全文获取类型
收费全文 | 766篇 |
免费 | 18篇 |
国内免费 | 33篇 |
专业分类
化学 | 768篇 |
晶体学 | 2篇 |
数学 | 5篇 |
物理学 | 42篇 |
出版年
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 10篇 |
2018年 | 10篇 |
2017年 | 8篇 |
2016年 | 6篇 |
2015年 | 9篇 |
2014年 | 12篇 |
2013年 | 39篇 |
2012年 | 13篇 |
2011年 | 13篇 |
2010年 | 9篇 |
2009年 | 16篇 |
2008年 | 14篇 |
2007年 | 19篇 |
2006年 | 13篇 |
2005年 | 8篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 453篇 |
2001年 | 56篇 |
2000年 | 16篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1982年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有817条查询结果,搜索用时 15 毫秒
71.
Ann M. Schmiedekamp Anthony Ginnetti Brian Piccione Kevin Cannon M. Dominic Ryan 《International journal of quantum chemistry》2007,107(6):1415-1429
Octahedral, six‐coordinate Co2+ can exist in two spin states: S = 3/2 and S = 1/2. The difference in energy between high spin (S = 3/2) and low spin (S = 1/2) is dependent on both the ligand mix and coordination stereochemistry. B3LYP calculations on combinations of neutral imidazole, NH3, and H2O ligands show that low‐spin isomers are stabilized by axial H2O ligands and in structures that also include trans pairs of equatorial NH3 and protonated imidazole ligands, spin crossover structures are predicted from spin state energy differences. Occupied Co d orbitals from the DFT calculations provide a means of estimating effective ligand strength for homoleptic and mixed ligand combinations. These calculations suggest that in a labile biological system, a spin crossover environment can be created. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
72.
Hydroxylated uroporphyrin I and urochlorin I derivatives formed by photochemical oxidation of uroporphyrinogen I were separated by high-performance liquid chromatography and fully characterized by electrospray ionization tandem mass spectrometry. The porphyrins and chlorins were identified by analysis of their product ion spectra with each hydroxylated derivative giving a characteristic collision-induced dissociation fragmentation pattern. The porphyrins and chlorins characterized were meso-hydroxyuroporphyrin I, alpha-hydroxypropionic acid uroporphyrin I, beta-hydroxypropionic acid uroporphyrin I, hydroxyacetic acid uroporphyrin I, trans-7-hydroxy-8-spirolactoneurochlorin I, cis-7-hydroxy-8-spirolactoneurochlorin I and trans- and cis-7,8-dihydroxyurochlorins I. 相似文献
73.
Farooq U Khan A Ahmad VU;Azhar-Ul-Haq Khan SS Farooq R Pervez A Shah MM 《Magnetic resonance in chemistry : MRC》2007,45(9):766-769
Two new tricyclic trans-clerodane diterpenoids trivially named as ballotenic acid A (1) and ballodiolic acid A (2) have been isolated from ethyl acetate fraction of Otostegia limbata. The structure assignments are based on (1)H and (13)C NMR spectra, 2D NMR (HMQC, HMBC, COSY, NOESY and NOE) techniques and comparison with the literature data. 相似文献
74.
Min Liu Jing‐Yi Ma Yanan Zhang Xiaolin Wang Hongna Zhao Aihua Du Man Yang Lingjie Meng Ming Deng Huichen Liu 《Biomedical chromatography : BMC》2016,30(3):288-293
A rapid and sensitive liquid chromatography–tandem mass spectrometric method was developed for the first time and validated for the determination of cefprozil diastereomers in human plasma. The plasma samples were prepared by protein precipitation using acetonitrile. Detection was performed using an electronic spray ion source in the negative ion mode, operating in the multiple reaction monitoring of the transitions m/z 388.0 to m/z 205.0 for cefprozil diastereomers and m/z 346.1 to m/z 268.1 for cephalexin (the internal standard). The calibration curves of cis‐cefprozil and trans‐cefprozil were linear in the ranges 0.125–16.0 µg/mL and 0.0403–1.72 µg/mL, respectively. The lower limits of quantification of cis‐ and trans‐cefprozil were 0.125 and 0.0403 µg/mL in human plasma, respectively. The intra‐ and inter‐day precisions of cis‐ and trans‐cefprozil were all <9.7%, and the accuracy ranged from 99.2 to 104.7% and from 100.6 to 102.2%, respectively. The validated method was successfully applied to a bioequivalence study of two cefprozil formulations in 24 healthy Chinese volunteers. The two cefprozil tablets were bioequivalent by measurement of cis‐, trans‐ and total cefprozil. We suggest that the bioequivalence of cefprozil formulations can be evaluated only using cis‐cefprozil as the analyte in future studies. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
75.
Fuzhou Wang Ryo Tanaka Zhengguo Cai Yuushou Nakayama Takeshi Shiono 《Macromolecular rapid communications》2016,37(16):1375-1381
α‐Diimine nickel complexes bearing bulky ortho‐sec‐phenethyl groups (bis{[N,N′‐(4‐methyl‐2,6‐di‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 1 ), bis{[N,N′‐(4,6‐dimethyl‐2‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 2 ), bis{[N,N′‐(4‐methyl‐2‐sec‐phenethylphenyl)imino]‐1,2‐dimethylethane}dibromonickel ( 3 )) and {bis[N,N′‐(2,4,6‐trimethylphenyl)imino]‐1,2‐dimethylethane}dibromidonickel ( 4 ) are used as a precatalyst for the polymerization of trans‐4‐octene upon activation with modified methylaluminoxane. These catalysts conduct chain‐walking polymerization of trans‐4‐octene to give polymers possessing propyl and butyl branches with high molecular weight and narrow molecular weight distribution. The branching structure depends on the nickel complex as well as the polymerization temperature, and the ratio of propyl branch was increased with increasing the bulkiness of the ligand and decreasing the polymerization temperature. Consequently, the most bulky 1 among the complexes used is found to polymerize trans‐4‐octene with high 1,5‐regioselectivity at −20 °C to give poly(1‐propylpentan‐1,5‐diyl).
76.
The title Schiff base compound,C19H19N3O3,a derivative of antipyrine,has been synthesized and characterized by elemental analysis,IR,and single-crystal X-ray diffraction.The crystal belongs to the monoclinic system,space group C2/c,with α = 27.935(5),b = 7.553(1),c =16.692(3) (A),β = 105.810(3)°,Z= 8,V= 3388.7(10) (A)3,Dc = 1.323 g/cm3,Mr = 337.37,λ(MoKα) =0.71073 (A),μ = 0.091 mm-1,F(000) = 1424,R = 0.0630 and wR = 0.1273.A total of 3513 unique reflections were collected,of which 2112 with I > 2σ(I) were observed.The molecular structure adopts a trans configuration about the C=N double bond.The pyrazoline and C(1)~C(6) pheny1 ring are approximately coplanar (mean deviation from the combined plane is 0.069(3) (A)),with the dihedral angle of 9.7(2)°.The preliminary biological tests show that the compound has high antibacterial activities. 相似文献
77.
LIN Hong-Wei 《结构化学》2007,26(7):773-776
A new Schiff base compound, C13H9Br2N3O2·CH3OH, isonicotinic acid [1-(3,5- dibromo-2-hydroxyphenyl)methylidene]hydrazide methanol, has been synthesized and characterized by elemental analysis, IR spectra, and single-crystal X-ray diffraction. The compound comprises a Schiff base moiety isonicotinic acid [1-(3,5-dibromo-2-hydroxyphenyl) methylidene]hydrazide and a methanol molecule. The crystal belongs to the triclinic system, space group P1 with a = 8.464(1), b = 9.511(2), c = 10.901(2) , α = 92.940(2), β = 110.456(2), γ = 96.040(2)o, Z = 2, V = 814.0(2) 3, Dc = 1.759 g/cm3, Mr = 431.09, λ(MoKα) = 0.71073 , μ = 4.994 mm-1, F(000) = 424, R = 0.0440 and wR = 0.1061. A total of 3284 unique reflections were collected, of which 2197 with I > 2σ(I) were observed. The molecule adopts a trans configuration about the C=N double bond. The dihedral angle between the benzene and pyridine rings is 22.0(4)o. The crystal structure is stabilized by intermolecular O-H···N and C-H···O hydrogen bonds, forming layers parallel to the ac plane. The preliminary biological tests show that the compound has potential antibacterial activities. 相似文献
78.
79.
A high‐performance liquid chromatographic method was developed for the analysis of 3'‐hydroxypterostilbene. This method involves the use of a Luna® C18 column with ultraviolet detection at 325 nm. The mobile phase consisted of acetonitrile, water and formic acid (50:50:0.01, v/v/v) with a flow rate of 0.8 mL/min. The calibration curves were linear over the range 0.5–100.0 µg/mL. The mean extraction efficiency was between 97.40 and 111.16%. The precision of the assay was 0.196–14.39% (RSD%), and within 15% at the limit of quantitation (0.5 µg/mL). The bias of the assay was <16% and within 15% at the limit of quantitation. This assay was successfully applied to pre‐clinical pharmacokinetic samples from rat urine and serum. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
80.