全文获取类型
收费全文 | 23532篇 |
免费 | 834篇 |
国内免费 | 124篇 |
专业分类
化学 | 15573篇 |
晶体学 | 243篇 |
力学 | 695篇 |
数学 | 2186篇 |
物理学 | 5793篇 |
出版年
2023年 | 131篇 |
2022年 | 406篇 |
2021年 | 568篇 |
2020年 | 428篇 |
2019年 | 433篇 |
2018年 | 382篇 |
2017年 | 360篇 |
2016年 | 675篇 |
2015年 | 577篇 |
2014年 | 815篇 |
2013年 | 1393篇 |
2012年 | 1699篇 |
2011年 | 1905篇 |
2010年 | 1206篇 |
2009年 | 1048篇 |
2008年 | 1605篇 |
2007年 | 1387篇 |
2006年 | 1431篇 |
2005年 | 1208篇 |
2004年 | 1072篇 |
2003年 | 899篇 |
2002年 | 849篇 |
2001年 | 528篇 |
2000年 | 474篇 |
1999年 | 331篇 |
1998年 | 210篇 |
1997年 | 205篇 |
1996年 | 277篇 |
1995年 | 190篇 |
1994年 | 203篇 |
1993年 | 205篇 |
1992年 | 159篇 |
1991年 | 125篇 |
1990年 | 127篇 |
1989年 | 97篇 |
1988年 | 68篇 |
1987年 | 75篇 |
1986年 | 51篇 |
1985年 | 74篇 |
1984年 | 50篇 |
1983年 | 42篇 |
1982年 | 63篇 |
1981年 | 62篇 |
1979年 | 46篇 |
1978年 | 45篇 |
1977年 | 34篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 36篇 |
1973年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
Synthesis and properties of amine‐containing poly(arylene ether sulfone) as an anion‐exchange matrix
Won‐Keun Son Hea‐Young Song Sang‐Hern Kim Hyung‐Joong Kim Whan‐Gi Kim 《Journal of polymer science. Part A, Polymer chemistry》2002,40(23):4281-4287
Poly(arylene ether sulfone) (PSF), showing good thermal stability and excellent mechanical properties, was synthesized as an anion‐exchange matrix. It was synthesized by the condensation polymerization between bisphenol A and 4,4′‐dichlorodiphenylsulfone. 1°‐Amine‐containing poly(arylene ether sulfone) (1°‐APSF) was synthesized by the reduction reaction of a nitrated PSF. Then, it was transferred to 3°‐amine‐containing poly(arylene ether sulfone) (3°‐APSF) by the alkylation of the amine of 1°‐APSF. The properties of PSF, 1°‐APSF, and 3°‐APSF were investigated by Fourier transform infrared, 1H NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The introduction of the 3°‐amine group into PSF increased the glass‐transition temperature but decreased thermooxidative stability. The ion‐exchange capacities of 1°‐APSF and 3°‐APSF were shown to be 2.24 and 2.86 mequiv/g, respectively. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4281–4287, 2002 相似文献
852.
Lee K Song H Kim B Park JT Park S Choi MG 《Journal of the American Chemical Society》2002,124(12):2872-2873
Reaction of Rh(6)(CO)(9)(dppm)(2)(mu(3)-eta(2),eta(2),eta(2)-C(60)) (1) with C(60) in refluxing chlorobenzene followed by treatment with CNR (R = CH(2)C(6)H(5)) at room temperature affords the first fullerene-metal sandwich complex Rh(6)(CO)(5)(dppm)(2)(CNR)(mu(3)-eta(2),eta(2),eta(2)-C(60))(2) (2). Compound 2 has been characterized by an X-ray diffraction study. Electrochemical study of 2 reveals six well-separated reversible redox couples localized at C(60) cages due to a strong electronic communication between the two C(60) centers via the Rh(6) cluster spacer. 相似文献
853.
Hwang IW Kamada T Ahn TK Ko DM Nakamura T Tsuda A Osuka A Kim D 《Journal of the American Chemical Society》2004,126(49):16187-16198
meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the F?rster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex. 相似文献
854.
The reaction of the Baylis-Hillman adducts 1b-f derived from o-nitrobenzaldehydes in trifluoroacetic acid in the presence of triflic acid (0.2 equiv.) afforded 3-substituted-4-hydroxyquinoline N-oxides 2b-e and 2a in good to moderate yields. The reaction mechanism was evidenced by the experiment with 1f, the Baylis-Hillman adduct of 2-nitrobenzaldehyde N-tosylimine, as the one involving N-hydroxyisoxazoline as the key intermediate. 相似文献
855.
The reaction of the anticancer active compound [Rh(2)(mu-O(2)CCH(3))(2)(bpy)(2)(CH(3)CN)(2)][BF(4)](2) (1) (bpy = 2,2'-bipyridine) with NaC(6)H(5)S under anaerobic conditions yields Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2).CH(3)OH (2), which was characterized by UV-visible, IR, and (1)H NMR spectroscopies as well as single-crystal X-ray crystallography. Compound 2 crystallizes as dark red platelets in the monoclinic space group C2/c with cell parameters a = 20.398(4) A, b = 11.861(2) A, c = 17.417(4) A, beta = 108.98 degrees, V = 3984.9(14) A(3), Z = 4. The main structural features are the presence of a [Rh(2)](4+) core with a Rh-Rh distance of 2.549(2) A bridged by two benzene thiolate ligands in a butterfly-type arrangement. The axial positions of the [Rh(2)](4+) core are occupied by two terminal benzene thiolates. Cyclic voltammetric studies of 2 reveal that the compound exhibits an irreversible oxidation at +0.046 V in CH(3)CN, which is in accord with the fact that the compound readily oxidizes in the presence of O(2). The fact that this unusual dirhodium(II/II) thiolate compound is formed under these conditions is an important first step in understanding the metabolism of dirhodium anticancer active compounds with thiol-containing peptides and proteins. 相似文献
856.
Theoretical insights into the mechanism of acetylcholinesterase-catalyzed acylation of acetylcholine
Acylation of acetylcholine (ACh) catalyzed by acetylcholinesterase (AChE) has been studied using high-level theoretical calculations on a model system that mimics the reaction center of the enzyme, and compared with uncatalyzed acylation reaction. The geometries of all the intermediates and transition states, activation energies, and solvent effects have been calculated. The calculations predict simultaneous formation of two short-strong hydrogen bonds (SSHB) in the rate-determining transition state structures [the first SSHB involves the hydrogen atom of Ser-200 (H(s)) and another involves the hydrogen atom of His-440 (H(h))]. In the intermediate states, the H-bond corresponding to H(h) involves SSHB, whereas the one corresponding to H(s) does not. 相似文献
857.
The multidentate dicarbollide ligand nido-7,8-(NMe2CH2)2-7,8-C2B9H11 has been prepared, structurally characterized, and employed in the preparation of the novel mono- and trimetallic titanium complexes [eta5:eta1-(NMe2CH2)C2B9H9CH2NMe2]Ti(NMe2)2 and [eta5:eta1-[(NMe2CH2)C2B9H9CH2NMe2]Ti(NMe2)]2-mu3-O-Ti(NMe2)2. 相似文献
858.
Jeong Hyun Yeum Byung Chul Ji Chul Joo Lee Ji Youl Lee Sang‐Seob Lee Sam Soo Kim Joon Ho Kim Won Seok Lyoo 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1103-1111
To prepare water‐soluble, syndiotacticity‐rich poly(vinyl alcohol) (PVA) microfibrils for various industrial applications, we synthesized syndiotacticity‐rich, low molecular weight PVA by the solution polymerization of vinyl pivalate (VPi) in tetrahydrofuran (THF) at low temperatures with 2,2′‐azobis(2,4‐dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of poly(vinyl pivalate) (PVPi). Effects of the initiator and monomer concentrations and the polymerization temperature were investigated in terms of the polymerization behaviors and molecular structures of PVPi and the corresponding syndiotacticity‐rich PVA. The polymerization rate of VPi in THF was proportional to the 0.91 power of the ADMVN concentration, indicating the heterogeneous nature of THF polymerization. The low‐temperature solution polymerization of VPi in THF with ADMVN proved to be successful in obtaining water‐soluble PVA with a number‐average degree of polymerization (Pn) of 300–900, a syndiotactic dyad content of 60–63%, and an ultimate conversion of VPi into PVPi of over 75%. Despite the low molecular weight of PVA with Pn = 800, water‐soluble PVA microfibrillar fibers were prepared because of the high level of syndiotacticity. In contrast, for PVA with Pn = 330, shapeless and globular morphologies were observed, indicating that molecular weight has an important role in the in situ fibrillation of syndiotacticity‐rich PVA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1103–1111, 2002 相似文献
859.
The substrate matching strategy is described as a new approach for effectively enhancing the lipase enantioselectivity in organic solvent. In the lipase-catalyzed transesterifications of 3a-c, higher enantioselectivities have been achieved using 1a-c, respectively, as the structurally matched acyl donors. 相似文献
860.
This paper discusses the kinetic simulation of TiCl4--coinitiated living carbocationic isobutylene (IB) polymerizations governed by dormant-active equilibria, using a mechanistic model. Two kinetic models were constructed from the same underlying mechanism: one using a commercial simulation software package (Predici®), and the other using the method of moments. Parameter estimation from experimental batch reactor data with Predici yielded a rate constant of propagation kp = 4.64 × 108 ± 2.75 × 108 L/mol s, with no constraints imposed. This agrees with kp data measured with diffusion clock and competition methods, but disagrees with kinetically obtained kp values. Estimation of rate constants with Predici® and the GREG parameter estimation software packages revealed that it was difficult to estimate the complete set of kinetic parameters, due to correlated effects of the parameters on model predictions. Estimability analysis confirmed that some of the strongly correlating parameters could not be estimated simultaneously using the available experimental data. Using kp = 6 × 108 ± 2.75 × 108 L/mol s measured by Mayr, and using starting estimates of other rate constants defined by experimentally observed correlations, yielded the set of rate constants required for the simulations. Both kinetic models yielded good agreement with experimental data, with the exception of Mw values that slightly diverged from the theoretically predicted ‘Mw−Mn = constant’ relationship. This may indicate the occurrence of a minor side reaction. However, the kp/k−1 = 17.5 L/mol average run length calculated from measured and simulated MWD data agrees well with earlier literature values. 相似文献