全文获取类型
收费全文 | 8098篇 |
免费 | 256篇 |
国内免费 | 64篇 |
专业分类
化学 | 5875篇 |
晶体学 | 46篇 |
力学 | 224篇 |
数学 | 970篇 |
物理学 | 1303篇 |
出版年
2023年 | 42篇 |
2022年 | 43篇 |
2021年 | 124篇 |
2020年 | 146篇 |
2019年 | 150篇 |
2018年 | 107篇 |
2017年 | 107篇 |
2016年 | 201篇 |
2015年 | 148篇 |
2014年 | 167篇 |
2013年 | 360篇 |
2012年 | 519篇 |
2011年 | 623篇 |
2010年 | 311篇 |
2009年 | 234篇 |
2008年 | 498篇 |
2007年 | 527篇 |
2006年 | 541篇 |
2005年 | 520篇 |
2004年 | 404篇 |
2003年 | 324篇 |
2002年 | 316篇 |
2001年 | 116篇 |
2000年 | 125篇 |
1999年 | 103篇 |
1998年 | 83篇 |
1997年 | 77篇 |
1996年 | 109篇 |
1995年 | 77篇 |
1994年 | 65篇 |
1993年 | 57篇 |
1992年 | 62篇 |
1991年 | 56篇 |
1990年 | 36篇 |
1989年 | 43篇 |
1988年 | 45篇 |
1987年 | 36篇 |
1985年 | 60篇 |
1984年 | 60篇 |
1983年 | 45篇 |
1982年 | 45篇 |
1981年 | 65篇 |
1980年 | 56篇 |
1979年 | 34篇 |
1978年 | 61篇 |
1977年 | 39篇 |
1976年 | 44篇 |
1975年 | 48篇 |
1974年 | 36篇 |
1973年 | 37篇 |
排序方式: 共有8418条查询结果,搜索用时 93 毫秒
101.
Abstract Experiments were performed to ascertain whether superoxide anion (O2 − ) was produced by the photodynamic activation of hematoporphyrin derivative (HPD). Three different systems were utilized to detect formation of O2 − , oxidation of epinephrine to adrenochrome, reduction of cytochrome c and reduction of nitro blue tetrazolium (NBT). The effects on these detectors under identical conditions for HPD + h ν were compared to those obtained with two O2 − generating systems, riboflavin + by and xanthine-xanthine oxidase, and to a singlet oxygen generating system, photoradiation of methylene blue. The results indicated that HPD + hv differed from the two O2 − generating systems in failing to reduce cytochrome c or NET, and that HPD + h ν was similar to the behavior of methylene blue + h ν . In addition, HPD + h ν but not the O2 − generating systems could inhibit mitochondrial cytochrome c oxidase activity. We conclude that the photodynamic activation of HPD does not produce O2 − as a major oxygen radical and that the effects of HPD + h ν on mitochondrial cytochrome c oxidase are not caused by O2 − . 相似文献
102.
Emtenäs H Carlsson M Pinkner JS Hultgren SJ Almqvist F 《Organic & biomolecular chemistry》2003,1(8):1308-1314
Optically active bicyclic beta-lactams were synthesized, starting from 2-H-delta 2-thiazolines and Meldrum's acid derivatives. Several methods to accomplish an ester hydrolysis without damaging the beta-lactam framework were investigated. A rapid CsOH saponification of the beta-lactam methyl esters was developed and protonation of the Cs-carboxylates by Amberlite (IR-120 H+) afforded a series of bicyclic beta-lactam carboxylic acids. Moreover, a convenient method for the synthesis of 2-H-delta 2-thiazolinecarboxylic acid methyl ester 2 was developed. Bicyclic beta-lactam carboxylic acids 7a-g and aldehydes 4a-d were screened for their affinity to the bacterial periplasmic chaperone PapD using a surface plasmon resonance technique. beta-Lactams substituted with large acyl substitutents showed better binding to the chaperone than the native C-terminal peptide PapG 8, demonstrating that bicyclic beta-lactams constitute a new class of potential bacterial chaperone inhibitors. 相似文献
103.
Garth R. Giesbrecht Gavin E. Collis John C. Gordon David L. Clark Brian L. Scott Ned J. Hardman 《Journal of organometallic chemistry》2004,689(13):2177-2185
Use of the bulky cyclopentadienyl ligand [η5-C5H2(SiMe3)3-1,2,4]− (Cp?) allows for the isolation of monomeric, mono-ring lanthanide species. As previously reported, (Cp?)K reacts with LaI3(THF)4 (THF=tetrahydrofuran) in THF/pyridine to form the mono-ring complex (Cp?)LaI2(py)3 (1) (py=pyridine); a minor product of this reaction is the bis-ring species (Cp?)2LaI(py) (2). The solid state structure of 2 reveals a monomeric compound containing a pseudo-tetrahedral metal center exhibiting no unusual intramolecular contacts. Addition of one equiv of KNHAr (Ar=2,6-iPr2C6H3) to complex 1 in THF generates the mono-anilido compound (Cp?)LaI(NHAr)(THF)2 (3), which may be converted to the more stable pyridine adduct (Cp?)LaI(NHAr)(py)2 (4) by the addition of pyridine to 3. An X-ray crystal structure of 3 indicated a trigonal bipyramidal metal center with the anilido group oriented trans to the iodide atom (N1-La1-I1=123.1(3)°). A structural study on the bis-pyridine adduct 4 revealed a similar Cs-symmetric structure with a slightly increased Nanilido-La-I angle of 132.1(2)°. Addition of KNHAr to the di-iodo bipyridine adduct (Cp?)LaI2(bipy)(py) (5), in which the two iodide atoms are cis-disposed, yields the mono-anilido complex (Cp?)LaI(NHAr)(bipy)(py) (6) (bipy=2,2′-bipyridine); this compound may also be prepared by the addition of bipy to (Cp?)LaI(NHAr)(py)2 (4). An X-ray diffraction study shows that the lanthanum center in 6 is octahedrally coordinated by a Cp? ring, an iodide, an anilido group, a pyridine molecule and two nitrogens of a bipy molecule. In this case, the anilido moiety and the iodide ligand are arranged in a cis fashion (Nanilido-La-I=111.2(2)°), resulting in a complex with C1 symmetry. Both (Cp?)LaI(NHAr)(py)2 (4) and (Cp?)LaI(NHAr)(bipy)(py) (6) are inactive as catalysts for the hydroamination/cyclization of 2-amino-hex-5-ene. 相似文献
104.
105.
Various phenylsulfonyl allene derivatives were prepared with double bonds tethered either to the alpha-position or the gamma-position of the allene. These substrates underwent a highly regio- and stereospecific thermal [2 + 2]-cycloaddition across the nonactivated cumulene double bond, forming distal cycloadducts (i.e., 57) in the case of alpha-tethered allenes and proximal adducts (i.e., 25) in the case of gamma-tethered allenes. The mechanistic rationale for the observed stereospecificity involves initial diradical formation, followed by a rapid ring closure to the more stable cis-fused ring system. The tether may be equipped with heteroatoms, allowing for the formation of fused heterocycles (e.g., 61), and the cycloaddition can be facilitated by the introduction of sterically bulky groups and/or by conformational rigidity to the tether. Other modes of cyclization were observed in the presence of sodium benzenesulfinate or Lewis acids, in which cases polar mechanisms prevail. The chemoselectivity is reversed for [4 + 2]-cycloadditions, which prefer instead to engage the vinyl sulfone moiety, independent of whether the tether is attached to the alpha- or gamma-position of the allene. 相似文献
106.
George P. Scott Abdelaziz M. R. Elghoul 《Journal of polymer science. Part A, Polymer chemistry》1970,8(8):2255-2263
Chain-transfer constants for radicals one to five units in length were determined for the methyl acrylate–isopropanethiol system. Effects of varying ester alkyl group, mercaptan solvent and initiator on three-unit diastereomer ratio were also investigated. It was found that the proportion of syndiotactic isomer increased as the size of either the ester alkyl group or the mercaptan increased, the latter result being equivalent to a penultimate effect. Polar solvents, on the other hand, favored the formation of the isotactic form. This effect was observed in the free-radical telomerization and to an even greater degree in the base-initiated anionic telomerization. Possible reasons for solvent effects are discussed. 相似文献
107.
As part of a fundamental study of the behaviour of mixed plastics during reprocessing and in service, blends of low density polyethylene (LDPE) and polyvinyl chloride (PVC) have been investigated. It was found that Young's modulus increased steadily from pure LDPE to pure PVC whereas both tensile strength at break and elongation at break passed through a minimum at about 5% PVC. Optical and scanning electron microscope studies have related this mechanical behaviour to morphological changes in the two phase system under stress. 相似文献
108.
Binoy K. Bordoloi Eli M. Pearce L. Blight B. R. Currell R. Merrall R. A. M. Scott C. Stillo 《Journal of polymer science. Part A, Polymer chemistry》1980,18(2):383-406
Liquid sulfur–olefinic reactions at 140°C forming crosslinked polysulfide polymers have been investigated. A systematic approach to the characterization and some structure–property studies on these new polymeric materials has been presented. The olefinic systems of interest are endo-and exo-dicyclopentadiene and an oligomeric alkenyl polysulfide. Equimolar S8–olefinic system copolymers are amorphous and they show no tendency for sulfur crystallization. A correlation has been drawn between chemical structure and glass transition temperature of the copolymers and terpolymers. Mechanical properties and, in particular, chemical stress relaxation of the crosslinked polysulfide polymers have been investigated. 相似文献
109.
The results of Fenske-Hall molecular orbital calculations are reported for the trigonal prismatic complexes Mo(S(2)C(2)H(2))(3) and Mo(S(2)C(6)H(4))(3). Both complexes exhibit a bend of the S-C-C-S ligand plane away from the S-Mo-S plane. A series of calculations which systematically follow the changes in electronic structure as the bend angle alpha is varied between 0 and 30 degrees indicates that the bend can be attributed to a second order Jahn-Teller distortion. The driving force for this distortion, which allows mixing between a set of ligand pi orbitals and the metal d(z)()()2 orbital, should be greatest for d(0) systems. In these systems the bent geometry leads to the stabilization of the doubly occupied HOMO. The driving force for ligand bending should be lower in systems having more or fewer electrons (e.g. Re(S(2)C(2)Ph(2))(3) or V(S(2)C(2)Ph(2))(3), respectively). While the steric bulk of the dithiolate ligands in the latter complexes may also influence the degree of ligand bending, this is probably a secondary effect. 相似文献
110.
Sternfeld T Thilgen C Hoffman RE Del Rosario Colorado Heras M Diederich F Wudl F Scott LT Mack J Rabinovitz M 《Journal of the American Chemical Society》2002,124(20):5734-5738
Reduction of the two "closed" [6,6] methanofullerenes, [6,6]C(61)H(2) (1) and [6,6]C(71)H(2) (5), to the corresponding hexaanions with lithium metal causes the bridgehead-bridgehead bonds to open, at least partially, and this change gives rise to diamagnetic ring currents in the resulting homoconjugated six-membered rings (6-MRs). These new ring currents shield the overlying hydrogen atoms on the methylene bridge and induce upfield shifts of 1.60 and 0.11 ppm in their (1)H NMR resonances, respectively. Analogous reduction of the already "open" [5,6]methanofullerenes, [5,6]C(61)H(2) (2) and [5,6]C(71)H(2) (3 and 4), only slightly enhances the shielding of the hydrogen atoms over the homoconjugated 6-MRs (upfield shifts of 0.13, 0.68, and 0.14 ppm, respectively) but leads to exceptionally strong diamagnetic ring currents in the homoconjugated five- membered rings (5-MRs), as evidenced by dramatic shielding of the hydrogen atoms situated over them (upfield shift of 5.01, 6.78, and 1.63 ppm, respectively). The strongest shielding is seen for the hydrogen atom sitting over the 5-MR at the pole of C(71)H(2)(6)(-) (delta = -0.255 ppm) indicating that the excess charge density is concentrated at the poles. 相似文献