全文获取类型
收费全文 | 26686篇 |
免费 | 606篇 |
国内免费 | 157篇 |
专业分类
化学 | 17905篇 |
晶体学 | 219篇 |
力学 | 681篇 |
综合类 | 1篇 |
数学 | 4480篇 |
物理学 | 4163篇 |
出版年
2021年 | 176篇 |
2020年 | 215篇 |
2019年 | 288篇 |
2018年 | 235篇 |
2017年 | 212篇 |
2016年 | 447篇 |
2015年 | 351篇 |
2014年 | 463篇 |
2013年 | 1361篇 |
2012年 | 1216篇 |
2011年 | 1538篇 |
2010年 | 792篇 |
2009年 | 634篇 |
2008年 | 1383篇 |
2007年 | 1441篇 |
2006年 | 1437篇 |
2005年 | 1453篇 |
2004年 | 1261篇 |
2003年 | 1062篇 |
2002年 | 1044篇 |
2001年 | 395篇 |
2000年 | 315篇 |
1999年 | 252篇 |
1998年 | 293篇 |
1997年 | 317篇 |
1996年 | 405篇 |
1995年 | 305篇 |
1994年 | 291篇 |
1993年 | 297篇 |
1992年 | 285篇 |
1991年 | 247篇 |
1990年 | 233篇 |
1989年 | 243篇 |
1988年 | 283篇 |
1987年 | 260篇 |
1986年 | 253篇 |
1985年 | 395篇 |
1984年 | 444篇 |
1983年 | 326篇 |
1982年 | 436篇 |
1981年 | 408篇 |
1980年 | 346篇 |
1979年 | 337篇 |
1978年 | 354篇 |
1977年 | 348篇 |
1976年 | 283篇 |
1975年 | 275篇 |
1974年 | 270篇 |
1973年 | 245篇 |
1972年 | 148篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Kadish KM Phan TD Giribabu L Shao J Wang LL Thuriere A Van Caemelbecke E Bear JL 《Inorganic chemistry》2004,43(3):1012-1020
Eleven different Ru(2)(4+) and Ru(2)(3+) derivatives are characterized by thin-layer FTIR and UV-visible spectroelectrochemistry under a CO atmosphere. These compounds, which were in-situ electrogenerated from substituted anilinopyridine complexes with a Ru(2)(5+) core, are represented as Ru(2)(L)(4)Cl where L = 2-CH(3)ap, ap, 2-Fap, 2,3-F(2)ap, 2,4-F(2)ap, 2,5-F(2)ap, 3,4-F(2)ap, 3,5-F(2)ap, 2,4,6-F(3)ap, or F(5)ap. The Ru(2)(5+) complexes do not axially bind CO while mono- and bis-CO axial adducts are formed for the Ru(2)(4+) and Ru(2)(3+) derivatives, respectively. Six of the eleven investigated compounds exist in a (4,0) isomeric form while five adopt a (3,1) geometric conformation. These two series of compounds thus provide a large enough number of derivatives to examine trends and differences in the spectroscopic data of the two types of isomers in their lower Ru(2)(4+) and Ru(2)(3+) oxidation states. UV-visible spectra of the Ru(2)(4+) derivatives and IR spectra of the Ru(2)(3+) complexes under CO are both isomer dependent, thus suggesting that these data can be used to reliably predict the isomeric form, i.e., (3,1) or (4,0), of diruthenium complexes containing four unsymmetrical substituted anilinopyridinate bridging ligands; this was confirmed by X-ray crystallographic data for seven compounds whose structures were available. 相似文献
892.
1-Ethyl-6-fluoro-1,4-dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxylic acid ( 8 ) has been prepared in large quantities by a highly efficient process. It has in turn been degraded to give 7-chloro-1-ethyl-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid ( 11 ). This intermediate has been reacted with piperazine to give the known antibacterial agent, enoxacin ( 12 ). 相似文献
893.
West Douglas X. Scovill John P. Silverton J. V. Bavoso Alfonso 《Transition Metal Chemistry》1986,11(4):123-131
Summary The ligand 3-azabicyclo[3.2.2]nonane-3-thiocarboxylic acid 2-[1-(2-pyridinyl)ethylidene]hydrazide (HL), which is observed in an unusual tautomeric form in the solid state, and its selenium analogue (HLSe) have been used to prepare a series of nickel(II) complexes. Compounds of the general formula [NiLX] (X=Cl, Br, NCS, N3, NO2 or NCSe) as well as [Ni(LSe)Cl] have been found to be diamagnetic, planar complexes. A single crystal study of [NiL(NCS)] shows the deprotonated ligand bound in a tridentate mannervia its pyridyl nitrogen, imine nitrogen and the thione sulphur atom with the nitrogen atom of the thiocyanato-ligand occupying the fourth coordination position. The solids prepared from the nickel(II) salts having tetrafluoroborate, nitrate and iodide ions approximate to octahedral symmetry and have neutral HL ligands coordinated in a bidentate fashionvia the pyridine and imine nitrogens with the remaining coordination sites being occupied by the anions or water molecules. The [NiL2] solid is also octahedral with the two deprotonated ligands bonding as tridentate groupsvia the same atoms as in the [NiLX] complexes. 相似文献
894.
Induction of Chromosome Aberrations and Delayed Genomic Instability by Photochemical Processes 总被引:1,自引:0,他引:1
Charles L. Limoli Joseph P. Day John F. Ward William F. Morgan 《Photochemistry and photobiology》1998,67(2):233-238
Exponentially growing cells cultured in medium containing bromodeoxyuridine, then exposed to UVA light in the presence of the dye Hoechst 33258, show significant levels of DNA strand breaks and base damage. This dye–bromodeoxyuridine–UVA photolysis treatment is markedly cytotoxic. We now demonstrate that exposure of cells to the agents used in photolysis leads directly to the formation of chromosome aberrations. Furthermore, we demonstrate that this photochemical treatment induces delayed chromosomal instability in clonal populations derived from single progenitor cells surviving photolysis. These results suggest that photolysis-induced DNA damage leads to chromosome rearrangements that could account for the observed cytotoxicity. Furthermore, in those cells surviving photolysis, the delayed effects of this treatment can be observed several generations after exposure and are manifested as compromised genomic integrity. 相似文献
895.
N.M.R. Spectroscopic Studies of 15N Labelled N-Phosphorylphosphazenes and Imidodiphosphoric Acid Derivatives 15N labelled compounds were prepared and investigated by means of 31P- and 15N-NMR spectroscopy. The chemical shift values δP and δN, and the coupling constants 1JPN and 2JPP are discussed and interpreted qualitatively by semiempirical quantumchemical calculations (CNDO/2) using POPLE 'S ΔE-model. 相似文献
896.
John Phillips 《Journal of Functional Analysis》1983,51(3):259-267
Let B be a unital C1-algebra and A = Γ(E) be the C1-algebra of sections of a bundle over the separable compact space X with fibre B and structure group Inn B. If B is the quotient of an AW1-algebra, then an exact sequence: 0 → Inn A → PInn A →ηH2(X, G), where PInn A is the group of pointwise inner automorphisms of A and G=H°(Z(B)^, ) is obtained. The map η is onto whenever A = C(X, B) and B is the quotient of a purely infinite AW1-algebra. An essential part of the analysis is the result that Ad: U(B) → Inn B is a fibre bundle if and only if the space of inner derivations of B is norm closed. These results extend and clarify previous joint work with I. Raeburn (Indiana Univ. Math. J.29 (1980), 799). 相似文献
897.
S M Holmes D F Schafer P T Wolczanski E B Lobkovsky 《Journal of the American Chemical Society》2001,123(43):10571-10583
Treatment of NaW2Cl7(THF)5 with 4 equiv of (t)Bu3SiNHLi afforded the C2 W(III) dimer [((t)Bu3SiNH)2WCl]2 (1, d(W triple bond W) = 2.337(2) A), which is a rare, primary amide M2X4Y2 species. Its degradation provided evidence of NH bond activation by the ditungsten bond. Addition of 2 equiv of (t)Bu3SiNHLi or TlOSi(t)Bu3 to 1 yielded H2 and hydride ((t)Bu3SiN)2((t)Bu3SiNH)WH (2, d(WH) = 1.67(3) A) or ((t)Bu3SiN)2((t)Bu3SiO)WH (3). Thermolysis (60 degrees C, 16 h) of 1 in py gave ((t)Bu3SiN)2WHCl(py) (4-py, 40-50%), ((t)Bu3SiN)2WCl2(py) (6-py, 10%), and ((t)Bu3SiN)2HW(mu-Cl)(mu-H)2W(NSi(t)Bu3)py2 (5-py2, 5%), whereas thermolysis in DME produced ((t)Bu3SiN)2WCl(OMe) (7, 30%), ((t)Bu3SiN)2WCl2 (6, 20%), and ((t)Bu3SiN)2HW(mu-Cl)(mu-H)2W(NSi(t)Bu3)DME (5-DME, 3%). Compound 7 was independently produced via thermolysis of 4-py and DME (-MeOEt, -py), and THF and ethylene oxide addition to hydride 2 gave ((t)Bu3SiN)2((t)Bu3SiNH)WO(n)Bu (8) and ((t)Bu3SiN)2((t)Bu3SiNH)WOEt (9), respectively. Dichloride 6 was isolated from SnCl4 treatment of 1 with the loss of H2. Sequential NH bond activations by the W2 core lead to "((t)Bu3SiN)2WHCl" (4) and subsequent thermal degradation products. Thermolysis of 1 in the presence of H2C=CH(t)Bu and PhC triple bond CPh trapped 4 and generated ((t)Bu3SiN)2W((neo)Hex)Cl (10) and a approximately 6:1 mixture of ((t)Bu3SiN)2WCl(cis-CPh=CPhH) (11-cis) and ((t)Bu(3)SiN)2WCl(trans-CPh=CPhH) (11-trans), respectively. Thermolysis of the latter mixture afforded ((t)Bu3SiNH)((t)Bu3SiN)WCl(eta2-PhCCPh) (12) as the major constituent. Alkylation of 1 with MeMgBr produced ((t)Bu3SiN)2W(CH3)2 (13), as did addition of 2 equiv of MeMgBr to 6. X-ray crystal structure determinations of 1, 2, 5-py2, 6-py, 11-trans, and 12 confirmed spectroscopic identifications. A general mechanism that features a sequence of NH activations to generate 4, followed by chloride metathesis, olefin insertion, etc., explains the formation of all products. 相似文献
898.
The reactions of [Co(η-C5H5)(L)I2] with Na[S2CNR2] (R = alkyl or phenyl) give [Co(η-C5H5)(I)(S2CNR2)] (I) when L = CO and [Co(η-C5H5)(L)(S2CNR2)]I (II) when L is a tertiary phosphine, phosphite or stibine, or organo-isocyanide ligand. In similar reactions [Co(η-C5H5)(CO)(C3F7)I] gives [Co(η-C5H5)(C3F7)(S2CNMe2)] and [Mn(η-MeC5H4)(CO)2(NO)]PF6 forms [Mn(η-MeC5H4)(NO)(S2CNR2)]. The iodide ligands in I may be displaced by L, to give II, or by other ligands such as [CN]?, [NCS]?, H2O or pyridine whilst SnCl2 converts it to SnCl2I. The iodide counter-anion in II may be replaced by others to give [BPh4]?, [Co(CO)4]? or [NO3]? salts. However [CN]? acts differently and displaces (PhO)3P from [Co(η-C5H5){P(OPh)3}(S2CNMe)]I to give [Co(η-C5H5)(CN)(S2CNMe2)] which may be alkylated reversibly by MeI and irreversibly by MeSO3F to [Co(η-C5H5)(CNMe)(S2CNMe2)]+ salts. Conductivity measurements suggest that solutions of I in donor solvents are partially ionized with the formation of [Co(η-C5H5)(solvent)(S2CNR2)]+ I? species. The IR and 1H NMR spectra of the various complexes are reported. They are consistent with pseudo-octahedral “pianostool” molecular structures in which the bidentate dithiocarbamate ligands are coordinated to the metal atoms through both sulphur atoms. 相似文献
899.
John Raymond O'Lear Larry Gene Wright John Nathan Louris R. Graham Cooks 《Journal of mass spectrometry : JMS》1987,22(6):348-358
A new scan is described which responds to ions that are intermediates in the dissociation of a mass-selected parent ion (mp) to give a mass-selected daughter ion (md). The scan gives a simple mass v. abundance output for ions which satisfy this condition. It is implemented here on a BEQQ hybrid mass spectrometer using, in sequence, collision-induced dissociation occurring at high energy in the first reaction region, and low-energy collisional activation in the collision quadrupole. The experiment provides information on reaction sequences not available from single scans of other types. In the several cases examined, it is demonstrated that, among many conceivable fragmentation routes connecting a parent ion with a particular fragment ion, only a few are significant. Examination of reaction intermediate spectra also appears to be a fruitful new approach to mechanistic questions, as illustrated by consideration of the behavior of several isomeric octanones. These new spectra also have analytical value: they show good signal-to-noise ratios and allow ready distinction between isobaric and isomeric ions. A comparison of the reaction intermediate spectrum with a daughter spectrum obtained by the B/E linked-scanning technique reveals the contributions of artifact peaks which result from poor parent ion mass resolution in the latter. Reaction intermediate spectra combine information from the daughter spectra of mp and the parent spectra of md and, as a specified portion of this data domain, have unique characteristics. 相似文献
900.
A series of 4-(2-keto-substituted)-3,4-dihydro-3-methyl-2H-1,3-benzoxazin-2-ones 1 (Table I) was synthesized by condensation of 3-alkyl-3,4-dihydro-4-hydroxy-2H-1,3-benzoxazin-2-ones 4 with ketones 5 having active alpha hydrogens. In the presence of alcoholic potassium borohydride, compounds 1 underwent reductive transacylation to give 1,3-oxazin-2-one derivatives 3 (Table III, a,b,c). When the other side of the ketone possessed substituents other than hydrogen, there were always also normal reduction products, i.e., secondary alcohols 2 (Table II) in addition to 3. 相似文献