全文获取类型
收费全文 | 9837篇 |
免费 | 261篇 |
国内免费 | 20篇 |
专业分类
化学 | 5840篇 |
晶体学 | 162篇 |
力学 | 267篇 |
数学 | 733篇 |
物理学 | 3116篇 |
出版年
2023年 | 94篇 |
2022年 | 211篇 |
2021年 | 162篇 |
2020年 | 177篇 |
2019年 | 240篇 |
2018年 | 207篇 |
2017年 | 233篇 |
2016年 | 319篇 |
2015年 | 214篇 |
2014年 | 402篇 |
2013年 | 782篇 |
2012年 | 548篇 |
2011年 | 658篇 |
2010年 | 418篇 |
2009年 | 405篇 |
2008年 | 463篇 |
2007年 | 442篇 |
2006年 | 335篇 |
2005年 | 281篇 |
2004年 | 229篇 |
2003年 | 163篇 |
2002年 | 183篇 |
2001年 | 117篇 |
2000年 | 125篇 |
1999年 | 87篇 |
1998年 | 57篇 |
1997年 | 56篇 |
1996年 | 92篇 |
1995年 | 105篇 |
1994年 | 98篇 |
1993年 | 93篇 |
1992年 | 120篇 |
1991年 | 86篇 |
1990年 | 79篇 |
1989年 | 92篇 |
1988年 | 87篇 |
1987年 | 90篇 |
1986年 | 74篇 |
1985年 | 104篇 |
1984年 | 119篇 |
1983年 | 82篇 |
1982年 | 98篇 |
1981年 | 97篇 |
1980年 | 102篇 |
1979年 | 90篇 |
1978年 | 102篇 |
1977年 | 84篇 |
1976年 | 56篇 |
1975年 | 61篇 |
1974年 | 74篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dheeraj K. SinghSunil K. Srivastava Prachi RaghuvanshRanjan K. Singh B.P. Asthana 《Vibrational Spectroscopy》2011,56(1):34-41
The Raman spectra of neat (C2H5)2CO (pentanone) and its binary mixtures with hydrogen donor solvent (CH3OH), [(C2H5)2CO + CH3OH] having different mole fractions of the reference system, (C2H5)2CO in the range 0.1-0.9 at a regular interval of 0.1 were recorded in the CO stretching region. In neat liquid, the Raman peak appears asymmetric. The asymmetric nature of the peak has been attributed to the CO stretching mode of the two conformers of (C2H5)2CO having C2 and C2v point groups and the corresponding bands at ∼1711 and ∼1718 cm−1, respectively. A careful analysis of the Iiso (isotropic component of the Raman scattered intensity) at different concentrations reveals that upon dilution with methanol, at mole fraction C = 0.6, an additional peak in the CO stretching region is observed at ∼1703 cm−1 which is attributed to the hydrogen bonding with methanol. A peculiar feature in this study is that upon dilution, the peak at ∼1718 cm−1 shows a minimum at C = 0.6, but on further dilution it shows a blue shift. However, the other peak at ∼1711 cm−1 shows a continuous red shift with dilution as well as a maximum at C = 0.7 in the linewidth vs. concentration plot, which is essentially due to competition between motional narrowing and diffusion phenomena. A significant amount of narrowing in the Raman band at ∼1718 cm−1 can be understood in terms of caging effect of the reference molecule by the solvent molecules at high dilution. A density functional theoretic (DFT) calculation on optimized geometries and vibrational frequencies of two conformers of neat (C2H5)2CO in C2 ad C2v forms and the complexes with one and two CH3OH molecules with both the conformers was performed. The experimental results and theoretical calculations together indicate a co-existence of two conformers as well as hydrogen bonded complex with methanol in the binary mixture, [(C2H5)2CO + CH3OH] at intermediate concentrations. 相似文献
992.
Various techniques have been adopted to impart a biological responsiveness to synthetic hydrogels for the delivery of therapeutic agents as well as the study and manipulation of biological processes and tissue development. Such techniques and materials include polyelectrolyte gels that swell and deswell with changes in pH, thermosensitive gels that contract at physiological temperatures, and peptide cross-linked hydrogels that degrade upon peptidolysis by cell-secreted enzymes. Herein we report a unique approach to photochemically deform and degrade disulfide cross-linked hydrogels, mitigating the challenges of light attenuation and low quantum yield, permitting the degradation of hydrogels up to 2 mm thick within 120 s at low light intensities (10 mW/cm(2) at 365 nm). Hydrogels were formed by the oxidation of thiol-functionalized 4-armed poly(ethylene glycol) macromolecules. These disulfide cross-linked hydrogels were then swollen in a lithium acylphosphinate photoinitiator solution. Upon exposure to light, photogenerated radicals initiate multiple fragmentation and disulfide exchange reactions, permitting and promoting photodeformation, photowelding, and photodegradation. This novel, but simple, approach to generate photoadaptable hydrogels portends the study of cellular response to mechanically and topographically dynamic substrates as well as novel encapsulations by the welding of solid substrates. The principles and techniques described herein hold implications for more than hydrogel materials but also for photoadaptable polymers more generally. 相似文献
993.
Mujtaba SF Dwivedi A Mudiam MK Ali D Yadav N Ray RS 《Photochemistry and photobiology》2011,87(5):1067-1076
The aim of this study was to analyze the photostability and phototoxicity mechanism of anthracene (ANT) in a human skin epidermal cell line (HaCaT) at ambient environmental intensities of sunlight/UV‐R (UV‐A and UV‐B). Photomodification of ANT under sunlight/UV‐R exposure produced two photoproducts, anthrone and 9,10 anthracenedione. Generation of 1O2, O2?? and ?OH was measured under UV‐R/sunlight exposure. Involvement of reactive oxygen species (ROS) was further substantiated by their quenching with free radical quenchers. Photodegradation of 2‐deoxyguanosine and linoleic acid peroxidation showed that ROS were mainly responsible for ANT phototoxicity. ANT generates significant amount of intracellular ROS in cell line. Maximum cell viability (85%) was reduced under sunlight exposure (30 min). Results of MTT assay accord NRU assay. ANT (0.01 μg mL?1) induced cell‐cycle arrest at G1 phase. RT‐PCR demonstrated constitutive inducible mRNA expression of CYP 1A1 and 1B1 genes. Photosensitive ANT upregulates CYP 1A1 (2.2‐folds) and 1B1 (4.1‐folds) genes. Thus, the study suggests that ROS and DNA damage were mainly responsible for ANT phototoxicity. ANT exposure may be deleterious to human health at ambient environmental intensities reaching the earth’s surface through sunlight. 相似文献
994.
Jackeray R Abid CK Singh G Jain S Chattopadhyaya S Sapra S Shrivastav TG Singh H 《Talanta》2011,84(3):952-962
Present work demonstrates the utilization of surface modified polycarbonate (PC) membrane as solid phase and antibody conjugated CdSe/ZnS quantum dots (QDs) as fluorescent label for the sensitive and selective detection of Salmonella typhi (S. typhi) in water in a period of 2.5 h. PC membrane was surface modified with glycine and activated by EDC/NHS for immobilization of S. typhi specific IgG. Antibody immobilized porous PC membrane was incubated with bacteria contaminated water for immunocapturing of S. typhi. Antibody conjugated QDs were also prepared by using carbodiimide chemistry. Both modified PC membrane and quantum dots were characterized by using various modern analytical tools. It was estimated that 1.95 molecules of QDs were successfully bio-conjugated per unit of IgG. PC membrane with captured bacteria was incubated with prepared IgG conjugated QDs for the formation of sandwich complex. Analysis of the regions of interest (ROI) in fluorescent micrographs showed that newly developed method based on PC and fluorescent QDs has 100 times higher detection sensitivity (100 cells/mL) as compared with detection using conventional dye (FITC) based methods. 相似文献
995.
Series of new tin complexes are synthesized by classical thermal and microwave‐irradiated techniques. The biologically potent ligands 3‐formyl‐4‐chlorocoumarin semicarbazone (L1H) and 3‐formyl‐4‐chlorocoumarin thiosemicarbazone (L2H), were prepared by the condensation of semicarbazide hydrochloride and thiosemicarbazide in ethanol with the particular ketone by using microwave as well as conventional methods. The tin(IV) complexes have been prepared by mixing Ph3SnCl/Me3SnCl/Me2SnCl2 in 1:1 and 1:2 molar ratios with monofunctional bidentate ligands. The structures of the ligands and their tin complexes were confirmed by the elemental analysis, melting point determinations, molecular weight determinations, IR, 1H NMR, 13C NMR, 119Sn NMR, UV, mass spectral and X‐ray powder diffraction studies. On the basis of these studies it is clear that the ligands coordinated to the metal atom in a monobasic bidentate mode, by X$^{\cap}$ N donor system. Thus, suitable trigonal bipyramidal geometry for penta‐coordinated state and octahedral geometry for hexa‐coordinated state have been suggested for the 1:1 and 1:2 metal compounds. Both the ligands and their complexes have been screened for their antimicrobial, pesticidal and nematicidal activities. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
996.
Fungal isolates (Aspergillus wentii 1, A. wentii 2, Penicillium citrinum, Penicillium granulatum) were selected to study their in vitro antioxidant potential by various assay procedures. Czapek–Dox’s medium was selected
for the growth of fungi as it supported the best antioxidant activity based on their EC50 values, P. citrinum was the best followed by P. granulatum, A. wentii 1, and A. wentii 2. The chromatographic analyses showed several compounds possessing antioxidant activity in the fungal extracts. Two such
compounds were partially purified from P. citrinum which demonstrated potent antioxidant activity, equally effective or better than some of the standard antioxidants. 相似文献
997.
The synthesis of Ni(dtc)2 [dtc = diethyldithiocarbamate] has been achieved by the interaction of NiL(ClO4)2 with sodium diethyldithiocarbamate. Although single crystal structure of this complex was already reported (R = 10.6%), we were able to refine crystal structure up to R = 2.99%. We also observed rare C-H?Ni anagostic interactions generally exhibited by d8 complexes which were overlooked previously. To investigate the structure of Ni(dtc)2 in solution, variable temperature NMR spectra in solution have also been recorded between 25 and −50 °C. Ni(dtc)2 was also tested for antibacterial and antifungal activities. It showed higher activity against the bacteria and fungi than the known antibiotics. 相似文献
998.
Kiran Kumar Varma ChakrahariRajendra Singh Dhayal Sundargopal Ghosh 《Polyhedron》2011,30(6):1048-1054
Pyrolysis of an in-situ generated intermediate, produced in the reaction of [Cp∗MoCl4], 1, (Cp∗ = η5-C5Me5) with [LiBH4·THF], with an excess of difuryl ditelluride in toluene at 90 °C yielded syn and anti isomers of [Cp∗Mo(O)(μ-Te)]2 (2, 3) and [Cp∗2Mo2O2(μ-O)(μ-Te)] (4, 5). In a similar fashion, dibenzyl diselenide yielded syn and anti isomers of [Cp∗Mo(O)(μ-Se)]2 (6, 7), along with the known nido-[(Cp∗Mo)2B4H8Se2]. Note that in parallel with 2-7, [(Cp∗Mo)2B5H9] was isolated as the major product in both cases. Compounds 2-7 have been isolated in modest yield as orange to brown crystalline solids. All the new compounds have been characterized in solution by mass, IR, 1H, 13C, 77Se and 125Te NMR spectroscopy, and the structural types were unequivocally established by crystallographic analysis of 2-4 and 7. 相似文献
999.
Singh P Toma FM Kumar J Venkatesh V Raya J Prato M Verma S Bianco A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(24):6772-6780
Single‐walled carbon nanotubes (SWCNTs) have been covalently functionalized with uracil nucleobase. The hybrids have been characterized by using complementary spectroscopic and microscopic techniques including solid‐state NMR spectroscopy. The uracil‐functionalized SWCNTs are able to self‐assemble into regular nanorings with a diameter of 50–70 nm, as observed by AFM and TEM. AFM shows that the rings do not have a consistent height and thickness, which indicates that they may be formed by separate bundles of CNTs. The simplest model for the nanoring formation likely involves two bundles of CNTs interacting with each other via uracil–uracil base‐pairing at both CNT ends. These nanorings can be envisaged for the development of advanced electronic circuits. 相似文献
1000.
Xu Z Singh NJ Kim SK Spring DR Kim KS Yoon J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(4):1163-1170
Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion-π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion-π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH(3)CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-π interactions and (C-H)(+)···F(-)-type ionic hydrogen bonds. (1)H NMR, (19)F NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F(-), a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F(-) as well as Gibbs free energy. The 2-F(-) complex is more stable than the 1-F(-) complex by 1.87 kcal mol(-1), which is attributable to the stronger anion-π interaction between F(-) and triethylbenzene. 相似文献