全文获取类型
收费全文 | 2184篇 |
免费 | 75篇 |
国内免费 | 13篇 |
专业分类
化学 | 1405篇 |
晶体学 | 20篇 |
力学 | 58篇 |
数学 | 274篇 |
物理学 | 515篇 |
出版年
2023年 | 12篇 |
2022年 | 19篇 |
2021年 | 43篇 |
2020年 | 27篇 |
2019年 | 28篇 |
2018年 | 31篇 |
2017年 | 33篇 |
2016年 | 53篇 |
2015年 | 60篇 |
2014年 | 58篇 |
2013年 | 96篇 |
2012年 | 123篇 |
2011年 | 154篇 |
2010年 | 82篇 |
2009年 | 99篇 |
2008年 | 151篇 |
2007年 | 153篇 |
2006年 | 139篇 |
2005年 | 102篇 |
2004年 | 82篇 |
2003年 | 71篇 |
2002年 | 53篇 |
2001年 | 53篇 |
2000年 | 49篇 |
1999年 | 34篇 |
1998年 | 28篇 |
1997年 | 26篇 |
1996年 | 38篇 |
1995年 | 31篇 |
1994年 | 23篇 |
1993年 | 26篇 |
1992年 | 22篇 |
1991年 | 13篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 21篇 |
1984年 | 22篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1934年 | 6篇 |
1930年 | 4篇 |
排序方式: 共有2272条查询结果,搜索用时 31 毫秒
991.
González-del Moral O Martín A Mena M Morales-Varela Mdel C Santamaría C 《Chemical communications (Cambridge, England)》2005,(29):3682-3684
Treatment of the mu3-ethylidyne complex [{TiCp*(mu-O)}3(mu3-CMe)](1), (Cp*=eta5-C5Me5) with alkali metal amides leads to the oxoheterometallocubane derivatives [M(mu3-O)3{(TiCp*)3(mu3-CCH2)}] [M = Li (2), Na (3), K (4), Rb (5), Cs (6)] containing the naked carbanion mu3-CCH2-; the addition of triphenylmethanol and tert-butanol to the compounds 2-6 gives rise to the oxoderivatives [{TiCp*(mu-O)}3(mu-CHMe)(OCR3)][R = Me (7), Ph (8)] which show a mu-ethylidene bridge on the surface model Ti3O3. 相似文献
992.
The reaction of cis-[Pt](CCPh)2 {[Pt]=(bipy)Pt, (bipy′)Pt; BIPY=2,2′-bipyridine, bipy′=4,4′-dimethyl-2,2′-bipyridine} with different copper(I) and silver(I) salts [M′X] (M′=Cu, Ag; X=inorganic or organic ligand) produces alkynyl-bridged (hetero)bi-, tri-, tetra- or pentametallic transition metal complexes. The structural aspects and reaction chemistry of such species and the preference for one coordination mode over another is discussed. The interconversion and mechanistical aspects in the formation of the latter complexes is also reported. 相似文献
993.
Determination of bisphenol A (BPA) in the presence of phenol by first-derivative fluorescence following micro liquid-liquid extraction (MLLE) 总被引:1,自引:0,他引:1
Bisphenol A (BPA) in the presence of phenol is determined using a method based on first-derivative spectrofluorimetry. The proposed method involves a micro liquid–liquid extraction of sodium chloride saturated water samples with diethyl ether followed by direct fluorimetric analysis of extracts. The excitation spectra of both compounds in diethyl ether are recorded between 200 and 290 nm, with the emission wavelength at 306 nm. The first-derivative spectra were calculated, measuring the analytical signal for BPA at 239 nm. The concentration range over which the method was applied was 0.5–10.0 μg·l−1 of BPA with relative standard deviations of 2.9% for a concentration of 4.0 μg·l−1 of BPA. The detection limit was 0.07 μg·l−1. The proposed method was applied satisfactorily to the determination of BPA in synthetic mixtures and water samples from different sources previously spiked with different amounts of these chemicals. Recovery values ranging from 93% to 112% were obtained for water samples. 相似文献
994.
Bouwkamp MW de Wolf J del Hierro Morales I Gercama J Meetsma A Troyanov SI Hessen B Teuben JH 《Journal of the American Chemical Society》2002,124(44):12956-12957
The tetraphenylborate salt of the decamethyl titanocene cation, [Cp*2Ti][BPh4] (1, Cp* = C5Me5), was prepared by reaction of Cp*2TiH with [Cp2Fe][BPh4] and by reaction of Cp*2TiMe with [PhNMe2H][BPh4]. The crystal structure of 1 shows that the Cp*2Ti cation has a bent metallocene structure with agostic interactions with the metal center of two adjacent methyl groups on one of the Cp* ligands. Compound 1 reacts readily with THF to give the adduct [Cp*2Ti(THF)][BPh4] (2). In fluorobenzene, 1 forms the eta1-fluorobenzene adduct [Cp*2Ti(eta1-FC6H5)][BPh4] (3), which was structurally characterized. In contrast to the thermal stability of 3, addition of alpha,alpha,alpha-trifluorotoluene to either 1 or 2 results in C-F activation to give Cp*2TiF2 and PhCF2CF2Ph as the main products. This reactivity toward benzylic C-F bonds is also reflected in the reactivity toward the fluorinated borate anions [B(C6F5)4]- and {B(3,5-(CF3)2C6H3]4}-: reaction of Cp*2TiMe with their [PhNMe2H]+ salts results in a stable complex for the former anion, whereas rapid C-F activation is observed for the latter. 相似文献
995.
The crystal structures of Co3[Co(CN)6]2, 12 H2O (a, = 10.210 ± 0.005 Å) and Cd3[Co(CN)6]2, 12 H2O (a = 10.590 ± 0.005 Å) have been determined by X-ray powder methods. According to the measured density the unit cell contains 1 1/3 formula units with 4 Co2+ (Cd2+) in 4a, 2 2/3 Co3+ in 4b, 16 C and 16 N in 24e, 8 H2OI near 24e, (96k) and 8 H2OII near 8 c (192 l). Structure factor calculations based on the space group Oh5 - F m 3 m lead to the following final values of the reliability index R: 0.038 (Co3[Co(CN)6]2, 12 H2O) and 0.037 (Cd3[Co(CN)6]2, 12 H2O). The interatomic distances for the cobaltous compound (in parentheses for the cadmium compound) are: Co3+-C: 1.88 Å (1.89); C-N: 1.15 Å (1.17); Co2+-N: 2.08 Å (2.24); Co2+-OI: 2.10 Å (2.27); shortest OI-H-OII-bonds: 2.89 Å (2.82). Co3+ is octahedrally coordinated by six carbon atoms, the divalent metal ion by four nitrogen atoms and two water molecules. The two different metal ions are connected by M2+-N-C-Co3-bonds to a threedimensional network. The infrared and electronic spectra are shown to be in agreement with the results of the structure analyses of these compounds. The observed positions of the OH-stretching vibrations lead to a hydrogenbond-length of 2.8–2.95 Å. 相似文献
996.
Effect of bacteria growth temperature on the distribution of supercoiled DNA and its thermal stability 总被引:4,自引:0,他引:4
Adamcík J Víglaský V Valle F Antalík M Podhradský D Dietler G 《Electrophoresis》2002,23(19):3300-3309
Changes in DNA supercoiling might be essential to generate the response of cellular machinery to temperature stress. The heat-induced structural transition for a topoisomer depends on the value of its specific linking difference. We detect only less negatively supercoiled DNA and an abundance of alternative irregular DNA forms at culture temperatures close to the growth limit of Escherichia coli. We show that the irregular forms are derived from regular plasmid DNAs and their population in the cells is temperature-dependent. Here, we show that it is possible to isolate and characterize individual DNA topoisomers directly from cells without a topoisomerase treatment. Temperature gradient gel electrophoresis (TGGE) and atomic force microscopy (AFM) were used to study the effect of bacteria growth temperature on the distribution of supercoiled DNA and its thermal stability. 相似文献
997.
Marrero-Ponce Y Castillo-Garit JA Olazabal E Serrano HS Morales A Castañedo N Ibarra-Velarde F Huesca-Guillen A Jorge E del Valle A Torrens F Castro EA 《Journal of computer-aided molecular design》2004,18(10):615-634
Summary In this work, the TOMOCOMD-CARDD approach has been applied to estimate the anthelmintic activity. Total and local (both atom and atom-type) quadratic indices and linear discriminant analysis were used to obtain a quantitative model that discriminates between anthelmintic and non-anthelmintic drug-like compounds. The obtained model correctly classified 90.37% of compounds in the training set. External validation processes to assess the robustness and predictive power of the obtained model were carried out. The QSAR model correctly classified 88.18% of compounds in this external prediction set. A second model was performed to outline some conclusions about the possible modes of action of anthelmintic drugs. This model permits the correct classification of 94.52% of compounds in the training set, and 80.00% of good global classification in the external prediction set. After that, the developed model was used in virtual in silicoscreening and several compounds from the Merck Index, Negwers handbook and Goodman and Gilman were identified by models as anthelmintic. Finally, the experimental assay of one organic chemical (G-1) by an in vivo test coincides fairly well (100) with model predictions. These results suggest that the proposed method will be a good tool for studying the biological properties of drug candidates during the early state of the drug-development process. 相似文献
998.
Hans U. Güdel 《Chemical physics letters》1975,36(3):328-330
Double excitations to 4Eg and 4A1g states in manganese pairs of KMgF3:Mn2+ have been studied spectroscopically. Three prominent sharp bands are observed in the low temperature absorption spectrum. These bands are displaced by about 400–500 cm?1 to higher energies from the expected electronic origins. It is proposed that the observed pair transitions gain their intensity through a vibronically induced exchange mechanism. With this mechanism the symmetric double excitations 6A1g6A1g → 4Egu4Egu, 4Egv4Egv and 4A1g4A1g become allowed in addition to the transition 6A1g6A1g → 4Egu4A1g. Analogy to the spectrum of the linear dinuclear chromium complex [(NH3)5CrOCr(NH3)5]4+, where the same mechanism has been postulated, is demonstrated. 相似文献
999.
del Mar Conejo M Fernández R Carmona E Andersen RA Gutiérrez-Puebla E Monge MA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4462-4471
The half-sandwich compounds [(eta(5)-C(5)Me(5))BeX] (X=Cl, 1 a; Br, 1 b), readily prepared from the reaction of the halides BeX(2) and M[C(5)Me(5)] (M=Na or K), are useful synthons for other (eta(5)-C(5)Me(5))Be organometallic compounds, including the alkyl derivatives [(eta(5)-C(5)Me(5))BeR] (R=Me, 2 a; CMe(3), 2 b; CH(2)CMe(3), 2 c; CH(2)Ph, 2 d). The latter compounds can be obtained by metathetical exchange of the halides 1 with the corresponding lithium reagent and exhibit NMR signals and other properties in accord with the proposed formulation. Attempts to make [(eta(5)-C(5)Me(5))BeH] have proved fruitless, probably due to instability of the hydride toward disproportionation into [Be(C(5)Me(5))(2)] and BeH(2). The half-sandwich iminoacyl [(eta(5)-C(5)Me(5))Be(C(NXyl)Cp')] and [(eta(5)-C(5)Me(4)H)Be(C(NXyl)Cp')]3, 6 where Xyl=C(6)H(3)-2,6-Me(2) and Cp'=C(5)Me(5) or C(5)Me(4)H, are formed when the beryllocenes [Be(C(5)Me(5))(2)], [Be(C(5)Me(4)H)(2)], and [Be(C(5)Me(5))(C(5)Me(4)H)] are allowed to react with CNXyl. Isolation of three different iminoacyl isomers from the reaction of the mixed-ring beryllocene [(eta(5)-C(5)Me(5))Be(eta(1)-C(5)Me(4)H)] and CNXyl, namely compounds 5 a, 5 b, and 6, provides compelling evidence for the existence in solution of different beryllocene isomers, generated in the course of two very facile processes that explain the solution dynamics of these metallocenes, that is the 1,5-sigmatropic shift of the Be(eta(5)-Cp') unit around the periphery of the eta(1)-Cp' ring, and the molecular inversion rearrangement that exchanges the roles of the two rings. 相似文献
1000.
J. Cardellach C. Estopa J. Font M. Moreno-Mañas R.M. Ortuño F. Sanchez-Ferrando S. Valle L. Vilamajo 《Tetrahedron》1982,38(15):2377-2394
Exhaustive approaches to the synthesis of racemic γ-heterosubstituted γ-methyl-α,β-butenolides are presented, starting mainly from C3 synthons (glyceraldehyde, glycidaldehyde, acrolein and 2,3-epoxypropyl ethers). Good general methods for the preparation of γ-hydroxymethyl-α,β-butenolide 2, several of its ether derivatives, as well as of γ-bromomethyl-α,β-butenolide 5, are given. The reactivities of these structurally simple but highly functionalized compounds, convenient synthons for more complex molecules, are preliminarily explored. 相似文献