全文获取类型
收费全文 | 2262篇 |
免费 | 109篇 |
国内免费 | 2篇 |
专业分类
化学 | 1434篇 |
晶体学 | 1篇 |
力学 | 63篇 |
数学 | 456篇 |
物理学 | 419篇 |
出版年
2023年 | 26篇 |
2022年 | 31篇 |
2021年 | 52篇 |
2020年 | 38篇 |
2019年 | 37篇 |
2018年 | 42篇 |
2017年 | 31篇 |
2016年 | 100篇 |
2015年 | 85篇 |
2014年 | 86篇 |
2013年 | 164篇 |
2012年 | 151篇 |
2011年 | 157篇 |
2010年 | 101篇 |
2009年 | 77篇 |
2008年 | 107篇 |
2007年 | 119篇 |
2006年 | 108篇 |
2005年 | 107篇 |
2004年 | 88篇 |
2003年 | 69篇 |
2002年 | 72篇 |
2001年 | 42篇 |
2000年 | 31篇 |
1999年 | 24篇 |
1998年 | 22篇 |
1997年 | 27篇 |
1996年 | 29篇 |
1995年 | 18篇 |
1994年 | 13篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 19篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 19篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1974年 | 11篇 |
1973年 | 5篇 |
1934年 | 11篇 |
1911年 | 5篇 |
排序方式: 共有2373条查询结果,搜索用时 15 毫秒
991.
Supramolecular reactions between crystalline materials as well as reactions between a crystalline material and a vapour can be used to generate new crystalline substances. These solvent-free processes can be exploited to prepare both hydrogen-bonded co-crystals and coordination networks. Solid-vapour reactions do not differ from solid-vapour uptake/release processes, and can also be used to prepare polymorphs and solvates. It is argued that solvent-less reactions involving molecular crystals represent a green route to supramolecular solid-state chemistry and crystal engineering. 相似文献
992.
A general geometric framework is devised in order to contain the presymplectic and Lagrangian formalisms as particular cases. We call these objectsconstrained dynamical systems, since their dynamics usually lead toconstraints. Their most elementary properties are studied, and several related structures, especially morphisms, are defined. In particular, a stabilization algorithm is performed. As a byproduct, the dynamics and constraints of the Lagrangian formalism (with the second-order condition) are intrinsically obtained. 相似文献
993.
Gobetto R Nervi C Chierotti MR Braga D Maini L Grepioni F Harris RK Hodgkinson P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7461-7471
Fumaric, malonic, maleic, and hydromuconic (HOOCCH2(CH)2CH2COOH) acids were used to prepare a series of hydrogen-bonded adducts or salts, depending on whether acid-base proton transfer takes place, with the dibase [N(muCH2CH2)3N] in various stoichiometric ratios. The resulting compounds have been investigated by using the 1H MAS, 15N, and 13C cross polarisation magic-angle spinning (CPMAS) methods and discussed in relation to X-ray diffraction studies to ascertain the nature of the O-HO, NH-O, and N+-HO- hydrogen bonds between the various species. In addition, two polymorphic forms of the malonic compound and a hydrate in the maleic case were examined. We also present the correlations between the chemical shifts of the hydrogen-bonded protons and those from the proton transfer reaction (acid-to-base) with the heavy atom distances. The dynamic behaviour in the solid-state of the [N(muCH2CH2)3N] adducts with fumaric 2:1, maleic 1:1 hydrate, and hydromuconic acids, and a malonate 2:1 polymorph adduct have been investigated by using variable-temperature 1H spin-lattice relaxation times. A substantial agreement between the activation energies obtained from fitting the T1 data and the results of potential energy barrier calculations demonstrates that the facile reorientation of the [N(muCH2CH2)3N] molecule occurs in several of the adducts. 相似文献
994.
Woflgang Bidell H. William Bosch Dario Veghini Hans-Ulrich Hund Joachim Dring Heinz Berke 《Helvetica chimica acta》1993,76(1):596-606
The tetrameric Cu(β-diketonate) alkoxide complex [Cu(thd)(OCH2CH2OCH3)]4 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate; 1a ) reacts with the alkaline earth metal alkoxides [M(OCH2CH2OCH3)2] (M = Ca, 2a ; M = Sr, 2b ; M = Ba, 2c ) to yield the heteronuclear compounds [Cu2M(thd)3(OCH2CH2OCH3)3] (M = Ca, 6a ; M = Sr, 6b ). These heterometallic complexes were also obtained in the reaction of 1a and the mixed Ca and Sr complexes of β-diketonate-alkoxide [Mx(thd)y(OCH2CH2OCH3)2x?y] (M = Ca, x = 7, y = 6, 3 ; M = Sr, x = 5, y = 3, 4 ), respectively. In comparison, 1a reacts with the analogous [Ba(thd)(OCH2CH2OCH3)] ( 5a ) to yield a[Ba2Cu2(thd)4(OCH3)4(HOCH2CH2OCH3)2] species ( 8a .) The in situ prepared mixed-ligand Ba Compounds [Ba(thd)OR)] (R = CH2CH2OCH2CH2OCH3, ( 5b ); R = CH2CH2CH2OCH3 ( 5c ) react with the corresponding Cu complexes [Cu(thd)(OR)]n (R = CH2CH2OCH2CH2OCH3), n = 4 ( 1b ); R = CH2CH2OCH2CH2OCH3 ( 8b ); R = CH2CH2CH2OCH3 ( 8c ). However, [Cu(hfd)(OCH2CH2OCH3)]4 (hfd = 1,1,1,5,5,5,-hexafluoroacetylacetonate; 1e ) is converted in the presence of 2a–c to the simple metathesis products [M(hfd)2] (M = Ca, Sr, Ba) and [Cu(OCH2CH2OCH3)2]. Crystalline [Ba2Cu2(hfd)2(thd)2(OCH2CH2CH2OCH3)4(HOCH2CH2CH2OCH3)2] ( 9 ) was isolated from the reaction of 1a with in situ prepared [Ba((hfd)OCH2CH2CH2OCH3)] ( 5d ) in 2-, methoxyethanol. X-Ray crystallographic structure determinations are reported for 6a , 6b , 8b , and 8c . 相似文献
995.
The complexes of the type [ReH(CO)5–n(PMe3)n] (n = 4, 3) were reacted with aldehydes, CO2, and RC?CCOOMe (R = H, Me) to establish a phosphine-substitutional effect on the reactivity of the Re–H bond. In the series 1–3 , benzaldehyde showed conversion with only 3 to afford a (benzyloxy)carbonyltetrakis(trimethylphosphine)rhenium complex 4 . Pyridine-2-carbaldehyde allowed reaction with all hydrides 1–3 . With 1 and 2 , the same dicarbonyl[(pyridin-2-yl)methoxy-O, N]bis(trimethylphosphine)rhenium 5b was formed with the intermediacy of a [(pyridin-2-yl)methoxy-O]-ligated species and extrusion of CO or PMe3, respectively. The analogous conversion of 3 afforded the carbonyl[(pyridin-2-yl)methoxy-O,N]tris(trimethylphosphine)rhenium ( 1 ) 7b . While 1 did not react with CO2, 2 and 3 yielded under relatively mild conditions the formato-ligated [Re(HCO2)(CO)(L)(PMe3)3] species ( 8 (L = CO) and 9 (L = PMe3)). Methyl propiolate and methyl butynoate were transformed, in the presence of 1 , to [Re{C(CO2Me)?CHR}(CO)3(PMe3)2] systems ( 10a (R = H), and 10b (R = Me)), with prevailing α-metallation and trans-insertion stereochemistry. Similarly, HC≡CCO2Me afforded with 2 and 3 , the α-metallation products [Re{C(CO2Me)?CH2}(CO)(L)(PMe3)3] 11 (L = CO) and 12 (L = PMe3). The methyl butyonate insertion into 2 resulted in formation of a mixture of the (Z)- and (E)-isomers of [Re{C(CO2Me)?CHMe} (CO)2(PMe3)3] ( 13a , b ). In the case of the conversion of 3 with MeC?CCO2Me, a Re–H cis-addition product [Re{(E)-C(CO2Me)?CHMe}(CO)(PMe3)4] ( 14 ) was selectively obtained. Complex 11 was characterized by an X-ray crystal-structure analysis. 相似文献
996.
Braga D Polito M Bracaccini M D'Addario D Tagliavini E Proserpio DM Grepioni F 《Chemical communications (Cambridge, England)》2002,(10):1080-1081
The bidentate sandwich ligand [Fe(eta 5-C5H(4)-1-C5H4N)2] has been prepared, structurally characterized and employed in the preparation of the novel supramolecular heterobimetallic metalla-macrocycles [Fe(eta 5-C5H(4)-1-C5H4N)2]Ag2(NO3)(2).1.5H2O, [Fe(eta 5-C5H(4)-1-C5H4N)2]Cu2(CH3COO)(4).3H2O and [Fe(eta 5-C5H(4)-1-C5H4N)2]Zn2Cl4. 相似文献
997.
A. Jaumà A. Escuer J.-A. Farrera J. M. Ribó 《Monatshefte für Chemie / Chemical Monthly》1996,127(10):1051-1062
Summary Several Mn, Cu, and Zn chelates of etiobiliverdin-IV-, mesobiliverdin-IX, and its dimethyl ester are studied. The results show that the chemical constitution of the Cu biliverdin chelate corresponds to a formal metal oxidation state of (II) coordinated to a neutral radical of the NH trideprotonated biliverdin. The reactivity of the Cu(II) bilindione chelates in nucleophilic solvents agrees with that expected for a neutral radical structure of the ligand; in CH3OH, they undergo oxidation towards dimethoxybilipurpurins. The magnetic behaviour of Cu(II) etiobiliverdinate-IV- in the solid state shows an intramolecular weak antiferromagnetic coupling d9Cu-to--radical (J=–23 cm–1) and an intermolecular weak antiferromagnetic coupling -radical-to--radical (J=–45 cm–1). The analogy of this magnetic behaviour to that of the cation radical of metalloporphyrins is discussed.
Reaktivität von Pyrrolpigmenten, 21. Mitt.: Struktur und Reaktivität von Cu(II)- und Zn(II)-Chelaten von Bilindionen
Zusammenfassung Einige Mn, Cu and Zn Chelate von Etiobiliverdin-IV-, Mesobiliverdin-IX und seinem Dimethylester werden untersucht. Im Komplex koordiniert ein Metallatom (mit der Formalladung II) mit einem Neutralradikal des dreifach NH-deprotonierten Bilindions. Die Reaktivität des Komplexes gegenüber nukleophilen Lösungmitteln entspricht erwartungsgemäß dem eines neutralen -Radikals; in CH3OH wird er zu Dimethoxybilipurpurin oxidiert. Bei Cu(II)-Etiobiliverdinat-IV beobachtet man im festen Zustand neben schwacher, antiferromagnetischer Kupplung zwischen d9Cu und -Radikal (J=–23 cm–1) auch eine schwache, intermolekulare, antiferromagnetische Wechselwirkung zwischen zwei -Radikalen (J=–45 cm–1). Dieses magnetische Verhalten wird dem des -Kationradikals von Metalloporphyrinen gegenübergestellt.相似文献
998.
Braga D Maini L Polito M Mirolo L Grepioni F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4362-4370
Manual grinding of the organometallic complex [Fe(eta(5)-C(5)H(4)COOH)(2)] with a number of solid bases, namely 1,4-diazabicyclo[2.2.2]octane, C(6)H(12)N(2), 1,4-phenylenediamine, p-(NH(2))(2)C(6)H(4), piperazine, HN(C(2)H(4))(2)NH, trans-1,4-cyclohexanediamine, p-(NH(2))(2)C(6)H(10), and guanidinium carbonate [(NH(2))(3)C](2)[CO(3)], generates quantitatively the corresponding adducts, [HC(6)H(12)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), [HC(6)H(8)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (2), [H(2)C(4)H(10)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)] (3), [H(2)C(6)H(14)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (4.2 H(2)O), and [C(NH(2))(3)](2)[Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (5.2 H(2)O), respectively. Crystallization from methanol in the presence of seeds of the ground sample allows the growth of single crystals of these adducts; therefore we were able to determine the structures of the adducts by single-crystal X-ray diffraction. This information was used in turn to identify and characterize the polycrystalline materials obtained by the grinding process. In the case of [HC(6)N(2)H(12)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), the base can be removed by mild treatment regenerating the starting dicarboxylic acid, while in all other cases decomposition is observed. The solid-solid processes described herein imply molecular diffusion through the lattice, breaking and reassembling of hydrogen-bonded networks, and proton transfer from acid to base. 相似文献
999.
Braga D D'Oria E Grepioni F Mota F Novoa JJ Rovira C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(5):1173-1180
The O-H...O interaction formed by the anions HCO(3)(-) and CO(3)(2-) has been investigated on the basis of data retrieved from the Inorganic Crystal Structure Database (ICSD) and by means of ab initio computations. It has been shown that the O-H...O separations associated with HCO(3)(-)...(3)(2-) interactions are shorter than those found in crystals containing hydrogen carbonate monoanions such as HCO(3)(-)...HCO(3)(-). Ab initio MP2/6-311G++(2d,2p) computations on the crystal Na(3)(HCO(3))(CO(3)).2H(2)O have shown that the interaction between the monoanion donor and the dianion acceptor, for example HCO(3)(-)...CO(3)(2-), is more repulsive than that between singly charged ions, for example HCO(3)(-)...HCO(3)(-), but is largely overcompensated for by anion-cation electrostatic attractions. The shortening of the (-)O-H...O(2-) interaction relative to the (-)O-H...O(-) interaction has been explained as a consequence of the increased charge compression, that is of the stronger cation-anion interactions established by the CO(3)(2-) dianions with respect to those established by monoanions, and does not reflect an increase in the strength of the (-)O-H ...O(-) interaction. To expand the structural sample in the crystal packing analysis, the structure of the novel mixed salt K(2)Na(HCO(3))(CO(3)).2H(2)O has been determined by single-crystal X-ray diffraction and compared with the structure of the salt Na(3)(HCO(3))(CO(3)).2H(2)O used in the computations. 相似文献
1000.
Summary A rapid, sensitive and selective high-performance liquid chromatographic method has been developed for the determination of
sphingosine in human serum. After precipitation with methanol, the samples were extracted using Carbopack B disposable columns;
the sphingosine was eluted with 0.05 M hydrochloric acid in methanol-dichloromethane (20∶80, v/v) and the extract evaporated
to dryness at 40°C. The sample residue was then reconstituted with methanol and reacted with o-phthaldialdehyde reagent to
produce a fluorescent compound. Separation was performed using an LC-18 column with 0.05 M phosphate buffer (pH 7)-methanol-acetonitrile
(15∶80∶5, v/v) as mobile phase. Fluorescence detection was performed with excitation and emission wavelengths of 340 and 455
nm, respectively. The serum extract was re-analyzed with a cyano LC column to minimize the possibility of false positive results.
The possible interference of compounds having a structure similar to that of sphingosine was evaluated. The mean recovery
of sphingosine was >94.5%. The limit of detection of the assay was 1 ng mL−1. The between-run and within-run coefficients of variation for replicate analyses were <4.0% and <3.4%, respectively. The
levels of free sphingosine in the serum of 40 normal subjects (20 male and 20 female) was investigated; the average level
was 81.6±41.1 ng mL−1 (mean ±S.D.) for males and 85.5±33.7 ng mL−1 for females. 相似文献