全文获取类型
收费全文 | 2647篇 |
免费 | 439篇 |
国内免费 | 329篇 |
专业分类
化学 | 1219篇 |
晶体学 | 13篇 |
力学 | 65篇 |
综合类 | 31篇 |
数学 | 311篇 |
物理学 | 1776篇 |
出版年
2024年 | 2篇 |
2023年 | 17篇 |
2022年 | 34篇 |
2021年 | 53篇 |
2020年 | 58篇 |
2019年 | 83篇 |
2018年 | 65篇 |
2017年 | 69篇 |
2016年 | 82篇 |
2015年 | 77篇 |
2014年 | 162篇 |
2013年 | 195篇 |
2012年 | 137篇 |
2011年 | 153篇 |
2010年 | 175篇 |
2009年 | 188篇 |
2008年 | 190篇 |
2007年 | 213篇 |
2006年 | 202篇 |
2005年 | 153篇 |
2004年 | 154篇 |
2003年 | 134篇 |
2002年 | 112篇 |
2001年 | 73篇 |
2000年 | 106篇 |
1999年 | 67篇 |
1998年 | 76篇 |
1997年 | 54篇 |
1996年 | 47篇 |
1995年 | 35篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 30篇 |
1990年 | 17篇 |
1989年 | 25篇 |
1988年 | 18篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有3415条查询结果,搜索用时 247 毫秒
71.
IntroductionThe water-gas shift(WGS)reaction is an impor-tant step in many important chemical processes[1].Re-cently,the renewed interest in the removal of CO bythe WGS reaction has grown significantly because of theincreasing attention to pure hydrogen p… 相似文献
72.
We describe the properties of a series of oligomeric polyfluorophores assembled on the DNA backbone. The 11 oligomers (oligodeoxyfluorosides, ODFs), 4-7 monomers in length, were composed of only two fluorescent monomers and a spacer in varied sequences, and were designed to test how fluorescent nucleobases can interact electronically to yield complexity in fluorescence emission. The monomer fluorophores were deoxyribosides of pyrene and perylene, which emit light in violet and blue wavelengths, respectively. The experiments show that simple variation in sequence and spacing can dramatically change fluorescence, yielding emission maxima ranging from 380 to 557 nm and visible colors from violet to orange-red. Fluorescence lifetime data, excitation spectra, and absorption data point to a number of multi-fluorophore electronic interactions, including pyrene-pyrene and perylene-perylene excimers, pyrene-perylene exciplexes, as well as monomer dye emissions, contributing to the final spectral outcomes. Thus, two simple fluorophores can be readily combined to give emissions over much of the visible spectrum, all requiring only a single excitation. The results demonstrate that fluorescent nucleobases in oligomeric form can act cooperatively as electronic units, and that fluorophore sequence in such oligomers is as important as fluorophore composition in determining fluorescence properties. 相似文献
73.
103Rh NMR Spectroscopic Evidence of Mixed Nonahalogenodirhodates(III), [Rh2ClnBr9–n]3?, n = 0–9 On heating a mixture of the tetrabutylammonium salts (TBA)3[Rh2Cl9] and (TBA)3[Rh2Br9] at 60°C in propylenecarbonate the complete system of the mixed nonahalogenodirhodates(III) [Rh2ClnBr9–n]3?, n = 0–9 is formed. In the 103Rh nmr spectra 40 different species have been detected, 16 with two equivalent 103Rh atoms each resulting in one singlet and 24 with inequivalent 103Rh atoms each pair giving two resonances. The signals of the geometric isomeres are not resolved. All 64 expected resonances are really observed. By additional measuring of the 103Rh nmr spectra of the fractions n = 0–4 separable by ion exchange chromatography on DEAE cellulose, and utilizing characteristic increments of chemical shifts the complete and unambiguous assignment of all signals is achieved. 相似文献
74.
A. A. Balandina V. A. Mamedov E. A. Khafizova Sh. K. Latypov 《Russian Chemical Bulletin》2006,55(12):2256-2264
The combined use of 2D NMR correlation methods and ab initio chemical shift calculations is efficient and, in some cases, virtually the only way to determine the structures of new organic
compounds. This approach enabled us to establish the structure of the major unusual product of the three-component reaction
of imidazo[1,5-a]quinoxalin-4-one, bis(2-chloroethyl)amine hydrochloride, and potassium carbonate in DMF.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2172–2179, December, 2006.
Dedicated to Professor A. V. Il’yasov on the occasion of his 70th birthday. 相似文献
75.
Paramagnetic metal ions bound to proteins generate a dipolar field that can be accurately probed by pseudocontact shifts (PCS) displayed by the protein's nuclear spins. PCS are highly useful for determining the coordinates of individual spins in the molecule and for rapid structure determinations of entire protein-protein and protein-ligand complexes. However, PCS measurements require reliable resonance assignments for the molecule in its paramagnetic state and in a diamagnetic reference state. This article discusses different approaches for pairwise resonance assignments, with emphasis on a strategy which exploits chemical exchange between the two states. 相似文献
76.
Reported here are two novel quasi-racemic (bis)indole alkaloids, cycloaplysinopsin A (5) and cycloaplysinopsin B (6), isolated from tropical Indo-Pacific (Comoros, Philippines) scleractinian corals of the family Dendrophylliidae. Although their structures suggest a Diels-Alder cycloaddition origin from aplysinopsin-type precursors, neither experiments, nor theory allowed us to clearly distinguish an enzymatic process with scarce enantioselection from the intrusion of an adventitious catalyst in the coral extracts, where the chiral environment caused a slight asymmetric induction. 相似文献
77.
水煤气变换反应的微观动力学分析──Cu基催化剂上H_2S中毒失活原因的 Monte Carlo 模拟研究 总被引:1,自引:0,他引:1
用BOCMP方法及MonteCarlo模拟技术,对H2S导致Cu基催化剂失活的原因进行了计算分析。研究结果表明,当原料气中存在H2S时,WGS反应的活化能明显高于无H2S时的活化能,随着表面H2S浓度的增大(θ=0,0.10,0.25),反应的活化能也逐渐变大(其大小比为1∶1.34∶3.3),究其原因可归结为H2S的存在使得反应物分子的吸附热减小,从而使H2O的解离吸附(WGS反应的速控步骤)活化能增大。 相似文献
78.
本文应用取代基效应(SCS)方法研究了乙烯-乙烯醇(EVA)共聚物,得到了羟基(-OH)在两种不同溶剂中的SCS参数:在苯酚+重水(80/20W/W)中参数为S_1=42.77±0.08ppm,S_2=7.155±0.06ppm,S_3(s)=-4.08±0.02ppm,S_3(t)=-3.09±0.02ppm,S_4=0.48±0.03ppm,S_5=0.26±0.05ppm,在以邻二氯苯为溶剂时参数为S_1=44.97±0.61ppm,S_2=7.40±0.00ppm,S_3(s)=-4.51±0.17ppm,S_3(t)=-3.13±0.00ppm,S_4=0.63±0.04ppm,S_5=0.36±0.00ppm,同时利用所得到的SCS参数对该共聚物的~(13)C NMR谱进行了归属。 相似文献
79.
离子性指数、极化效应指数烷烃^13C NMR化学位移的 关系研究 总被引:5,自引:0,他引:5
定义了烷烃分子中碳原子的离子性指数(INI),用离子性指数(INI)、极化效应指数(PEI)及N^i~H(i=αβΥ)结构信息参数研究了烷烃的^13CNMR化学位移模型,结果表明,烷烃^13CNMR化学位移(CS)可用下式来定量描述:CS=194.6156-37.7394(INI)+98.6505(ΣPEI)+27.1630(INI/ΣPEI)-652.9106(ΣPEI/INI)+0.7735N^α~H+2.2468N^β~H-0.1742N^γ~H。用上式估算了304个碳原子的化学位移,平均绝对误差仅为0.77δ,标准差0.9860δ,预测值与实验值非常吻合。 相似文献
80.