全文获取类型
收费全文 | 758篇 |
免费 | 15篇 |
国内免费 | 3篇 |
专业分类
化学 | 414篇 |
晶体学 | 11篇 |
力学 | 34篇 |
数学 | 121篇 |
物理学 | 196篇 |
出版年
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 11篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 15篇 |
2015年 | 13篇 |
2014年 | 21篇 |
2013年 | 45篇 |
2012年 | 38篇 |
2011年 | 33篇 |
2010年 | 17篇 |
2009年 | 21篇 |
2008年 | 32篇 |
2007年 | 38篇 |
2006年 | 20篇 |
2005年 | 29篇 |
2004年 | 36篇 |
2003年 | 23篇 |
2002年 | 34篇 |
2001年 | 15篇 |
2000年 | 15篇 |
1998年 | 6篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 16篇 |
1977年 | 5篇 |
1976年 | 11篇 |
1974年 | 9篇 |
1973年 | 4篇 |
1972年 | 9篇 |
1970年 | 5篇 |
1969年 | 6篇 |
1968年 | 7篇 |
排序方式: 共有776条查询结果,搜索用时 8 毫秒
1.
Condensation of bis-(2-chloroethyl)phosphoramidic dichloride with 3′-amino-3′-deoxy-N,N-dimethyladenosine afforded the 2′,3′-cyclicphosphorodiamidate (III). By an improved synthesis, methyl 3-amino-3-deoxy-β-D-ribofuranoside was obtained as a model compound for conversion to the analogous 2,3-cyclicphosphorodiamidate (XII). Existence of the latter as two diastereomers due to phosphorus asymmetry was shown by nmr analysis, using comparison with the 5-(O-p-nitrobenzoate) (XIII) as a basis for assignments. 相似文献
2.
3.
The spontaneous release of tumor cell antigens from the cell surface into the circulation has been proposed as a mechanism whereby tumors may escape the immune response of the host. In this study we have found that Ehrlich ascites tumor cells after removal from the host (mouse) spontaneously release significant amounts of cell surface components during incubation for 1 h in cold isotonic buffer. Immunodiffusion studies revealed that immunoglobulin G (IgG) and a complement component (C3) are included in this spontaneously released material. These surface-bound humoral immune components are apparently released in the form of a high-molecular-weight aggregate (cell coat particle) as shown by ultracentrifugation and ultrafiltration experiments. Precipitation of IgG from the cell coat particle preparation with antibodies directed against mouse IgG followed by detergent gel electrophoresis of the immune precipitate revealed five major bands in addition to the heavy and light chains of IgG. These results suggest that host IgG is tightly bound to several other components at the cell surface, perhaps in the form of immune complexes. IgG is localized on the tumor cell surface in a highly heterogenous pattern with the appearance of patches and caps in some cells as shown by immunofluorescence analysis. The possibility that humoral immune components bind to the tumor cell surface and result in the shedding of high-molecular-weight aggregates of cell surface antigens into extracellular fluids is discussed. 相似文献
4.
A simple and efficient preparation of a number of hybrid linear‐branched PEG esters are described. The polymers are generated by direct coupling of PEG–carboxylic acids and a variety of pentaerythritol ethoxylates using carbon tetrabromide catalyst. 相似文献
5.
Tayler AB Holland DJ Sederman AJ Gladden LF 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,212(1):1-10
Although biomolecular dynamics has been investigated using NMR for at least 40 years, only in the past 20 years have internal motions been characterized at atomic resolution throughout proteins and nucleic acids. This development was made possible by multidimensional heteronuclear NMR approaches that provide near complete sequential signal assignments of uniformly labeled biomolecules. Recent methodological advances have enabled characterization of internal dynamics on timescales ranging from picoseconds to seconds, both in solution and in the solid state. The size, complexity and functional significance of biomolecules investigated by NMR continue to grow, as do the insights that have been obtained about function. In this article I review a number of recent advances that have made such studies possible, and provide a few examples of where NMR either by itself or in combination with other approaches has paved the way to a better understanding of the complex relationship between dynamics and biomolecular function. Finally, I discuss prospects for further advances in this field. 相似文献
6.
The characteristic features of ion transmission properties in a radio-frequency (RF) electric-field driven ion funnel using the SIMION ion trajectory simulation package is presented. A user program applying the Douglas ion-neutral collisional drag coefficient model is incorporated to properly account for the ion focusing and transport effect of the background gas under the effect of the driving RF and a superimposed DC field. The simulated m/z transmission window compares favorably with the experimental results reported by Smith et al. RF amplitude and pressure dependence of experimentally observed m/z transmission windows are also examined, and an approximated effective potential model based on Gerlich's equation is proposed to interpret the low-m/z cutoff behavior. A modified ion funnel configuration is described. 相似文献
7.
Burdick Brent A. Esders Theodore W. Schaeffer James R. Lynn Shirley 《Applied biochemistry and biotechnology》1987,16(1):145-156
Several hydrophilic polymeric thiols were prepared from aminoactivated polymeric supports by reaction with N-acetylhomocysteinethiolactone.
Supports include agaroses, cellulose, Glycophase™ controlled-pore glass, and Matrex™ acrylic beads. Thiol content in these
polymers was 3–72 μmol SH/g dry polymer. Several were effective solid-phase activators of the sulfhydryl-dependent enzyme
creatine phosphokinase at concentrations comparable to that of monomeric thiol required for enzyme activation. The kinetic
activation curves for the polymeric and the monomeric (thioglucose) activators were similar, suggesting unhindered interaction
of the enzyme with the polymeric activator. 相似文献
8.
Reactions of the ionized enol tautomer of acetanilide: elimination of HNCO via a novel rearrangement
Heydorn LN Carter LM Bowen RD Terlouw JK 《European journal of mass spectrometry (Chichester, England)》2003,9(4):343-350
The reactions of ionised acetanilide, C(6)H(5)NH(=O)CH(3)(.+), and its enol, C(6)H(5)NH(OH)=CH(2)(.+), have been studied by a combination of tandem mass spectrometric and computational methods. These two isomeric radical cations have distinct chemistries at low internal energies. The keto tautomer eliminates exclusively CH(2)=C=O to give ionised aniline. In contrast, the enol tautomer loses H-N=C=O, via an unusual skeletal rearrangement, to form predominantly ionised methylene cyclohexadiene. Hydrogen atom loss also occurs from the enol tautomer, with the formation of protonated oxindole. The mechanisms for H-N=C=O and hydrogen atom loss both involve cyclisation; the former proceeds via a spiro transition state formed by attachment of the methylene group to the ipso position, whereas the latter entails the formation of a five-membered ring by attachment to the ortho position. The behaviour of labelled analogues reveals that these two processes have different site selectivities. Hydrogen atom loss involves a reverse critical energy and is subject to an isotope effect. Surprisingly, attempts to promote the enolisation of ionised acetanilide by proton-transport catalysis were unsuccessful. In a reversal of the usual situation for ionised carbonyl compounds, ionised acetanilide is actually more stable than its enol tautomer. The enol tautomer was resistant to proton-transport catalysed ketonisation to ionised acetanilide, possibly because the favoured geometry of the encounter complex with the base molecule is inappropriate for facilitating tautomerisation. 相似文献
9.
The lithiated (organo)sulfonylacetonitrile complex [MeSO(2)CHCNLi.THF] (3) has been prepared and structurally characterized in order to demonstrate that well-known molecular aggregates of s-block metals may be used as building blocks in the controlled assembly of complex supramolecular architectures. The solid state structure of 3 can be described as a novel basket-weaved, 2-D network, composed of (SO(2)Li)(2) "dimeric" rings joined via "interdimer" donation of nitrile units. 相似文献
10.
Mladenova G Singh G Acton A Chen L Rinco O Johnston LJ Lee-Ruff E 《The Journal of organic chemistry》2004,69(6):2017-2023
The photochemical pinacol reaction of a series of nonsymmetrical 9-fluorenyl-substituted vic-diols was investigated and compared with their acid-catalyzed thermal reaction. Unlike the thermal reaction, the radiation-induced processes involve only fluorenyl cations, as is reflected in differences of product distribution between the two reactions. From the product studies, substituent migratory aptitudes are reversed in the photochemical process, suggesting that kinetic control takes place under neutral conditions unlike the acid-catalyzed thermal reactions. The presence of fluorenyl cation intermediates and their lifetimes were established by laser flash spectroscopy studies. 相似文献