全文获取类型
收费全文 | 78595篇 |
免费 | 14295篇 |
国内免费 | 5872篇 |
专业分类
化学 | 68937篇 |
晶体学 | 739篇 |
力学 | 3126篇 |
综合类 | 364篇 |
数学 | 7228篇 |
物理学 | 18368篇 |
出版年
2024年 | 132篇 |
2023年 | 914篇 |
2022年 | 1499篇 |
2021年 | 1794篇 |
2020年 | 2802篇 |
2019年 | 3951篇 |
2018年 | 2296篇 |
2017年 | 1919篇 |
2016年 | 5115篇 |
2015年 | 5240篇 |
2014年 | 5679篇 |
2013年 | 6924篇 |
2012年 | 6701篇 |
2011年 | 5900篇 |
2010年 | 5341篇 |
2009年 | 5119篇 |
2008年 | 4875篇 |
2007年 | 3996篇 |
2006年 | 3525篇 |
2005年 | 3307篇 |
2004年 | 2883篇 |
2003年 | 2451篇 |
2002年 | 3073篇 |
2001年 | 2265篇 |
2000年 | 2058篇 |
1999年 | 1357篇 |
1998年 | 859篇 |
1997年 | 762篇 |
1996年 | 829篇 |
1995年 | 695篇 |
1994年 | 648篇 |
1993年 | 560篇 |
1992年 | 522篇 |
1991年 | 438篇 |
1990年 | 371篇 |
1989年 | 286篇 |
1988年 | 246篇 |
1987年 | 228篇 |
1986年 | 174篇 |
1985年 | 185篇 |
1984年 | 146篇 |
1983年 | 122篇 |
1982年 | 87篇 |
1981年 | 67篇 |
1980年 | 56篇 |
1979年 | 38篇 |
1978年 | 31篇 |
1976年 | 31篇 |
1975年 | 34篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ji Duck Kim 《Tetrahedron letters》2005,46(7):1079-1082
Diastereoselective synthesis of 1,2-amino alcohols was achieved from a highly diastereoselective allylic amination reaction of α-hydroxy allyl ethers using chlorosulfonyl isocyanate. Diastereoselectivities varied depending on the stereochemistry of the ethers used and the stability of the carbocation intermediate obtained during the reaction. We propose that this CSI reaction is the results of either a SNi or SN1 mechanism, according to the stability of the carbocation intermediate. 相似文献
992.
A pair of novel heptentriol stereoisomers, hept‐6‐ene‐2,4,5‐triols 2 and 3 , were isolated from the culture broth of the ascomycete Daldinia concentrica (Bolton : Fries ) Cesati & De Notaris , besides three known compounds, i.e., 2,3‐dihydro‐5‐hydroxy‐2‐methyl‐4H‐1‐benzopyran‐4‐one ( 1 ), 3,5‐dihydroxy‐2‐(1‐oxobutyl)‐cyclohex‐2‐en‐1‐one ( 4 ), and pyroglutamic acid (=5‐oxo‐L ‐proline; 5 ). Their structures were determined by spectroscopic means, including 2D‐NMR (HMQC, HMBC, 1H,1H‐COSY). 相似文献
993.
Abdullah A. Al‐Karim Al‐Shara'ey 《中国化学会会志》2004,51(3):547-552
6‐Aryl‐5‐cyano‐4‐pyrimidinone‐2‐thion derivatives 1a‐c reacted with methyl iodide (1:2) to give the corresponding 2‐S,N‐dimethyl pyrimidine‐4‐one derivatives 2a‐c . Compounds 2a‐c were in turn, reacted with hydrazine hydrate to give the sulfur free reaction products 3a‐c . These reaction products were taken as the starting materials for the synthesis of several new heterocyclic derivatives. Reaction of 3a‐c with acetic anhydride and formic acid gave pyrimido triazines 4a‐c and 7a‐c , respectively. Their reactions with active methylene containing reagents gave the corresponding 2‐(1‐pyrazonyl)‐N‐methyl pyrimidine derivatives 9a‐c and 10a‐c , respectively. Their reactions with aromatic aldehydes afforded the corresponding 2‐hydrazono pyrimidine derivatives 11a‐c . The structure of these reactions products were established based on both elemental analysis and spectral data studies. 相似文献
994.
Two new cyclopeptides, named arenariphilin A ( 1 ) and arenariphilin B ( 2 ), were isolated from the whole plants of Arenaria oreophila. Their structures were determined as cyclo‐(Thr‐Gly) ( 1 ) and cyclo‐(Ser1‐Gly ‐Ser2‐Ile ‐Phe1‐Phe2) ( 2 ) on the basis of spectral data, especially by 2D‐NMR. 相似文献
995.
Elisabete Curti Douglas de Britto Srgio Paulo Campana‐Filho 《Macromolecular bioscience》2003,3(10):571-576
N,N,N‐trimethylchitosan (TMC) was prepared by reacting purified chitosan with iodomethane, in the presence of sodium hydroxide, water and sodium iodide, at room temperature. The reaction medium was N‐methyl‐2‐pyrrolidone. Different samples of TMC were obtained by adding to the reaction medium a fixed volume (5.5 mL) of aqueous NaOH solutions at different concentrations (15, 20, 30 and 40 wt.‐%) and carrying out the reactions for 9 or 24 h. The features observed in the 1H and 13C NMR spectra of these chitosan derivatives, in respect of the chemical shift, number and relative intensity of the signals, depended strongly on the excess of NaOH and H3CI added to the reaction medium, but when the lowest excess was employed, the characteristics of the derivative were not affected by the reaction time to the same extent. The average degree of quaternization of these N‐methylated derivatives of chitosan ranged from 10.5% to 44.8%, according to the reaction conditions. Increasing the excess of NaOH, in reactions carried out for 9 h, resulted in TMC samples with progressively higher content of trimethylated sites however, the reaction yields were correspondingly lower and O‐alkylation was favored in these cases.
996.
New microporous particles with large pore size (mean pore diameter of 820 nm) are successfully prepared from a mixture of cellulose and konjac glucomannan (RC-KGM3) in 1.5 M NaOH-0.65 M thiourea aqueous solution by coagulating with 5 weight percentage (wt%) CaCl(2), and then 2 wt% HCl aqueous solution. A preparative size-exclusion chromatographic (SEC) column packed with the gel particles is used for the fractionation of a dextran in water. The exclusion limit and fractionation range of the stationary phase are molecular masses of 125 yen 104 g/mol and 5.6 yen 104 to 125 yen 104 g/mol, respectively. The dextran [dextran 50, weight-average molecular mass (M(w)) = 40.1 yen 104 g/mol, polydispersity index (d) = 3.5] is fractionated by the preparative SEC column to obtain six fractions, and four of them are refractionated twice by the same preparative SEC column. The refractionated samples F-3-3 and F-4-3 are characterized by analytical SEC combined with laser light scattering and light scattering to obtain M(w) of 91.8 and 61.9 yen 104 g/mol, as well as d of 1.3 and 1.4, respectively. The results indicate that the fractions having narrow molecular mass distribution are satisfactorily prepared with the SEC column. The described SEC column can be successfully used to fractionate polymers in aqueous solution. 相似文献
997.
Qing Z Wen-Chen Z Xiao-Xuan W Yang M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,62(4-5):769-772
From the high-order perturbation formulas of EPR parameters (zero-field splitting D, g factors gparallel, gperpendicular and hyperfine structure constants Aparallel, Aperpendicular) based on the two spin-orbit coupling parameter model for 3d3 ions in trigonal symmetry, the EPR parameters of Cr3+ and V2+ ions in HfS2 crystals are calculated. From the calculations, it is found that the local trigonal distortion angle theta of impurity center in HfS2:Cr3+ is smaller than that in HfS2:V2+. The dominant cause of the small zero-field splitting |D| and g-anisotropy |Deltag|=|gparallel-gperpendicular| in HfS2:Cr3+ (compound to HfS2:V2+) is due to the small local trigonal distortion angle theta rather than to the small impurity-ligand distance R in HfS2:Cr3+. 相似文献
998.
An improved method has been developed for the determination of acrylamide in infant powdered milk and baby foods in jars, a particular class of foodstuffs which represent an important source of nutrition for young infants and babies. This method uses isotope dilution liquid chromatography coupled to a tandem mass spectrometer with electrospray ionization and is significantly more sensitive than previous published methods with a limit of quantification estimated at 1 microg kg(-1). The new method offers effective sample preparation procedures including defatting with petroleum ether, extraction with aqueous solution of sodium chloride, further liquid-liquid extraction with ethyl acetate and clean-up by solid-phase extraction (SPE) with HLB 200 mg cartridges. The analytical method was well validated and good results were obtained with respect to repeatability (RSD < 5%) and recovery (86-97%) which fulfilled the requirements defined by European Union (EU) legislation. The acrylamide level in infant powdered milk and baby foods in jars were 3.01-9.06 microg kg(-1) and 6.80-124.93 microg kg(-1), respectively. Especially, this new method is successfully applied to the trace quantification of acrylamide in infant/baby foods, the content of which is less than 10 microg kg(-1). 相似文献
999.
1000.