全文获取类型
收费全文 | 2161篇 |
免费 | 41篇 |
国内免费 | 14篇 |
专业分类
化学 | 1231篇 |
晶体学 | 11篇 |
力学 | 47篇 |
数学 | 223篇 |
物理学 | 704篇 |
出版年
2022年 | 15篇 |
2021年 | 13篇 |
2020年 | 13篇 |
2019年 | 28篇 |
2018年 | 18篇 |
2017年 | 13篇 |
2016年 | 35篇 |
2015年 | 27篇 |
2014年 | 40篇 |
2013年 | 87篇 |
2012年 | 100篇 |
2011年 | 129篇 |
2010年 | 54篇 |
2009年 | 39篇 |
2008年 | 97篇 |
2007年 | 106篇 |
2006年 | 118篇 |
2005年 | 79篇 |
2004年 | 83篇 |
2003年 | 67篇 |
2002年 | 77篇 |
2001年 | 53篇 |
2000年 | 64篇 |
1999年 | 34篇 |
1998年 | 18篇 |
1997年 | 36篇 |
1996年 | 40篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 38篇 |
1992年 | 39篇 |
1991年 | 21篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 37篇 |
1987年 | 32篇 |
1986年 | 23篇 |
1985年 | 27篇 |
1984年 | 27篇 |
1983年 | 25篇 |
1982年 | 22篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 25篇 |
1977年 | 23篇 |
1976年 | 29篇 |
1975年 | 26篇 |
1974年 | 23篇 |
1973年 | 30篇 |
排序方式: 共有2216条查询结果,搜索用时 0 毫秒
991.
Abbott LC Batchelor SN Oakes J Gilbert BC Whitwood AC Lindsay Smith JR Moore JN 《The journal of physical chemistry. A》2005,109(12):2894-2905
The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals. 相似文献
992.
Paul A. Liddell John P. Sumida Alisdair N. Macpherson Lori Noss Gilbert R. Seely Kristine N. Clark Ana L. Moore Thomas A. Moore Devens Gust 《Photochemistry and photobiology》1994,60(6):537-541
Abstract Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn -C60 ) and free base (P-C60 ) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1 C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pzn bull; +-C60 bull; -. In toluene, P-1 C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1 C60 underoes photoinduced electron transfer to give P• + -C60 bull; -. The electron transfer rate constant is −2 × 1011 s−1 . 相似文献
993.
McGrath MJ Fletcher MT König WA Moore CJ Cribb BW Allsopp PG Kitching W 《The Journal of organic chemistry》2003,68(10):3739-3748
A suite of allenic hydrocarbons, previously unknown as a molecular class from insects, has been characterized from several Australian melolonthine scarab beetles. The allenes are represented by the formula CH(3)(CH(2))(n)()CH=*=CH(CH(2))(7)CH(3) with n being 11-15, 17 and 19, and thus, all have Delta(9,10)-unsaturation. These structures have been confirmed by syntheses and comparisons of spectral and chromatographic properties with those of the natural components. The enantiomers of (+/-)-Delta(9.10)-tricosadiene and Delta(9,10)-pentacosadiene were separable on a modified beta-cyclodextrin column (gas chromatography), and the natural Delta(9,10)-tricosadiene (n = 11) and Delta(9,10)-pentacosadiene (n = 13) were shown to be of >85% ee. Syntheses of nonracemic allenes of known predominating chirality were acquired using both organotin chemistry and sulfonylhydrazine intermediates, and comparisons then demonstrated that the natural allenes were predominantly (R)-configured. 相似文献
994.
Jayven S. Moore Sotiris S. Xantheas Jay W. Grate Thomas W. Wietsma Enrico Gratton Andreas E. Vasdekis 《Journal of Polymer Science.Polymer Physics》2016,54(1):98-103
We recently demonstrated Solvent Immersion Imprint Lithography (SIIL), a rapid benchtop microsystem prototyping technique, including polymer functionalization, imprinting and bonding. Here, we focus on the realization of planar polymer sensors using SIIL through simple solvent immersion without imprinting. We describe SIIL's impregnation characteristics, including an inherent mechanism that not only achieves practical doping concentrations, but their unexpected 2‐fold enhancement compared to the immersion solution. Subsequently, we developed and characterized optical sensors for detecting molecular O2. To this end, a substantially high dynamic range is reported, including its control through the immersion duration, a manifestation of SIIL's modularity. Overall, SIIL exhibits the potential of improving the operating characteristics of polymer sensors, while significantly accelerating their prototyping, as it requires a few seconds of processing and no need for substrates or dedicated instrumentation. These are critical for O2 sensing as probed by way of example here, as well as any polymer permeable reactant. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 98–103 相似文献
995.
996.
Andrew J. Y. Jones James N. Blaza Hannah R. Bridges Benjamin May Anthony L. Moore Judy Hirst 《Angewandte Chemie (International ed. in English)》2016,55(2):728-731
Complex I is a crucial respiratory enzyme that conserves the energy from NADH oxidation by ubiquinone‐10 (Q10) in proton transport across a membrane. Studies of its energy transduction mechanism are hindered by the extreme hydrophobicity of Q10, and they have so far relied on native membranes with many components or on hydrophilic Q10 analogues that partition into membranes and undergo side reactions. Herein, we present a self‐assembled system without these limitations: proteoliposomes containing mammalian complex I, Q10, and a quinol oxidase (the alternative oxidase, AOX) to recycle Q10H2 to Q10. AOX is present in excess, so complex I is completely rate determining and the Q10 pool is kept oxidized under steady‐state catalysis. The system was used to measure a fully‐defined KM value for Q10. The strategy is suitable for any enzyme with a hydrophobic quinone/quinol substrate, and could be used to characterize hydrophobic inhibitors with potential applications as pharmaceuticals, pesticides, or fungicides. 相似文献
997.
The vibrational relaxation of pure HF(υ = 3 and υ = 4) has been studied by pumping HF directly from υ = 0 to υ = 4. The relaxation rates of υ = 3 and υ = 4 were determined to be k3T = (2.8 ± 0.4) × 10?11 cm3 molecule?1 s?1 and k4T = (7.2. ± 0.5) × 10?11 cm3 molecule?1 s?1 at 293 K. It is shown that sigle quantum energy transfer can account for all the vibrational relaxation. 相似文献
998.
Induction and long-term maintenance of Thy-1 positive T lymphocytes: derivation from continuous bone marrow cultures 总被引:1,自引:0,他引:1
In the present study we investigated the presence of T-lymphocyte progenitors in the long-term murine bone marrow culture system described by Dexter: mature Thy-1 antigen-bearing T lymphocytes are lost in these cultures after a few days. By culturing nonadherent cells from such cultures in the presence of a supernatant of concanavalin A-stimulated spleen cells, a source of T-cell growth factor, we found that Thy-1 positive blast cells proliferated together with a second population of Thy-1 negative cells. These two populations of cells have been maintained in long-term in vitro cultures by passaging the cells in fresh conditioned medium at regular intervals. Moreover, we have been able to establish pure cultures of the Thy-1 bearing blast cells after separating them from the non-T cells using their adherence property to plastic surfaces. Long-term cultures of T lymphocytes can thus be established from long-term marrow cultures as well as from the spleen, thymus or fresh bone marrow. 相似文献
999.
UV-irradiated DNA is less susceptible to restriction by Type II endonucleases than unirradiated DNA presumably due to photolesions formed in the recognition sites. Previous reported studies have used 254 nm radiation or 313 nm plus acetophenone, both treatments which introduce pyrimidine dimers in preference to other photolesions. To assess the effect of a longer wavelength, at which the ratio of pyrimidine dimer formation to the formation of other photolesions is reduced, two different DNAs were irradiated with UV of either 254 or 313 nm and restricted with suitable restriction endonucleases. Restriction patterns were analysed for novel fragments resulting from UV-induced alteration of enzyme recognition sites. EcoRI restriction of 254 nm irradiated lambda DNA produced six novel bands, only three of which were observed following restriction of 313 nm irradiated lambda. These three represented the largest fragments resulting from single site blocks. Novel fragments involving adjacent site blocks observed at 254 nm were not found with 313 nm radiation. Comparison of 254 nm irradiated pSV2 gpt to that irradiated at 313 nm, both restricted with Dral, revealed a more complex pattern. Although all sites were singly blocked by radiation of both wavelengths, multiple site blocks produced by 313 nm radiation did not occur in the order predicted by the 254 nm radiation dose response. These data suggest that certain sites in pSV2 gpt may be more refractory to multiple site blocks than others when irradiated at 313 nm. 相似文献
1000.