全文获取类型
收费全文 | 2667篇 |
免费 | 115篇 |
国内免费 | 2篇 |
专业分类
化学 | 1810篇 |
晶体学 | 8篇 |
力学 | 121篇 |
数学 | 435篇 |
物理学 | 410篇 |
出版年
2023年 | 19篇 |
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 43篇 |
2019年 | 42篇 |
2018年 | 60篇 |
2017年 | 56篇 |
2016年 | 66篇 |
2015年 | 80篇 |
2014年 | 100篇 |
2013年 | 185篇 |
2012年 | 178篇 |
2011年 | 188篇 |
2010年 | 109篇 |
2009年 | 73篇 |
2008年 | 166篇 |
2007年 | 152篇 |
2006年 | 132篇 |
2005年 | 185篇 |
2004年 | 163篇 |
2003年 | 118篇 |
2002年 | 85篇 |
2001年 | 37篇 |
2000年 | 29篇 |
1999年 | 25篇 |
1998年 | 17篇 |
1997年 | 9篇 |
1996年 | 21篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 19篇 |
1992年 | 22篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 20篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 16篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1976年 | 15篇 |
1974年 | 14篇 |
1973年 | 10篇 |
1948年 | 14篇 |
排序方式: 共有2784条查询结果,搜索用时 15 毫秒
71.
Cetinkaya B Demir S Ozdemir I Toupet L Sémeril D Bruneau C Dixneuf PH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(10):2323-2330
The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me(3)C(6)H(2)CH(2))NCH(2)CH(2)N(R)Cdbond;](2) (2a: R=CH(2)CH(2)OMe, 2 b R=CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R=CH(2)Mes) group on the nitrogen atom, with [RuCl(2)(arene)](2) (arene=p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(II) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta(1)-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene,eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl(2)(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl(2)(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HCtbond;CCPh(2)OH, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl(2)[eta(1)-CN(CH(2)C(6)H(2)Me(3)-2,4,6)CH(2)CH(2)N- (CH(2)CH(2)OMe)](C(6)Me(6))] 3 with a monodentate carbene ligand and [RuCl(2)[eta(1)-CN[CH(2)(eta(6)-C(6)H(2)Me(3)-2,4,6)]CH(2)CH(2)N-(CH(2)C(6)H(2)Me(3)-2,4,6)]] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography. 相似文献
72.
Bis(cyclodisilazane-1-yl) dimethylsilanes — Synthesis and Reactions The monolithium derivate of trisilazan-1-yl-cyclodisilazane 1 reacts with F3SiN (SiMe3)2 with substitution. The silyl-bridged cyclodisilazanes 3–6 are formed in the reaction of the dilithium derivate of 1 with fluoro- and chlorosilanes. Using lithiumamide and lithiummethanolate a controlled exchange of one fluoro atom of 4 occurs ( 7,8 ). 9 and 10 are formed by hydrolysis of 4 . The aminofunctional compounds 11 und 12 are obtained in the reaction of 5 and 6 with NH3. The dispirocyclus 13 is formed in the reaction of 8 with tert.-butyllithium. The reaction of dilithiated 1 with 4 gives the spirocyclus 14 . The crystal structure of 14 is discussed. 相似文献
73.
Yockot D Moreau V Demailly G Djedaïni-Pilard F 《Organic & biomolecular chemistry》2003,1(10):1810-1818
The synthesis of branched beta-cyclodextrins substituted with mannosyl mimetic derivatives at one primary hydroxy group is described. It was shown that the self-inclusion phenomenon observed for the target compounds in water did not preclude the inclusion properties of the cyclodextrin moiety. 相似文献
74.
75.
Marie-Thrse Le Bris 《Journal of heterocyclic chemistry》1989,26(2):429-433
The structure of 7-aminobenzoxazinones was modified by enclosing their amino nitrogen atom in a julolidyl ring. This rigidization was expected to enhance the fluorescence performances in this series. Several fluorescent dyes and styryl derivatives were prepared and their spectral characteristics were investigated. Comparison with homologous benzoxazinones with a flexible amino group shows that rigidization does not improve the quantum yield and the photochemical stability, in contrast with the results reported for other classes of dyes, like coumarins or rhodamines. 相似文献
76.
A miniaturised technique to analyse and detect heterocyclic aromatic amines (HAs) using micro solid-phase extraction (SPE) coupled on-line (in-capillary) to capillary electrophoresis (CE) separation with nanospray (nESI) mass spectrometry (MS) detection has been developed. HAs are mutagenic and carcinogenic compounds formed at low levels in protein-rich food during cooking. Due to the low concentrations of HAs and the high complexity of the matrix in which they exist, sensitive and selective analytical methods are required for quantification. SPE was performed on a packed bed of C18 particles inside the CE capillary, which minimised the dead volume. The on-line coupling of SPE, CE and nESI-MS reduced the time for extraction and identification to less than half an hour, which will allow for screening of several samples per day. The new technique provides short analysis time, low sample and solvent consumption, and HAs in standard solutions were easily detected at 12–17 fmol injections, and in spiked urine samples at 750–810 fmol injections. 相似文献
77.
Linde C Koliaï N Norrby PO Akermark B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2568-2573
The substrate electronic effects on the selectivity in the catalytic epoxidation of para-substituted cis stilbenes 2a-i were investigated by using (R,R)-[N,N'-bis(3,5-di-tBu-salicylidene)-1,2-cyclohexanediamine]manganese(III) chloride 1 in benzene as the catalyst with iodosobenzene as the terminal oxidant. A Hammett study of the selectivity results reveals a stronger electrophilic character than previously assumed in the (salen)Mn-catalyzed reaction. In general, the best correlations with the experimental values were obtained by using the Hammett sigma + values, which gave rho = -1.37 for the rate of cis-epoxide formation and rho = -0.43 for the rate of the stepwise process leading to the corresponding trans product. The reaction involves two separate pathways as indicated also by the competitive breakdown of the intermediate on the path to trans epoxide for methoxy-substituted substrates. The asynchronicity in the concerted pathway leading to cis epoxide is apparent for 4-methoxy-4'-nitrostilbene, which yields cis epoxide with 75% ee entirely as a result of electronic effects. 相似文献
78.
The [Ru(bipy)(2)(1)](PF(6))(2) (bipy refers to 2,2'-bipyridine) complex, comprising a ruthenium(II) tris(2,2'-bipyridine) luminophore covalently linked to a di[(o-triethyleneglycoxy)phenyl]amine crown ether 1, has been synthesized and fully characterized. The photophysical properties of this metal complex have been examined in solution at ambient temperature. Luminescence from the metal complex is enhanced significantly in the presence of various adventitious cations, including protons. In particular, Li(+) cations bind to the crown ether, as evidenced by (1)H NMR and luminescence spectroscopy. Cation binding serves to decrease the rate of reductive quenching of the triplet state of the metal complex, thereby increasing the extent of luminescence. The solution-phase conformation of [Ru(bipy)(2)(1)](PF(6))(2), with and without encapsulated Li(+), has been examined by 2-D NMR and by molecular dynamics simulations. 相似文献
79.
Jean Ravez Annie Perron Jean Pierre Chaminade Paul Hagenmuller Loïc Rivoallan 《Journal of solid state chemistry》1974,10(3):274-281
Two series of phases with tetragonal bronze-like structure and composition BaxLi5?2xT5O15 (T = Nb, Ta) have been isolated in the systems BaNb2O6LiNbO3 and BaTa2O6LiTaO3. All these phases show ferroelectric-paraelectric transitions. The Curie temperature increases with the lithium content. The value of TC for Ba2.03Li0.94Nb5O15 is the highest ever observed for this type of structure: the obtained phases are potentially good materials for the harmonic generation of the 0.53-μm radiation. The optical yield of the niobate Ba2.14Li0.71Nb5O15 is about 2.5 times that of Ba2NaNb5O15 and 250 times that of the K.D.P. The crystallographic and dielectric data of the system Ba2.14Li0.71Nb5O15Ba2.14Li0.71Ta5O15 characterize three domains, which are respectively antiferroelectric, ferroelectric, and paraelectric. The Curie temperature and the optical yield decrease with increasing tantalum content. 相似文献
80.
Isobe H Tanaka T Nakanishi W Lemiègre L Nakamura E 《The Journal of organic chemistry》2005,70(12):4826-4832
The reaction of [60]fullerene with a variety of a secondary aliphatic amines in 20% v/v dimethyl sulfoxide in chlorobenzene under an atmospheric pressure of molecular oxygen allows regioselective introduction of four amino groups and one epoxide group around one pentagon of the fullerene molecule in good to high yield. This new synthesis of tetraaminofullerene expoxide can be carried out with a simple procedure on a multigram scale at room temperature and affords a variety of functionalized fullerene derivatives. Near-infrared analysis of a mixture of [60]fullerene and piperidine in a deaerated dimethyl sulfoxide/chlorobenzene mixture indicated equilibrium formation of [60]fullerene radical anion (C60*-) that persists at least for 2 weeks at room temperature but reacts immediately with molecular oxygen to give the tetraaminofullerene expoxide. The Benesi-Hildebrand analysis of the concentration dependency of the near-infrared absorption indicated that a [C60*- piperidine*+] radical ion pair is formed with an equivalent constant of K = 0.62 +/- 0.02 M(-1) at 25 degrees C. This and other lines of evidence suggest that the oxygenative amination reaction involves C60-mediated reduction of molecular oxygen by the amine. 相似文献