全文获取类型
收费全文 | 6282篇 |
免费 | 167篇 |
国内免费 | 49篇 |
专业分类
化学 | 4676篇 |
晶体学 | 67篇 |
力学 | 145篇 |
数学 | 666篇 |
物理学 | 944篇 |
出版年
2023年 | 26篇 |
2022年 | 34篇 |
2021年 | 49篇 |
2020年 | 76篇 |
2019年 | 95篇 |
2018年 | 65篇 |
2017年 | 46篇 |
2016年 | 129篇 |
2015年 | 109篇 |
2014年 | 121篇 |
2013年 | 285篇 |
2012年 | 378篇 |
2011年 | 473篇 |
2010年 | 221篇 |
2009年 | 194篇 |
2008年 | 399篇 |
2007年 | 450篇 |
2006年 | 442篇 |
2005年 | 436篇 |
2004年 | 399篇 |
2003年 | 313篇 |
2002年 | 282篇 |
2001年 | 95篇 |
2000年 | 70篇 |
1999年 | 74篇 |
1998年 | 80篇 |
1997年 | 57篇 |
1996年 | 86篇 |
1995年 | 62篇 |
1994年 | 60篇 |
1993年 | 45篇 |
1992年 | 43篇 |
1991年 | 48篇 |
1990年 | 43篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 34篇 |
1986年 | 35篇 |
1985年 | 56篇 |
1984年 | 53篇 |
1983年 | 48篇 |
1982年 | 48篇 |
1981年 | 48篇 |
1980年 | 46篇 |
1979年 | 44篇 |
1978年 | 51篇 |
1977年 | 43篇 |
1976年 | 37篇 |
1975年 | 24篇 |
1974年 | 25篇 |
排序方式: 共有6498条查询结果,搜索用时 31 毫秒
991.
Jiang MY Shin JY Patrick BO Dolphin D 《Dalton transactions (Cambridge, England : 2003)》2008,(19):2598-2602
A meso-palladioporphyrin intermediate was isolated from a Heck reaction between an iodoporphyrin and a non-activated olefin using a Pd(PPh3)2Cl2/Et3N system; its structure was characterized by NMR, MS and X-ray crystallography. Studies on its formation indicate that the Pd(II) catalyst was reduced in situ by Et3N with the assistance of water. The catalytic activity of the intermediate was confirmed by stoichiometric and catalytic reactions using a more reactive olefin, ethyl acrylate. 相似文献
992.
Kilpin KJ Henderson W Nicholson BK 《Dalton transactions (Cambridge, England : 2003)》2008,(29):3899-3906
The pyridyl-2-alkylsulfonamides C(5)H(4)N(CH(2))(n)NHSO(2)R (n = 1,2; R = Me, Ph or p-C(6)H(4)Me) and 8-(p-tosylamino)quinoline undergo facile cycloauration reactions with H[AuCl(4)] in water, giving metallacyclic complexes coordinated through the pyridyl (or quinolyl) nitrogen atom and the deprotonated nitrogen of the sulfonamide group. The complexes have been fully characterised by NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of two derivatives reveal the presence of non-planar sulfonamide nitrogen atoms. The complexes show low activity against P388 murine leukaemia cells, possibly as a result of their ease of reduction with mild reducing agents. 相似文献
993.
Variability of UV irradiance in Europe 总被引:1,自引:1,他引:0
Seckmeyer G Pissulla D Glandorf M Henriques D Johnsen B Webb A Siani AM Bais A Kjeldstad B Brogniez C Lenoble J Gardiner B Kirsch P Koskela T Kaurola J Uhlmann B Slaper H den Outer P Janouch M Werle P Gröbner J Mayer B de la Casiniere A Simic S Carvalho F 《Photochemistry and photobiology》2008,84(1):172-179
994.
Wulff BC Schick JS Thomas-Ahner JM Kusewitt DF Yarosh DB Oberyszyn TM 《Photochemistry and photobiology》2008,84(2):317-321
Nonmelanoma skin cancer resulting from UVB exposure is a large and growing problem in the United States. Production of reactive oxygen species (ROS) during the UVB-induced inflammatory response results in the formation of oxidative DNA adducts such as 8-hydroxy-2-deoxyguanine (8-oxo-dG), which have been shown to contribute to the development of this cancer. The 8-oxoguanine DNA glycosylase (OGG1) enzyme repairs 8-oxo-dG adducts, suggesting that enhancing its activity in the skin might increase 8-oxo-dG repair thus preventing skin cancer development. We therefore used the SKH-1 murine model to examine the effect of topically applied OGG1 on UVB-induced skin cancer development. Mice were exposed three times weekly to UVB followed immediately by topical treatment with a formulation of liposome-encapsulated OGG1 enzyme for 25 weeks. While this treatment did not affect UVB-induced tumor multiplicity, it did reduce tumor size and dramatically reduced tumor progression, as indicated by tumor grade. These results suggest that oxidative DNA damage contributes to the progression of UVB-induced skin tumors and that a topical formulation containing OGG1, perhaps in conjunction with other DNA repair enzymes such as T4 endonuclease V, could be used in populations at high risk for skin cancer development. 相似文献
995.
Spectroscopy, binding to liposomes and production of singlet oxygen by porphyrazines with modularly variable water solubility 总被引:1,自引:0,他引:1
Sholto A Lee S Hoffman BM Barrett AG Ehrenberg B 《Photochemistry and photobiology》2008,84(3):764-773
Three novel classes of porphyrazine-like structures were synthesized to form modular structures in which lipophilicity and water solubility can be tuned. Subtle modification of solubility is an important criterion in selecting a compound for biological photosensitization. The general structure takes the form H2[pz(AnB4-n)], where the core is a porphyrazine (pz) group, A is a pyrrole ring with two sulfide linkages (SR moieties) and B is a pyrrole fused with a 4,7-bis(isopropyloxy)benzo group, with n=4, 3 and 2. These molecules possess their longest wavelength absorption band between 700 and 810 nm, hence laser beams of higher tissue penetration depth could be used to illuminate them in photodynamic therapy (PDT). Armed with absorption bands in the far-red and near-infrared (near-IR), and a capability to tune the solubility, these molecules could make for better sensitizers because of optimized uptake by lipidic membranes and better optical properties. We tested several derivatives of the A4, A3B and A2B2 structures for their singlet oxygen quantum yields in methanol and in liposomes, using 9,10-dimethyl anthracene (DMA) as a singlet oxygen target. Singlet oxygen quantum yields in liposomes ranged from 0.01 to 0.44, with the A2B2 group showing the most promise. In the binding assay to find the equilibrium binding constant, Kb, we detected fluorescence changes due to a change in environment. Peripheral long-chain moieties (the R group in the SR moieties) dominate lipid binding. These moieties range in the hydrophobicity that they induce from C8H17 and benzene, which rendered the molecule totally insoluble in water, to polyethylene glycol (PEG) and carboxylate groups, which imparted water solubility. Each molecule had between 4 and 8 such identical chains. Chains bearing an ether or ester link resulted in measurable equilibrium constants, with a higher Kb for ether substituents. Results for Kb ranged from 0.23 to 26.52 (mg mL(-1))(-1). A delicate balance exists between water solubility and good partitioning to membranes. In general, a higher oxygen-to-carbon ratio in the chains improves binding. Fewer chains and a centrally coordinated zinc ion further improve binding and singlet oxygen production. 相似文献
996.
Diffey B 《Photochemistry and photobiology》2008,84(2):371-375
Determining the variability of solar UV exposure of different members of a population by direct measurement demands high compliance over an extended period of time by a large number of people. An alternative approach is to model the variables that affect personal exposure and this is the basis of the method reported here, which uses a random sampling technique to explore variability of exposure at different times of the year by habitués. It is shown that there are large variations in daily personal erythemal exposure, more so for indoor workers living in northern Europe than those resident in Florida, which are due not only to seasonal changes in ambient, but just as importantly to seasonal variation in behavior. Not surprisingly, holiday and summer weekend exposure account for the largest daily UV doses. Northern Europeans who take their summer vacation in Florida can double their exposure during this period compared with holidaying at home and this illustrates just how important sun protection measures should be during recreational exposure in areas of high insolation if the annual UV burden is to be sensibly controlled. 相似文献
997.
998.
Kevin J. Coutinho Ron S. Dickson George A. Koutsantonis Brian W. Skelton Allan H. White Prof. Dr. 《无机化学与普通化学杂志》2008,634(4):669-674
Syntheses of the array of heterobimetallic complexes [(OC)3M(μ‐PPh2)2(μ‐O‐C(CHMe(CH2)2PPh2)RhL], M = Cr, Mo, W, L = tBuNC, are described, extending the previous study of the counterpart array for L = CO. A single crystal X‐ray structure determination is reported for the M = Mo adduct, enabling comparison with its previously reported L = CO counterpart, for which an improved redetermination is also reported. In the present complex the tBuNC ligand is found to be much more weakly bound (Rh‐C 2.026(5) Å) than the carbonyl group it displaces (Rh‐C 1.945(2) Å) with concomitant minor impact on the remainder of the rhodium ambience. 相似文献
999.
Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers: 2. Slip and interfacial water structure 总被引:2,自引:0,他引:2
We discuss the structure of water at hydrophobic interfaces from the standpoint of its impact on electrokinetic phenomena in microfluidic devices fabricated from hydrophobic polymers such as Teflon or Zeonor. Water structuring at hydrophobic interfaces has been described as a source of interfacial charge (see Part 1, this issue), and dewetting phenomena, whether via depletion layers or nanobubbles, contribute to slip and enhanced apparent electrokinetic potentials. Issues concerning the impact of hydrodynamic slip and the role of diffuse interfacial structures are discussed. These issues are coupled with each other and with interfacial charge concerns, providing challenges for measurements of individual parameters. 相似文献
1000.
Crow B Bishop M Kovalcik K Norton D George J Bralley JA 《Biomedical chromatography : BMC》2008,22(4):394-401
A rapid and high-throughput method for the determination of urinary levels of the oxidative stress biomarker, 8-hydroxy-2'-deoxyguanosine (8-OH-dG), has been developed and validated using liquid chromatography combined with electrospray ionization tandem mass spectrometry (LC-MS/MS). The assay features a cheap and readily available non-isotopic internal standard, a single-step filtration sample preparation, and a total analysis time of 6 min including column re-equilibration. The method was validated based on linearity, accuracy (100-106%), precision (CV < 7%), sample preparation stability (< or =5%, 72 h). Intra-laboratory patient ranges were established comparing children and adults (n = 345). 相似文献