全文获取类型
收费全文 | 4560篇 |
免费 | 121篇 |
国内免费 | 16篇 |
专业分类
化学 | 2879篇 |
晶体学 | 13篇 |
力学 | 236篇 |
数学 | 797篇 |
物理学 | 772篇 |
出版年
2023年 | 50篇 |
2022年 | 138篇 |
2021年 | 132篇 |
2020年 | 109篇 |
2019年 | 97篇 |
2018年 | 82篇 |
2017年 | 91篇 |
2016年 | 185篇 |
2015年 | 143篇 |
2014年 | 139篇 |
2013年 | 245篇 |
2012年 | 258篇 |
2011年 | 286篇 |
2010年 | 171篇 |
2009年 | 149篇 |
2008年 | 279篇 |
2007年 | 268篇 |
2006年 | 221篇 |
2005年 | 236篇 |
2004年 | 162篇 |
2003年 | 156篇 |
2002年 | 112篇 |
2001年 | 49篇 |
2000年 | 47篇 |
1999年 | 44篇 |
1998年 | 32篇 |
1997年 | 31篇 |
1996年 | 52篇 |
1995年 | 44篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 27篇 |
1990年 | 28篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 39篇 |
1986年 | 23篇 |
1985年 | 39篇 |
1984年 | 38篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 26篇 |
1980年 | 27篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 28篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1858年 | 11篇 |
排序方式: 共有4697条查询结果,搜索用时 46 毫秒
981.
A mixture of the red calcareous algae Lithothamnion corallioides CROUAN and Lithothamnion calcareum (PALLAS ) ARESCHOUGH , from Brittany waters, gives, on ethanolic extraction, the novel icosanoids ethyl 13-hydroxy-arachidonate (=(+)-all-Z-ethyl 13-hydroxyicosa-5,8, 11,14,-tetraenoate; (+)- 1 )), ethyl 13-hydroxytimnodonate (=(+)-(all-Z)-ethyl 13-hydroxyicosa-5,8,11,14,17-pentaenoate ((+)- 2 ), and (5Z,9E,11E,14E)-ethyl 8-hydroxy-13-oxoicosa-5,9,11,14-tetraenoate ( 3 ). Moreover, the ethyl esters of four known icosatetraenoic acids are contained in the above algae. Structural elucidations of these metabolites are based mainly on NMR data. 相似文献
982.
A complete screening and confirmation analytical method for the direct determination of six endogenous (cortisol, cortisone, deoxycorticosterone, tetrahydrocortisol, tetrahydrocortisone, tetrahydro-S) and 17 synthetic (amcinonide, betamethasone, desoximethasone, dexamethasone, fludrocortisone, flumethasone, flunisolide, flucinolone acetonide, flucinonide, fluprednisolone, flurandrenolide, fluorometholone, 6-methylprednisolone, prednisolone, prednisone, triamcinolone, triamcinolone acetonide) glucocorticoids in human urine by gas chromatography with mass spectrometric detection (GC-MS) is presented.The analytical technique comprises a pre-treatment procedure and the instrumental analysis of the trimethylsilyl (TMS) derivatives, performed by GC-MS (quadrupole) with electron impact (EI) ionization. The derivatization yields obtained by two different derivatizing mixtures, namely N-methyl-N(trimethylsilyl)trifluoroacetamide (MTSFA):NH4I:dithioerythritol (DTE) 1000:2:4 (usually indicated as TMSiodine); and N-trimethylsilylimidazole (TMSim):N,O-bis(trimethylsilyl)acetamide (BSA):trimethylchlorosilane (TMCS) 3:3:2, both under direct thermal heating and with microwave (MW) irradiation, were evaluated, also as a function of the temperature, of the MW power and of the incubation time.The highest yields of the derivatization process were obtained, for most of the compounds here considered, by a two-step procedure: a microwave-assisted derivatization stage (40 min in a microwave oven at 900 W emitted power), followed by a traditional heat transfer derivatization (1.5 h in a thermostated bath at 70 °C) with the derivatization mixture TMSim:BSA:TMCS 3:3:2. In these operating conditions, diagnostic EI-MS spectra of all considered glucocorticoids were obtained. Limits of detection (LOD) of synthetic glucocorticoids in urine ranged from 3 to 25 μg/l. The effectiveness of the method for the determination of glucocorticoids in urine was evaluated on spiked urine samples and on real samples obtained from patients under pharmacological treatment with synthetic glucocorticoids.Apart from the clinical monitoring of glucocorticoids in urine, the method can be applied as a complete screening + confirmation analytical protocol in antidoping tests for the detection of illicit administration of glucocorticoids by the athletes. 相似文献
983.
Angela M BernardAngelo Frongia Francesco SecciGiovanna Delogu Jean OllivierPier P Piras Jacques Salaün 《Tetrahedron》2003,59(47):9433-9440
Mixtures of (E and Z)-2-cyclobutylidenepropyl sulfonates, readily available from α,α-disubstituted cyclobutanones arising from suitable cyclopropane derivatives ring expansion, underwent regioselective and stereospecific reduction by formate anion to offer, through π-1,1-trimethyleneallylpalladium complexes formed upon treatment with palladium(0), a new and convenient entry to the diastereomeric four-membered ring monoterpenoids (±)-fragranol and (±)-grandisol. 相似文献
984.
A critical comparison of phase-selective fundamental harmonic a.c. anodic stripping voltammetry and differential pulse anodic stripping voltammetry for the determination of Pb, Cd, Cu and Zn in sea-water is reported. Differential pulse anodic stripping voltammetry was found to be slightly more sensitive than the a.c. technique, but the effect of the charging current could be more effectively eliminated by the latter, especially in the determination of zinc(II) and copper(II). The detection limits for both techniques were found to be about 10–10-10–8
M for all four elements. The precision, expressed as relative standard deviation, was of the order of 2–5% for the fundamental harmonic a.c. method and 5–8% for differential pulse voltammetry. The accuracy (expressed as recovery) was 95–105% for the former and 90–110% for the latter. 相似文献
985.
Michele D'Ambrosio Antonio Guerriero Francesco Pietra Ccile Debitus 《Helvetica chimica acta》1996,79(1):51-60
Leucascandrolide A ((+)- 1 ), a doubly O-bridged 18-membered macrolide of a new type, i.e., showing little C1-branching vs. extensive 1,3-dioxygenation and a peculiar side chain, was isolated from a calcareous sponge of a new genus, Leucascandra caveolata BOROJEVIC and KLAUTAU from the Coral Sea. Transesterification of (+)- 1 gave the methyl ester 3 , derived from the side chain, and the 5-hydroxy derivative (+)- 2 , derived from the macrolide portion and with the natural configuration at C(5) (axial). Mosher's MTPA esters 4 and 5 obtained from (+)- 2 showed scattered Δδ = (δ(S) ? δ(R)) data. However, inversion of the configuration at C(5) led, via ketone (+)- 6 , to the less encumbered 5-equatorial hydroxy derivative (+)- 7 , whose MTPA esters 8 and 9 gave consistent Δδ data, allowing the assignment of the absolute configuration of (+)- 7 , and hence of (+)- 1 . The structural novelty of (+)- 1 and its powerful antifungal and cytotoxic activities are likely to renew interest in calcareous sponges, previously limited to scarcely biologically active 2-aminoimidazoles. 相似文献
986.
Angelo Albert Francesco Paolo Colonna Maurizio Guerra Bianca F. Bonini Germana Mazzanti Zoltàn Dinya Gian Franco Pedulli 《Journal of organometallic chemistry》1981,221(1):47-55
The radical adducts resulting from the reaction between group IVB organometallic radicals MR3 (M = Si, Ge, Sn; R = alkyl or aryl) and a number of thioketones of the chromone and flavone series have been investigated by ESR spectroscopy. The spectral parameters obtained for these species have been compared with those of similar adducts of the corresponding ketones. Both classes of radicals adopt an orthogonal conformation, and the differences between the former and the latter paramagnetic species can be accounted for by the different electronegativity of oxygen and sulphur. the effects of substitution of the heterocyclic oxygen with a sulphur or selenium atom on the spin density distribution are likewise explained. INDO calculations have been carried out on the model systems CH2XSiH3 (X = O, S). 相似文献
987.
Akbayeva DN Gonsalvi L Oberhauser W Peruzzini M Vizza F Brüggeller P Romerosa A Sava G Bergamo A 《Chemical communications (Cambridge, England)》2003,(2):264-265
The new water soluble ruthenium complexes [(C5R5)RuCl(PTA)2] (R = H, Me; PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesised and characterised. Their evaluation as regioselective catalysts for hydrogenation of unsaturated ketones in aqueous biphasic conditions and as cytotoxic agents towards the TS/A adenocarcinoma cell line is briefly presented. 相似文献
988.
Francesco Pergola Maria R. Moncelli Rolando Guidelli 《Journal of Electroanalytical Chemistry》1980,107(2):295-306
A polarographic investigation of CuII electroreduction from solutions of benzotriazole (BTA) of pH from 1 to 3 at a dropping mercury electrode shows the presence of two successive adsorption waves of equal height (waves Ic and IIIc) due, respectively, to the formation and to the subsequent dissolution of an adsorbed film of a CuI compound. The dependence of the half-wave potential of wave IIIc, which is polarographically reversible, upon pH and BTA concentration indicates that the adsorbed film has the composition [CuI(BTA?)], where BTA? denotes the deprotonated anionic form of benzotriazole. Moreover, the maximum height of wave IIIc indicates that this film is one monolayer thick. A third cathodic wave (wave IIc), lying between waves Ic and IIIc, stems from the reduction to the metal state of the CuII ions diffusing from the bulk solution. Copper(0) oxidation at dropping amalgam electrodes in BTA solutions yields two successive adsorption waves (waves Ia and IIa). Wave Ia is due to the same electrode process, Cu0 (Hg)+BTA?CuI(BTA?)+H++e, responsible for the cathodic wave IIIc. On the other hand, wave IIa is due to the formation of roughly two adsorbed monolayers of [CuI(BTA?)] upon that formed along wave Ia. 相似文献
989.
The ionization enthalpy of benzoic acid has been measured calorimetrically at 25°C in H2ODMSO mixtures ranging from pure water to a maximum DMSO molar ratio XDMSO = 0.80. With the increase of DMSO content, the ionization becomes more and more endothermic, and for XDMSO = 0.8 the ionization enthalpy is about 6 kcal mol?1 higher than in water. By also measuring the solution enthalpy of crystalline benzoic acid in the mixtures, it has been shown that the solvation of the undissociated molecule is the main cause for the increase of the dissociation enthalpy. A comparison has been made between the relative enthalpies of benzoic and hydroxide ions in H2ODMSO mixtures. 相似文献
990.
Francesco Pegoraro 《Communications in Mathematical Physics》1975,42(1):41-63
We consider a theorem due to Michel [1] which relates the invariance properties in peculiar directions in a linear space on which we represent a Lie groupG to the extremal points of an arbitrary smoothG-invariant function. The group we are interested in isSO(4) and we apply the mathematical results to the following problems:
- mixed linear Stark Zeeman effect in a hydrogen atom,
- perturbation of a finite Robertson-Walker metric,
- gas evolutions preserving angular momentum and vorticity.