全文获取类型
收费全文 | 2820篇 |
免费 | 102篇 |
国内免费 | 16篇 |
专业分类
化学 | 1945篇 |
晶体学 | 13篇 |
力学 | 76篇 |
数学 | 409篇 |
物理学 | 495篇 |
出版年
2023年 | 18篇 |
2022年 | 49篇 |
2021年 | 65篇 |
2020年 | 49篇 |
2019年 | 41篇 |
2018年 | 44篇 |
2017年 | 37篇 |
2016年 | 99篇 |
2015年 | 81篇 |
2014年 | 80篇 |
2013年 | 163篇 |
2012年 | 167篇 |
2011年 | 205篇 |
2010年 | 108篇 |
2009年 | 116篇 |
2008年 | 178篇 |
2007年 | 176篇 |
2006年 | 178篇 |
2005年 | 160篇 |
2004年 | 151篇 |
2003年 | 117篇 |
2002年 | 120篇 |
2001年 | 38篇 |
2000年 | 38篇 |
1999年 | 32篇 |
1998年 | 21篇 |
1997年 | 19篇 |
1996年 | 38篇 |
1995年 | 16篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 20篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 29篇 |
1984年 | 21篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1980年 | 23篇 |
1979年 | 25篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 6篇 |
排序方式: 共有2938条查询结果,搜索用时 15 毫秒
61.
Claudio Bernardi 《Annali di Matematica Pura ed Applicata》1980,126(1):253-266
Summary This paper is concerned with the Stone space X of a direct product
of infinitely many Boolean algebras. In paragraph 2, after recalling that X is the Stone-ech compactification of the sum (disjoint union)
of the Stone spaces of the algebras Bi, we exhibit a compactification of
which is not a Stone space and we give a method to construct all the «Stone compactifications» of
(the corresponding Boolean algebras are easily characterised). In paragraph 3, a set of ultrafilters of B (the «decomposable» ultrafilters) are introduced: this set properly contains
, but, as is shown in paragraph 5, there are direct products that admit nondeeomposable ultrafilters (this is the case iff the set {Card Bi: i I } is not bounded by a natural number). In paragraph 4, among other things, we prove, for the set of decomposable ultrafilters, a weak form of countable compactness, in the sense that every countable clopen cover has a finite subcover; then, we deduce that the set of decomposable ultrafilters is pseudocompact, while obviously
is not. Lastly, in paragraph 6, we give a second characterisation of the Stone space of B, showing that every ultrafilter of B can be obtained by iterating in a suitable way the procedure which leads to the construction of decomposable ultrafilters.
Lavoro eseguito nell'ambito dell'attività del Comitato Nazionale per le Scienze Matematiche del C.N.R. 相似文献
Lavoro eseguito nell'ambito dell'attività del Comitato Nazionale per le Scienze Matematiche del C.N.R. 相似文献
62.
Hélio A. Stefani Claudio M.P. Pereira Roberta B. Almeida Karla P. Guzen 《Tetrahedron letters》2005,46(40):6833-6837
The 4-halo-3,5-dimethyl pyrazoles have been synthetisized in good yields in short reaction times in the absence of a catalyst by reaction of 3,5-dimethyl pyrazoles with N-halosuccinimides (NBS, NCS and NIS) under ultrasound irradiation. Finally, the halogenation of pyrazoles with Br2, ICl and I2 was showed in similar conditions. 相似文献
63.
Marcelo O. Santiago Alzir A. Batista Marcio P. de Araújo Claudio L. Donnici Icaro de S. Moreira Eduardo E. Castellano Javier Ellena Sauli dos Santos Jr Salete L. Queiroz 《Transition Metal Chemistry》2005,30(2):170-175
A series of cis-{RuCl2(PPh3)2[4,4-(X)2-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH2Cl2 solution by 31P{1H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)2-2,2-bipy ligands. 相似文献
64.
Elena Khomko Leonid Mashlyakovskiy Claudio Tonelli 《Journal of polymer science. Part A, Polymer chemistry》2004,42(21):5354-5371
The effect of catalyst dibutyltin dilaurate (DBTDL) on the kinetics of urethane formation reactions of α,ω‐bis(hydroxy)‐terminated fluoropolyethers Fomblin® Z‐DOL TXs (FPEs) of various molecular weights and poly(oxyethylene) glycol PEG‐400 with isophorone diisocyanate (IPDI) in hexafluoroxylene (HFX) and tetrahydrofuran (THF) at 40 °C and NCO:OH = 2:1 have been studied in a broad range of catalyst (0.10–9.00) ×10?4 M and total reagents (10.0–60.1 wt %) concentrations. The rate of tin‐catalyzed second‐order reactions (with respect to diol and diisocyanate) was found to be proportional to the square root of catalyst concentration [DBTDL]0.5 both in low polar (HFX) and polar (THF) solvents. Effect of catalyst saturation was revealed for all the reaction systems at higher DBTDL concentrations as well as the appearance of the limiting catalyst concentrations Clim below which the rates of reaction were close to zero. Based on these findings new effective rate coefficients have been derived k = kcat/(C ? C) that are independent of the total reagent concentration in the range of 10.0–60.1 wt % ([OH] = 0.10–0.91 equiv/L). This new approach highlights that the rate of the tin‐catalyzed urethane formation reactions of α,ω‐bis(hydroxy)‐terminated fluoropolyethers Z‐DOL TXs with IPDI in HFX at 40 °C and NCO:OH = 2:1 increases significantly with increasing MW of FPE from 776 up to 3405. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5354–5371, 2004 相似文献
65.
Alpha-amino acids are soluble in acetonitrile when treated with phosphazene bases. As a result, the protection/deprotection events that are usually required for peptide coupling reactions can be minimized. This is illustrated in the synthesis of the important angiotensin-converting enzyme (ACE) inhibitor enalapril. [reaction: see text] 相似文献
66.
Dalvit C Ardini E Flocco M Fogliatto GP Mongelli N Veronesi M 《Journal of the American Chemical Society》2003,125(47):14620-14625
High-throughput screening is usually the method of drug-lead discovery. It is now well accepted that, for a functional assay, quality is more important than quantity. The ligand-based or protein-based NMR screening methodologies for detecting compounds binding to the macromolecular target of interest are now well established. A novel and sensitive NMR method for rapid, efficient, and reliable biochemical screening is presented. The method named 3-FABS (three fluorine atoms for biochemical screening) requires the labeling of the substrate with a CF(3) moiety and utilizes (19)F NMR spectroscopy for the detection of the starting and enzymatically modified substrates. The method allows for high-quality screening of large compound or natural product extract collections and for measuring their IC(50) values. Applications of this technique to the screening of inhibitors of the Ser/Thr kinase AKT1 and the protease trypsin are presented. In addition, an interesting application of 3-FABS to functional genomics is also presented. 相似文献
67.
G. Giraudi Claudio Baggiani Antonella Cosmaro Emilio Santià Adriano Vanni 《Fresenius' Journal of Analytical Chemistry》1998,360(2):235-240
A very selective polyclonal antiserum against 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (p,p′-DDT) was obtained by a careful choice of the haptenic structure (2,2-bis(4-chlorophenyl)-ethanol hemisuccinate). This hapten
was conjugated to BSA to prepare the immunogen. The effects of different types of solid phases on the equilibrium reaction
between the hapten on solid phase and the polyclonal antiserum were evaluated to obtain a fine tuning of the antiserum performances
in terms of specificity for p,p′-DDT and sensitivity to low levels of this pesticide. The calibration curves obtained show that it is possible to set up
a sensitive assay for p,p′-DDT, employing a p,p′-dichlorodiphenylacetic acid-based solid phase, with a detection limit of 0.12 ng/mL and a working range of about 0.21–40
ng/mL. Selectivity towards several p,p′-DDT-related substances was good (o,p-DDT 17%, p,p′-DDD 1.2% o,p-DDD 6.3%, p,p′-DDE 6.7%).
Received: 4 November 1996 / Revised: 9 June 1997 / Accepted: 16 June 1997 相似文献
68.
Effendy Marchetti F Pettinari C Pettinari R Ricciutelli M Skelton BW White AH 《Inorganic chemistry》2004,43(6):2157-2165
Adducts of the ligand bis(1,2,4-triazol-1-yl)methane (tz(2)(CH(2))) of the form AgX:tz(2)(CH(2)):ER(3):MeCN (1:1:1:x) (X = NO(3), R = Ph, E = P, As, or Sb, x = 1 or 2; X = NO(2), ClO(4), O(3)SCF(3), E = P, R = Ph, x = 0, 1 or 2; X = NO(3), ClO(4), E = P, R = cy, x = 1; X = ClO(4), E = As, R = Ph, x = 2) and AgNO(3):tz(2)(CH(2)):P(o-tolyl)(3) (2:2:1) have been synthesized and characterized in the solid state and in solution by analyses, spectral (IR, far-IR, (1)H and (13)C NMR, ESI MS data) data, and conductivity measurements. In the one-dimensional polymers (characterized by X-ray studies) AgNO(3):tz(2)(CH(2)):PPh(3):CH(3)CN (1:1:1:1), AgClO(4):tz(2)(CH(2)):PPh(3):CH(3)CN (1:1:1:2), AgNO(3):tz(2)(CH(2)):AsPh(3): CH(3)CN (1:1:1:2), and AgNO(3):tz(2)(CH(2)):SbPh(3):CH(3)CN (1:1:1:2), the silver atom can be regarded as four-coordinate, the tz(2)(CH(2)) ligands behaving as bridging groups rather than chelates, with no pair of ligands being dominant, quasi-trans, in their interactions. The AgNO(3):tz(2)(CH(2)):P(o-tolyl)(3) (2:2:1) adduct is a two-dimensional polymer containing two independent silver atoms, one four-coordinated unsymmetrically by a pair of triazolyl rings, one P(o-tolyl)(3), and a unidentate nitrate and the second by a quasi-symmetrical O(2)NO chelate and a pair of equivalent triazolyl rings. 相似文献
69.
Dalvit C Mongelli N Papeo G Giordano P Veronesi M Moskau D Kümmerle R 《Journal of the American Chemical Society》2005,127(38):13380-13385
NMR-based binding and functional screening performed with FAXS (fluorine chemical shift anisotropy and exchange for screening) and 3-FABS (three fluorine atoms for biochemical screening) represents a potential alternative approach to high-throughput screening for the identification of novel potential drug candidates. The major limitation of this method in its current status is its intrinsic low sensitivity that limits the number of tested compounds. One approach for overcoming this problem is the use of a cryogenically cooled (19)F probe that reduces the thermal noise in the receiver circuitry. Sensitivity improvement in the two screening techniques achieved with the novel cryogenic (19)F probe technology permits an increased throughput, detection of weaker binders and inhibitors (relevant in a fragment-based lead discovery program), detection of slow binders, and reduction in protein and substrate consumption. These aspects are analyzed with theoretical simulations and experimental quantitative performance evaluation. Application of 3-FABS combined with the cryogenic (19)F probe technology to rapid screening at very low enzyme concentrations and the current detection limits reached with this approach are also presented. 相似文献
70.
Dalvit C Hadden DT Sarver RW Ho AM Stockman BJ 《Combinatorial chemistry & high throughput screening》2003,6(5):445-453
High-throughput ligand-based proton NMR screening performed in the presence of a spy molecule and a control molecule is a valuable tool for identifying drug leads. A limitation of the technique is represented by the severe overlap encountered in the screening of large chemical mixtures. An approach for overcoming this overlap problem is the use of multi-selective R(1) filtered and COSY or TOCSY experiments. Application of this methodology to compounds binding to the Sudlow site I of human serum albumin is presented. The screening is performed by simply monitoring the intensity of two signals. The precise measurement of the relative intensity of the two resonances permits determination of the binding constant of the NMR-hit. For a simple competition binding mechanism, the rapidly-derived NMR binding constants are in good agreement with the values derived from full-titration ITC and fluorescence spectroscopy measurements. 相似文献