全文获取类型
收费全文 | 2144篇 |
免费 | 43篇 |
国内免费 | 8篇 |
专业分类
化学 | 1199篇 |
晶体学 | 4篇 |
力学 | 70篇 |
数学 | 265篇 |
物理学 | 657篇 |
出版年
2021年 | 12篇 |
2019年 | 16篇 |
2018年 | 15篇 |
2017年 | 11篇 |
2016年 | 31篇 |
2015年 | 22篇 |
2014年 | 34篇 |
2013年 | 87篇 |
2012年 | 84篇 |
2011年 | 108篇 |
2010年 | 70篇 |
2009年 | 48篇 |
2008年 | 131篇 |
2007年 | 127篇 |
2006年 | 99篇 |
2005年 | 93篇 |
2004年 | 89篇 |
2003年 | 63篇 |
2002年 | 75篇 |
2001年 | 51篇 |
2000年 | 56篇 |
1999年 | 20篇 |
1998年 | 22篇 |
1997年 | 24篇 |
1996年 | 48篇 |
1995年 | 62篇 |
1994年 | 41篇 |
1993年 | 46篇 |
1992年 | 41篇 |
1991年 | 27篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 24篇 |
1987年 | 27篇 |
1986年 | 17篇 |
1985年 | 20篇 |
1984年 | 24篇 |
1983年 | 20篇 |
1982年 | 21篇 |
1981年 | 32篇 |
1980年 | 21篇 |
1979年 | 23篇 |
1978年 | 24篇 |
1977年 | 23篇 |
1976年 | 29篇 |
1975年 | 25篇 |
1974年 | 17篇 |
1973年 | 21篇 |
1971年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有2195条查询结果,搜索用时 31 毫秒
101.
IR and Raman spectra of MCl2(meso-2,3-diaminobutane), (M = Pd, Pt), have been recorded down to liquid nitrogen temperature or lower. It is shown that correlation coupling occurs between closely spaced hydrogen-bonded pairs of molecules, which form a “super molecule,” but not between all eight molecules in the unit cell. The lattice mode region is also understood in outline on the basis of motions of the “super molecules” Methyl torsional modes appear to be near 140 cm?1. 相似文献
102.
This paper describes the validation of an isocratic HPLC method for the assay of voriconazole in tablets. The method employs
a Merck LiChrospher? 100 RP-8 (125 × 4.6 mm I.D., 5 μm particle size) column, with a mobile phase of methanol : triethylamine solutions 0.6 %,
pH 6.0 (50:50, v/v) and UV detection at 255 nm. A linear response (r > 0.9999) was observed in the range of 20.0–100.0 μg mL−1. The method showed good recoveries (average 100.4%) and the relative standard deviation intra and inter-day were ≤ 1.0 %.
Validation parameters as specificity and robustness were also determined. The method can be used for both quality control
assay of voriconazole in tablets and for stability studies as the method separates voriconazole from its degradation products
and tablet excipients. 相似文献
103.
104.
M. Paneli E. Rosenberg M. Grasserbauer M. Ceulemans F. Adams 《Analytical and bioanalytical chemistry》1997,357(6):756-762
In order to evaluate the effectiveness of recent restrictions of Austrian government for the consumption of leaded gasoline,
ionic organolead species were determined in 13 sampling sites of the Austrian and Slovakian Danube-basin in 4 bimonthyl sampling
campaigns. Speciation analysis was performed using a rapid and sensitive method based on Grignard derivatization of the ionic
organolead species and a GC-MIP-AED coupling for separation and detection. The operational variables were optimized for chromatographic
resolution and detection limits. 100 ml of sample were used for the analyses and the detection limits for trimethyllead were
0.5 ng Pb l–1 and for triethyllead 0.85 ng Pb l–1. In general, absence of organolead induced pollution has been observed for most of the sampling locations and campaigns.
Only the part of the river Danube between Vienna (Austria) and Bratislava (Slovakia), which is loaded by various intensive
anthropogenic sources, showed in two campaigns the occurrence of very low trimethyl- and triethyllead concentrations, ranging
between 1.2–12.0 ng Pb l–1 and 1.6–5.8 ng Pb l–1, respectively.
Received: 21 March 1996 / Revised: 6 May 1996 / Accepted: 9 May 1996 相似文献
105.
The reaction of Pt2Ru4(CO)18, 1 with 1,8-bis(phenylethynyl)naphthalene, 2 has yielded two metal carbonyl cluster complexes: Ru2(CO)6[-
2-C10H6C4Ph2], 3 (60% yield) and Ru2Pt(CO)6[-
2-C10H6C4Ph2]2, 4 (8% yield). Both compounds were characterized by a single crystal X-ray diffraction analysis. Both products were formed as a result of fragmentation of the Pt2Ru4 cluster of 1. Compound 3 contains two ruthenium atoms. They are bridged by a tricyclic C10H6C4Ph2 ligand formed by the coupling of the two -carbon atoms of the alkyne groups. The -carbon atoms of the alkynes are -bonded to one of the ruthenium atoms to form a metallacycle and this entire group is -bonded to the second ruthenium atom. Compound 4 contains two ruthenium atoms with a platinum atom between them. This molecule contains two tricyclic C10H6C4Ph2 ligands similar to that in 3, and two metallacycles formed by coordination of the -carbon atoms of both ligands to the platinum atom. One ligand is -bonded to each of the ruthenium atoms. 相似文献
106.
Protein kinases are important enzymes controlling the majority of cellular signaling events via a transfer of the gamma-phosphate of ATP to a target protein. Even after many years of study, the mechanism of this reaction is still poorly understood. Among many factors that may be responsible for the 1011-fold rate enhancement due to this enzyme, the role of the conserved aspartate (Asp166) has been given special consideration. While the essential presence of Asp166 has been established by mutational studies, its function is still debated. The general base catalyst role assigned to Asp166 on the basis of its position in the active site has been brought into question by the pH dependence of the reaction rate, isotope measurements, and pre-steady-state kinetics. Recent semiempirical calculations have added to the controversy surrounding the role of Asp166 in the catalytic mechanism. No major role for Asp166 has been found in these calculations, which have predicted the reaction process consisting of an early transfer of a substrate proton onto the phosphate group. These conclusions were inconsistent with experimental observations. To address these differences between experimental results and theory with a more reliable computational approach and to provide a theoretical platform for understanding catalysis in this important enzyme family, we have carried out first-principles structural and dynamical calculations of the reaction process in cAPK kinase. To preserve the essential features of the reaction, representations of all of the key conserved residues (82 atoms) were included in the calculation. The structural calculations were performed using the local basis density functional (DFT) approach with both hybrid B3LYP and PBE96 generalized gradient approximations. This kind of calculation has been shown to yield highly accurate structural information for a large number of systems. The optimized reactant state structure is in good agreement with X-ray data. In contrast to semiempirical methods, the lowest energy product state places the substrate proton on Asp166. First-principles molecular dynamics simulations provide additional support for the stability of this product state. The latter also demonstrate that the proton transfer to Asp166 occurs at a point in the reaction where bond cleavage at the PO bridging position is already advanced. This mechanism is further supported by the calculated structure of the transition state in which the substrate hydroxyl group is largely intact. A metaphoshate-like structure is present in the transition state, which is consistent with the X-ray structures of transition state mimics. On the basis of the calculated structure of the transition state, it is estimated to be 85% dissociative. Our analysis also indicates an increase in the hydrogen bond strength between Asp166 and substrate hydroxyl and a small decrease in the bond strength of the latter in the transition state. In summary, our calculations demonstrate the importance of Asp166 in the enzymatic mechanism as a proton acceptor. However, the proton abstraction from the substrate occurs late in the reaction process. Thus, in the catalytic mechanism of cAPK protein kinase, Asp166 plays a role of a "proton trap" that locks the transferred phosphoryl group to the substrate. These results resolve prior inconsistencies between theory and experiment and bring new understanding of the role of Asp166 in the protein kinase catalytic mechanism. 相似文献
107.
J. E. Adams R. M. Stratt 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(Z1):323-325
Just what can be learned about cluster dynamics (and, more generally, about solvation dynamics) from spectral studies of small clusters that have been doped with a chromophore is still an open question. In the present work we suggest a novel procedure for calculating the shift in the electronic absorption spectrum of a chromophore deriving from the attachment to or the incorporation in a cluster. The particular system of interest here is benzene·Ar
n
, for which experimental results are readily available although their interpretation has been a point of controversy. In addition, since the present formalism is equally applicable to a chromophore isolated in a bulk phase (either liquid or solid), we are able to venture an explanation for the apparent observation that the spectral shift of cluster-isolated benzene does not approach the asymptotic values characteristic of the bulk-isolated species. 相似文献
108.
109.
Straub PB Jaffe DE Glass HD Adams MR Brown CN Charpak G Cooper WE Crittenden JA Finley DA Gray R Hemmi Y Hsiung YB Hubbard JR Jonckheere AM Jöstlein H Kaplan DM Lederman LM Luk KB Maki A Mangeot P McCarthy RL Miyake K Plaag RE Rutherfoord JP Sakai Y Santiard JC Sauli F Smith SR Yoshida T Young KK 《Physical review letters》1992,68(4):452-455
110.