全文获取类型
收费全文 | 6117篇 |
免费 | 267篇 |
国内免费 | 25篇 |
专业分类
化学 | 3983篇 |
晶体学 | 46篇 |
力学 | 349篇 |
数学 | 1122篇 |
物理学 | 909篇 |
出版年
2023年 | 30篇 |
2022年 | 163篇 |
2021年 | 166篇 |
2020年 | 136篇 |
2019年 | 161篇 |
2018年 | 121篇 |
2017年 | 94篇 |
2016年 | 242篇 |
2015年 | 192篇 |
2014年 | 196篇 |
2013年 | 363篇 |
2012年 | 346篇 |
2011年 | 352篇 |
2010年 | 263篇 |
2009年 | 238篇 |
2008年 | 312篇 |
2007年 | 327篇 |
2006年 | 304篇 |
2005年 | 256篇 |
2004年 | 237篇 |
2003年 | 171篇 |
2002年 | 192篇 |
2001年 | 90篇 |
2000年 | 86篇 |
1999年 | 70篇 |
1998年 | 44篇 |
1997年 | 54篇 |
1996年 | 61篇 |
1995年 | 48篇 |
1994年 | 43篇 |
1993年 | 50篇 |
1992年 | 54篇 |
1991年 | 41篇 |
1990年 | 25篇 |
1989年 | 25篇 |
1988年 | 28篇 |
1987年 | 42篇 |
1986年 | 21篇 |
1985年 | 44篇 |
1984年 | 57篇 |
1983年 | 48篇 |
1982年 | 37篇 |
1981年 | 58篇 |
1980年 | 64篇 |
1979年 | 51篇 |
1978年 | 37篇 |
1977年 | 35篇 |
1976年 | 31篇 |
1975年 | 24篇 |
1973年 | 16篇 |
排序方式: 共有6409条查询结果,搜索用时 15 毫秒
61.
Roberto Grisorio Giuseppe Romanazzi Piero Mastrorilli Domenico Acierno Giuseppe Ciccarella 《Tetrahedron》2006,62(4):627-634
A series of novel bifluorene based systems was synthesised by a convergent approach by means of a Suzuki cross-coupling between 7,7′-bis-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-9,9,9′,9′-tetraoctyl-2,2′-bifluorene and suitable aryl-bromides. All the oligomers have been characterized by 1H, 13C NMR, FT-IR, UV-vis, PL spectroscopy and mass analyses. In particular, it has been demonstrated that the presence of strong electron donor (amines) or withdrawing (carboxylic esters) groups causes a bathochromic shift of the optical properties with respect to those of unsubstituted molecules. The effects of these functional groups on the HOMO-LUMO energy levels were investigated by cyclic voltammetry. Remarkably, the LUMO energy level of 7,7′-bis-[5′-carbodecaoxy-2,2′-bithiophen-5-yl]-9,9,9′,9′-tetraoctyl-2,2′-bifluorene (−3.07 eV) is strongly influenced by the presence of the ester functional group. 相似文献
62.
Benati L Leardini R Minozzi M Nanni D Spagnolo P Strazzari S Zanardi G 《Organic letters》2002,4(18):3079-3081
[reaction: see text] Aryl- and alkyl-derived azidoacyl radicals, generated from thiolesters by intramolecular homolytic substitution at the sulfur, can undergo five- and six-membered cyclization onto the azido moiety to give cyclized lactams. 相似文献
63.
Pablo Ríos Javier Borge Francisco Fernndez de Crdova Giuseppe Sciortino Agustí Lleds Amor Rodríguez 《Chemical science》2021,12(7):2540
The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.A bis(boryl)nickel complex promotes the facile and reversible activation of H2 through a cooperative mechanism that involves the metal and both boryl moieties in a concerted five-center process. 相似文献
64.
Novel synthesis of (1H)‐pyridin‐2‐one, pyrazolo[1,5‐a]pyrimidine and isoxazole derivatives incorporating N‐methylphthalimide moiety are reported. Reaction of enaminone 2 with malononitrile affords 4. Condensation of 2 with cyanothioacetamide or benzoylacetonitrile affords compounds 6 and 7 respectively. Reaction of 2 with hydrazine hydrate afford 2,3‐dihydrophthalazine‐1,4‐dione ( 10 ). Condensation of 2 with hydroxylamine and 3‐aminopyrazole derivatives affords compounds 12 and 15a,b respectively. Antimicrobial and antifungal activity were determined for representative compounds and most of them showed moderate activity as antimicrobial agents, while compounds 2 and 7 show strong activity against Aspergillus niger. The structure of the newly synthesized compounds was elucidated by elemental analyses and 1H nmr spectra and some cases by 13C nmr investigation. 相似文献
65.
Manetta AC Di Giuseppe L Giammarco M Fusaro I Simonella A Gramenzi A Formigoni A 《Journal of chromatography. A》2005,1083(1-2):219-222
A new HPLC method with fluorescence detection using pyridinium hydrobromide perbromide as a post-column derivatising agent has been developed to determine aflatoxin M1 in milk and cheese. The detection limits were 1 ng/kg for milk and 5 ng/kg for cheese. The calibration curve was linear from 0.001 to 0.1 ng injected. The method includes a preliminary C18-SPE clean-up and the average recoveries of Aflatoxin M1 from milk and cheese, spiked at levels of 25-75 ng/kg and 100-300 ng/kg, respectively, were 90 and 76%; the precision (RSDr) ranged from 1.7 to 2.6% for milk and from 3.5 to 6.5% for cheese. The method is rapid, easily automatable and therefore useful for accurate and precise screening of aflatoxin M1 in milk and cheese. 相似文献
66.
Studies of the complexes of pyridinecarboxylic acids with divalent metal ions as a function of the position of the carboxyl groups were extended. The thermal properties of the complexes of quinoline acid (pyridine-2,3-dicarboxylic acid) with several divalent metal ions were determined by thermogravimetry (TG) and differential thermal analysis (DTA). A correlation between these compounds and others obtained by reaction between the studied metal ions with similar acids (lutidinic acid (pyridine-2,4-dicarboxylic acid) and isocinchomeronic acid (pyridine-2,5-di-carboxylic acid) is discussed in terms of the position of the carboxyl group far from the aza group. The thermal stability of the metal complexes is in the order Mn(II) > Fe(II) > Zn(II) ? Co(II) > Ni(II) > Cu(II). 相似文献
67.
Spoto G Gribov E Bordiga S Lamberti C Ricchiardi G Scarano D Zecchina A 《Chemical communications (Cambridge, England)》2004,(23):2768-2769
Cu(I) ions in Cu-ZSM-5 form Cu+(H2) complexes, stable at room temperature and sub-atmospheric H2 pressure, which do not have any homogeneous analogue except for matrix-isolated [Cu(eta2-H2)Cl]. Comparison with the unstable Na+(H2) adducts formed in the parent Na-ZSM-5 zeolite allow the conclusion that the Cu(I)/H2 bond is governed by sigma-pi overlap forces. 相似文献
68.
Piperidine-, morpholine-4-, N-methylpiperazine-4- and thiornorpholine-4-carbodithioate complexes of chromium(III), manganese(III), tin(II) and lead(II) are prepared and characterized by chemical analyses, spectroscopic methods (I.R. and electronic spectra), magnetic susceptibilities, conductivity measurements and mass spectra. The complexes are of the type M(R2dtc)n, where n is the oxidation number of the metal ion. Where possible a tentative stereochemistry of the complexes is discussed on the basis of the results obtained. In all the complexes the dithiocarbamate ligands show bidentate behaviour. 相似文献
69.
Celebre G De Luca G Longeri M Pileio G Emsley JW 《The Journal of chemical physics》2004,120(15):7075-7084
The proton NMR spectra of two (13)C-labeled isotopomers of styrene dissolved in two liquid crystalline solvents have been obtained and analyzed to yield four sets each of 24 dipolar couplings. These couplings were then used to investigate the structure of the ring and the ene fragments of the molecule, and the position of the maximum, phi(0), in the ring-ene bond rotational probability distribution. To do this, the effect on the dipolar couplings of small-amplitude vibrational motion was taken into account using vibrational wave functions calculated by molecular orbital and density functional methods. It is concluded that the NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. The value of phi(0) is found to be 18.0 degrees +/-0.2 degrees for the two solutions, compared with a value of 27 degrees calculated by the molecular method MP2/6-31G(*). 相似文献
70.
J. Al Andari A. M. Diamy J. C. Legrand R. I. Ben-Aim 《Plasma Chemistry and Plasma Processing》1993,13(1):103-116
Electron temperature and ion density are measured in an air microwave-induced plasma (2450 MHz) by means of a floating double probe. A 'cinetic scheme for ion formation and decay is set up, and a relationship between atomic oxygen and ion densities is obtained. From this relationship an order-of-magnitude of atomic oxygen concentration in the discharge is derived and compared with results obtained by optical actinometry in another work. 相似文献