全文获取类型
收费全文 | 2506篇 |
免费 | 116篇 |
国内免费 | 16篇 |
专业分类
化学 | 2000篇 |
晶体学 | 14篇 |
力学 | 39篇 |
综合类 | 16篇 |
数学 | 228篇 |
物理学 | 341篇 |
出版年
2023年 | 21篇 |
2022年 | 14篇 |
2021年 | 40篇 |
2020年 | 46篇 |
2019年 | 64篇 |
2018年 | 45篇 |
2017年 | 29篇 |
2016年 | 58篇 |
2015年 | 60篇 |
2014年 | 61篇 |
2013年 | 135篇 |
2012年 | 168篇 |
2011年 | 164篇 |
2010年 | 83篇 |
2009年 | 105篇 |
2008年 | 194篇 |
2007年 | 160篇 |
2006年 | 159篇 |
2005年 | 162篇 |
2004年 | 141篇 |
2003年 | 110篇 |
2002年 | 112篇 |
2001年 | 45篇 |
2000年 | 38篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 22篇 |
1996年 | 23篇 |
1995年 | 14篇 |
1994年 | 21篇 |
1993年 | 18篇 |
1992年 | 13篇 |
1991年 | 10篇 |
1990年 | 25篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 28篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1975年 | 4篇 |
1974年 | 8篇 |
排序方式: 共有2638条查询结果,搜索用时 281 毫秒
81.
Chiral resolution of native DL-tartaric acid was achieved by ion-pair capillary electrophoresis (CE) using an aqueous-ethanol background electrolyte with (1R,2R)-(-)-1,2-diaminocyclohexane (R-DACH) as a chiral counterion. Factors affecting chiral resolution and migration time of tartaric acid were studied. By increasing the viscosity of the background electrolyte and the ion-pair formation, using organic solvents with a lower relative dielectric constant, resulted in a longer migration time. The optimum conditions for both high resolution and short migration time of tartaric acid were found to be a mixture of 65% v/v ethanol and 35% v/v aqueous solution containing 30 mM R-DACH and 75 mM phosphoric acid (pH 5.1) with an applied voltage of -30 kV at 25 degrees C, using direct detection at 200 nm. By using this system, the resolution (Rs) of racemic tartaric acid was approximately 1. The electrophoretic patterns of tartaric and malic acids suggest that two carboxyl groups and two hydroxyl groups of tartaric acid are associated with the enantioseparation of tartaric acid by the proposed CE method. 相似文献
82.
Motokawa M Kobayashi H Ishizuka N Minakuchi H Nakanishi K Jinnai H Hosoya K Ikegami T Tanaka N 《Journal of chromatography. A》2002,961(1):53-63
Reduction of through-pore size and skeleton size of a monolithic silica column was attempted to provide high separation efficiency in a short time. Monolithic silica columns were prepared to have various sizes of skeletons (approximately 1-2 microm) and through-pores (approximately 2-8 microm) in a fused-silica capillary (50-200 microm I.D.). The columns were evaluated in HPLC after derivatization to C18 phase. It was possible to prepare monolithic silica structures in capillaries of up to 200 microm I.D. from a mixture of tetramethoxysilane and methyltrimethoxysilane. As expected, a monolithic silica column with smaller domain size showed higher column efficiency and higher pressure drop. High external porosity (> 80%) and large through-pores resulted in high permeability (K = 8 x 10(-14) -1.3 x 10(-12) m2) that was 2-30 times higher than that of a column packed with 5-mirom silica particles. The monolithic silica columns prepared in capillaries produced a plate height of about 8-12 microm with an 80% aqueous acetonitrile mobile phase at a linear velocity of 1 mm/s. Separation impedance, E, was found to be as low as 100 under optimum conditions, a value about an order of magnitude lower than reported for conventional columns packed with 5-microm particles. Although a column with smaller domain size generally resulted in higher separation impedance and the lower total performance, the monolithic silica columns showed performance beyond the limit of conventional particle-packed columns under pressure-driven conditions. 相似文献
83.
G. Mairle Liu Ken Pao G. J. Wagner K. T. Knöpfle H. Riedesel 《Zeitschrift für Physik A Hadrons and Nuclei》1981,301(2):157-163
The reactions22Ne(d, t)21Ne and22Ne(d, τ)21F have been investigated in a parallel experiment atE d=52MeV. Energy spectra of tritons andτ particles have been measured up to excitation energies of 18MeV in both21Ne and21F nuclei. Thel values could be determined, and spectroscopic factors have been obtained by a DWBA analysis of the measured angular distributions. Together with the results from a preceding \({}^{22}Ne(\vec d, \tau ){}^{21}F\) experiment, several spins could be determined in21F. From a comparison of the triton andτ particle spectra,T=3/2 states in21Ne could be identified and on this basis the new information on spins of21F states could be transferred to their analog states in21Ne. The completeT < andT > parts of the (2s, 1d) strengths could be observed. The 1p strength is highly fragmented. Of theT < part only 60% of the 1p1/2 and 40% of the lp3/2 strength could be found, but the totalT > part of the 1p strength could be localized. The spectroscopic results for positive parity states are in qualitatively good agreement with shell model predictions. 相似文献
84.
1-Chlorovinyl p-tolyl sulfoxides were synthesized from several kinds of cyclic ketones and chloromethyl p-tolyl sulfoxide in good yields. Treatment of the 1-chlorovinyl p-tolyl sulfoxides with cyanomethyllithium at −78°C to room temperature gave spirocyclic enaminonitriles in high yields. Acidic treatment of the enaminonitriles afforded spiro[4.n]alkenones in good yields. By using an unsymmetrical cyclic ketone, α-tetralone, and optically active chloromethyl p-tolyl sulfoxide, this procedure afforded enantiomerically pure spiro[4.5]decenone in good yield with excellent asymmetric induction from the sulfoxide chiral center. By using this method a formal total synthesis of a racemic spirocyclic sesquiterpene, acorone, was realized. 相似文献
85.
86.
Density functional theory is used to study the hydrogen bonding pattern in cytosine, which does not contain alternating proton donor and acceptor sites and therefore is unique compared with the other pyrimidines. Complexes between various small molecules (HF, H(2)O, and NH(3)) and four main binding sites in (neutral and (N1) anionic) cytosine are considered. Two complexes (O2(N1) and N3(N4)) involve neighboring cytosine proton acceptor and donor sites, which leads to cooperative interactions and bidendate hydrogen bonds. The third (less stable) complex (N4) involves a single cytosine donor. The final (O2-N3) complex involves two cytosine proton acceptors, which leads to an anticooperative hydrogen bonding pattern for H(2)O and NH(3). On the neutral surface, the anticooperative O2-N3 complex is less stable than those involving bidentate hydrogen bonds, and the H(2)O complex cannot be characterized when diffuse functions are included in the (6-31G(d,p)) basis set. On the contrary, the anionic O2-N3 structure is the most stable complex, while the HF and H(2)O N3(N4) complexes cannot be characterized with diffuse functions. B3LYP and MP2 potential energy surface scans are used to consider the relationship between the water N3(N4) and O2-N3 complexes. These calculations reveal that diffuse functions reduce the conversion barrier between the two complexes on both the neutral and anionic surfaces, where the reduction leads to a (O2-N3) energy plateau on the neutral surface and complete (N3(N4)) complex destabilization on the anionic surface. From these complexes, the effects of hydrogen bonds on the (N1) acidity of cytosine are determined, and it is found that the trends in the effects of hydrogen bonds on the (N1) acidity are similar for all pyrimidines. 相似文献
87.
The concentrations of microbial cells in suspensions can be measured with a graphite electrode modifed with adsorbed 4,4′-bipyridine, by using cyclic volummetry or differential pulse volummetry. The relationship between the peak current and the cell concentration for Saccharomyces cerevisiae is linear over the range 0.03 × 108—2.0 × 108 cells ml?1. Yeast, gram-positive bacteria and gram-negative bacteria gave different peak potentials, which may be useful for classification purposes. Electron transfer between the microbial cells and the 4,4′-bipyridine-modified electrode is mediated by coenzyme A in the cell wall. 相似文献
88.
Tadataka Zaima Yoshiharu Matsunaga Keiryo Mitsuhashi 《Journal of heterocyclic chemistry》1983,20(1):1-4
A novel and convenient synthesis of 1H -pyrrole-2,3,4,5-tetracarboxylates is described. Photocyclization of 1,1′-bis(methoxycarbonyl)divinylamine with acetylenedicarboxylates gave 7-azabicyclo[2.2.1]hept-2-ene-1,2,3,4-tetracarboxylates 2a-i , which on melting split ethylene off to yield 1H-pyrrole-2,3,4,5-tetracarboxylates 3a-i quantitatively. 相似文献
89.
[reaction: see text] A copper fluoride-catalyzed cyanomethylation that can be applied to a wide range of ketones and aldehydes was developed using TMSCH(2)CN as a nucleophile. The reaction was extended to a conceptually more advanced copper alkoxide-catalyzed direct addition of alkylnitriles to aldehydes, which can act as a surrogate direct catalytic aldol reaction of esters. These reactions can be applied to the first catalytic enantioselective cyanomethylation of ketones and direct catalytic enantioselective cyanomethylation of aldehydes. 相似文献
90.