全文获取类型
收费全文 | 10860篇 |
免费 | 1904篇 |
国内免费 | 1261篇 |
专业分类
化学 | 7747篇 |
晶体学 | 66篇 |
力学 | 683篇 |
综合类 | 77篇 |
数学 | 1094篇 |
物理学 | 4358篇 |
出版年
2024年 | 44篇 |
2023年 | 278篇 |
2022年 | 388篇 |
2021年 | 443篇 |
2020年 | 486篇 |
2019年 | 434篇 |
2018年 | 355篇 |
2017年 | 302篇 |
2016年 | 527篇 |
2015年 | 471篇 |
2014年 | 563篇 |
2013年 | 808篇 |
2012年 | 994篇 |
2011年 | 979篇 |
2010年 | 647篇 |
2009年 | 639篇 |
2008年 | 715篇 |
2007年 | 630篇 |
2006年 | 562篇 |
2005年 | 486篇 |
2004年 | 369篇 |
2003年 | 330篇 |
2002年 | 273篇 |
2001年 | 217篇 |
2000年 | 219篇 |
1999年 | 261篇 |
1998年 | 218篇 |
1997年 | 197篇 |
1996年 | 213篇 |
1995年 | 180篇 |
1994年 | 147篇 |
1993年 | 125篇 |
1992年 | 105篇 |
1991年 | 76篇 |
1990年 | 84篇 |
1989年 | 57篇 |
1988年 | 48篇 |
1987年 | 45篇 |
1986年 | 29篇 |
1985年 | 28篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
运用三足四齿配体三(2-甲基吡啶)胺(TPA)或三(2-甲基苯丙咪唑)胺(TBA),得到两个双核铁(III)配合物,[Fe2L2(μ2-O)(μ2-p-NH2-C6H4COO)]3+
(L = TPA, 1 和 L = TBA,
2)。两个配合物均为单斜晶系,空间群为P2(1)/c.晶胞参数 1: a =
1.4529(4), b = 1.6622(5), c = 2.0625(6) nm, β= 100.327(5)º,
V = 4.900(3) nm3, z = 4, F(000) = 2344, 分子量Mr = 1142.91, Dc
= 1.549 g/cm3, R1 = 0.0544,
R2 = 0.0962. 2: a = 1.3378(4), b = 2.1174(7), c =
2.4351(7) nm, β= 97.315(6)º, V = 6.842(4) nm3,
z = 4, F (000) = 3116, 分子量Mr = 1505.08, Dc
= 1.444 g/cm3, R1 = 0.0793, R2 = 0.1623. 在两个双核铁(III)配合物中,中心的三价铁和配体TPA或TBA上的四个氮原子和两个氧原子通过不同的桥形成一个畸变的八面体构型。 相似文献
162.
Stroh JG Loulakis P Lanzetti AJ Xie J 《Journal of the American Society for Mass Spectrometry》2005,16(1):38-45
Limited proteolysis is an important and widely used method for analyzing the tertiary structure and determining the domain boundaries of proteins. Here we describe a novel method for determining the N- and C-terminal boundary amino acid sequences of products derived from limited proteolysis using semi-specific and/or non-specific enzymes, with mass spectrometry as the only analytical tool. The core of this method is founded on the recognition that cleavage of proteins with non-specific proteases is not random, but patterned. Based on this recognition, we have the ability to determine the sequence of each proteolytic fragment by extracting a common association between data sets containing multiple potential sequences derived from two or more different mass spectral molecular weight measurements. Proteolytic product sequences derived from specific and non-specific enzymes can be accurately determined without resorting to the conventional time-consuming and laborious methods of SDS-PAGE and N-terminal sequencing analysis. Because of the sensitivity of mass spectrometry, multiple transient proteolysis intermediates can also be identified and analyzed by this method, which allows the ability to monitor the progression of proteolysis and thereby gain insight into protein structures. 相似文献
163.
Summary A sensitive LC method for the detection of amino acids and oligopeptides with pre-column fluorescence derivatization has been
developed. Glycine, glycylglycine, triglycine, glutathione, glutamic acid, and cysteine were separated on a reversed-phase
C18 column with methanol-water-triethylamine eluent, derivatization and chromatographic conditions were optimized. The six derivatives
were eluted in 20 min with good reproducibility. The relative standard derviations (n=6) at an analytical concentration of
2×10−6 M are <5%. Detection limits (signal-to-noise ratio=3) for the six derivatives are 23–68 fmol. 相似文献
164.
Graft chitosan derivatives (CMCTS‐g‐MAAS and CMCTS‐g‐AAS) were prepared by the graft copolymerization of methacrylic acid sodium (MAAS) and acrylic acid sodium (AAS) onto the etherification product of chitosan‐carboxymethyl chitosan (CMCTS). Their antioxidant activity was estimated as superoxide anion scavengers by chemiluminescence techniques. The derivatives with low grafting percentages have a relatively low 50% inhibition concentration (IC50), which could be related to the fact that they have different contents of hydroxyl and amino groups in the polymer chains.
165.
166.
Jiaze Xie Jan-Niklas Boyn Alexander S. Filatov Andrew J. McNeece David A. Mazziotti John S. Anderson 《Chemical science》2020,11(4):1066
Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH4, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt4−. Capping of TTFtt4− with Bu2Sn2+ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1′-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical–radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.Capping TTFtt enables facile transmetalation in three different oxidation states. 相似文献
167.
Under the catalysis of 1 mol % Pd(PPh3)4, the reaction of 4,4-disubstituted 2,3-allenamides and organic iodides in toluene afforded iminolactones stereospecifically in >90% yields using K2CO3 (2 equiv)-5 mol % TBAB as the base. A similar reaction with 4-monosubstituted 2,3-allenamides afforded gamma-hydroxy-gamma-lactams in relatively lower yields. The N/O-attack selectivity may be determined by the steric effect at the 4-position of 2,3-allenamides. 相似文献
168.
Nee MJ Osterwalder A Neumark DM Kaposta C Cibrián Uhalte C Xie T Kaledin A Bowman JM Carter S Asmis KR 《The Journal of chemical physics》2004,121(15):7259-7268
Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated. 相似文献
169.
Let R be a commutative ring with identity and I0 an ideal of R.We introduce and study the c-weak global dimension c-w.gl.dim(R/I0) of the factor ring R/I0.Let T be a w-linked extension of R,and we also introduce the wR-weak global dimension wR-w.gl.dim(T) of T.We show that the ring T with wR-w.gl.dim(T) =0 is exactly a field and the ring T with wR-w.gl.dim(T) ≤ 1 is exactly a PwRMD.As an application,we give an upper bound for the w-weak global dimension of a Cartesian square (RDTF,M).More precisely,if T is w-linked over R,then w-w.gl.dim(R) ≤ max{wR-w.gl.dim(T) + w-fdR T,c-w.gl.dim(D) + w-fdn D}.Furthermore,for a Milnor square (RDTF,M),we obtain w-w.gl.dim(R) ≤ max{wR-w.gl.dim(T) + w-fdR T,w-w.gl.dim(D) + w-fdR D}. 相似文献
170.
Heck reaction is one of the most important carbon-carbon bond forming reactions with wide applications in organic synthesis.Considerable advances of enantiosele... 相似文献