全文获取类型
收费全文 | 1194篇 |
免费 | 43篇 |
国内免费 | 3篇 |
专业分类
化学 | 1036篇 |
晶体学 | 5篇 |
力学 | 19篇 |
数学 | 60篇 |
物理学 | 120篇 |
出版年
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 12篇 |
2020年 | 15篇 |
2019年 | 9篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 26篇 |
2015年 | 31篇 |
2014年 | 24篇 |
2013年 | 56篇 |
2012年 | 86篇 |
2011年 | 103篇 |
2010年 | 46篇 |
2009年 | 36篇 |
2008年 | 72篇 |
2007年 | 108篇 |
2006年 | 89篇 |
2005年 | 88篇 |
2004年 | 62篇 |
2003年 | 49篇 |
2002年 | 43篇 |
2001年 | 21篇 |
2000年 | 18篇 |
1999年 | 14篇 |
1998年 | 27篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1975年 | 3篇 |
1973年 | 5篇 |
1972年 | 4篇 |
1969年 | 2篇 |
1968年 | 8篇 |
1967年 | 3篇 |
排序方式: 共有1240条查询结果,搜索用时 15 毫秒
951.
A functional coordination polymer decorated with urea hydrogen-bonding donor groups has been designed for optimal binding of sulfate; self-assembly of a tripodal tris-urea linker with Ag2SO4 resulted in the formation of a 1D metal-organic framework that encapsulates SO4(2-) anions via twelve complementary hydrogen bonds, which represents the highest coordination number observed for sulfate in a natural or synthetic host. 相似文献
952.
Sarah R. Yong Stephen G. Pyne Alison T. Ung Brian W. Skelton Peter Turner 《Tetrahedron》2005,61(34):8120-8129
The phosphine-catalyzed [3+2]-cycloaddition of the 2-methylene γ-lactams 4 and 5 and the acrylate 6 with the ylides derived from the ethyl ester, the amide or the chiral camphor sultam derivative of 2-butynoic acid (7a-c) give directly, or indirectly after reductive cyclization, spiro-heterocyclic products. The acid 32 underwent Curtius rearrangement and then acid hydrolysis to give two novel spiro-cyclic ketones, 41 and 42. 相似文献
953.
The coordination and bonding of equatorial hydroxide, carbonyl, cyanide (CN-), and isocyanide (NC-) ligands with uranyl dication, [UO2]2+, has been studied using density functional theory with relativistic effective core potentials. Good agreement is seen between experimental and calculated geometries of [UO2(OH)4]2-. Newly predicted ground-state structures of [UO2(OH)5]3-, [UO2(CO)4]2+, [UO2(CO)5]2+, [UO2(CN)4]2-, [UO2(CN)5]3-, [UO2(NC)4]2-, and [UO2(NC)5]3- are reported. Four-coordinate uranyl isocyanide complexes are the predicted gas-phase species while five-coordinate uranyl cyanide complexes are energetically favorable in aqueous solution. Small energy differences between cyanide and isocyanide complexes indicate the energetic feasibility of mixed cyanide and isocyanide complexes. A D2d uranyl tetrahydroxide is the dominant gas-phase and aqueous species, but formation of uranyl carbonyl complexes is seen to be exothermic in the gas-phase and endothermic in aqueous solution. 相似文献
954.
Ab initio extensive configuration interaction calculations were carried out on the π-electron states of benzene. Among the three π → π*(e1g → e2u) singlet states, 1B2u(S1). 1B1u(S2), and 1E1u(S3), the π* orbital was found to be velence-like in S1 and S2, but diffuse in S3. All three corresponding triplet states, 3B1u(T1) and 3B2u(T3), were found to be valence-like. The valence-like 1E2g(S4) and 3E2g(T4) states were found to have significant double-excitation character, and were estimated to lie somewhat above S3 and T3, respectively. No low-lying S5 and T5 states were found. Several low-lying Rydberg states were identified. 相似文献
955.
Michael G. Hamilton Catrin E. Hughes Alison M. Irving Christopher M. Vogels Stephen A. Westcott 《Journal of organometallic chemistry》2003,680(1-2):143-147
The hydroboration of allyl sulfonamides (4-H3CC6H4SO2NRCH2CH=CH2: R=H, 1; Ph, 2; Bz, 3) with catecholborane (HBcat) using different rhodium catalysts has been examined using multinuclear NMR spectroscopy. Reactions give complex product distributions, regardless of the choice of catalyst, arising from a competing isomerization reaction. This isomerization reaction can be used with N-substituted allyl sulfonamides 2 and 3 to give the corresponding enamines (4-H3CC6H4SO2CH=CH2CH3), which in turn react with HBcat to give regioselective formation of one isomer (4-H3CC6H4SO2NRCH2CH2(Bcat)CH3). 相似文献
956.
Alison M. Green Andrew C. Clark G. R. Scollary 《Analytical and bioanalytical chemistry》1997,358(6):711-717
Stripping potentiometry has been employed for the measurement of the labile and total copper concentrations in white wine. Conditions have been optimised for a supporting electrolyte consisting of 1.0 mol/L hydrochloric acid and 0.5 mol/L calcium chloride. Mercury(II) was found to be a more suitable oxidant than O2 and, for the determination of the labile concentrations, this required the use of medium exchange. With lead, O2 proved to be an efficient oxidant for the stripping step and this allowed the direct measurement in the wine without the need for medium exchange. It was observed that red wine rapidly binds added lead, demonstrating that wine has a high complexation capacity for lead. A similar complexation effect was also observed for white wine, but the degree of lead complexation was less than that for red wine. The impact of this lead complexation effect for the use of standard additions is discussed. 相似文献
957.
l-Penicillamine (Pen) has been investigated as a ligand for metalloprotein design by examining the binding of Co(II) to the sequence NH(2)-KL(Pen)EGG.(Pen)IG(Pen)GA(Pen).GGW-CONH(2). For comparison, we have studied Co(II) binding to the analogous sequence with Cys ligands, the ferredoxin maquette ligand IGA that was originally designed to bind a [4Fe-4S] cluster. The Co(II) affinity and UV-vis spectroscopic properties of IGA indicate formation of a pseudotetrahedral tetrathiolate ligated Co(II). In contrast, IGA-Pen showed formation of a pseudotetrahedral complex with Co(II) bound by three Pen ligands and an exogenous H(2)O. EXAFS data on both Co(II) complexes confirms not only the proposed primary coordination spheres but also shows six Co(II)-C(beta) methyl group distances in Co(II)-IGA-Pen. These results demonstrate that ligand sterics in simple peptides can be designed to provide asymmetric coordination spheres such as those commonly observed in natural metalloproteins. 相似文献
958.
Summary Copper(II) and nickel(II) complexes of 10-methyl-10-nitro-1,4,8,12-tetra-azacyclopentadecane have been prepared by reaction of the appropriate metal complex of 1,10-diamino-4,7-diazadecane (3,2,3-tet) with formaldehyde and nitroethane in the presence of triethylamine as a base. The copper(II) complex decomposes in hydrochloric acid solutions to give 15-nitro-2,6,9,13-tetra-azahexadecan-1-ol and the kinetics of this reaction have been studied in detail. 相似文献
959.
Mohamed A. Saleh Yehia A. Hafez Foad E. Abdel‐Hay Wagdy I. Gad 《Journal of heterocyclic chemistry》2003,40(6):973-978
The reaction of 3‐N‐(2‐mercapto‐4‐oxo‐4H‐quinazolin‐3‐yl)acetamide ( 1 ) with hydrazine hydrate yielded 3‐amino‐2‐methyl‐3H‐[1,2,4]triazolo[5,1‐b]quinazolin‐9‐one ( 2 ). The reaction of 2 with o‐chlorobenzaldehyde and 2‐hydroxy‐naphthaldehyde gave the corresponding 3‐arylidene amino derivatives 3 and 4 , respectively. Condensation of 2 with 1‐nitroso‐2‐naphthol afforded the corresponding 3‐(2‐hydroxy‐naphthalen‐1‐yl‐diazenyl)‐2‐methyl‐3H‐[1,2,4]triazolo[5,1‐b]quinazolin‐9‐one ( 5 ), which on subsequent reduction by SnCl2 and HCl gave the hydrazino derivative 6. Reaction of 2 with phenyl isothiocyanate in refluxing ethanol yielded thiourea derivative 7. Ring closure of 7 subsequently cyclized on refluxing with phencyl bromide, oxalyl dichloride and chloroacetic acid afforded the corresponding thiazolidine derivatives 8, 9 and 10 , respectively. Reaction of 2‐mercapto‐3‐phenylamino‐3H‐quinazolin‐4‐one ( 11 ) with hydrazine hydrate afforded 2‐hydrazino‐3‐phenylamino‐3H‐quinazolin‐4‐one ( 12 ). The reactivity 12 towards carbon disulphide, acetyl acetone and ethyl acetoacetate gave 13, 14 and 15 , respectively. Condensation of 12 with isatin afforded 2‐[N‐(2‐oxo‐1,2‐dihydroindol‐3‐ylidene)hydrazino]‐3‐phenylamino‐3H‐quinazolin‐4‐one ( 16 ). 2‐(4‐Oxo‐3‐phenylamino‐3,4‐dihydroquinazolin‐2‐ylamino)isoindole‐1,3‐dione ( 17 ) was synthesized by the reaction of 12 with phthalic anhydride. All isolated products were confirmed by their ir, 1H nmr, 13C nmr and mass spectra. 相似文献
960.
A series of fluorinated β-diketones, RfC(O)CH2C(O)Rf′ (Rf=C6F13, Rf′=CF3; Rf=Rf′=C6F13, C7F15), have been prepared in reasonable yields by a two-step synthesis. On reaction with appropriate metal substrates, deprotonation and concurrent coordination of the perfluoroalkyl-derivatised β-diketonate ligands affords a range of fluorous metal complexes which have been characterised by elemental analysis, mass spectrometry, IR and NMR spectroscopies. The structures of [Cu(L-L)2(H2O)2] {L-L=CF3C(O)CHC(O)C6F13, C6F13C(O)CHC(O)C6F13} and [Cu(PPh3)2{C7F15C(O)CHC(O)C7F15}] have been determined by single-crystal X-ray diffraction. 相似文献