全文获取类型
收费全文 | 894篇 |
免费 | 71篇 |
国内免费 | 1篇 |
专业分类
化学 | 800篇 |
晶体学 | 1篇 |
力学 | 12篇 |
数学 | 108篇 |
物理学 | 45篇 |
出版年
2024年 | 1篇 |
2023年 | 7篇 |
2022年 | 16篇 |
2021年 | 20篇 |
2020年 | 40篇 |
2019年 | 23篇 |
2018年 | 12篇 |
2017年 | 15篇 |
2016年 | 53篇 |
2015年 | 45篇 |
2014年 | 70篇 |
2013年 | 65篇 |
2012年 | 88篇 |
2011年 | 84篇 |
2010年 | 60篇 |
2009年 | 53篇 |
2008年 | 46篇 |
2007年 | 51篇 |
2006年 | 46篇 |
2005年 | 39篇 |
2004年 | 33篇 |
2003年 | 18篇 |
2002年 | 23篇 |
2001年 | 16篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1971年 | 1篇 |
1932年 | 1篇 |
1916年 | 1篇 |
1915年 | 1篇 |
排序方式: 共有966条查询结果,搜索用时 0 毫秒
41.
Stefanie K. Nguyen Supaporn Sophonputtanaphoca Eugene Kim Michael H. Penner 《Applied biochemistry and biotechnology》2009,158(2):352-361
A low, but significant, fraction of the carbohydrate portion of herbaceous biomass may be composed of fructose/fructosyl-containing
components (“fructose equivalents”); such carbohydrates include sucrose, fructooligosaccharides, and fructans. Standard methods
used for the quantification of structural-carbohydrate-derived neutral monosaccharide equivalents in biomass are not particularly
well suited for the quantification of fructose equivalents due to the inherent instability of fructose in conditions commonly
used for hemicellulose/cellulose hydrolysis (>80% degradation of fructose standards treated at 4% sulfuric acid, 121°C, 1 h).
Alternative time, temperature, and acid concentration combinations for fructan hydrolysis were considered using model fructans
(inulin, β-2,1, and levan, β-2,6) and a grass seed straw (tall fescue, Festuca arundinacea) as representative feedstocks. The instability of fructose, relative to glucose and xylose, at higher acid/temperature combinations
is demonstrated, all rates of fructose degradation being acid and temperature dependent. Fructans are shown to be completely
hydrolyzed at acid concentrations well below that used for the structural carbohydrates, as low as 0.2%, at 121°C for 1 h.
Lower temperatures are also shown to be effective, with corresponding adjustments in acid concentration and time. Thus, fructans
can be effectively hydrolyzed under conditions where fructose degradation is maintained below 10%. Hydrolysis of the β-2,1
fructans at temperatures ≥50°C, at all conditions consistent with complete hydrolysis, appears to generate difructose dianhydrides.
These same compounds were not detected upon hydrolysis of levan, sucrose, or straw components. It is suggested that fructan
hydrolysis conditions be chosen such that hydrolysis goes to completion; fructose degradation is minimized, and difructose
dianhydride production is accounted for. 相似文献
42.
Dr. Robert J. Wilson Dr. Lies Broeckaert M. Sc. Fabian Spitzer Priv.‐Doz. Dr. Florian Weigend Prof. Dr. Stefanie Dehnen 《Angewandte Chemie (International ed. in English)》2016,55(39):11775-11780
Reaction of the binary Zintl anion (Sn2Sb2)2? with the β‐diketiminato complex [LCu(NCMe)] (L=nacnac=[(N(C6H3iPr2‐2,6)C(Me))2CH]?) in ethylenediamine or DMF affords the ternary cluster dimer {[CuSn5Sb3]2?}2 ( 1 ) as its [K(crypt‐222)]+ salt. The chemical formulation of 1 is supported by energy‐dispersive X‐ray spectroscopy (EDX) and quantum chemical calculations. Each monomeric part of the dimer represents a trimetallic “[CuSn5Sb3]2?” cluster, with an architecture in between a tricapped trigonal prism and a capped square antiprism. As shown by quantum chemical investigations, the presence of Sb atoms and, in particular, of Cu atoms in the cluster skeleton makes the monomeric unit behave like an inhomogeneous superatom, which clearly prefers to dimerize, thereby producing a relatively short, yet virtually non‐bonding Cu???Cu distance. 相似文献
43.
Niklas Rinn Dr. Jens P. Eußner Willy Kaschuba Dr. Xiulan Xie Prof. Dr. Stefanie Dehnen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(9):3094-3104
Reactions of R1SnCl3 (R1=CMe2CH2C(O)Me) with (SiMe3)2Se yield a series of organo‐functionalized tin selenide clusters, [(SnR1)2SeCl4] ( 1 ), [(SnR1)2Se2Cl2] ( 2 ), [(SnR1)3Se4Cl] ( 3 ), and [(SnR1)4Se6] ( 4 ), depending on the solvent and ratio of the reactants used. NMR experiments clearly suggest a stepwise formation of 1 through 4 by subsequent condensation steps with the concomitant release of Me3SiCl. Furthermore, addition of hydrazines to the keto‐functionalized clusters leads to the formation of hydrazone derivatives, [(Sn2(μ‐R3)(μ‐Se)Cl4] ( 5 , R3=[CMe2CH2CMe(NH)]2), [(SnR2)3Se4Cl] ( 6 , R2=CMe2CH2C(NNH2)Me), [(SnR4)3Se4][SnCl3] ( 7 , R4=CMe2CH2C(NNHPh)Me), [(SnR2)4Se6] ( 8 ), and [(SnR4)4Se6] ( 9 ). Upon treatment of 4 with [Cu(PPh3)3Cl] and excess (SiMe3)2Se, the cluster fragments to form [(R1Sn)2Se2(CuPPh3)2Se2] ( 10 ), the first discrete Sn/Se/Cu cluster compound reported in the literature. The derivatization reactions indicate fundamental differences between organotin sulfide and organotin selenide chemistry. 相似文献
44.
Thomas P J Linsinger Alexander Bernreuther Philippe Corbisier Marta Dabrio Håkan Emteborg Andrea Held Andrée Lamberty Gaida Lapitajs Marina Ricci Gert Roebben Stefanie Trapmann Franz Ulberth Hendrik Emons 《Accreditation and quality assurance》2007,12(3-4):167-174
The potential approaches for third-party assessment of reference material producers are revisited and the activities of the
Reference Materials (RM) Unit of the Institute for Reference Materials and Measurements (IRMM) to obtain accreditation to
ISO Guide 34 and ISO 17025 are described. Accreditation was related to the Unit as all matrix RM activities of the institute
are concentrated there. A management system was established that allows sufficient flexibility to be applicable to a wide
range of RMs while being precise enough to ensure compliance with ISO Guides 30, 31 and especially 34 and 35. Accreditation
was achieved in 2004 with independent scopes for testing and RM production and was confirmed and extended in 2005. The key
aspects of the RM Unit's management system for RM production are presented.
Presented at BERM-10, April 2006, Charleston, SC, USA 相似文献
45.
The use of temperature as a variable in liquid chromatography enables the facile alteration of eluotropic strength without the need to change solvent composition. The ability to change eluotropic strength via temperature alone means that thermal gradients can be used to mimic the effects of solvent gradients but without many of the unwanted effects of changes in solvent composition. Here we illustrate the use of thermal gradients as a means of controlling chromatographic separations using either constant flow or, with the flow rate increased to maintain isobaric conditions, constant pressure, performed using columns packed with 1.7 μm particles. A model is described that can be used to used to predict flow, pressure and temperature under gradient conditions. Practical experimental factors such as the need for post column cooling and the use of frit restrictors in order to obtain optimum results are described. 相似文献
46.
47.
48.
49.
Leonard S Van Schepdael A Iványi T Lázár I Rosier J Vanstockem M Vermeersch H Hoogmartens J 《Electrophoresis》2005,26(3):627-632
A capillary electrophoretic (CE) method was developed for the separation of diastereoisomers of a new human immunodeficiency virus (HIV) protease inhibitor TMC114. In total 16 isomers of this drug have been synthesized (eight pairs of enantiomers). We succeeded in the separation of the eight diastereoisomers, but no enantiomers could be separated. Because of the high similarity and water-insolubility of these isomers, the separation is a real challenge. Different CE modes were tried out: capillary zone electrophoresis (CZE), nonaqueous capillary electrophoresis (NACE), micellar electrokinetic capillary chromatography (MEKC), and microemulsion electrokinetic capillary chromatography (MEEKC). Only MEEKC offered resolution of these compounds. 相似文献
50.
Two libraries of alpha-substituted alkynes has been prepared on solid phase using a sequential Sonogashira/Nicholas reaction approach. The scope of nucleophiles in the Nicholas reaction on solid phase has been investigated, including carbon, oxygen, nitrogen, sulfur, fluoride, and hydride nucleophiles. The conditions for the reaction sequence have been optimized in terms of Lewis acid, catalyst for the Sonogashira step, temperature, reaction time, and decomplexation method, enabling the five-step sequence to be performed in 1 day. 相似文献