全文获取类型
收费全文 | 800篇 |
免费 | 31篇 |
国内免费 | 2篇 |
专业分类
化学 | 512篇 |
晶体学 | 19篇 |
力学 | 17篇 |
数学 | 54篇 |
物理学 | 231篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 39篇 |
2021年 | 22篇 |
2020年 | 39篇 |
2019年 | 33篇 |
2018年 | 24篇 |
2017年 | 44篇 |
2016年 | 38篇 |
2015年 | 22篇 |
2014年 | 39篇 |
2013年 | 78篇 |
2012年 | 44篇 |
2011年 | 61篇 |
2010年 | 20篇 |
2009年 | 27篇 |
2008年 | 37篇 |
2007年 | 39篇 |
2006年 | 28篇 |
2005年 | 24篇 |
2004年 | 20篇 |
2002年 | 13篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 2篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有833条查询结果,搜索用时 62 毫秒
821.
H. R. Manjunath S. Naveen C. S. Ananda Kumar S. B. Benaka Prasad M. V. Deepa Naveen M. A. Sridhar J. Shashidhara Prasad K. S. Rangappa 《Journal of Structural Chemistry》2011,52(5):959-963
The title compound 1′-Benzyl-8-(4-fluorobenzyl)-8-azaspiro[bicyclo[3.2.1] octane-3,4′-imidazolidine]-2′,5′-dione, C23H23FN3O2 is synthesized and the structure is investigated by X-ray diffraction studies. The compound crystallizes in the triclinic
crystal class in the P1 space group. The hydantoin ring adopts a planar conformation and is affected by the π conjugation. The pyrrolidine and piperidine
rings in the bicyclo octane moiety adopt envelope and chair conformations respectively. The structure exhibits both inter-
and intramolecular hydrogen bonds of the type N-H...O, C-H...O, and C-H...N. The oxygen atom in the hydantoin ring simultaneously
accepts two hydrogen bonds to form a three-centered hydrogen bonding pattern. 相似文献
822.
Shubham Sharma Virender Singh Vaishali Rakesh Kumar Rahul Jamra Naveen Banyal Jyoti 《Journal of heterocyclic chemistry》2024,61(2):232-284
The synthesis of pyrazole derivative using α,β-unsaturated carbonyl compounds has attracted increasing attention of the synthetic organic chemist community. Interestingly, the simplicity of the synthetic method, high reactivity, and ease of incorporating diversity into the desired prototype have contributed a lot toward the exploration of α,β-unsaturated carbonyl compounds by various research groups. Due to the tremendous pharmacological significance of pyrazole derivatives, their synthesis has been one of the leading research frontiers in recent years. As prime examples, sildenafil, zometapin, Celebrex, and rimonabant have been successfully commercialized in the market to treat various life-challenging diseases. Considering the great profile of α,β-unsaturated carbonyl compound in the synthesis of biologically active pyrazole derivatives, this review incorporates contemporary literature (2011–2022) on the synthesis of pyrazole and its derivatives using α,β-unsaturated carbonyl compound as a starting precursor. 相似文献
823.
Shalendra Kumar Ravi Kumar P. Thakur K.H. Chae S.K. Sharma Alimuddin 《Journal of magnetism and magnetic materials》2008,320(14):e121-e124
The electronic structure of Mg0.95Mn0.05Fe2−2xTi2xO4 (0x0.8) compound is investigated using near edge X-ray absorption fine structure, (NEXAFS) spectroscopy measurements, carried out at O K, Fe and Ti L3,2-edges at room temperature. The O K-edge spectra indicate that the Fe 3d orbitals have been considerably modified and a new spectral feature start dominating in the pre-edge region at higher Ti doping. The Fe 2p NEXAFS spectra exhibit a mixed valent Fe2+/Fe3+ states apart from the conversion of Fe3+ to Fe2+ with the substitution of Ti ions. The Ti L3,2-edge spectra indicate that Ti ions remain unchanged at 4+ state. These variations in the host electronic structure due to Ti substitution are consistent with the dielectric and transport properties of the material. 相似文献
824.
S. Naveen Sridhar M. Anandalwar J. Shashidhara Prasad Dinesh Manvar Arun Mishra Anamik Shah 《Journal of chemical crystallography》2008,38(4):315-319
Abstract 2,6-Dimethyl-5-(phenylcarbamoyl)-4-(3-nitrophenyl)-1,4-dihydropyridine-2-carboxylate was synthesized in two steps and was
characterized spectroscopically and confirmed by X-ray diffraction studies. The molecule crystallizes in the monoclinic crystal
class in the spacegroup P21/c with cell parameters a = 10.5960(6) ?, b = 10.2450(7) ?, c = 19.5790(11) ?, β = 107.448(3)° and Z = 4. The 1,4-dihydropyridine ring in the structure adopts a flattened boat conformation.
Graphical Abstract 2,6-Dimethyl-5-(phenylcarbamoyl)-4-(3-nitrophenyl)-1,4-dihydropyridine-2-carboxylate was synthesized in two steps and was
characterized spectroscopically and confirmed by X-ray diffraction studies. The molecule crystallizes in the monoclinic crystal
class in the spacegroup P21/c with cell parameters a = 10.5960(6) ?, b = 10.2450(7) ?, c = 19.5790(11) ?, β = 107.448(3)° and Z = 4. The 1,4-dihydropyridine ring in the structure adopts a flattened boat conformation.
相似文献
825.
Liquid crystal thermography and pressure drop measurements have been carried out to study the heat transfer and frictional characteristics in a rectangular duct with solid ribs (C1), converging slit-ribs (C2), and alternate solid-slit ribs (C3) mounted transversely on the bottom wall, where C2 carries a continuous converging-slit in the flow direction. Effect of rib configurations, and rib pitch to height ratios (6, 8, 10, and 12) has been investigated at Re of 9400, 26160, 42500, and 58850. Results show that converging-slit considerably enhances the heat transfer rate in the downstream vicinity, and help in obviating the local hot spot formation.
Abbreviations: LCT: Liquid crystal thermography; HTC: Heat transfer coefficient; LHI: Laser holographic interferometry; NST: Naphthalene sublimation technique; IR: Infrared; TPF: Thermo-hydraulic performance; PIV: Particle image velocimetry. 相似文献
826.
Ajay Kumar Sandeep Kaur Sukhvinder Dhiman Prithvi Pal Singh Gaurav Bhatia Sharad Thakur Hardeep Singh Tuli Upendra Sharma Subodh Kumar Abdulmajeed G. Almutary Abdullah M. Alnuqaydan Arif Hussain Shafiul Haque Kuldeep Dhama Satwinderjeet Kaur 《Molecules (Basel, Switzerland)》2022,27(11)
Onosma bracteata Wall. is an important medicinal and immunity-enhancing herbs. This plant is commonly used in the preparation of traditional Ayurvedic drugs to treat numerous diseases. Inspired by the medicinal properties of this plant, the present study aimed to investigate the antiproliferative potential and the primary molecular mechanisms of the apoptotic induction against human osteosarcoma (MG-63) cells. Among all the fractions isolated from O. bracteata, ethyl acetate fraction (Obea) showed good antioxidant activity in superoxide radical scavenging assay and lipid peroxidation assay with an EC50 value of 95.12 and 80.67 µg/mL, respectively. Silica gel column chromatography of ethyl acetate (Obea) fraction of O. bracteata yielded a pure compound, which was characterized by NMR, FTIR, and HR-MS analysis and was identified as 1,2-benzene dicarboxylic acid, bis (2-methyl propyl) ester (BDCe fraction). BDCe fraction was evaluated for the antiproliferative potential against human osteosarcoma MG-63, human neuroblastoma IMR-32, and human lung carcinoma A549 cell lines by MTT assay and exhibited GI50 values of 37.53 μM, 56.05 μM, and 47.12 μM, respectively. In MG-63 cells, the BDCe fraction increased the level of ROS and simultaneously decreased the mitochondria membrane potential (MMP) potential by arresting cells at the G0/G1 phase, suggesting the initiation of apoptosis. Western blotting analysis revealed the upregulation of p53, caspase3, and caspase9 while the expressions of p-NF-κB, p-Akt and Bcl-xl were decreased. RT-qPCR studies also showed upregulation in the expression of p53 and caspase3 and downregulation in the expression of CDK2, Bcl-2 and Cyclin E genes. Molecular docking analysis displayed the interaction between BDCe fraction with p53 (−151.13 kcal/mol) and CDK1 (−133.96 kcal/mol). The results of the present work suggest that the BDCe fraction has chemopreventive properties against osteosarcoma (MG-63) cells through the induction of cell cycle arrest and apoptosis via Akt/NF-κB/p53 pathways. This study contributes to the understanding of the utilization of BDCe fraction in osteosarcoma treatment. 相似文献
827.
Piyush Kashyap Mamta Thakur Nidhi Singh Deep Shikha Shiv Kumar Poonam Baniwal Yogender Singh Yadav Minaxi Sharma Kandi Sridhar Baskaran Stephen Inbaraj 《Molecules (Basel, Switzerland)》2022,27(19)
The recent coronavirus disease (COVID-19) outbreak in Wuhan, China, has led to millions of infections and the death of approximately one million people. No targeted therapeutics are currently available, and only a few efficient treatment options are accessible. Many researchers are investigating active compounds from natural plant sources that may inhibit COVID-19 proliferation. Flavonoids are generally present in our diet, as well as traditional medicines and are effective against various diseases. Thus, here, we reviewed the potential of flavonoids against crucial proteins involved in the coronavirus infectious cycle. The fundamentals of coronaviruses, the structures of SARS-CoV-2, and the mechanism of its entry into the host’s body have also been discussed. In silico studies have been successfully employed to study the interaction of flavonoids against COVID-19 Mpro, spike protein PLpro, and other interactive sites for its possible inhibition. Recent studies showed that many flavonoids such as hesperidin, amentoflavone, rutin, diosmin, apiin, and many other flavonoids have a higher affinity with Mpro and lower binding energy than currently used drugs such as hydroxylchloroquine, nelfinavir, ritonavir, and lopinavir. Thus, these compounds can be developed as specific therapeutic agents against COVID-19, but need further in vitro and in vivo studies to validate these compounds and pave the way for drug discovery. 相似文献
828.
829.
Suneetha Devpura Jagdish S. Thakur Janet M. Poulik Rajah Rabah Vaman M. Naik Ratna Naik 《Journal of Raman spectroscopy : JRS》2013,44(3):370-376
Raman spectroscopy is a nondestructive technique that can provide information at the molecular level about the biochemicals in tissues. We have investigated the cellular regions in neuroblastoma and ganglioneuroma using Raman spectroscopy and compared their spectral characteristics with those of the corresponding normal adrenal gland. Thin sections from both the frozen and the corresponding formalin‐fixed paraffin‐processed (FFPP) tissues were studied in conjunction with the pathological examination of the tissues. Investigation of the spectral data shows that the normal adrenal gland tissues have higher levels of carotenoids, lipids, and cholesterol compared with the neuroblastoma and ganglioneuroma frozen tissues. However, in comparison with the frozen tissues, the FFPP tissues show a significant alteration of several biochemicals, including the complete removal of carotenoids, lipids, and cholesterol in the adrenal tissues. A quantitative analysis using chemometric methods of principal component analysis and discriminant function analysis of the Raman spectral data obtained from the frozen tissues show a clear‐cut classification among pathological groups with high sensitivity and specificity. We have validated the classification results of the FFPP tissues against a training set data obtained from the archived FFPP tissues of nine other patients. The validation process correctly identified and grouped the data with the training set of normal adrenal gland (>97% of the time) and neuroblastoma (100% of the time) tissues, whereas the validation was not so strong for ganglioneuroma. This study shows that Raman spectroscopy combined with chemometric methods can be successfully used to distinguish neuroblastoma and ganglioneuroma at cellular level in frozen tissue sections. This study also shows that formalin fixation and paraffinization/deparaffinization of tissues can alter their biochemical composition. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
830.
Raj N. Patel Dharmik M. Patel Nileshkumar B. Rathod Dinesh G. Thakur Sachinkumar D. Patel Srinu Tothadi Subhash Chandra Ghosh 《European journal of organic chemistry》2023,26(34):e202300669
A cobalt-catalyzed, N,O-bidentate directing group-assisted C−H bond functionalization of benzamides with maleimides was developed for the facile access to isoindolone spirosuccinimides in good to excellent yields. This C−H bond activation and spirocyclization employing pyridine N-oxide as directing group provided very good substrate scope and tolerated various functional groups. Furthermore, the mechanistic investigation revealed that the C−H bond activation is the rate-determining step of this reaction. 相似文献