全文获取类型
收费全文 | 33296篇 |
免费 | 948篇 |
国内免费 | 249篇 |
专业分类
化学 | 22977篇 |
晶体学 | 226篇 |
力学 | 785篇 |
数学 | 5473篇 |
物理学 | 5032篇 |
出版年
2022年 | 287篇 |
2021年 | 430篇 |
2020年 | 519篇 |
2019年 | 486篇 |
2018年 | 377篇 |
2017年 | 372篇 |
2016年 | 840篇 |
2015年 | 744篇 |
2014年 | 813篇 |
2013年 | 1740篇 |
2012年 | 1880篇 |
2011年 | 2369篇 |
2010年 | 1145篇 |
2009年 | 1010篇 |
2008年 | 2029篇 |
2007年 | 2037篇 |
2006年 | 2021篇 |
2005年 | 1877篇 |
2004年 | 1606篇 |
2003年 | 1395篇 |
2002年 | 1299篇 |
2001年 | 423篇 |
2000年 | 401篇 |
1999年 | 364篇 |
1998年 | 338篇 |
1997年 | 360篇 |
1996年 | 495篇 |
1995年 | 354篇 |
1994年 | 299篇 |
1993年 | 276篇 |
1992年 | 274篇 |
1991年 | 245篇 |
1990年 | 209篇 |
1989年 | 212篇 |
1988年 | 246篇 |
1987年 | 204篇 |
1986年 | 188篇 |
1985年 | 358篇 |
1984年 | 355篇 |
1983年 | 246篇 |
1982年 | 327篇 |
1981年 | 329篇 |
1980年 | 304篇 |
1979年 | 289篇 |
1978年 | 264篇 |
1977年 | 234篇 |
1976年 | 212篇 |
1975年 | 217篇 |
1974年 | 217篇 |
1973年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Clarke DJ Murray E Hupp T Mackay CL Langridge-Smith PR 《Journal of the American Society for Mass Spectrometry》2011,22(8):1432-1440
Noncovalent protein–ligand and protein–protein complexes are readily detected using electrospray ionization mass spectrometry
(ESI MS). Furthermore, recent reports have demonstrated that careful use of electron capture dissociation (ECD) fragmentation
allows covalent backbone bonds of protein complexes to be dissociated without disruption of noncovalent protein–ligand interactions.
In this way the site of protein–ligand interfaces can be identified. To date, protein–ligand complexes, which have proven
tractable to this technique, have been mediated by ionic electrostatic interactions, i.e., ion pair interactions or salt bridging.
Here we extend this methodology by applying ECD to study a protein–peptide complex that contains no electrostatics interactions.
We analyzed the complex between the 21 kDa p53-inhibitor protein anterior gradient-2 and its hexapeptide binding ligand (PTTIYY).
ECD fragmentation of the 1:1 complex occurs with retention of protein–peptide binding and analysis of the resulting fragments
allows the binding interface to be localized to a C-terminal region between residues 109 and 175. These finding are supported
by a solution-phase competition assay, which implicates the region between residues 108 and 122 within AGR2 as the PTTIYY
binding interface. Our study expands previous findings by demonstrating that top-down ECD mass spectrometry can be used to
determine directly the sites of peptide–protein interfaces. This highlights the growing potential of using ECD and related
top-down fragmentation techniques for interrogation of protein–protein interfaces. 相似文献
992.
Cerruti CD Touboul D Guérineau V Petit VW Laprévote O Brunelle A 《Analytical and bioanalytical chemistry》2011,401(1):75-87
Mass spectrometry imaging of lipids using MALDI–TOF/TOF mass spectrometers is of growing interest for chemical mapping of
organic compounds at the surface of tissue sections. Many efforts have been devoted to the best matrix choice and deposition
technique. Nevertheless, the identification of lipid species desorbed from tissue sections remains problematic. It is now
well-known that protonated, sodium- and potassium-cationized lipids are detected from biological samples, thus complicating
the data analysis. A new sample preparation method is proposed, involving the use of lithium salts in the matrix solution
in order to simplify the mass spectra with only lithium-cationized molecules instead of a mixture of various cationized species.
Five different lithium salts were tested. Among them, lithium trifluoroacetate and lithium iodide merged the different lipid
adducts into one single lithium-cationized species. An optimized sample preparation protocol demonstrated that the lithium
trifluoroacetate salt slightly increased desorption of phosphatidylcholines. Mass spectrometry images acquired on rat brain
tissue sections by adding lithium trifluoroacetate showed the best results in terms of image contrast. Moreover, more structurally
relevant fragments were generated by tandem mass spectrometry when analyzing lithium-cationized species. 相似文献
993.
Maintaining the predictive abilities of multivariate calibration models by spectral space transformation 总被引:1,自引:0,他引:1
Du W Chen ZP Zhong LJ Wang SX Yu RQ Nordon A Littlejohn D Holden M 《Analytica chimica acta》2011,690(1):64-70
In quantitative on-line/in-line monitoring of chemical and bio-chemical processes using spectroscopic instruments, multivariate calibration models are indispensable for the extraction of chemical information from complex spectroscopic measurements. The development of reliable multivariate calibration models is generally time-consuming and costly. Therefore, once a reliable multivariate calibration model is established, it is expected to be used for an extended period. However, any change in the instrumental response or variations in the measurement conditions can render a multivariate calibration model invalid. In this contribution, a new method, spectral space transformation (SST), has been developed to maintain the predictive abilities of multivariate calibration models when the spectrometer or measurement conditions are altered. SST tries to eliminate the spectral differences induced by the changes in instruments or measurement conditions through the transformation between two spectral spaces spanned by the corresponding spectra of a subset of standardization samples measured on two instruments or under two sets of experimental conditions. The performance of the method has been tested on two data sets comprising NIR and MIR spectra. The experimental results show that SST can achieve satisfactory analyte predictions from spectroscopic measurements subject to spectrometer/probe alteration, when only a few standardization samples are used. Compared with the existing popular methods designed for the same purpose, i.e. global PLS, univariate slope and bias correction (SBC) and piecewise direct standardization (PDS), SST has the advantages of implementation simplicity, wider applicability and better performance in terms of predictive accuracy. 相似文献
994.
Four novel benzamido-functionalized prolinamides have been prepared and tested as organocatalysts for enantioselective aldol reaction of aldehydes and cyclic ketones in water. In particular, prolinamide derived from achiral ethylene diamine was the best catalyst leading to anti aldols in excellent diastereomeric (up to 98/2) and enantiomeric (up to 99/1) ratios, thereby showing that lateral amide functionalities might be a key issue for facilitating "in water" chemistry. These catalysts are cheaper and easier to prepare than those previously described. 相似文献
995.
A critical factor limiting the rates of photocatalytic reactions, including water splitting, on oxide semiconductors is the high rate of charge-carrier recombination. In this contribution, we demonstrate that this issue can be alleviated significantly by combining a semiconductor photocatalyst with tailored plasmonic-metal nanostructures. Plasmonic nanostructures support the formation of resonant surface plasmons in response to a photon flux, localizing electromagnetic energy close to their surfaces. We present evidence that the interaction of localized electric fields with the neighboring semiconductor allows for the selective formation of electron/hole (e(-)/h(+)) pairs in the near-surface region of the semiconductor. The advantage of the formation of e(-)/h(+) pairs near the semiconductor surface is that these charge carriers are readily separated from each other and easily migrate to the surface, where they can perform photocatalytic transformations. 相似文献
996.
The quadratic vibronic coupling model is an important computational tool for simulating photoelectron spectra involving strongly coupled electronic states in polyatomic molecules. However, recent work has indicated the need for higher order terms, with most of the initial studies focusing on molecules with symmetry-required degeneracies. In this study we report an extension of our approach for constructing fully quadratic representations of bound electronic states coupled by conical intersections, which allows for the inclusion of higher order terms, demonstrated here employing a quartic expansion. Procedures are developed that eliminate unphysical behavior for large displacements, a problem likely to be an endemic to anharmonic expansions. Following work on representing dissociative electronic states, Lagrange multipliers are used to constrain the constructed representation to reproduce exactly the energy, energy gradients, and∕or derivative couplings at specific points, or nodes, in nuclear coordinate space. The approach is illustrated and systematically studied using the four lowest electronic states of triazolyl, (CH)(2)N(3). 相似文献
997.
We present a systematic study on the excited electron-bubble states in superfluid (4)He using a time-dependent density functional approach. For the evolution of the 1P bubble state, two different functionals accompanied with two different time-development schemes are used, namely an accurate finite-range functional for helium with an adiabatic approximation for electron versus an efficient zero-range functional for helium with a real-time evolution for electron. We make a detailed comparison between the quantitative results obtained from the two methods, which allow us to employ with confidence the optimal method for suitable problems. Based on this knowledge, we use the finite-range functional to calculate the time-resolved absorption spectrum of the 1P bubble, which in principle can be experimentally determined, and we use the zero-range functional to real-time evolve the 2P bubble for several hundreds of picoseconds, which is theoretically interesting due to the break down of adiabaticity for this state. Our results discard the physical realization of relaxed, metastable configurations above the 1P state. 相似文献
998.
David Adeyemi Janes Mokgadi James Darkwa Chimezie Anyakora Grace Ukpo Charlotta Turner Nelson Torto 《Chromatographia》2011,73(9-10):1015-1020
Electrospun polystyrene (PS) nanofibers (130–500 nm) incorporating a potassium salt of imidazole-1-carbodithioate were evaluated as potential sorbents for the pre-concentration of a model organochlorine pesticide; 1,1-dichloro-2,2bis-(4-chlorophenyl)ethylene (DDE). The efficiencies of DDE (0.25–1.0 μg L?1) adsorption by the nanofiber sorbent followed by desorption employing pressurized hot water extraction (PHWE) were investigated and monitored using gas chromatography with electron capture detection (GC-ECD). Parameters such as time, temperature and pressure of extraction, sample volume, DDE concentration and sorbent mass were optimized. The maximum adsorption of DDE (0.50 μg L?1) on electrospun PS and carbodithioate incorporated PS nanofibers was at 43.7 and 94.6%, respectively, in 20 min. Incorporation of carbodithiote doubled the adsorption efficiency of PS and achieved LOD of 0.000234 μg L?1 for DDE. The optimal DDE desorption on the PHWE system was 93.8% in 10 min. It would seem that the use of electrospun nanofibers as sorbent material with subsequent desorption by PHWE has great potential and thus warrants further investigations. This approach as it uses water as an extraction solvent for an organochlorine pesticide provides an opportunity to eliminate organic solvents, especially for procedures aimed at monitoring organic pollutants in the environment. 相似文献
999.
C.T. Mortimer M.P. Wilkinson J. Burgess R.D.W. Kemmitt N. Morton 《Journal of organometallic chemistry》1979,171(3):369-371
The enthalpy, ΔH = ?64.7 ± 4 kJ mol?1, for the reaction Pt(PPh3)2(η-C2H4)(s) + pcbd(g) → Pt(PPh3)2(η-pcbd)(s) + C2H4(g) where pcbd is 3-phenylcyclobutene-1,2-dione, , has been measured calorimetrically. The Ptolefin bond in this complex is slightly stronger than that in Pt(PPh3)2(η-PhCHCH2). 相似文献
1000.
- and -dialkylvinyloxyboranes, prepared directly from cyclohexyl ethyl ketone, undergo stereo-selective aldol condensations to provide - -β-hydroxy-α-methylketones, respectively. 相似文献