全文获取类型
收费全文 | 15887篇 |
免费 | 564篇 |
国内免费 | 49篇 |
专业分类
化学 | 10679篇 |
晶体学 | 238篇 |
力学 | 545篇 |
综合类 | 1篇 |
数学 | 1268篇 |
物理学 | 3769篇 |
出版年
2024年 | 65篇 |
2023年 | 173篇 |
2022年 | 446篇 |
2021年 | 437篇 |
2020年 | 476篇 |
2019年 | 537篇 |
2018年 | 497篇 |
2017年 | 471篇 |
2016年 | 680篇 |
2015年 | 485篇 |
2014年 | 778篇 |
2013年 | 1318篇 |
2012年 | 1161篇 |
2011年 | 1207篇 |
2010年 | 795篇 |
2009年 | 622篇 |
2008年 | 772篇 |
2007年 | 795篇 |
2006年 | 617篇 |
2005年 | 542篇 |
2004年 | 409篇 |
2003年 | 350篇 |
2002年 | 291篇 |
2001年 | 174篇 |
2000年 | 162篇 |
1999年 | 114篇 |
1998年 | 86篇 |
1997年 | 115篇 |
1996年 | 116篇 |
1995年 | 89篇 |
1994年 | 82篇 |
1993年 | 112篇 |
1992年 | 120篇 |
1991年 | 96篇 |
1990年 | 84篇 |
1989年 | 82篇 |
1988年 | 68篇 |
1987年 | 57篇 |
1986年 | 53篇 |
1985年 | 75篇 |
1984年 | 66篇 |
1983年 | 71篇 |
1982年 | 53篇 |
1981年 | 49篇 |
1980年 | 64篇 |
1979年 | 74篇 |
1978年 | 57篇 |
1977年 | 80篇 |
1976年 | 49篇 |
1975年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Thirupathi Dongala Naresh Kumar Katari Ashok Kumar Palakurthi Sreekantha B. Jonnalagadda 《Biomedical chromatography : BMC》2019,33(10)
We developed novel stability‐indicating HPLC method for simultaneous estimation of 14 impurities in excedrin tablet, a formulation with a combination of acetaminophen, aspirin, and caffeine. In addition, a new impurity that was generated through degradation of aspirin at high temperatures during the accelerated stability conditions was positively identified and confirmed, using liquid chromatography–mass spectrometry technique. The HPLC method was optimized using the Inertsustain C18, 250 × 4.6 mm, 5.0 μm column, employing simple gradient method. Forced degradation studies were performed under acidic, basic, oxidative and thermal conditions to prove the scope and stability‐indicating the nature of the method. The optimized method was validated as per the International Conference on Harmonization guidelines. The HPLC method showed linearity from LOQ concentration to 21 μg mL?1. Precision and intermediate precision values were <5% RSD. The validated HPLC method is currently applied for the routine testing of excedrin tablet formulations in quality control laboratories. 相似文献
992.
Macrocyclization of bis-indole quinolines for selective stabilization of G-quadruplex DNA structures
The recognition of G-quadruplex (G4) DNA structures as important regulatory elements in biological mechanisms, and the connection between G4s and the evolvement of different diseases, has sparked interest in developing small organic molecules targeting G4s. However, such compounds often lack drug-like properties and selectivity. Here, we describe the design and synthesis of a novel class of macrocyclic bis-indole quinolines based on their non-macrocyclic lead compounds. The effects of the macrocyclization on the ability to interact with G4 DNA structures were investigated using biophysical assays and molecular dynamic simulations. Overall, this revealed compounds with potent abilities to interact with and stabilize G4 structures and a clear selectivity for both G4 DNA over dsDNA and for parallel/hybrid G4 topologies, which could be attributed to the macrocyclic structure. Moreover, we obtained knowledge about the structure–activity relationship of importance for the macrocyclic design and how structural modifications could be made to construct improved macrocyclic compounds. Thus, the macrocyclization of G4 ligands can serve as a basis for the optimization of research tools to study G4 biology and potential therapeutics targeting G4-related diseases.Macrocyclization improves the selectivity, affinity, and ability to stabilize G4 DNA structures. 相似文献
993.
Abhishek Maurya Arun Kumar Mahato Nikita Chaudhary Neha Kesharwani Payal Kachhap Vivek Kumar Mishra Chanchal Haldar 《应用有机金属化学》2020,34(4):e5508
Two vanadium (IV) complexes [VIVO(Haeae-sal)(MeOH)]+ ( 1 ) and [VIVO(Haeae-hyap)(MeOH)]+ ( 2 ) were prepared by reacting [VO(acac)2] with ligands [H2aeae-sal] ( I ) and [H2aeae-hyap] ( II ) respectively. Condensation of 2-(2-aminoethylamino)ethanol with salicylaldehyde and 2-hydroxyacetophenone produces the ligands ( I ) and ( II ) respectively. Both vanadium complexes 1 and 2 are sensitive towards aerial oxygen in solution and rapidly convert into vanadium(V) dioxido species. Vanadium(V) dioxido species crystalizes as the dimeric form in the solid-state. Single-crystal XRD analysis suggests octahedral geometry around each vanadium center in the solid-state. To access the benefits of heterogeneous catalysis, vanadium(V) dioxido complexes were anchored into the polymeric chain of chloromethylated polystyrene. All the synthesized neat and supported vanadium complexes have been studied by a number of techniques to confirm their structural and functional properties. Bromoperoxidase activity of the synthesized vanadium(V) dioxido complexes 3 and 4 was examined by carrying out oxidative bromination of salicylaldehyde and oxidation of thioanisole. In the presence of hydrogen peroxide, 3 shows 94.4% conversion ( TOF value of 2.739 × 102 h−1) and 4 exhibits 79.0% conversion (TOF value of 2.403 × 102 h−1) for the oxidative bromination of salicylaldehyde where 5-bromosalicylaldehyde appears as the major product. Catalysts 3 and 4 also efficiently catalyze the oxidation of thioanisole in the presence of hydrogen peroxide where sulfoxide is observed as the major product. Covalent attachment of neat catalysts 3 and 4 into the polymer chain enhances substrate conversion (%) and their catalytic efficiency increases many folds, both in the oxidative bromination and oxidation of thioether. Polymer supported catalysts 5 displayed 98.8% conversion with a TOF value of 1.127 × 104 h−1 whereas catalyst 6 showed 95.7% conversion with a TOF value of 4.675 × 103 h−1 for the oxidative bromination of salicylaldehyde. These TOF values are the highest among the supported vanadium catalysts available in the literature for the oxidative bromination of salicylaldehyde. 相似文献
994.
Dr. Subrat Kumar Barik Shou-Chih Huo Chun-Yen Wu Tzu-Hao Chiu Jian-Hong Liao Dr. Xiaoping Wang Dr. Samia Kahlal Prof. Jean-Yves Saillard Prof. C. W. Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(46):10471-10479
Although atomically precise polyhydrido copper nanoclusters are of prime interest for a variety of applications, they have so far remained scarce. Herein, this work describes the synthesis of a dithiophosphate-protected copper(I) hydride-rich nanocluster (NC), [Cu30H18{S2P(OnPr)2}12] ( 1H ), fully characterized by various spectroscopic methods and single-crystal X-ray diffraction. The X-ray structure of 1H reveals an unprecedented central Cu12 hollow icosahedron. Six faces of this icosahedron are capped by Cu3 triangles, the whole Cu30 core being wrapped by twelve dithiophosphate ligands and the whole cluster has ideal S6 symmetry. The locations of the 18 hydrides in 1H were ascertained by a single-crystal neutron diffraction study. They are composed of three types: capping μ3-H, interstitial μ4-H (seesaw) and μ5-H ligands (square pyramidal), in good agreement with the DFT simulations. The numbers of hydrides and ligand resonances in the 1H NMR spectrum of 1H are in line with their coordination environment in the solid state, retaining the S6 symmetry in solution. Furthermore, two new Se-protected polyhydrido copper nanoclusters, [Cu30H18{Se2P(OR)2}12] ( 2H : R=iPr 3H : R=iBu) were synthesized from their sulfur relative 1H via ligand displacement reaction and their X-ray structures feature the exceptional case where both the NC shape and size are fully conserved during the course of ligand exchange. DFT and TD-DFT calculations allow understanding the bonding and optical properties of clusters 1H – 3H . In addition, the reaction of 1H with [Pd(PPh3)2Cl2] in the presence of terminal alkynes led to the formation of new bimetallic Cu−Pd alloy clusters [PdCu14H2{S2P(OnPr)2}6(C≡CR)6] ( 4 : R=Ph; 5 : R = C6H4F). 相似文献
995.
Maxime Balestrat Abhijeet Lale Andr Vinícius Andrade Bezerra Vanessa Proust Eranezhuth Wasan Awin Ricardo Antonio Francisco Machado Pierre Carles Ravi Kumar Christel Gervais Samuel Bernard 《Molecules (Basel, Switzerland)》2020,25(22)
The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000–1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiNxCy nanocrystals (x + y = 1) are homogeneously formed “in situ” in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si3N4, TiNxCy (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere. 相似文献
996.
A. Sri Hari Kumar V. N. Kalevaru A. Qiao A. Alshammari N. Lingaiah Ch. Sailu P. S. Sai Prasad A. Martin 《Kinetics and Catalysis》2013,54(5):615-619
Oxidative dehydrogenation of ethane (ODHE) to ethylene was investigated over a series of alumina supported molybdophosphoric acid (MPA) catalysts. The MPA was transformed into surface Mo oxides on Al2O3 when subjected to calcination at 600°C. The catalysts were characterized by N2-adsorption, XRD, FT-IR spectroscopy and TPR techniques. The results showed that MPA loading and the source of Mo precursor had a clear influence on the catalytic performance. The evaluation of the catalysts for ODHE at temperatures between 450 and 550°C revealed superior ethane conversion (X~24%) and ethylene selectivity (S = ca. 65%) over 20 wt % MPA/Al2O3 catalyst. The transformation of MPA into finely dispersed Mo oxides on Al2O3 appeared to be responsible for this improved performance. 相似文献
997.
Pitchiah Esakki Karthik Chinnaiah Jeyabharathi Shanmugam Senthil Kumar Kanala Lakshmi Narasimha Phani 《Journal of Solid State Electrochemistry》2013,17(12):3055-3061
It is shown that the gold surface is catalytically deactivated and smoothened upon removal of the Prussian blue (PB)–gold nanocomposite formed on the gold surface. Atomic force microscopy proves surface smoothening after PB removal. The voltammetric responses of Ru(NH3)6Cl3 on the smoothened surface remain unaffected, but the reactions that involve multistep and inner-sphere electron transfer are affected on the smoothened surface as exemplified by hydroquinone, ferrous oxalate redox reactions, and oxygen reduction. These effects are attributed to catalytic deactivation as a consequence of removal of the active sites. Figure
It is shown that the gold surface is catalytically deactivated and smoothened upon removal of the Prussian blue (PB)–gold nanocomposite formed on the gold surface. Atomic force microscopy proves surface smoothening after PB removal. The voltammetric responses of Ru(NH3)6.Cl3 on smoothened surface remain unaffected, but the reactions that involve multistep and inner-sphere electron transfer are affected on the smoothened surface as exemplified by hydroquinone, ferrous oxalate redox reactions, and oxygen reduction. These effects are attributed to catalytic deactivation as a consequence of removal of the active sites. Graphical abstract shows the Au surface smoothening as a consequence of Prussian blue-gold nanocomposite (Au-PB) formation and removal 相似文献
998.
Richa Upadhyay Jitendra Kumar Chaurasia Kavindra Nath Tiwari Karuna Singh 《Applied biochemistry and biotechnology》2013,171(8):2153-2164
Phyllanthus fraternus is widely used in the cure of various liver diseases and possess antiviral properties especially against hepatitis virus. In the present study, evaluation of the antioxidant activity of stem and calli induced from stem has been done by different assays. Extraction was done by standard method in water and ethanol. Total antioxidant capacity was measured by 1, 1-diphenyl-2-picrylhydrazyl free radical scavenging method. Lipid peroxidation was measured in terms of thiobarbituric acid-reactive substances (TBARS) by using egg yolk homogenates as lipid-rich media, and superoxide radical scavenging activity was measured using riboflavin–light–nitro blue tetrazolium assay. Reducing power was determined on the basis of Fe3+–Fe2+ transformation in the presence of the extract. In addition to the antioxidant activity, polyphenolic compounds like total phenolics and flavonoids were also measured by spectroscopic method. Results showed that the ethanolic extract of stem is more potent in antioxidant activity than its aqueous extract and ethanolic extract of calli. A significant correlation between antioxidant capacity and polyphenolic content and reducing potential was observed, indicating that phenolic compounds and reducers present in extract are major contributors to the antioxidant potential. Thus, this plant extract could be used as a potent natural antioxidant. 相似文献
999.
Kumar H Mukherjee B Lin ST Dasgupta C Sood AK Maiti PK 《The Journal of chemical physics》2011,134(12):124105
Experiments and computer simulations demonstrate that water spontaneously fills the hydrophobic cavity of a carbon nanotube. To gain a quantitative thermodynamic understanding of this phenomenon, we use the recently developed two phase thermodynamics method to compute translational and rotational entropies of confined water molecules inside single-walled carbon nanotubes and show that the increase in energy of a water molecule inside the nanotube is compensated by the gain in its rotational entropy. The confined water is in equilibrium with the bulk water and the Helmholtz free energy per water molecule of confined water is the same as that in the bulk within the accuracy of the simulation results. A comparison of translational and rotational spectra of water molecules confined in carbon nanotubes with that of bulk water shows significant shifts in the positions of the spectral peaks that are directly related to the tube radius. 相似文献
1000.
Dokuburra Chanti BabuJondoss Jon Paul Selavam Dorigondla Kumar ReddyVanam Shekhar Yenamandra Venkateswarlu 《Tetrahedron》2011,67(21):3815-3819
A stereoselective total synthesis of (−)-cleistenolide (1) derived from d-(−)-isoascorbic acid has been described. The new synthetic strategy involves highly diastereoselective reduction, one-pot protection of required benzoyl, acetyl groups, and the RCM reaction by using Grubbs catalyst are the key steps with considerable yields. 相似文献