全文获取类型
收费全文 | 2386篇 |
免费 | 74篇 |
国内免费 | 16篇 |
专业分类
化学 | 1595篇 |
晶体学 | 16篇 |
力学 | 43篇 |
数学 | 466篇 |
物理学 | 356篇 |
出版年
2022年 | 24篇 |
2021年 | 28篇 |
2020年 | 26篇 |
2019年 | 36篇 |
2018年 | 23篇 |
2017年 | 25篇 |
2016年 | 37篇 |
2015年 | 31篇 |
2014年 | 42篇 |
2013年 | 107篇 |
2012年 | 83篇 |
2011年 | 131篇 |
2010年 | 79篇 |
2009年 | 64篇 |
2008年 | 115篇 |
2007年 | 92篇 |
2006年 | 110篇 |
2005年 | 106篇 |
2004年 | 118篇 |
2003年 | 102篇 |
2002年 | 87篇 |
2001年 | 20篇 |
2000年 | 33篇 |
1999年 | 23篇 |
1997年 | 20篇 |
1996年 | 52篇 |
1994年 | 29篇 |
1993年 | 21篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 43篇 |
1984年 | 34篇 |
1983年 | 28篇 |
1982年 | 32篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 27篇 |
1978年 | 35篇 |
1977年 | 32篇 |
1976年 | 22篇 |
1975年 | 29篇 |
1974年 | 34篇 |
1973年 | 30篇 |
1970年 | 20篇 |
1968年 | 19篇 |
排序方式: 共有2476条查询结果,搜索用时 31 毫秒
991.
992.
Claude Montignoul Marie-Josée Richard Christian Vigne Louis Giral 《Journal of heterocyclic chemistry》1984,21(5):1489-1497
The synthesis of β-imidazolylketones 1 and 2 is described. The compounds 1 are prepared by reaction of Mannich bases with the imidazole. Treatment of the imidazole with haloketones or α-ethylenic ketones or the reaction β-kétol with N,N'-thinoyldiimidazole lead to β-imidazolylketones 2 . 相似文献
993.
J. Ronald Hass Maurice M. Bursey Louis A. Levy Donald J. Harvan 《Journal of mass spectrometry : JMS》1979,14(6):319-325
Unimolecular and collision induced decompositions of the major ions of selected polychlorinated biphenyls in the field free region between the magnetic and electric sectors of a reversed Nier-Johnson instrument were studied. Loss of a single chlorine atom is associated with a wide range of kinetic energy releases but still can be correlated by a single reaction mechanism. Loss of two chlorines is interpreted as a rapid sequential loss from isomerized molecular ions for all but one compound. The decompositions which metastable ions undergo are not always the same as those of high energy ions in the source. Correlations between substituent positions and kinetic energy release can be made for the [M]+·→[M? Cl]+ and [M? Cl2]+· processes. 相似文献
994.
The solvation transfer coefficients which characterize the changes of ion reactivity with phosphoric acid concentration have been calculated for cadmium from the constants of the successive chloride complexes, and for silver and diethyldithiophosphate from potentiometric measurements. They evidence the strong desolvation of the cadmium species in concentrated phosphoric acid media, causing a remarkable increase of its reactivity. They allow the results of liquid-liquid extraction, precipitation and flotation reactions to be correctly interpreted and their changes to be foreseen when the reagents are modified. 相似文献
995.
[reaction: see text] The acid-mediated Prins/pinacol and the triple domino reactions Diels-Alder/Prins/pinacol were used to construct highly functionalized bicyclo[m.n.1]alkanones 19-29 and 33a-c possessing various ring sizes from ketals 8-18 and 31a-c in 44-96% yields. This approach proves to be highly efficient and reliable to generate high molecular complexity in a single step. 相似文献
996.
De-Liang Long Darren Orr Georg Seeber Paul Kögerler Louis J. Farrugia Leroy Cronin 《Journal of Cluster Science》2003,14(3):313-324
Two new metastable polyoxovanadate-based cluster compounds have been isolated and crystallographically characterized with nuclearities of {V16} (1) and {V18} (2). The {V16} cluster represents a new framework type and incorporates two protons into the cluster framework whereas the {V18} framework has been previously characterised, and the oxidation states of the {V18} cluster can be assigned as {VIV
15VV
3}. Compound K10[H2V16O38]13H2O (1) crystallizes in the monoclinic space group P21/c, a=12.12820(10), b=38.2302(3), c=12.35400(10) Å, =115.0470(10)°, V=5189.43(7) Å3, D
c=2.624 gcm–3. 10086 unique reflection and 702 refined parameters were used in structure refinement. R1=0.039, R2=0.109 (all data). From the same preparation the new compound K11[V18O42(SO4)]20H2O (2) was also isolated and crystallographically characterized. Compound 2 crystallizes in the monoclinic space group P21/n, a=12.7854(3), b=20.2812(5), c=13.2386(4) Å, =115.3400(10)°, V=3102.53(14) Å3, D
c=2.650 gcm–3. 7115 unique reflections and 462 refined parameters were used in structure refinement. R1=0.046, R2=0.121 (all data). 相似文献
997.
Zhou Z Steigerwald M Hybertsen M Brus L Friesner RA 《Journal of the American Chemical Society》2004,126(11):3597-3607
All-electron static and time-dependent DFT electronic calculations, with complete geometrical optimization, are performed on tubular molecules up to C(210)H(20) that are finite sections of the (5,5) metallic single wall carbon nanotube with hydrogen termination at the open ends. We find pronounced C-C bond reconstruction at the tube ends; this initiates bond alternation that propagates into the tube centers. For the especially low band gap molecules C(120)H(20), C(150)H(20), and C(180)H(20), alternation increases, and a second nearly isoenergic structural isomer of different alternation is found. A small residual C-C bond alternation and band gap may be present in the infinite tube. The van Hove band gap forms quickly with length, while the metallic Fermi point (at the crossing of linear bands) forms very slowly with length. There are no end-localized states at energies near the Fermi energy. The HOMO-LUMO gap and the lowest singlet excited state, whose energies show a periodicity with length as previously calculated, are optically forbidden. However, each molecule shows an intense visible "charge transfer" transition, not present in the infinite tube, whose energy varies smoothly with length; this transition should be an identifying signature for these molecules. The static axial polarizability per unit length increases rapidly with N as the "charge transfer" transition moves into the infrared; this indicates increasing metallic character. However, the ionization potential, electron affinity, chemical hardness, and relative energetic stability all show the length periodicity seen in the HOMO-LUMO gap, in contrast to the optical "charge transfer" transition and the static axial polarizability. These periodicities, due to a one-dimensional quantum size effect as originally modeled by Coulson in 1938, nevertheless cancel in the calculated Fermi energy, which varies smoothly toward a predicted bulk work function near 3.9 eV. A detailed study of C(190)H(20) with up to eight extra electrons or holes shows the total energy is closely fit by a simple classical charging model, as is commonly applied to metallic clusters. 相似文献
998.
A method is described for the production of screen-printed graphite electrodes and also for similar electrodes chemically modified with the electrocatalyst cobalt phthalocyanine. Using cyclic voltammetry, the electrochemical behaviour of these electrodes towards ascorbic acid, reduced glutatione and coenzyme A (CoA-SH) was investigated. The modified electrodes were found to give significant decreases in the over-potential required for the oxidation of these species at carbon electrodes. The useful electrochemical window for the unmodified carbon film electrodes was ?1.08 V to +0.85 V vs. SCE, using 1 μA background current cut-off points.Amperometry in stirred solutions was used to investigate the hydrodynamic behaviour of the electrodes and their calibration performance. The limits of detection for ascorbic acid and reduced glutathione at the modified films were 5 × 10?8 and 1 × 10?7 M, respectively. The calibration graphs were also linear up to 2 mM concentrations of both analytes. Using differential-pulse voltammetry, linear calibration graphs were obtained for both species up to 2.5 mM. This technique was also used to assess the reproducibility of the electrode manufacture; the coefficient of variation was 2.8% for 1.49 mM ascorbic acid and 6.9% for 0.92 mM reduced glutathione. 相似文献
999.
A concise route to a key intermediate in the total synthesis of guanacastepene A is described. The main features include the simultaneous construction of the seven- and six-membered rings, using a tandem ring-closing metathesis and a stereoselective introduction of the oxygenated function at the C5 position. [reaction: see text] 相似文献
1000.
The formylation of 1-methylfluorene according Rieche yields 1-methyl-2-fluorene-carbaldehyde, the constitution of which being proved by three independent ways. 相似文献