全文获取类型
收费全文 | 2931篇 |
免费 | 112篇 |
国内免费 | 21篇 |
专业分类
化学 | 2114篇 |
晶体学 | 21篇 |
力学 | 61篇 |
数学 | 487篇 |
物理学 | 381篇 |
出版年
2023年 | 26篇 |
2022年 | 62篇 |
2021年 | 101篇 |
2020年 | 65篇 |
2019年 | 60篇 |
2018年 | 61篇 |
2017年 | 49篇 |
2016年 | 107篇 |
2015年 | 95篇 |
2014年 | 110篇 |
2013年 | 180篇 |
2012年 | 185篇 |
2011年 | 254篇 |
2010年 | 156篇 |
2009年 | 127篇 |
2008年 | 184篇 |
2007年 | 204篇 |
2006年 | 150篇 |
2005年 | 146篇 |
2004年 | 130篇 |
2003年 | 94篇 |
2002年 | 89篇 |
2001年 | 34篇 |
2000年 | 29篇 |
1999年 | 25篇 |
1998年 | 16篇 |
1997年 | 22篇 |
1996年 | 24篇 |
1995年 | 21篇 |
1994年 | 25篇 |
1993年 | 23篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 16篇 |
1982年 | 19篇 |
1981年 | 12篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1912年 | 2篇 |
排序方式: 共有3064条查询结果,搜索用时 15 毫秒
61.
Eduardo Corts Corts Cristina A. Corts Romero Olivia García Mellado 《Journal of heterocyclic chemistry》2002,39(6):1321-1324
A series of eleven new 2‐methylthio‐3H‐7‐[(o‐; m‐ and p‐substituted) phenoxy]‐4‐(p‐substituted‐phenyl)‐[1,5]benzodiazepines, which have potentially useful pharmacological activities, has been synthesized by condensing the 4‐[(o‐; m‐ and p‐R1)phenoxy]‐1,2‐phenylendiamines with 3,3‐dimercapto‐1‐(p‐R2‐phenyl)‐2‐propen‐1‐one. Afterward the lH‐[1,5]benzodiazepine‐2‐thiones obtained were treated with sodium hydride and methyl iodide. The structure of all products was corroborated by ir, 1H nmr, 13C nmr and ms. 相似文献
62.
Andrés R Galakhov MV Gómez-Sal MP Martín A Mena M del Carmen Morales-Varela M Santamaría C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(4):805-811
The photochemical treatment of mu(3)-alkylidyne complexes [[TiCp*(mu-O)](3)(mu(3)-CR)] (R=H (1), Me (2), Cp*=eta(5)-C(5)Me(5)) with the amines (2,6-Me(2)C(6)H(3))NH(2), Et(2)NH, and Ph(2)NH and the imine Ph(2)C=NH leads to the partial hydrogenation of the alkylidyne moiety that is supported on the organometallic oxide, [Ti(3)Cp*O(3)], and the formation of new oxoderivatives [[TiCp*(3)(mu-CHR)(R'NR")] (R"=2,6-Me(2)C(6)H(3), R'=H, R=H (3), Me (4); R'=R"=Et, R=H (5), Me (6); R'=R"=Ph, R=H (7), Me (8)) and [[TiCp*(mu-O)](3)(mu-CHR)(N=CPh(2))] (R=H (9), R=Me (10)), respectively. A sequential transfer hydrogenation process occurs when complex 1 is treated with tBuNH(2), which initially gives the mu-methylene [[TiCp*(mu-O)](3)(mu-CH(2))(HNtBu)] (11) complex and finally, the alkyl derivative [[TiCp*(mu-O)](3)(mu-NtBu)Me] (12). Furthermore, irradiation of solutions of the mu(3)-alkylidyne complexes 1 or 2 in the presence of diamines o-C(6)H(4)(NH(2))(2) and H(2)NCH(2)CH(2)NH(2) (en) affords [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(6)H(4)NH)] (13) and [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(2)H(4)NH)] (14) by either methane or ethane elimination, respectively. In the reaction of 1 with en, an intermediate complex [[TiCp*(mu-O)](3)(mu-CH(2))(NHCH(2)CH(2)NH(2))] (15) is detected by (1)H NMR spectroscopy. Thermal treatment of the complexes 4-10 quantitatively regenerates the starting mu(3)-alkylidyne compounds and the amine R'(2)NH or the imine Ph(2)C=NH; however, heating of solutions of 3 or 4 in [D(6)]benzene or a equimolecular mixture of both at 170 degrees C produces methane, ethane, or both, and the complex [[TiCp*(mu-O)](3)[mu(3)-eta(2)-NC(6)H(3)(Me)CH(2)]] (16). The molecular structure of 8 has been established by single-crystal X-ray analysis. 相似文献
63.
Lenice de Loureno Marques Ernesto Schulz Lang Herton Fenner Eduardo Ernesto Castellano 《无机化学与普通化学杂志》2005,631(4):745-748
[Hg(sulfamethoxazolato)2]·2DMSO ( 1 ) and [Cu2(CH3COO)4(sulfa‐methoxazole)2] ( 2 ) can be obtained by the reaction of sulfamethoxazole with mercury acetate or copper acetate in methanol. The structures of the two complexes were characterized by single crystal X—ray diffractometry. Compound 1 consists of sulfamethoxazolato ligands bridging the metal ions building an unidimensional chain. Two solvent dimethylsulfoxide molecules are involved via N‐H···O hydrogen bridges. The mercury atom shows a linear primary coordination arrangement formed by two trans deprotonated sulfonamidic nitrogen atoms. The overall coordination around the metal atom may be regarded as a strongly distorted octahedron when the interactions of mercury with four sulfonamidic oxygen atoms [bond distances of 2.761(4) Å—2.971(4) Å] are also considered to build an equatorial plane and the N1 and N1′ atoms [bond distance of 2.037(5) Å] occupy the apical positions. Compound 2 is a dinuclear complex in which the copper ions are bridged by four syn‐syn acetate ligands which are related by a symmetry centre located in the centre of the complex. Each copper atom presents a nearly octahedral coordination where the equatorial plane is formed by four oxygen atoms and an isoxazolic nitrogen atom and the second copper atom occupy the apical positions. 相似文献
64.
The interactions between PSS-co-BVE copolymers and ionic surfactants (anionic and cationic) in aqueous solution have been investigated using pyrene as a photophysical probe. Static and dynamic fluorescence determinations have been used to obtain information about the microenvironments formed between both species. Micropolarity studies using the I1/I3 ratio of the vibronic bands of pyrene and the behavior of the I(E)/I(M) ratio between the monomer and excimer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and the oppositely charged surfactants lead to the formation of induced premicelles at surfactant concentrations lower than the cmc of the surfactants. This aggregation process is assumed to be due to electrostatic attraction. At the same concentration, the excimer-to-monomer emission ratio shows its first peak. At higher surfactant concentrations, near the cmc, micelles with the same properties as those found in pure aqueous solution are formed. On the other side, systems containing an anionic surfactant do not show this behavior at low concentrations. There is no apparent dependence of the cac on the composition of the polymer, reinforcing the assumption that the electrostatic interactions induce the formation of the premicelles. The values of the cac's follow the same trend as for the cmc's, DTAC>DTAB>CTAC. The polarity of the induced premicelles, as measured by the I1/I3 ratio, also indicates that the microdomains formed by the longer chain surfactants are more hydrophobic than those of the shorter chain surfactants, as also happens with real micelles. 相似文献
65.
We prove some facts concerning surfaces of minimal area bounding regions of prescribed volume in n. The main result we prove is that the mean curvature of such a surface is constant, if possibly a discontinuous function of the enclosed volume. The boundary behaviour of the solutions is also discussed. 相似文献
66.
Marcelo O. Santiago Alzir A. Batista Marcio P. de Araújo Claudio L. Donnici Icaro de S. Moreira Eduardo E. Castellano Javier Ellena Sauli dos Santos Jr Salete L. Queiroz 《Transition Metal Chemistry》2005,30(2):170-175
A series of cis-{RuCl2(PPh3)2[4,4-(X)2-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH2Cl2 solution by 31P{1H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)2-2,2-bipy ligands. 相似文献
67.
Hsu FF Turk J Rhoades ER Russell DG Shi Y Groisman EA 《Journal of the American Society for Mass Spectrometry》2005,16(4):491-504
We report negative-ion electrospray tandem mass spectrometric methods for structural characterization of cardiolipin (CL), a four-acyl-chain phospholipid containing two distinct phosphatidyl moieties, of which structural assignment of the fatty acid residues attached to the glycerol backbones performed by low-energy CAD tandem mass spectrometry has not been previously described. The low-energy MS2-spectra of the [M - H]- and [M - 2H]2- ions obtained with ion-trap or with tandem quadrupole instrument combined with ion-trap MS3-spectra or with source CAD product-ion spectra provide complete structural information for CL characterization. The MS2-spectra of the [M - H]- ions contain two sets of prominent fragment ions that comprise a phosphatidic acid, a dehydrated phosphatidylglycerol, and a (phosphatidic acid + 136) anion. The substantial differences in the abundances of the two distinct phosphatidic anions observed in the MS2-spectra of the [M -H]- ions lead to the assignment of the phosphatidyl moieties attached to the 1' or 3' position of central glycerol. Upon further collisional dissociation, the MS3-spectra of the phosphatidic anions provide information to identify the fatty acyl substituents and their position in the glycerol backbone. The MS2-spectra of the [M - 2H]2- ions obtained with TSQ or ITMS contain complementary information to confirm structural assignment. The applications of the above methods in the differentiation of cardiolipin isomers and in the identification of complex cardiolipin species consisting of multiple molecular structures are also demonstrated. 相似文献
68.
Saez IM Goodby JW Richardson RM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(13):2758-2764
A hexadecamer, first-generation, octasilsesquioxane liquid-crystalline dendrimer was synthesized by a platinum-catalyzed hydrosilylation reaction of the parent first-generation vinyl octasilsesquioxane dendrimer with a modified, laterally substituted mesogen. The structure and purity of the octasilsesquioxane substrate was confirmed by 1H, 13C, and 29 Si NMR spectroscopy, microanalysis, and size exclusion chromatography (SEC). The mesogenic substrate was found to exhibit only a chiral nematic phase, whereas the resulting hexadecamer dendrimer displays enantiotropic chiral nematic, disordered hexagonal columnar, and disordered rectangular columnar phases, with a glass transition below room temperature. The lateral or side-on attachment of the mesogen to the dendritic core was found to be a key design feature in the formation of the mesophases. 相似文献
69.
Danil de Namor AF Chahine S Castellano EE Piro OE 《The journal of physical chemistry. A》2005,109(30):6743-6751
The solvent control on the ability of a partially substituted lower rim calix(4)arene derivative 5,11,17,23,tetra-tert-butyl[25,27-bis(hydroxy)-26,28-bis(ethylthioethoxy)]-calix(4)arene, 1 to host soft metal cations (Hg(II) and Ag(I)) is demonstrated through 1H NMR, electrochemical (conductance measurements), and thermodynamic characterization of the complexation process in a wide variety of solvents. Solvent-ligand interactions were assessed from 1H NMR measurements involving 1 and various solvents in CDCl3. Thus, the formation of a 1:1 1-CH3CN adduct is reported. As far as metal cations are concerned, depending on the medium their complexation with 1 was only observed for Hg(II) and Ag(I). Thus, in acetonitrile, 1 is more selective for Hg(II) relative to Ag(I) by a factor of 2.2 x 10(3). In methanol the selectivity is reversed to an extent that the affinity of 1 for Ag(I) is 1.4 x 10(3) higher than that for Hg(II). However, 1 is unable to recognize selectively these cations in N,N-dimethylformamide while in propylene carbonate the ability of 1 to interact with these cations is lost. An outstanding feature of thermodynamics emerges when an assessment is made of the ligand effect on the complexation of these cations and analogues calix(4)arene derivatives. Thus, in acetonitrile the thermodynamics of cation complexation by the hydrophilic cavity of a calix(4)arene containing mixed pendant groups is built up from thermodynamic data for the same process involving derivatives with common functionalities at the narrow rim. This is a unique example of the additive contribution of pendant arms in the field of thermodynamics of calixarene chemistry. 相似文献
70.
R. Saez PucheM. Norton T. R. White W. S. Glaunsinger 《Journal of solid state chemistry》1983,50(3):281-293
The magnetic susceptibility of the semiconducting lanthanide cuprates Nd2CuO4, Pr2CuO4, Eu2CuO4, and Sm2CuO4 has been measured in the range 4–300 K. Below 300 K, the Cu2+ ions are ordered antiferromagnetically in the CuO2 planes of these compounds, and the exchange interactions involving the Ln3+ ions are relatively weak. The suceptibility of the Ln3+ ions obeys the Curie-Weiss law at elevated temperatures, but deviations from this law occur at lower temperatures. An attempt is made to account for these deviations by fitting theoretical expressions for the susceptibility of isolated Ln3+ ions under the influence of a cubic crystal field to the experimental data. Excellent agreement is obtained for Nd3+ and Eu3+ over the entire temperature range and for Pr3+ and Sm3+ at elevated temperatures. Deviations at lower temperatures for the latter two ions may be due to structural changes, exchange interactions involving the Ln3+ ions, or possibly oxygen nonstoichiometry. The susceptibility parameters derived by fitting the theoretical expressions to the experimental data are also discussed. It is concluded that these compounds form an interesting new series of planar Cu2+-ion antiferromagnets. 相似文献