全文获取类型
收费全文 | 4470篇 |
免费 | 126篇 |
国内免费 | 17篇 |
专业分类
化学 | 3136篇 |
晶体学 | 45篇 |
力学 | 114篇 |
数学 | 219篇 |
物理学 | 1099篇 |
出版年
2022年 | 25篇 |
2021年 | 46篇 |
2020年 | 38篇 |
2019年 | 50篇 |
2018年 | 33篇 |
2017年 | 34篇 |
2016年 | 81篇 |
2015年 | 75篇 |
2014年 | 89篇 |
2013年 | 200篇 |
2012年 | 232篇 |
2011年 | 265篇 |
2010年 | 129篇 |
2009年 | 125篇 |
2008年 | 228篇 |
2007年 | 211篇 |
2006年 | 227篇 |
2005年 | 212篇 |
2004年 | 209篇 |
2003年 | 192篇 |
2002年 | 198篇 |
2001年 | 136篇 |
2000年 | 112篇 |
1999年 | 75篇 |
1998年 | 56篇 |
1997年 | 59篇 |
1996年 | 60篇 |
1995年 | 49篇 |
1994年 | 60篇 |
1993年 | 59篇 |
1992年 | 65篇 |
1991年 | 41篇 |
1990年 | 45篇 |
1989年 | 52篇 |
1988年 | 43篇 |
1987年 | 45篇 |
1986年 | 43篇 |
1985年 | 86篇 |
1984年 | 81篇 |
1983年 | 27篇 |
1982年 | 39篇 |
1981年 | 38篇 |
1980年 | 41篇 |
1979年 | 67篇 |
1978年 | 48篇 |
1977年 | 54篇 |
1976年 | 49篇 |
1975年 | 32篇 |
1974年 | 26篇 |
1973年 | 18篇 |
排序方式: 共有4613条查询结果,搜索用时 15 毫秒
151.
Lewis acid catalysis in supercritical carbon dioxide (CO(2)) was investigated. While solubility of most organic materials is low in scCO(2), poly(ethylene glycol) derivatives or perfluoroalkylbenzenes were found to work as surfactants to dissolve organic materials in scCO(2). In the presence of these molecules, Lewis acid catalyzed organic reactions such as aldol-, Mannich-, and Friedel-Crafts-type reactions proceeded smoothly in scCO(2). Formation of emulsions was observed in these reactions, and the systems were studied in detail. 相似文献
152.
Bis(substituted-2,3-naphthalocyaninato)europium(III) complexes: bis(octakis(dodecylthio)-2,3-naphthalocyaninato)europium(III) (Eu[2,3-Nc(SC12H25)8]2, 1) and bis(tetra-tert-butyl-2,3- naphthalocyaninato)europium(III) (Eu[2,3-Nc(t-Bu)4]2, 2) have been synthesized by cyclic tetramerization of naphthalonitriles with Eu(acac)3.H2O in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing n-octanol. These compounds were characterized by UV-visible, magnetic circular dichroism (MCD), near-IR, IR, EPR, and mass spectroscopies. The absorption and MCD spectra of 1 showed splitting of the Q band, with peaks at 700 and 784 nm, red shifted from the Q band of 2 at 763 nm. The absorption and MCD spectral band deconvolution calculations of complex 1 gave two A terms in the Q-band region. The A terms are assigned to 2A2-->2E1 transitions. Cyclic voltammograms of 1 and 2 showed reversible oxidation couples at E1/2 = -0.28 V (for 2) and -0.25 V (for 1) vs ferrocenium/ferrocene (Fc+/Fc). The second oxidation exhibited a complicated behavior for both complexes. The reduction couples for 2 were observed at E1/2 = -0.61, -1.64, -1.97, and -2.42 V, and for 1 they were observed at E1/2 = -0.62, -1.60, -1.86, and -2.27 V vs Fc+/Fc. Spectral changes observed on chemical oxidation and reduction of the complexes are presented, and the behaviors of 1 and 2 are compared. 相似文献
153.
Two- or three-component aza Diels-Alder reactions of Danishefsky's diene with imines or aldehydes and amines in water took place smoothly under neutral conditions in the presence of a catalytic amount of an alkaline salt such as sodium triflate to afford dihydro-4-pyridones in high yields. 相似文献
154.
S. Tomaru S. Zembutsu M. Kawachi M. Kobayashi 《Journal of inclusion phenomena and macrocyclic chemistry》1984,2(3-4):885-890
Second harmonic generation (S.H.G.) in inclusion complexes between dimethyl -cyclodextrin (dimethyl -CD) and nitroaniline derivatives occurs. This is due to the destruction of the centrosymmtric crystal structure in nitroaniline derivatives caused by forming inclusion complexes. S.H.G. intensity of dimethyl -CD complex with N-methyl-nitroaniline is 5.5 times as large as that of urea. 相似文献
155.
Substoichiometric isotope dilution analysis has been applied for the determination of trace elements in liquid samples, of
carrier content in radioisotope solution and of concentration of organic reagent. Cu in mineral acids and in ZnSe single crystal
was determined by the substoichiometric extraction with dithizone. The values of 1.8 and 0.018 ppm in nitric acid and distilled
one and of 1.4 and 0.44 ppm in ZnSe were obtained. Cu and P carrier contents in64Cu and32P solutions were determined by the substoichiometric extractions with dithizone in CCl4 and with molybdic acid into MIBK in the series of the solutions adding various amounts of Cu or P carrier. An analogous method
has been applied for the determination of dithizone and diethyldithiocarbamate solutions. The method was also applied for
the determination of60Co radioactivity in environmental samples. The analytical result of water samples is described. 相似文献
156.
This new method overcomes problems of conventional analytical methodologies such as light scattering and sampling reproducibility issues. We used this method for mechanistic studies of catalytic reactions under heterogeneous conditions. Direct-type hydroxymethylation reactions and Mukaiyama-type hydroxymethylation reactions both catalyzed by a scandium–bipyridine ligand complex under micellar conditions were employed as examples of heterogeneous reactions. For direct-type hydroxymethylation reactions, initial reaction rate assays revealed first-order dependency on both substrate and catalyst. On the other hand, Mukaiyama-type hydroxymethylation reactions showed first-order rate dependency on substrate, zero-order on catalyst and saturation kinetics on formaldehyde.A direct and quantitative method for monitoring heterogeneous organic reactions has been developed by using direct analysis in real time mass spectrometry (DART-MS) with an isotope-labeled reaction product as an internal standard. 相似文献
157.
Araguspongines B, C, D, E, F, G, H, and J, new vasodilative bis-1-oxaquinolizidine alkaloids from an okinawan marine sponge, Xestospongia sp 总被引:1,自引:0,他引:1
Nine new vasodilative alkaloids, araguspongines A, B (1), C (2), D (3), E (4), F (5), G (6), H (7), and J (8), were isolated from an Okinawan marine sponge, Xestospongia sp. On the basis of chemical and physicochemical evidence, the absolute stereostructures of araguspongines B, D, E, F, G, H, and J were determined respectively as 1, 3, 4, 5, 6, 7, 8, and the relative stereostructure of araguspongine C was determined as 2 having two 1-oxaquinolizidine moieties. Araguspongines B, D, and E each comprised a pair of the enantiomers, 1a and 1b, 3a and 3b, and 4a and 4b, respectively. 相似文献
158.
A high performance liquid chromatographic (HPLC) method for the quantification of 2-hydroxyfluorene (2-OHF) in normal human urine was established using deuterated 2-hydroxyfluorene (2-OHF-d9) as an internal standard with column-switching and fluorescence detection. The 2-OHF-d9 was synthesized by the metabolism of deuterated fluorene with cytochrome P450. The analytes were cleaned up on an ODS pre-column, via column-switching, and separated on an alkylamide-type reversed phase column. The internal standard eluted immediately prior to non-deuterated 2-OHF on the HPLC system and had nearly the same fluorescence characteristics as the non-deuterated 2-OHF. The detection limit was 0.03 nmol l(-1) (S/N = 3) and the calibration range of urine sample was from 0.2 to 50 nmol l(-1). The urine sample treatment involved enzymatic hydrolysis followed by solid phase extraction using a Sep-Pak C18 cartridge. 2-OHF was observed in the form of conjugates such as glucuronide and/or sulfate in human urine, and urinary metabolites were completely hydrolyzed for 2 h with beta-glucuronidase/aryl sulfatase. The proposed method was used to determine urinary 2-OHF in smokers and non-smokers, and showed that the urinary concentrations of 2-OHF in smokers were significantly higher than those in non-smokers (P < 0.01). Thus, the data suggest that urinary 2-OHF might be a sensitive and specific biological marker for the assessment of the exposure to polycyclic aromatic hydrocarbons. 相似文献
159.
Ihori Y Yamashita Y Ishitani H Kobayashi S 《Journal of the American Chemical Society》2005,127(44):15528-15535
Catalytic enantioselective Mannich-type reactions of silicon enolates with aldimines were investigated using chiral zirconium catalysts prepared from Zr(O(t)Bu)(4), N-methylimidazole, and newly designed multidentate BINOL derivatives. These new multidentate BINOL ligands were designed on the basis of an assumed transition state structure of a chiral zirconium catalyst derived from two molecules of (R)-6,6'-Br(2)-BINOL. Not only tetradentate BINOL 4 but also tridentate BINOL derivatives were found to be effective, and high enantioselectivities were attained. In a structural study of the most effective zirconium complex prepared from tridentate ligand 6e, several NMR experiments and DFT calculations were carried out. Consequently, the structure of an active catalyst and plausible mechanism of asymmetric induction were elucidated. 相似文献
160.
The title reaction was carried out with the help of the transient response method over MgO. It was concluded that no catalyst reduction occurred over this catalyst and the reaction proceeded through the reaction between adsorbed nitrous oxide and adsorbed carbon monoxide without participation of MgO oxygen.
MgO. , - MgO.相似文献