全文获取类型
收费全文 | 4284篇 |
免费 | 142篇 |
国内免费 | 21篇 |
专业分类
化学 | 3112篇 |
晶体学 | 17篇 |
力学 | 72篇 |
数学 | 564篇 |
物理学 | 682篇 |
出版年
2021年 | 38篇 |
2020年 | 50篇 |
2019年 | 53篇 |
2018年 | 34篇 |
2017年 | 24篇 |
2016年 | 83篇 |
2015年 | 108篇 |
2014年 | 78篇 |
2013年 | 149篇 |
2012年 | 197篇 |
2011年 | 258篇 |
2010年 | 176篇 |
2009年 | 125篇 |
2008年 | 243篇 |
2007年 | 234篇 |
2006年 | 227篇 |
2005年 | 214篇 |
2004年 | 161篇 |
2003年 | 125篇 |
2002年 | 137篇 |
2001年 | 83篇 |
2000年 | 91篇 |
1999年 | 85篇 |
1998年 | 65篇 |
1997年 | 73篇 |
1996年 | 75篇 |
1995年 | 68篇 |
1994年 | 72篇 |
1993年 | 84篇 |
1992年 | 68篇 |
1991年 | 34篇 |
1990年 | 42篇 |
1989年 | 32篇 |
1988年 | 38篇 |
1987年 | 48篇 |
1986年 | 43篇 |
1985年 | 67篇 |
1984年 | 30篇 |
1983年 | 32篇 |
1982年 | 47篇 |
1981年 | 24篇 |
1980年 | 40篇 |
1979年 | 25篇 |
1978年 | 37篇 |
1977年 | 44篇 |
1976年 | 22篇 |
1974年 | 28篇 |
1973年 | 32篇 |
1972年 | 26篇 |
1971年 | 25篇 |
排序方式: 共有4447条查询结果,搜索用时 0 毫秒
91.
Mariana Antunes Vieira Bernhard Welz Adilson Jos Curtius 《Spectrochimica Acta Part B: Atomic Spectroscopy》2002,57(12):251-2067
A simple and reliable method for the determination of arsenic in sediment and in coal without sample digestion, based on hydride generation from slurry samples is proposed. After grinding the samples to a particle size of 50 μm, the sample powder was mixed with aqua regia and hydrofluoric acid in an ultrasonic bath for 30 min. After diluting the mixture with hydrochloric acid, the slurry was allowed to stand for 48 h, and an aliquot was used for hydride generation with sodium borohydride. More than 80% of the arsenic was leached to the aqueous phase under these conditions, except for one sediment sample with very high silica content. The generated arsine was collected in a graphite tube, treated with 0.5 mg of iridium as a permanent modifier, and the arsenic determination was carried out by electrothermal atomic absorption spectrometry. The same tube could be used for at least 160 cycles without any re-treatment. The greatest advantage of the method was that only a minimum of reagents and sample handling were required, reducing the risks of contamination and/or analyte loss. However, the addition calibration technique had to be used in order to obtain results within the 95% confidence level for 11 certified reference materials, 5 sediments, 5 coals and one coal fly ash. One certified sediment slurry was spiked with the analyte and the resulting addition calibration curve was used for the analysis of the certified sediments. Similarly, one certified coal was used to obtain the addition calibration curve for the coal and coal fly ash samples. The recoveries of the certified values, except for one sediment, were between 91 and 115%. The limits of detection in the samples were 0.54 and 0.7 μg g−1 for the coal and sediment samples, respectively, obtained for 1 ml of slurry containing 1 mg of sample. 相似文献
92.
93.
Schlegel HB 《Journal of computational chemistry》2003,24(12):1514-1527
Potential energy surfaces form a central concept in the application of electronic structure methods to the study of molecular structures, properties, and reactivities. Recent advances in tools for exploring potential energy surfaces are surveyed. Methods for geometry optimization of equilibrium structures, searching for transition states, following reaction paths and ab initio molecular dynamics are discussed. For geometry optimization, topics include methods for large molecules, QM/MM calculations, and simultaneous optimization of the wave function and the geometry. Path optimization methods and dynamics based techniques for transition state searching and reaction path following are outlined. Developments in the calculation of ab initio classical trajectories in the Born-Oppenheimer and Car-Parrinello approaches are described. 相似文献
94.
The reaction of 2,6‐pyridinedicarboxylic acid ( 1 , LH2) with CeCl3·7H2O and Sm(NO3)3·6H2O in the presence of triethylamine led to the coordination polymer complexes [M(L)(LH)(H2O)2]·4H2O [M = Ce ( 2 ) and Sm ( 3 )]. Both complexes were characterized by elemental analyses, IR spectroscopy and the crystal structures of 2 and 3 . Crystal data for 2 at ?80 °C: monoclinic, space group P21/c, a = 1404.6(1), b = 1122.1(1), c = 1296.1(1) pm, β = 102.09(1)°, Z = 4, R1 = 0.0217 and for 3 at ?80 °C: monoclinic, space group P21/c, a = 1395.1(1), b = 1120.1(1), c = 1282.8(1) pm, β = 102.71(1)°, Z = 4, R1 = 0.019. 相似文献
95.
Bernhard Schrader 《Angewandte Chemie (International ed. in English)》1973,12(11):884-908
Raman spectorscopy is—like infrared spectroscopy—a method for the study of vibrations of molecules and crystals. The two methods are complementary: if a vibration results in a change of the polarizability of a molecule, it is Raman active; if a change in the molecular dipole moment results, it is infrared active Vibrations of nonpolar groups and totally symmetrical vibrations of molecules are often only Raman active. IR and Raman spectra together give information about the symmetries and structures of molecules and crystals and about the properties of chemical bonds and intermolecular interactions. Until about 10 years ago Raman spectra could only be recorded on relatively large amounts of essentially colorless substances. After the advent of laser light sources the situation changed completely. The amount of sample substance required is now in the region of milli- and micrograms. Gases, liquids and solid samples, especially air-sensitive and reactive substances, single crystals, crystal needles and filaments as well as aqueous solutions can be readily investigated. The identification of molecules and the elucidation of molecular structures, biochemical analysis, and control of evnivornmental pollution are important aplications of Raman spectroscopy. Raman spectroscopy now constitutes an additional powerful tool in instrumental analysis 相似文献
96.
3β,16α-Diacetoxy-20-(5-methyl-2-pyridyl)-pregna-5,20-diene (2) available from 3β-acetoxy-pregna-5,16-dien-20-one (1) was transformed into solafloridine (11) and 25-iso-solafloridine (13). 相似文献
97.
Reductive dehalogenation of dichloride
with magnesium affords the new stable diazacyclooctatetraene (1,5-diazocine)
. There is strong evidence for the intermediate formation of a 2, 6-diaza-4,8 - dicyanosemibullvalene
. 相似文献
98.
Extensive classical trajectory calculations are reported for the system F + HD → HF + D or DF + H, over a wide variety of initial conditions. The results of these computations are used to assess the utility of the information theoretic approach to the prediction of the initial-state dependence of the branching ratio, ΓHF/DF. Two distinct forms for the information theoretic branching ratio are considered, corresponding to two different choices of constraints upon the transition probability matrix. The constraints in question are the “model” constraint employed by Kaplan and Levine, and the “direct” constraint, a generalisation of that introduced by Levine and Kosloff. The two forms of information theory give satisfactory predictions under selected (differing) initial conditions. Neither form is clearly superior. In particular, both fail to predict the observed trend of ΓHF/DF with initial rotational state J, for the case of thermally averaged initial translational energies. This failure is linked to an important “interbranch constraint” not incorporated in the theory, stemming from the different activation energies for formation of the two products. 相似文献
99.
Reaction of the hydroaromatic compounds () and () with lithium-diisopropylamide followed by phenylselenenyl chloride gives the selenides () and () resp. (), which form exclusively the phenols () resp. () after oxidation with 3-chloroperbenzoic acid in the presence of 3,5-dimethoxyaniline (). 相似文献
100.
1,3-Dimethyluracil (1,3-DimeU) reacts with trans-[(CH(3)NH(2))(2)Pt(H(2)O)(2)](+) to give trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(H(2)O)]X (X = NO(3)(-), 1a, ClO(4)(-), 1b) and subsequently with NaCl to give trans-(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)Cl (2) or with NH(3) to yield trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(NH(3))]ClO(4) (3). In a similar way, (dien)Pt(II) forms [dienPt(1,3-DimeU-C5)](+) (4). Reactions leading to formation of 1 and 4 are slow, taking days. In contrast, Hg(CH(3)COO)(2) reacts fast with 1,3-DimeU to give (1,3-DimeU-C5)Hg(CH(3)COO) (5). Both 1-methyluracil (1-MeUH) and uridine (urdH) react with (dien)Pt(II) initially at N(3) and subsequently with either (dien)Pt(II) or Hg(CH(3)COO)(2) also at C(5) to give the diplatinated species 7 and 9 or the mixed PtHg complex 8. C(5) binding of either Pt(II) or Hg(II) is evident from coupling of uracil-H(6) with either (195)Pt or (199)Hg nuclei and (3)J values of 47-74 Hz (for Pt compounds) and 185-197 Hz (for Hg compounds). J values of Pt compounds are influenced both by the ligands trans to the uracil C(5) position and by the number of metal entities bound to a uracil ring. Both 2 and 5 were X-ray structurally characterized. 2: monoclinic system, space group P2(1)/c, a = 15.736(6) ?, b = 11.481(6) ?, c = 25.655 (10) ?, beta = 145.55(3) degrees, V = 2621.9(28) ?(3), Z = 4. 5: monoclinic system, space group P2(1)/c, a = 4.905(2) ?, b = 18.451(6) ?, c = 11.801(5) ?, beta = 94.47(3) degrees, V = 1064.77(72) ?(3), Z = 4. 相似文献