全文获取类型
收费全文 | 2644篇 |
免费 | 102篇 |
国内免费 | 31篇 |
专业分类
化学 | 1589篇 |
晶体学 | 26篇 |
力学 | 45篇 |
数学 | 632篇 |
物理学 | 485篇 |
出版年
2022年 | 25篇 |
2021年 | 33篇 |
2020年 | 46篇 |
2019年 | 44篇 |
2018年 | 23篇 |
2017年 | 23篇 |
2016年 | 61篇 |
2015年 | 57篇 |
2014年 | 69篇 |
2013年 | 133篇 |
2012年 | 146篇 |
2011年 | 126篇 |
2010年 | 105篇 |
2009年 | 84篇 |
2008年 | 141篇 |
2007年 | 131篇 |
2006年 | 146篇 |
2005年 | 123篇 |
2004年 | 102篇 |
2003年 | 79篇 |
2002年 | 97篇 |
2001年 | 36篇 |
2000年 | 49篇 |
1999年 | 37篇 |
1998年 | 33篇 |
1997年 | 29篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 40篇 |
1993年 | 42篇 |
1992年 | 50篇 |
1991年 | 28篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 16篇 |
1986年 | 20篇 |
1985年 | 30篇 |
1984年 | 27篇 |
1983年 | 22篇 |
1982年 | 27篇 |
1981年 | 25篇 |
1980年 | 29篇 |
1979年 | 18篇 |
1978年 | 35篇 |
1977年 | 18篇 |
1976年 | 16篇 |
1975年 | 22篇 |
1974年 | 24篇 |
1973年 | 15篇 |
1970年 | 17篇 |
排序方式: 共有2777条查询结果,搜索用时 0 毫秒
41.
Roybal JE Pfenning AP Storey JM Gonzales SA Turnipseed SB 《Journal of AOAC International》2003,86(5):930-934
A simple liquid chromatographic (LC) method is presented for the determination of diminazene (DZ) in raw bovine milk. DZ is extracted from raw milk by chilled aqueous centrifugation and is isolated from milk components on a cyano solid-phase extraction column. DZ is eluted by using a methanol-ion pairing reagent. A Phenomenex LUNA CN column and an acetonitrile-buffered mobile phase with a counter ion are used for gradient LC. The LC effluent is monitored at a detection wavelength of 372 nm by using a deuterium lamp. Under the parameters described, the retention time of DZ is 8-10 min with a peak area response of 6.5 mAU/ng. The method demonstrated excellent precision over all levels tested (25-400 ppb) with an overall average recovery of 90.4 +/- 14.5%. The method is applicable to the monitoring of milk for DZ residues at the 25 ppb level with a limit of quantitation of 10 ppb. 相似文献
42.
Steve Scheiner Paul Redfern Eric A. Hillenbrand 《International journal of quantum chemistry》1986,29(4):817-827
Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A-H-B lead not only to a lowering of the energy of the A-HB state relative to AH-B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH-B and A-HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH-B and A-HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies. 相似文献
43.
44.
45.
An ab initio molecular orbital calculation has been carried out for three different conformations of 1,3 propanediol, one of which permits intramolecular H-bond studied by ab initio quantum mechanical methods. The ΔE for H-bonding formation is compated to be 0.9 kcal/mole and the charge redistributions and molecular orbital energy changes are compared to those found in intermolecular H-bonds. 相似文献
46.
47.
Spontaneous Nef reaction of primary and secondary 3-aryl-2-(diethoxyphosphoryl)-4-nitroalkanoic acids has been observed for the first time. The reaction provides a general and effective, highly diastereoselective synthesis of 3-(diethoxyphosphoryl)-1-hydroxysuccinimides and 2-(diethoxyphosphoryl)-4-oxoalkanoic acids. 相似文献
48.
Photoelectron spectroscopic investigations indicate that (i) monocyclic diaziridines prefer a trans-conformation of the nitrogen-lone pairs, and (ii) the interaction between these is comparable to that exhibited by open-chain analogues, i.e. alkyl-substituted hydrazines. The chemical experience concerning the particular case of ? NH? NH? →? N?N? oxidation of diaziridines can therefore not be accounted for by a prevailing unfavorable ‘lone pair’ interaction. Thermochemical as well as entropy arguments may serve as an explanation. A rather unsatisfactory agreement between predictions from various theoretical procedures and experimental results for the ionization potentials of ‘lone pair’ MO electrons is noted. 相似文献
49.
Gaunt AJ May I Helliwell M Richardson S 《Journal of the American Chemical Society》2002,124(45):13350-13351
The reaction between PW9O349- and NpO2+ has yielded the first structurally characterized neptunyl(V) polyoxometalate complex, [Na2(NpO2)2(A-PW9O34)2]14-. This complex is isostructural with the uranyl(VI) analogue, and there is also spectroscopic evidence for its existence in solution. The complex is readily extracted into toluene, and this may have significance in the sequestering and/or separation of the neptunyl ion in terms of nuclear waste management. 相似文献
50.
Shi G Wu JT Li Y Geleziunas R Gallagher K Emm T Olah T Unger S 《Rapid communications in mass spectrometry : RCM》2002,16(11):1092-1099