全文获取类型
收费全文 | 5708篇 |
免费 | 239篇 |
国内免费 | 43篇 |
专业分类
化学 | 4019篇 |
晶体学 | 35篇 |
力学 | 136篇 |
数学 | 1066篇 |
物理学 | 734篇 |
出版年
2024年 | 10篇 |
2023年 | 44篇 |
2022年 | 132篇 |
2021年 | 162篇 |
2020年 | 105篇 |
2019年 | 128篇 |
2018年 | 106篇 |
2017年 | 82篇 |
2016年 | 192篇 |
2015年 | 174篇 |
2014年 | 203篇 |
2013年 | 345篇 |
2012年 | 397篇 |
2011年 | 442篇 |
2010年 | 285篇 |
2009年 | 288篇 |
2008年 | 355篇 |
2007年 | 371篇 |
2006年 | 341篇 |
2005年 | 329篇 |
2004年 | 241篇 |
2003年 | 178篇 |
2002年 | 181篇 |
2001年 | 76篇 |
2000年 | 60篇 |
1999年 | 55篇 |
1998年 | 42篇 |
1997年 | 46篇 |
1996年 | 71篇 |
1995年 | 40篇 |
1994年 | 38篇 |
1993年 | 37篇 |
1992年 | 45篇 |
1991年 | 33篇 |
1990年 | 20篇 |
1989年 | 28篇 |
1988年 | 22篇 |
1987年 | 21篇 |
1986年 | 25篇 |
1985年 | 35篇 |
1984年 | 26篇 |
1983年 | 22篇 |
1982年 | 29篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1974年 | 7篇 |
排序方式: 共有5990条查询结果,搜索用时 15 毫秒
121.
José Luis Baeza 《Tetrahedron letters》2007,48(21):3689-3693
To investigate the ability of 2-alkyl-2-carboxy-azetidines (Azx) to induce reverse turns when incorporated into peptides, RCO-Azx-l-Ala-NHMe dipeptide derivatives were selected as simplified tetrapeptide models, in which the azetidine residue is incorporated at the i + 1 position. Molecular modelling, 1H NMR and FTIR studies showed the high tendency of the model tetrapeptides to adopt γ-turn conformations, indicating that these azetidine-containing amino acids could serve as general γ-turn promoters. 相似文献
122.
Gámiz-Gracia L Cuadros-Rodrı́guez L Almansa-López E Soto-Chinchilla JJ Garcı́a-Campaña AM 《Talanta》2003,60(2-3):523-534
A new formal strategy in the multidimensional optimisation of the experimental variables affecting the chemiluminescence (CL) detection in flow injection analysis (FIA) is proposed here. The strategy implies several steps, being the most significant: selection of the variables to be studied and their experimental domain; use of a screening design to detect significant variables and interactions into the experimental region; study of the main effect of variables and second-order interactions; and finally application of a Draper-Lin small composite design (orthogonal) to obtain the optimum values of the significant variables. The methodology is applied to the determination of methylamine by FIA based on the use of the peroxyoxalate CL (PO-CL) reaction. Considering the high number of experiments required due to the different chemical and instrumental variables to be taken account and their adequate compatibility to obtain maximum sensitivity, the methodology offers a rigorous study of the main effects and interactions, achieving a reduction of experimental work. 相似文献
123.
Branco LC Serbanovic A da Ponte MN Afonso CA 《Chemical communications (Cambridge, England)》2005,(1):107-109
The combination of ionic liquids (ILs) as solvents in the asymmetric Sharpless dihydroxylation (AD) with the use of scCO(2) in the separation process allows a very simple, efficient, clean and robust system for the reuse of the AD catalytic system, which also does not need the use of organic solvents either for the reaction or for the separation of products and allows the isolation of the diol, in high yield and enantiomeric excess and basically without contamination with osmium. 相似文献
124.
Let Ω be an open subset of , N ? 3, containing 0. We consider the solutions of ?Δu(x) + g(u(x)) = f(x) in Ω-{0}, where g is nondecreasing and f is bounded and we study the possible singularities at 0: when u(x) = o(|x|1 ? N) we prove that u is isotropic near 0 and show that either it is a C1 function in Ω (removable singularity) or |x|N ? 2u(x) → c, c ≠ 0 (weak singularity) or |x|N ? 2 |u(x) |→ + ∞ (strong singularity). We also characterize the g's for which solutions with a weak singularity exist and improve a previous removability result of H. Brézis and L. Véron (Arch. Rational Mech. Anal.23 (1979), 153–166). 相似文献
125.
126.
The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed. 相似文献
127.
We report on a global potential energy hypersurface for the upper sheet of the lowest triplet state of H3+. The analytic representation is based on the double many-body expansion theory. The ab initio data points, calculated with a large cc-pV5Z basis, are represented with a root mean square deviation of only 5.54 cm(-1) in the energy region below the H(+)+2H(2S) dissociation threshold. The quasi-bound vibronic states supported by this surface have also been calculated. 相似文献
128.
In this work we have studied the stabilizing effect of a natural antioxidant, sugar cane bagasse-lignin, on the photo-oxidation of a commercial sample of butadiene rubber. This stabilizer was investigated in various concentrations, in the pure form, and associated with a diamine or a phosphite stabilizer. Stabilization of butadiene rubber with this lignin associated with the diamine produced a material with a photo-chemical stability comparable to that of a stabilized commercial sample. 相似文献
129.
Liu JA Petzold CJ Ramirez-Arizmendi LE Perez J Kenttämaa H 《Journal of the American Chemical Society》2005,127(37):12758-12759
Laser-induced acoustic desorption combined with mass spectrometry has been used to demonstrate that phenyl radicals can attack dinucleoside phosphates at both the sugar and base moieties, that purine bases are more susceptible to the attack than pyrimidine bases, and that the more electrophilic the radical, the more efficient the damage to dinucleoside phosphates. 相似文献
130.
Eulogio Jimenez Luis Romani Maria Inmaculada Paz Andrade Geneviève Roux-Desgranges Jean-Pierre E. Grolier 《Journal of solution chemistry》1986,15(11):879-890
Molar excess volumes V
E
at 25°C have been determined by vibrating-tube densimetry, as a function of mole fraction x for different series of an alkanoate (H
2m+1
C
m
COOC
n
H
2n+1
)+cyclohexane. Three types of alkanoates were investigated, i.e., methanoates (m=0, with n=3 and 4), ethanoates (m=1, with n=2, 3, and 4) and propanoates (m=2, with n=1, 2, and 3). In addition, a Picker flow calorimeter was used to obtain molar excess heat capacities C
p
E
at constant pressure at the same temperature. V
E
is positive for all systems and rather symmetric, with V
E
(x=0.5) amounting to almost identical values in a series of mixtures containing an alkanoate isomer of same formula (say C4H8O2, C5H10O2, or C6H12O2). The composition dependence of C
p
E
is rather unusual in that two more or less marked minima are observed for most of the mixtures, especially when the alkanoate is a methanoate or an ethanoate. These results are discussed in terms of possible changes in conformation of both the ester and cyclohexane. 相似文献