–180, 1990) in classical Reverse Mathematics. This further validates the status of WWKL0 as one of the ‘Big’ systems of Reverse Mathematics. In the context of ERNA’s Reverse Mathematics (Sanders in J Symb Log 76(2):637–664, 2011), we show that the Dirac Delta Theorem is equivalent to the Universal Transfer Principle. Since the Universal Transfer Principle corresponds to WKL, it seems that, in ERNA’s Reverse Mathematics, the principles corresponding to WKL and WWKL coincide. Hence, ERNA’s Reverse Mathematics is actually coarser than classical Reverse Mathematics, although the base theory has lower first-order strength. 相似文献
897.
Sam Werner 《Foundations of Physics》2012,42(1):122-139
On the 25th anniversary of Berry’s historic papers on the geometric phase, I discuss here our neutron interferometry experiment in which this phase is clearly separated from the dynamical phase. The connection of this experiment to the observation of the sign reversal of the wave function of a fermion during a 2π precession in a magnetic field by three groups independently in 1975 is discussed. 相似文献
898.
S. Ouir G. Fortas S. Sam H. Aliouat A. Manseri Y. Belaroussi N. Gabouze A.Y. Khereddine 《Journal of Physics and Chemistry of Solids》2012,73(3):391-395
In this work, we report on cathodic deposition of FeNi thin films into porous silicon (PS) formed on n-type Si. Macroporous and mesoporous silicon layers were formed at constant potential or current density. The electrodeposited thin films were characterized by Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). The magnetic properties of the FeNi layers were investigated by hysteresis loops measurements. SEM images of the FeNi films indicate that tubular and granular forms were obtained depending on the porous silicon surface morphology. Moreover, the FeNi films compositions are found to depend on the porous silicon microstructure. Finally, the XRD spectra of the deposited films show the presence of FeNi (111) and FeNi (220) peaks. The FeNi (111) peak has been shown for all polarization potentials, in agreement with results reported in the literature. 相似文献
899.
Hoang Anh T. Phan Sam G. Giannakoulias Taylor M. Barrett Chunxiao Liu E. James Petersson 《Chemical science》2021,12(32):10825
Aberrant levels of cathepsin L (Cts L), a ubiquitously expressed endosomal cysteine protease, have been implicated in many diseases such as cancer and diabetes. Significantly, Cts L has been identified as a potential target for the treatment of COVID-19 due to its recently unveiled critical role in SARS-CoV-2 entry into the host cells. However, there are currently no clinically approved specific inhibitors of Cts L, as it is often challenging to obtain specificity against the many highly homologous cathepsin family cysteine proteases. Peptide-based agents are often promising protease inhibitors as they offer high selectivity and potency, but unfortunately are subject to degradation in vivo. Thioamide substitution, a single-atom O-to-S modification in the peptide backbone, has been shown to improve the proteolytic stability of peptides addressing this issue. Utilizing this approach, we demonstrate herein that good peptidyl substrates can be converted into sub-micromolar inhibitors of Cts L by a single thioamide substitution in the peptide backbone. We have designed and scanned several thioamide stabilized peptide scaffolds, in which one peptide, RS1A, was stabilized against proteolysis by all five cathepsins (Cts L, Cts V, Cts K, Cts S, and Cts B) while inhibiting Cts L with >25-fold specificity against the other cathepsins. We further showed that this stabilized RS1A peptide could inhibit Cts L in human liver carcinoma lysates (IC50 = 19 μM). Our study demonstrates that one can rationally design a stabilized, specific peptidyl protease inhibitor by strategic placement of a thioamide and reaffirms the place of this single-atom modification in the toolbox of peptide-based rational drug design.Information on the effects of sidechain and backbone modification on the activity of cathepsin (Cts) L, V, K, S, and B was used to design a thioamide peptide that is inert to all Cts and selectively inhibits Cts L. 相似文献
900.
Tracey Bray Sam Damiris Andrew Grace Graeme Moad Michael O'Shea Ezio Rizzardo Gary van Diepen 《Macromolecular Symposia》1998,129(1):109-118
The maleation of conventional and metallocene linear low density polyethylenes by reactive extrusion has been explored with a view to defining the conditions necessary for a robust process that provides both high grafting efficiencies (>80%) and minimal degradation or cross-linking. The dependence of grafting efficiency on various operating parameters (maleic anhydride level, maleic anhydride:initiator ratio, throughput rate, direction of screw rotation, temperature) has been established. Literature methods for characterization of the grafted product based on FTIR or 1H NMR analysis have been critically examined with respect to their ability to distinguish between single unit and oligomeric maleic anhydride grafts and found to yield ambiguous results. 相似文献