全文获取类型
收费全文 | 1021篇 |
免费 | 27篇 |
国内免费 | 10篇 |
专业分类
化学 | 733篇 |
晶体学 | 5篇 |
力学 | 24篇 |
数学 | 103篇 |
物理学 | 193篇 |
出版年
2024年 | 4篇 |
2023年 | 5篇 |
2022年 | 21篇 |
2021年 | 42篇 |
2020年 | 19篇 |
2019年 | 23篇 |
2018年 | 33篇 |
2017年 | 25篇 |
2016年 | 30篇 |
2015年 | 26篇 |
2014年 | 33篇 |
2013年 | 83篇 |
2012年 | 54篇 |
2011年 | 54篇 |
2010年 | 51篇 |
2009年 | 50篇 |
2008年 | 53篇 |
2007年 | 64篇 |
2006年 | 46篇 |
2005年 | 48篇 |
2004年 | 58篇 |
2003年 | 33篇 |
2002年 | 41篇 |
2001年 | 11篇 |
2000年 | 9篇 |
1999年 | 13篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 10篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1962年 | 2篇 |
排序方式: 共有1058条查询结果,搜索用时 15 毫秒
961.
The rates of the acid hydrolysis of potassium ethyl malonate in presence of KCl, NaCl, Li2SO4 and MgSO4 in water and in 50% by weight of dioxane—water mixture were measured. The concentration of the added salts covered the range 0–1M. A negative salt effect on the rate of the half-ester hydrolysis was observed. The applicability ofBrønsted's equation was tested and deviations found were explained as due among other reasons, to the operation of an iondipole type of interaction. The correlation between the hydration numbers as well as the radii of the cations of the added electrolytes and the reaction rate were discussed. The activation energy was found to increase with an increase of the concentration of electrolyte due to the relative changes in solvation of the transition state as a result of hydration of the electrolytes added. The thermodynamic properties of the activated complex were calculated and discussed as functions of the salt concentration. A remarkable linear compensation between H
and S
was found.
Der kinetische Salz-Effekt bei der sauren Hydrolyse von Kaliumethylmalonat in Wasser und in 50% Dioxan—Wasser
Zusammenfassung Es wurde die Kinetik der Hydrolyse von Kaliumethylmalonat in Gegenwart von KCl, NaCl, Li2SO4 und MgSO4 (jeweils 0–1M) in Wasser und in 50% (w/w) Dioxan—Wasser untersucht. Es wurde ein negativer Salz-Effekt bezüglich der Hydrolysegeschwindigkeit des Halbesters beobachtet. Es werden sowohl Korrelationen zwischen Hydratationszahlen bzw. Ionenradien und der Reaktionsgeschwindigkeit, als auch zwischen Aktivierungsenergie und Elektrolytkonzentration diskutiert. Die thermodynamischen Parameter werden als Funktion der Salzkonzentration untersucht.相似文献
962.
Soler M Wernsdorfer W Abboud KA Huffman JC Davidson ER Hendrickson DN Christou G 《Journal of the American Chemical Society》2003,125(12):3576-3588
963.
Khalil MM Ali SA Ramadan RM 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2001,(5):1017-1024
Reaction of Ru3(CO)12, with 2-(2'-pyridyl)benzimidazole (HPBI) resulted in the formation of Ru(CO)3(HPBI) (I) complex. In presence of pyridine or dipyridine, the two derivatives [Ru(CO)3(HPBI)].Py (II) and [Ru(CO)3(HPBI)].dpy (III) were isolated. The corresponding reactions of Os3(CO)12 yielded only one single product; Os(CO)2(HPBI)2 (IV). Spectroscopic studies of these complexes revealed intramolecular metal to ligand CT interactions. Reactions of RuCl3 with HPBI gave three distinct products; [Ru(HPBI)2Cl2]Cl (V), [Ru(HPBI)(dipy)Cl2]C1 (VI) and [Ru(PBI)2(py)2]Cl (VII). The UV-vis studies indicated the presence of intramolecular ligand to metal CT interactions. Electrochemical investigation of the complexes showed some irreversible, reversible and quasi-reversible redox reactions due to tautomeric interconversions through electron transfer. 相似文献
964.
Three dodecanuclear Mn clusters [Mn12O10(OMe)3(OH)(O2CC6H3F2)16(MeOH)2].8MeOH (1), [Mn12O10(OMe)4(O2CBu(t))16(MeOH)2] (2), and [Mn12O12(O2CBu(t))16(MeOH)4] (3) synthesized by reductive aggregation reactions are reported. Clusters 1 and 2 possess a central alkoxide-bridged planar Mn4 topology, whereas 3 is a new high-symmetry member of the normal Mn12 family. Complexes 1 and 2 crystallize in the monoclinic space groups C2/c and P2(1)/n, respectively. Both consist of four Mn(IV) and eight Mn(III) ions held together by 10 mu3-O2- ions, and either (i) one mu-OH- and three mu-MeO- groups for 1 or (ii) four mu-MeO- groups for 2. Complex 3 crystallizes in the orthorhombic space group Aba2 and possesses the normal Mn12 structure but with terminal MeOH molecules. The cyclic voltammogram (CV) of 1 exhibits no reversible redox processes. Variable-temperature, solid-state dc and ac magnetic susceptibility measurements on 1 and 2 reveal that they possess S = 5 and 9 ground states, respectively. In addition, ac susceptibility measurements on complex 1 in a zero dc field in the temperature range 1.8-10 K and in a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display a nonzero frequency-dependent out-of-phase (chi(M)') signal at temperatures below 3 K, with the peak maxima lying at temperatures below 1.8 K. For complex 2, two frequency dependent chi(M)' signals are seen, one in the higher temperature range of 3-5 K and a second at lower temperatures with its peak maxima at temperatures below 1.8 K. Single-crystal magnetization vs dc field scans down to 0.04 K for 1.8MeOH and 2 show hysteresis behavior at <1 K, confirming that both complexes are new examples of SMMs. 相似文献
965.
Sabry Khalil 《Mikrochimica acta》1999,130(3):181-184
Ion-association complexes of Naphazoline HCl (I), Tolazoline HCl (II) and Xylometazoline HCl (III) with [Co(NO2)6]3– and [Fe(CN)6]3– were precipitated and the excess of the unreacted iron or cobalt comples was determined. A new method using atomic emission and atomic absorption spectrometry for the determination of the above drags in pure solutions and in pharmaceutical preparations is given. The drugs have been determined in the ranges 0.98–14.76, 0.78–11.80 and 1.12–16.80 g/ml solutions of I, II and III. respectively, using [Co(NO2)6]3–], with mean relative standard deviations of 0.4–1.5% and 1.92%–19.68, 1.52–5.68 and 2.24–22.4 g/ml solutions of I, II and III, respectively using [Fe(CN)6]3– with mean relative standard deviations of 0.6–1.6%. The recovery values of 98.12–101.26% indicate high precision and accuracy. 相似文献
966.
Mohammad Nikpassand Manouchehr Mamaghani Khalil Tabatabaeian Maryam Kupaei Abiazi 《Molecular diversity》2009,13(3):389-393
A facile and convenient protocol was developed for the regioselective synthesis of 1,5-diarylpyrazoles using Baylis–Hillman
adducts over KSF catalyst in high yields (70–90%) and low reaction times. 相似文献
967.
J.R. Schwesyg H.A. Eggert K. Buse E. Śliwińska S. Khalil M. Kaiser K. Meerholz 《Applied physics. B, Lasers and optics》2007,89(1):15-17
Stable suspensions of iron- or copper-doped lithium niobate nanocrystals in heptane are prepared by high-energy ball milling
of iron- and copper-doped bulk crystals for 25 h. The distribution of particle sizes is determined by means of dynamic light
scattering with more than 90% of the crystals in the range of 10–25 nm. The optical absorption of the suspensions suggests
a changed environment of the dopants.
PACS 77.84.Nh; 78.67.Bf; 81.07.-b 相似文献
968.
Frank Seela Khalil Shaikh Henning Eickmeier 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(7):o489-o491
The title compound, C14H16N4O4, adopts the anti conformation at the glycosylic bond [χ−117.1 (5)°]. The sugar pucker of the 2′‐deoxyribofuranosyl moiety is C2′‐endo–C3′‐exo, 2T3 (S‐type). The orientation of the exocyclic C4′—C5′ bond is +sc (gauche). The propynyl group is linear and coplanar with the nucleobase moiety. The structure of the compound is stabilized by several hydrogen bonds (N—H⋯O and O—H⋯O), leading to the formation of a multi‐layered network. The nucleobases, as well as the propynyl groups, are stacked. This stacking might cause the extraordinary stability of DNA duplexes containing this compound. 相似文献
969.
M. Sebawe Abdalla E. M. Khalil A. S.-F. Obada 《International Journal of Theoretical Physics》2017,56(8):2363-2375
The problem of the codirectional Kerr coupler has been considered several times from different point of view. In the present paper we introduce the interaction between a two-level atom and the codirectional Kerr nonlinear coupler in terms of su\(\left (2\right ) \) Lie algebra. Under certain conditions we have adjusted the Kerr coupler and consequently we have managed to handle the problem. The wave function is obtained by using the evolution operator where the Heisnberg equation of motion is invoked to get the constants of the motion. We note that the Kerr parameter χ as well as the quantum number j plays the role of controlling the atomic inversion behavior. Also the maximum entanglement occurs after a short period of time when χ = 0. On the other hand for the entropy and the variance squeezing we observe that there is exchange between the quadrature variances. Furthermore, the variation in the quantum number j as well as in the parameter χ leads to increase or decrease in the number of fluctuations. Finally we examined the second order correlation function where classical and nonclassical phenomena are observed. 相似文献
970.
Here a simple and sensitive fluorescent assay for detecting Cefixime based on inner filter effect (IFE) has been proven, which is conceptually different from the previously reported CEF fluorescent assays. In this sensing platform, fluorescent carbon dots (CDs) were prepared by one-pot synthesis and was directly used as fluorophore in IFE. The method is based on the complexation reaction between cefixime and palladium ion in the presence of acidic buffer solution (pH 4). The Pd(II)-CEFcomplex was capable of functioning as a powerful absorber in IFE to influence the excitation of fluorophore (CDs). Production Pd(II)-CEFcomplex induced the absorption band transition from 310 to 400 nm, which resulted in the complementary overlap with the excitation spectra of CDs. Due to the competitive absorption, the excitation of CDs was significantly weakened, resulting in the quenching of CDs. The present IFE-based sensing strategy showed a good linear relationship from 0.2 × 10?6 M to 8 × 10?6 M (R2 = 0.987) and provided an exciting detection limit of 0.5 × 10?7 (3δ/slope). The proposed method has been successfully applied for the determination of cefixime in raw milk and human urine samples. 相似文献