全文获取类型
收费全文 | 43472篇 |
免费 | 2537篇 |
国内免费 | 4552篇 |
专业分类
化学 | 33213篇 |
晶体学 | 230篇 |
力学 | 504篇 |
综合类 | 727篇 |
数学 | 6760篇 |
物理学 | 9127篇 |
出版年
2023年 | 556篇 |
2022年 | 643篇 |
2021年 | 1864篇 |
2020年 | 1358篇 |
2019年 | 1341篇 |
2018年 | 1065篇 |
2017年 | 1072篇 |
2016年 | 1377篇 |
2015年 | 1381篇 |
2014年 | 1619篇 |
2013年 | 2972篇 |
2012年 | 2082篇 |
2011年 | 2132篇 |
2010年 | 1830篇 |
2009年 | 2443篇 |
2008年 | 2523篇 |
2007年 | 2700篇 |
2006年 | 2111篇 |
2005年 | 1421篇 |
2004年 | 1335篇 |
2003年 | 1276篇 |
2002年 | 3814篇 |
2001年 | 1127篇 |
2000年 | 830篇 |
1999年 | 659篇 |
1998年 | 652篇 |
1997年 | 494篇 |
1996年 | 550篇 |
1995年 | 499篇 |
1994年 | 509篇 |
1993年 | 517篇 |
1992年 | 511篇 |
1991年 | 354篇 |
1990年 | 279篇 |
1989年 | 256篇 |
1988年 | 272篇 |
1987年 | 207篇 |
1986年 | 215篇 |
1985年 | 322篇 |
1984年 | 243篇 |
1983年 | 144篇 |
1982年 | 299篇 |
1981年 | 469篇 |
1980年 | 433篇 |
1979年 | 470篇 |
1978年 | 373篇 |
1977年 | 280篇 |
1976年 | 244篇 |
1974年 | 78篇 |
1973年 | 151篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Two new homosecoiridoids, named loniaceticiridoside (1) and lonimalondialiridoside (2), were isolated from an aqueous extract of the flower buds of Lonicera japonica. Their structures including the absolute configuration were determined by extensive spectroscopic studies, especially by 2D NMR and CD data analysis. A proposed biosynthetic pathway and preliminary investigations of the biological activity of compounds 1 and 2 are also discussed. 相似文献
52.
The analysis of vitamin D status, with special emphasis on 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, is gaining interest in clinical studies due to the classical and non-classical effects attributed to this prohormone. In this research, the influence of the two steps preceding determination (viz. sample collection and preparation) on the quantitative analysis of vitamin D and its more important metabolites has been studied. Two preparation approaches, deproteination and solid-phase extraction (SPE), have been evaluated in terms of sensitivity to delimit their application, thus establishing that detection of 1,25-dihydroxyvitamin D cannot be addressed by protein precipitation. Concerning sample collection, serum and plasma reported high accuracy (above 83.3%) for vitamin D and metabolites, while precision, expressed as relative standard deviation, was below 12.9% for all analytes in both samples. Statistical analysis revealed that serum and plasma provided similar physiological levels for vitamin D3, 24,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3, while significantly different levels were obtained for 1,25-dihydroxyvitamin D3, always higher in plasma than in serum. Sample collection and treatment have proved to be significant in the analysis of vitamin D and its relevant metabolites. 相似文献
53.
《Comptes Rendus Chimie》2015,18(6):668-677
The combination of basil oil, natural antifungal, and nystatin has the potential to prevent the extension of topical fungal infections towards systemic infections. The aim of this study was to develop formulations based on basil oil and nystatin with the desired antifungal and antioxidant activity and low toxicity by using lipid nanocarriers. The synthesized nanocarriers showed spherical and homogeneous particles with main diameters less than 150 nm, as determined by TEM. The scanning calorimetric study revealed an imperfect crystallization in the core of lipid nanocarriers. Quantitative results suggested that basil oil concentration affects encapsulation efficiency. The prepared nanocarriers guaranteed an increased nystatin encapsulation by using 3% basil oil content. Chemiluminescence assay proved that the protective activity against oxygen free radicals was influenced by nystatin concentration. The in vitro antifungal studies revealed a better activity of the nanocarriers loaded with 1% nystatin in comparison with 0.5% loading. 相似文献
54.
Maia I. Kelly Tyler J. Bechtel D. Rajasekhar Reddy Erome D. Hankore Jon R. BeckCliff I. Stains 《Analytica chimica acta》2015
Inhibitors of Rho-associated protein kinase (ROCK) enzymatic activity have been shown to reduce the invasive phenotype observed in metastatic hepatocellular carcinoma (HCC). We describe the design, synthesis, and evaluation of a direct probe for ROCK activity utilizing a phosphorylation-sensitive sulfonamido-oxine fluorophore, termed Sox. The Sox fluorophore undergoes an increase in fluorescence upon phosphorylation of a proximal amino acid via chelation-enhanced fluorescence (CHEF, ex. = 360 nm and em. = 485 nm), allowing for the direct visualization of the rate of phosphate addition to a peptide substrate over time. Our optimal probe design, ROCK-S1, is capable of sensitively reporting ROCK activity with a limit of detection of 10 pM and a high degree of reproducibility (Z’-factor = 0.6 at 100 pM ROCK2). As a proof-of-principle for high-throughput screening (HTS) we demonstrate the ability to rapidly assess the efficacy of a 78 member, small molecule library against ROCK2 using a robotics platform. We identify two previously unreported ROCK2 inhibitor scaffolds, PHA665752 and IKK16, with IC50 values of 3.6 μM and 247 nM respectively. Lastly, we define conditions for selectively monitoring ROCK activity in the presence of potential off-target enzymes (PKCα, PKA, and PAK) with similar substrate specificities. 相似文献
55.
A new asymmetric Salamo‐based ligand H2L was synthesized using 3‐tert‐butyl‐salicylaldehyde and 6‐methoxy‐2‐[O‐(1‐ethyloxyamide)]‐oxime‐1‐phenol. By adjusting the ratio of the ligand H2L and Cu (II), Co (II), and Ni (II) ions, mononuclear, dinuclear, and trinuclear transition metal (II) complexes, [Cu(L)], [{Co(L)}2], and [{Ni(L)(CH3COO)(CH3CH2OH)}2Ni] with the ligand H2L possessing completely different coordination modes were obtained, respectively. The optical spectra of ligand H2L and its Cu (II), Co (II) and Ni (II) complexes were investigated. The Cu (II) complex is a mononuclear structure, and the Cu (II) atom is tetracoordinated to form a planar quadrilateral structure. The Co (II) complex is dinuclear, and the two Co (II) atoms are pentacoordinated and have coordination geometries of distorted triangular bipyramid. The Ni (II) complex is a trinuclear structure, and the terminal and central Ni (II) atoms are all hexacoordinated, forming distorted octahedral geometries. Furthermore, optical properties including UV–Vis, IR, and fluorescence of the Cu (II), Co (II), and Ni (II) complexes were investigated. Finally, the antibacterial activities of the Cu (II), Co (II), and Ni (II) complexes were explored. According to the experimental results, the inhibitory effect was found to be enhanced with increasing concentrations of the Cu (II), Co (II), and Ni (II) complexes. 相似文献
56.
Zhong-Hui Pu Juan Liu Min Luo Qin-Mei Zhou Xiao-Fang Xie 《Natural product research》2019,33(10):1399-1405
Two new (1–2) and six known (3–8) nucleoside alkaloids were isolated from the rhizomes of Ligusticum striatum DC. Compounds 1 and 2 (liguadenosines A and B) were unusual N-10 substituted adenosine derivatives. Their structures were elucidated by extensive spectroscopic analyses and ECD calculation. Most of them significantly inhibited the abnormal increase in platelet aggregation induced by ADP at concentrations of 50 and 100 μM. Particularly, the inhibitory effect of 3 was equivalent to aspirin. 相似文献
57.
New thiazole derivatives were synthesized and fully characterized, then coordinated with PtCl4 salt. Also, the newly synthesized Pt(IV) complexes were investigated analytically (elemental and thermogravimetric analyses), spectrally (infrared, UV–visible, mass, 1H NMR, 13C NMR, X‐ray diffraction) as well as theoretically (kinetics, modeling and docking). The data extracted led to the establishment of the best chemical and structural forms. Octahedral geometry was the only formula proposed for all complexes, which is favorable for d6 systems. The molecular ion peaks from mass spectral analysis coincide with all analytical data, confirming the molecular formula proposed. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) allowed discrimination of features between crystalline particles and other amorphous morphology. By applying Gaussian09 as well as HyperChem 8.2 programs, the best structural forms were obtained, as well as computed significant parameters. Computed parameters such as softness, hardness, surface area and reactivity led us towards application in two opposing pathways: tumor inhibition and oxidation activation. The catalytic oxidation for CO was conducted over PtO2, which was yielded from calcination of the most reactive complex. The success of catalytic role for synthesized PtO2 was due to its particulate size and surface morphology, which were estimated from XRD patterns and SEM images, respectively. The antitumor activity was tested versus HCT‐116 and HepG‐2 cell lines. Mild toxicity was recorded for two of the derivatives and their corresponding complexes. This degree of toxicity is more favorable in most cases, due to exclusion of serious side effects, which is coherently attached with known antitumor drugs. 相似文献
58.
Andrey A. Dobrynin 《Discrete Mathematics》2019,342(1):74-77
Distance between two vertices is the number of edges in a shortest path connecting them in a connected graph . The transmission of a vertex is the sum of distances from to all the other vertices of . If transmissions of all vertices are mutually distinct, then is a transmission irregular graph. It is known that almost no graphs are transmission irregular. Infinite families of transmission irregular trees of odd order were presented in Alizadeh and Klav?ar (2018). The following problem was posed in Alizadeh and Klav?ar (2018): do there exist infinite families of transmission irregular trees of even order? In this article, such a family is constructed. 相似文献
59.
Yanmin Yu 《Molecular physics》2019,117(9-12):1360-1366
ABSTRACTPotential energy surfaces and molecular dynamics of the intramolecular 1, 3-dipolar cycloaddition and ene reaction of a nitrile oxide with an alkene were performed in the gas phase and in dichloromethane with density functional theory. One hundred trajectories were propagated in the gas phase and in dichloromethane, respectively. Twenty percent of the trajectories in the gas phase involve bicyclic intermediate and the mean time gap is 472fs. A dynamically stepwise reaction is observed. In dichloromethane, more reactive trajectories were obtained and the time gap is larger than that in the gas phase. 相似文献
60.