全文获取类型
收费全文 | 3656篇 |
免费 | 102篇 |
国内免费 | 19篇 |
专业分类
化学 | 2263篇 |
晶体学 | 24篇 |
力学 | 46篇 |
数学 | 394篇 |
物理学 | 475篇 |
无线电 | 575篇 |
出版年
2023年 | 18篇 |
2022年 | 31篇 |
2021年 | 50篇 |
2020年 | 61篇 |
2019年 | 45篇 |
2018年 | 60篇 |
2017年 | 42篇 |
2016年 | 102篇 |
2015年 | 90篇 |
2014年 | 133篇 |
2013年 | 182篇 |
2012年 | 303篇 |
2011年 | 361篇 |
2010年 | 226篇 |
2009年 | 226篇 |
2008年 | 294篇 |
2007年 | 244篇 |
2006年 | 223篇 |
2005年 | 216篇 |
2004年 | 195篇 |
2003年 | 138篇 |
2002年 | 78篇 |
2001年 | 62篇 |
2000年 | 37篇 |
1999年 | 51篇 |
1998年 | 20篇 |
1997年 | 27篇 |
1996年 | 25篇 |
1995年 | 18篇 |
1994年 | 15篇 |
1993年 | 24篇 |
1992年 | 18篇 |
1991年 | 16篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有3777条查询结果,搜索用时 10 毫秒
71.
Francisco Palacios Ana M. Ochoa de Retana José I. Gil José M. Alonso 《Tetrahedron》2004,60(40):8937-8947
A simple and efficient regioselective synthesis of 4-oxazole-phosphine oxides 11 and -phosphonates 12 from 2H-azirine-phosphine oxides 1 and -phosphonates 6 is described. The key step for the synthesis of oxazoles 11 is a base-mediated ring closure of vinylogous α-aminophosphorus compounds derived from phosphine oxides 4 and from phosphonates 8. These derivatives 4 and 8 are obtained by reaction of functionalized azirines 1 and 6 with acyl chlorides 2 and subsequent acid-catalyzed ring opening of N-acylaziridine-phosphine oxides 3 and -phosphonates 7. Regioselective thermal ring cleavage of N-acylaziridine-phosphine oxides 3 leads α-chloro-β-(N-acylamido)-phosphine oxides 13 and their treatment with bases gives 5-oxazole-phosphine oxides 16. 相似文献
72.
Palacios F Aparicio D Ochoa De Retana AM de los Santos JM Gil JI Alonso JM 《The Journal of organic chemistry》2002,67(21):7283-7288
A simple and efficient asymmetric synthesis of 2H-azirine-2-phosphine oxides 3 is described. The key step is a solid-phase bound achiral or chiral amine-mediated Neber reaction of beta-ketoxime tosylates derived from phosphine oxides 1. Reaction of 2H-azirines 3 and 11 with carboxylic acids 4 gives phosphorylated ketamides 5 and 12. Ring closure of ketamides 5 and 12 with triphenylphosphine and hexachloroethane in the presence of triethylamine leads to the formation of phosphorylated oxazoles 8 and 13. 相似文献
73.
[reaction: see text] Tetrasubstituted pyrazines containing two phosphonate groups 2 in positions 2 and 5 and trisubstituted pyrazines containing a phosphonate 5 or a phosphine oxide group 7 in position 2 are obtained by thermal treatment of 2H-azirine-2-phosphonates 1 and -phosphine oxides 6. These pyrazines can also be prepared from beta-ketoxime tosylates 9 and 10 or from oxime derived from phosphine oxide 11. 相似文献
74.
E. Pinilla Gil L. Calvo Blazquez R. M. Garcia-Monco Carra A. Sanchez Misiego 《Fresenius' Journal of Analytical Chemistry》1989,335(8):1002-1004
Summary A differential pulse polarographic method for the determination of oxytetracycline in urine and human serum in acid media (HClO4 of pH 2) is proposed. The effects of the amount of sample taken and the concentration of HClO4 present were investigated. The detection limit was 5.5×10–6 mol/l. The standard deviation of the determination of 5.5×10–5 mol/l of oxytetracycline in 2 ml of urine was 1.7×10–6 mol/l and that of the determination of 5.5×10–5 mol/l of oxytetracycline in 2 ml of human serum was 1.9×10–6 mol/l.
Bestimmung von Oxytetracyclin in Urin und Humanserum durch Differential-Pulspolarography相似文献
75.
An expression for the atom-atom poiarizability between the1s orbitals of geminal or vicinal protons has been derived by perturbation theory using thePople-Santry Theory of electron systems. The coupling constants calculated on this basis agree well with these obtained from full molecular orbital calculations. Expressions have been obtained for the variation of vicinal coupling constants with dihedral angle in ethane and the HCC bond angle and C-C bond length in ethylene, and these agree well with corresponding expressions obtained using the VB method.
Zusammenfassung Es wurde ein Ausdruck für die Atom-Atom-Polarisierbarkeit zwischen den1s Orbitalen geminaler bzw. vicinaler Protonen abgeleitet mittels Störungsrechnung unter Benutzung derPople-Santry-Theorie für-Elektronensysteme. Die auf dieser Basis berechneten Koppelungskonstanten stimmen gut mit denen aus einer vollständigen MO Rechnung überein. Es wurden Ausdrücke erhalten für die Änderung der vicinalen Koppelungskonstanten mit dem Diederwinkel in Äthan sowie dem HOC Bindungswinkel und der C-C Bindungsiänge in Äthylen, die gut mit den entsprechenden Ausdrücken aus der VB Methode übereinstimmen.
Résumé A l'aide de la théorie dePople etSantry sur les systèmes d'électrons, nous dérivons de la théorie des perturbations une expression pour la polarisabiiité atome-atome entre les orbitales1s des protons géminaux ou vicinaux. Les constantes de couplage ici calculées s'accordent bien à celles obtenues des calculs OM complets. Les expressions obtenues pour la variation du couplage vicinal avec l'angle dihédrique dans l'éthane, et l'angle HCC et la distance C-C dans l'éthylène, s'accordent bien aux formules correspondantes de la méthode de la mésomérie.相似文献
76.
The reactions F + H2 → HF + H, HF → H + F, F → F+ + e? and F + e? → F? were used as simple test cases to assess the additivity of basis set effects on reaction energetics computed at the MP4 level. The 6-31G and 6-311G basis sets were augmented with 1, 2, and 3 sets of polarization functions, higher angular momentum polarization functions, and diffuse functions (27 basis sets from 6-31Gd, p) to 6-31 ++ G(3df, 3pd) and likewise for the 6-311G series). For both series substantial nonadditivity was found between diffuse functions on the heavy atom and multiple polarization functions (e.g., 6-31 + G(3d, 3p) vs. 6-31 + G(d, p) and 6-31G(3d, 3p)). For the 6-311G series there is an extra nonadditivity between d functions on hydrogen and multiple polarization functions. Provided that these interactions are taken into account, the remaining basis set effects are additive to within ±0.5 kcal/mol for the reactions considered. Large basis set MP4 calculations can also be estimated to within ±0.5 kcal/mol using MP2 calculations, est. EMP4(6-31 ++ G(3df, 3pd)) ≈ EMP4(6-31G(d, p)) + EMP2(6-31 ++ G(3df, 3pd)) – EMP2(6-31G(d, p)) or EMP4(6-31 + G(d, p) + EMP2(6-31 ++ G(3df, 3pd)) – EMP2(6-31 + G(d, p)) and likewise for the 6-311G series. 相似文献
77.
Ethanol production from steam-explosion pretreated wheat straw 总被引:1,自引:0,他引:1
Ignacio Ballesteros Ma José Negro José Miguel Oliva Araceli Cabañas Paloma Manzanares Mercedes Ballesteros 《Applied biochemistry and biotechnology》2006,130(1-3):496-508
Bioconversion of cereal straw to bioethanol is becoming an attractive alternative to conventional fuel ethanol production
from grains. In this work, the best operational conditions for steam-explosion pretreatment of wheat straw for ethanol production
by a simultaneous saccharification and fermentation process were studied, using diluted acid [H2SO4 0.9% (w/w)] and water as preimpregnation agents. Acid-or water-impregnated biomass was steam-exploded at different temperatures
(160–200°C) and residence times (5, 10, and 20 min). Composition of solid and filtrate obtained after pretreatment, enzymatic
digestibility and ethanol production of pretreated wheat straw at different experimental conditions was analyzed. The best
pretreatment conditions to obtain high conversion yield to ethanol (approx 80% of theoretical) of cellulose-rich residue after
steam-explosion were 190°C and 10 min or 200°C and 5 min, in acid-impregnated straw. However, 180°C for 10 min in acid-impregnated
biomass provided the highest ethanol yield referred to raw material (140 L/t wheat straw), and sugars recovery yield in the
filtrate (300 g/kg wheat straw). 相似文献
78.
Regioselectivity in acylation of oligosaccharides catalyzed by the metalloprotease thermolysin 总被引:2,自引:0,他引:2
Investigation of the acylation scope of carbohydrates by metalloprotease thermolysin immobilized on Celite as biocatalyst has been carried out. The reactions were performed in DMSO, a good solvent for carbohydrates, where the enzyme has previously shown its activity in transesterifications of sucrose, maltose and maltose-containing oligosaccharides. Surprisingly, no reaction was observed for glucose or the glucose-containing disaccharides, trehalose and lactose. In contrast, laurate monoesters of several sucrose-containing tri- and tetrasaccharides were synthetized through a one step transesterification using vinyl laurate as the acylating agent. Enzyme regioselectivity was accurately determined by HPLC/MS and the structure of the main regioisomers was established by a combination of NMR experiments. The preferred position of acylation in all cases was the 2-OH of the α-d-glucopyranose moiety linked 1→2 to the β-d-fructofuranose unit. These results correlate with the regioselectivity observed in the case of the disaccharide sucrose. A general carbohydrate binding motif for catalysis by thermolysin is proposed. 相似文献
79.
Gil Kalai 《Israel Journal of Mathematics》1983,45(4):337-351
Let
(n, k) be the class of all simplicial complexesC over a fixed set ofn vertices (2≦k≦n) such that: (1)C has a complete (k−1)-skeleton, (2)C has precisely (
k
n−1
)k-faces, (3)H
k
(C)=0. We prove that for
,H
k−1(C) is a finite group, and our main result is:
. This formula extends to high dimensions Cayley’s formula for the number of trees onn labelled vertices. Its proof is based on a generalization of the matrix tree theorem. 相似文献
80.
Takehiko Yamato Fumika Kitajima Jeong Tae Gil 《Journal of inclusion phenomena and macrocyclic chemistry》2005,53(3):257-262
The lower rim functionalized hexahomotrioxacalix[3]arene triamide 4 with cone-conformation was synthesized from triol 1 by a stepwise reaction. The different extractability for alkali metal ions, transition metal ions, and alkyl ammonium ions from water into dichloromethane is discussed. Due to the strong intramolecular hydrogen bonding between the neighboring NH and CO groups in triamide 4, its affinity to metal cations was weakened. Triamide 4 shows a single selectivity to n-BuNH3+. The anion complexation of triamide 4 was also studied by 1H NMR titration experiments. Triamide 4 binds halides through the intermolecular hydrogen bonding among the NH hydrogens of amide in a 1:1 fashion in CDCl3. The association constants calculated from these changes in chemical shifts of the amide protons are Ka = 223 M−1 for Cl− and Ka = 71.7 M−1 for Br−. Triamide 4 shows a preference for Cl− complexation than Br− complexation. 相似文献