全文获取类型
收费全文 | 418篇 |
免费 | 6篇 |
国内免费 | 2篇 |
专业分类
化学 | 280篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 31篇 |
物理学 | 103篇 |
出版年
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 2篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2016年 | 12篇 |
2015年 | 8篇 |
2013年 | 17篇 |
2012年 | 23篇 |
2011年 | 24篇 |
2010年 | 17篇 |
2009年 | 11篇 |
2008年 | 21篇 |
2007年 | 15篇 |
2006年 | 17篇 |
2005年 | 17篇 |
2004年 | 18篇 |
2003年 | 5篇 |
2002年 | 8篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 9篇 |
1996年 | 7篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1972年 | 4篇 |
1970年 | 2篇 |
1966年 | 3篇 |
1965年 | 1篇 |
排序方式: 共有426条查询结果,搜索用时 0 毫秒
1.
A one-dimensional bulk reaction model for the oxidation of nickeltitanium is formulated, with preferential oxidation of titaniumbeing included. The modelling is directed at the better understandingof the dominant mechanisms involved in the oxidation processand their significance for the biocompatibility of the alloy.Two different regimes for the relative diffusivities of oxygenand the metals are investigated. By assuming fast bulk reactions,different asymptotic structures emerge in different parameterregimes and the resulting models take the form of moving boundaryproblems. Different profiles of nickel concentration are obtained:in particular a nickel-rich layer (observed in practice) ispresent below the oxide/metal interface for the case when oxygenand the metals diffuse at comparable rates. 相似文献
2.
The use of temporary tethers in facilitating meta photocycloaddition reactions between phenol and allyl alcohol derivatives has been investigated. The merits of silicon, carbonate and methylene acetal tethers were assessed, whilst considering strategies for the preparation of the natural products gymnomitrol and gelsemine. The photoadducts were epoxidised, and then subjected to acid catalysed fragmentation with concomitant cleavage of the tether. Depending on whether water or methanol was used during the fragmentation stage of the methylene tethers, the methylene group was either removed altogether or transformed into a MOM group. 相似文献
3.
The super-macromolecular complex, succinate:quinone oxidoreductase (SQR, Complex II, succinate dehydrogenase) couples the oxidation of succinate in the matrix / cytoplasm to the reduction of quinone in the membrane. This function directly connects the Krebs cycle and the aerobic respiratory chain. Until the recent first report of the structure of SQR from Escherichia coli (E. coli) the structure-function relationships in SQR have been inferred from the structures of the homologous QFR, which catalyses the same reaction in the opposite direction. The structure of SQR from E. coli, analogous to the mitochondrial respiratory Complex II, has provided new insight into SQR's molecular design and mechanism, revealing the electron transport pathway through the enzyme. Comparison of the structures of SQR, QFR and other related flavoproteins shows how common amino acid residues at the interface of two domains facilitate the inter-conversion of succinate and fumarate. Additionally, the structure has provided a possible explanation as to why certain organisms utilise both SQR and QFR despite the fact that both can catalyse the inter-conversion of succinate and fumarate, in vitro and in vivo. Here we review how this structure has advanced our knowledge of this important enzyme and compare the structural information to other members of the Complex II superfamily and related flavoproteins. 相似文献
4.
The carbobenzyloxy (cbz) protecting group is evaluated for it's potential to enhance the resolution of chiral amine enantiomers using high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). A series of cbz derivatives of commercially available racemates was prepared and analyzed by enantioselective chromatography using a variety of mobile phases and polysaccharide and Pirkle-type chiral stationary phases (CSPs). The cbz-derivatized product consistently demonstrated enhanced chiral resolution under HPLC and SFC conditions. Improved selectivity and resolution combined with an automated preparative HPLC or SFC system can lead to the rapid generation of highly purified enantiomers of desirable starting materials, intermediates or final products. 相似文献
5.
6.
The determination of medium and long-lived nuclides can be combined with short-lived ones if a medium or long irradiation is made prior to the short irradiation and radiochemical processing. Thus, an RNAA method previously developed for determination of iodine based on the reaction127I(n,)128I (T
1/2=25 m) using oxygen flask ignition of the irradiated sample, followed by solvent extraction with an iodine-iodide redox cycle, was combined with an overnight preirradiation to induce the235U fission product133I (T
1/2=20.8 h). By reactivating the sample, cooled 1–2 days after the first irradiation, for few minutes both128I and133I could be quantified in the separated iodine fraction. Non-combustible inorganic materials (e.g., sediment, soil, etc.) can be successfully ignited after mixing with excess cellulose powder. Chemical yields for iodine were determined spectrophotometrically in the organic phase, while homogeneously spiked Whatman cellulose powder was used as uranium standard. Mercury is also released on ignition and collected in the absorbing solution, from where it was separated by toluene extraction. Its chemical yield was determined for each aliquot using203Hg tracer and counting on an LEPD. Results for some suitable SRMs are presented, and the general features of the double irradiation technique discussed. 相似文献
7.
Kevin J. Sheehy Lorraine M. Bateman Niko T. Flosbach Martin Breugst Peter A. Byrne 《Chemical science》2020,11(35):9630
The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the 1H–15N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG‡0) and ΔrG° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of ΔrG° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG‡0) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.Marcus theory enables rationalisation and quantification of selectivities in reactions of ambident nucleophiles for which the HSAB principle cannot operate. 相似文献
8.
H. J. Byrne L. Akselrod C. Thomsen A. Mittelbach S. Roth 《Applied Physics A: Materials Science & Processing》1993,57(4):299-302
The resonant Raman scattering of C60 crystallites is monitored as a function of excitation intensity at 514.5 nm. At low intensities, a strong line at 1468 cm–1 is observed. No feature at 1459 cm–1 is observable. With increasing intensities, the 1468 cm–1 line shifts continuously and reversibly to lower frequencies. The mode softening is nonlinearly dependent on the input intensity and is accompanied by a nonlinear increase in the Raman intensity. The spectral changes are discussed in terms of a nonlinear reduction in force constant and increase in bond polarisability as a result of an increased intermolecular delocalisation of the -electron cloud. The nonlinear changes are associated with the nonlinear luminescence and photoconductive response observed in fullerenes and are contrasted to the irreversible phototransformation recently discussed. 相似文献
9.
Determination of Uranium and Thorium in Zircon,Apatite, and Fluorite: Application to Laser (U-Th)/He Thermochronology 总被引:8,自引:0,他引:8
N. J. Evans J. P. Byrne J. T. Keegan L. E. Dotter 《Journal of Analytical Chemistry》2005,60(12):1159-1165
We have developed a methodology for (U-Th)/He thermochronology on a variety of mineral species. With many laboratories initiating research in the area of (U-Th)/He thermochronology, we recognize that there may be interest in a review of analytical procedures for uranium and thorium determination in single crystals of apatite, zircon, rutile, and fluorite. Uranium and thorium are both determined by inductively coupled plasma mass-spectrometry using an isotope dilution method. While standard and spike solutions can be purchased, their isotopic composition and the concentration of the standard solution need to be verified. Digestion procedures for apatite and fluorite are relatively straightforward, but zircon decomposition requires the use of pressure vessels or fusion. Matrix effects are shown to have an insignificant effect on isotope ratios, although isobaric interferences, particularly of PtAr+ on U isotopes, can be a problem. We include complete thermochronology datasets for replicate analysis of Durango apatite, Yucca Mountain fluorite, and an Australian megacryst zircon. 相似文献
10.
A simple modification of the West?? extraction procedure for methylmercury and its determination by gas chromatography (GC) is presented. The cysteine clean-up step has been modified, with use of cysteine-impregnated paper instead of cysteine solution. Methylmercury bromide is extracted from the sample into toluene and is selectively adsorbed on the cysteine paper. Interfering compounds are washed from the paper with toluene. The isolated methylmercury is set free with sulphuric acid containing bromide, extracted into benzene and determined by GC. The modification of the extraction procedure results in good recovery and reproducibility for various biological and environmental samples, good sensitivity with a detection limit of 0.1 ng/g, avoidance of difficulties arising from emulsion formation, cleaner chromatograms, and faster analysis. It is particularly suitable for determination of low levels of MeHg. 相似文献