全文获取类型
收费全文 | 4517篇 |
免费 | 665篇 |
国内免费 | 382篇 |
专业分类
化学 | 3252篇 |
晶体学 | 51篇 |
力学 | 237篇 |
综合类 | 23篇 |
数学 | 527篇 |
物理学 | 1474篇 |
出版年
2024年 | 7篇 |
2023年 | 110篇 |
2022年 | 171篇 |
2021年 | 180篇 |
2020年 | 165篇 |
2019年 | 146篇 |
2018年 | 140篇 |
2017年 | 108篇 |
2016年 | 201篇 |
2015年 | 204篇 |
2014年 | 248篇 |
2013年 | 279篇 |
2012年 | 346篇 |
2011年 | 386篇 |
2010年 | 239篇 |
2009年 | 234篇 |
2008年 | 281篇 |
2007年 | 258篇 |
2006年 | 234篇 |
2005年 | 198篇 |
2004年 | 168篇 |
2003年 | 152篇 |
2002年 | 127篇 |
2001年 | 103篇 |
2000年 | 90篇 |
1999年 | 114篇 |
1998年 | 100篇 |
1997年 | 90篇 |
1996年 | 90篇 |
1995年 | 78篇 |
1994年 | 54篇 |
1993年 | 33篇 |
1992年 | 37篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 9篇 |
1978年 | 2篇 |
1976年 | 7篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有5564条查询结果,搜索用时 0 毫秒
101.
A novel support with artificially created recognition for the selective removal of proteins and for affinity chromatography 总被引:4,自引:0,他引:4
Summary Acrylamide and N,N-methylenebisacrylamide were copolymerized in the presence of a protein to form a gel which was pressed through a sieve. The gel particles obtained were packed into a chromatographic tube. The experimental conditions for the polymerization are such that the pores of the gel particles are large enough to permit the protein to diffuse out of the particles, so that the entrapped protein can be removed from the bed by washing with an aqueous solution. However the interaction with the matrix is so strong that the protein can be desorbed only by a buffer containing 0.5 M sodium chloride or by a 10% solution of acetic acid containing 10% SDS. When a sample containing the protein present during the polymerization was applied to the column along with other proteins this protein was the only one adsorbed. The technique worked selectively with hemoglobin, cytochrome C and transferrin. 相似文献
102.
The self‐assembly of NiCl2·6H2O with a diaminodiamide ligand 4,8‐diazaundecanediamide (L‐2,3,2) gave a [Ni(C9H20N4O2)(Cl)(H2O)] Cl·2H2O ( 1 ). The structure of 1 was characterized by single‐crystal X‐ray diffraction analysis. Structural data for 1 indicate that the Ni(II) is coordinated to two tertiary N atoms, two O atoms, one water and one chloride in a distorted octahedral geometry. Crystal data for 1: orthorhombic, space group P 21nb, a = 9.5796(3) Å, b = 12.3463(4) Å, c = 14.6305(5) Å, Z = 4. Through NH···Cl–Ni (H···Cl 2.42 Å, N···Cl 3.24 Å, NH···Cl 158°) and OH···Cl–Ni contacts (H···Cl 2.36 Å, O···Cl 3.08 Å, OH···Cl 143°), each cationic moiety [Ni(C9H20N4O2) (Cl)(H2O)]+ in 1 is linked to neighboring ones, producing a charged hydrogen‐bonded 1D chainlike structure. Thermogrametric analysis of compound 1 is consistent with the crystallographic observations. The electronic absorption spectrum of Ni(L‐2,3,2)2+ in aqueous solution shows four absorption bands, which are assigned to the 3A2g → 3T2g, 3T2g → 1Eg, 3T2g → 3T1g, and 3A2g → 3T1g transitions of triplet‐ground state, distorted octahedral nickel(II) complex. The cyclic volammetric measurement shows that Ni2+ is more easily reduced than Ni(L‐2,3,2)2+ in aqueous solution. 相似文献
103.
Ruojie Tao Shuangquan Zang Chongzhen Mei Qinglun Wang Benyong Lou Jingyang Niu Yanxiang Cheng Daizheng Liao 《Journal of Cluster Science》2003,14(4):459-469
Two heterometallic trinuclear complexes {[Cu(oxbp)]2Co(H2O)2}1.5DMF0.5H2O (complex 1) and {[Cu(oxbm)]2Co(H2O)2}2DMF (complex 2) were obtained from the self-organization of two new dissymmetrical oxamidato-bridged copper(II) building blocks [Cu(oxbp)]– and [Cu(oxbm)]–[H3oxbp=N-benzoato-N'-(3-aminopropyl)oxamido, H3oxbm=N-benzoato-N'-(2-amino-2-methylethyl)oxamido, DMF=dimethylformamide]. The crystal structure of complex 1 has been determined. Complex 1 crystallize in triclinic system, space group P-1, a=8.0609(16) Å, b=10.661(2) Å, c=22.279(5) Å, =85.32(3), =86.64(3), =70.90(3), and Z=1. The crystal structure of complex 1 consists of neutral trinuclear complex units, and hydrogen bond involved DMF and water molecules. Through the hydrogen bonds, weak coordination and CuCu weak interactions, complex 1 features a 2-D supramolecular structure. Magnetic susceptibility measurements (5–100 K) indicate that the central Co(II) and terminal copper metal ions are antiferromagnetically coupled with J=–28.09 and J=–29.70 cm–1 for complex 1 and 2, respectively. 相似文献
104.
离子交换树脂负载Ni—B无定形合金催化剂的制备与性能 总被引:2,自引:0,他引:2
制备了一种新型离子交换树脂负载的Ni-B无定形合金催化剂.用3种树脂作为催化剂载体,即弱酸型阳离子树脂D152,强酸型阳离子树脂D72及强碱型阴离子树脂D261.用XPS、TEM和ICP等技术对催化剂进行了表征.结果表明,催化剂上镍与带有功能基的载体树脂之间有着很强的相互作用,但其强度与所带的功能基有关.3种催化剂Ni2p3/2的XPS谱图上,NiB/D152催化剂的氧化态峰最小,而NiB/D261的氧化态峰最大.异丙醇脱氢反应活性实验证实了这一结果. 相似文献
105.
Huang HC Lee JF Chao HP Yeh PW Yang YF Liao WL 《Journal of colloid and interface science》2005,286(1):127-133
The influence of natural organic matter (NOM) constituents on contaminant distribution coefficients was evaluated by determining the Koc values of aromatic and aliphatic organic compounds (solutes) with clays modified with both aromatic- and aliphatic-rich organic constituents. The studied compounds consisted of naphthalene, phenanthrene, n-pentane, and 2,3,4-trimethylmethane; the solid samples comprised two clays with little organic content, kaolinite and Ca-montmorillonite. Two aliphatic surfactants and three aromatic dyes, sorbed to the clays, served as reference NOM constituents. For solutes of comparable water solubilities, the organic-carbon normalized distribution coefficients (Koc) of the aliphatic solutes between sorbed aliphatic organic matter and aqueous solution slightly exceed those of the aromatic solutes. By contrast, the aromatic solutes exhibited higher Koc values than did the aliphatic compounds with sorbed aromatic-rich organic matter. The difference in Koc values could be attributed to either comparable solubility parameters or the difference in the chemical structure between nonionic organic solutes and specific components of the simulated NOM. The much higher Koc values observed for the aromatic solutes indicate that the NOM composition is a major factor determining the NOC environmental distribution. 相似文献
106.
Bin Liu Yan Ouyang Qing-Lun Wang Dai-Zheng Liao Zong-Hui Jiang Shi-Ping Yan 《Transition Metal Chemistry》2005,30(8):964-967
Two novel oxamidato-bridged Mn[Cu(PMoxd)]3(ClO4)2 (1) Ni[Cu(PMoxd)]3(ClO4)2 (2) tetranuclear complexes were prepared and characterized by i.r., e.p.r., electronic spectra, cyclic voltammograms, and magnetic
properties. The magnetic analysis was carried out by means of the theoretical expression of the magnetic susceptibility deduced
from the spin Hamiltonian H=−2JSM(SCu1+ SCu2 + SCu3) (M=Mn, Ni), leading to J=−20.4 cm−1; −121.1 cm−1 for complexes (1) and (2) respectively. Magnetic measurements indicate that the overall magnetic behavior of the tetranuclear species are antiferromagnetic. 相似文献
107.
Ye Xu Dr. Huifeng Yao Lijiao Ma Ling Hong Jiayao Li Qing Liao Yunfei Zu Jingwen Wang Mengyuan Gao Prof. Long Ye Prof. Jianhui Hou 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(23):9089-9095
Decreasing the energy loss is one of the most feasible ways to improve the efficiencies of organic photovoltaic (OPV) cells. Recent studies have suggested that non-radiative energy loss ( ) is the dominant factor that hinders further improvements in state-of-the-art OPV cells. However, there is no rational molecular design strategy for OPV materials with suppressed . Herein, taking molecular surface electrostatic potential (ESP) as a quantitative parameter, we establish a general relationship between chemical structure and intermolecular interactions. The results reveal that increasing the ESP difference between donor and acceptor will enhance the intermolecular interaction. In the OPV cells, the enhanced intermolecular interaction will increase the charge-transfer (CT) state ratio in its hybridization with the local exciton state to facilitate charge generation, but simultaneously result in a larger . These results suggest that finely tuning the ESP of OPV materials is a feasible method to further improve the efficiencies of OPV cells. 相似文献
108.
Two new triterpene lactones, polysperlactones A ( 2 ) and B ( 3 ), were isolated from the stems of Kadsura polysperma, together with the known compounds heteroclitalactone D ( 1 ) and schisanlactone E ( 4 ). Their structures were elucidated by spectroscopic methods, including 2D‐NMR and HR‐MS techniques. The configuration of 1 was confirmed by X‐ray analysis. Compounds 2 and 3 are members of a rare class of 3,4‐secolanostane metabolites with ring‐expanded or cyclized structures, respectively. 相似文献
109.
High-performance liquid chromatography (HPLC) was combined with electrospray ionization tandem mass spectrometry (ESI-MS/MS) to develop a sensitive and selective method for the quantitative measurement of N(7)-(2-hydroxyethyl)guanine (N(7)-HEG) adducts in DNA obtained from ethylene oxide-exposed biological samples. Selected reaction monitoring (SRM) was used as the detection mode while the fragmentation product ion at m/z 152 generated from the precursor protonated N(7)-HEG (m/z 196) was monitored. The detection limits for N(7)-HEG were estimated by twofold serial dilution and determined to be 4 fmol in neat standard solution and 16 fmol when a matrix effect is considered. When the mass spectrometer was operated in the selected ion monitoring mode using only the first quadrupole (without MS/MS function), the detection limits increased to 128 fmol and 1 pmol (when matrix effect is considered), respectively. A good linear correlation (R(2) = 0.999) was observed for signal intensities obtained by injecting 16 fmol--33 pmol of N(7)-HEG into the HPLC/ESI-MS/MS system. Hep G2 cells were incubated for 8 h with medium containing various concentrations of ethylene oxide (ranging from 0.05 to 5.0 mM). A dose-response relationship was established, indicating that the adduct formation increases with the exposure level. The method shows potential, although the detection limit needs to be lowered for practical applications, for use in monitoring N(7)-HEG formation in other biological systems. 相似文献
110.
A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), phthalocyanine (Pc), porphycene (Pn), dibenzoporphycene (DBPn), and hemiporphyrazine (HPz) with iron (Fe) has been carried out using a density functional theory (DFT) method. The difference in the core size and shape of the macrocycle has a substantial effect on the electronic structure and properties of the overall system. The ground states of FeP and FePc were identified to be the 3A2g [(d(xy))2(d(z)2)2(d(pi))2] state, followed by 3E(g) [(d(xy))2(d(z)2)1(d(pi))3]. For FePz, however, the 3E(g)-3A2g energy gap of 0.02 eV may be too small to distinguish between the ground and excited states. When the symmetry of the macrocycle is reduced from D4h to D2h, the degeneracy of the d(pi) (d(xz), d(yz)) orbitals is removed, and the ground state becomes 3B2g [(d(xy))2(d(z)2)1(d(yz))2(d(xz))1] or 3B3g [...(d(yz))1(d(xz))2] for FePn, FeDBPn, and FeHPz. The calculations also show how the change of the macrocycle can influence the axial ligand coordination of pyridine (Py) and CO to the Fe(II) complexes. Finally, the electronic structures of the mono- and dipositive and -negative ions for all the unligated and ligated iron macrocycles were elucidated, which is important for understanding the redox properties of these compounds. The differences in the observed electrochemical (oxidation and reduction) properties between metal porphycenes (MPn) and metal porphyrins (MP) can be accounted for by the calculated results (orbital energy level diagrams, ionization potentials, and electron affinities). 相似文献