全文获取类型
收费全文 | 4956篇 |
免费 | 643篇 |
国内免费 | 468篇 |
专业分类
化学 | 2473篇 |
晶体学 | 48篇 |
力学 | 109篇 |
综合类 | 21篇 |
数学 | 280篇 |
物理学 | 1485篇 |
无线电 | 1651篇 |
出版年
2024年 | 15篇 |
2023年 | 45篇 |
2022年 | 107篇 |
2021年 | 111篇 |
2020年 | 161篇 |
2019年 | 126篇 |
2018年 | 150篇 |
2017年 | 158篇 |
2016年 | 229篇 |
2015年 | 216篇 |
2014年 | 345篇 |
2013年 | 357篇 |
2012年 | 264篇 |
2011年 | 345篇 |
2010年 | 267篇 |
2009年 | 296篇 |
2008年 | 312篇 |
2007年 | 305篇 |
2006年 | 317篇 |
2005年 | 244篇 |
2004年 | 264篇 |
2003年 | 228篇 |
2002年 | 171篇 |
2001年 | 137篇 |
2000年 | 119篇 |
1999年 | 135篇 |
1998年 | 82篇 |
1997年 | 99篇 |
1996年 | 68篇 |
1995年 | 63篇 |
1994年 | 61篇 |
1993年 | 38篇 |
1992年 | 39篇 |
1991年 | 29篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 22篇 |
1987年 | 20篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 26篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有6067条查询结果,搜索用时 15 毫秒
941.
The LiRh-type crystal structure of the equiatomic intermetallic compound IrZn turns out to be a competitor to the otherwise energetically favored CsCl-type structures of congeneric transition metal zinc phases, thus enlightening the structural impact of element-specific factors beyond the Hume-Rothery concept. 相似文献
942.
Kate Lauder Silvia Anselmi Dr. James D. Finnigan Dr. Yuyin Qi Dr. Simon J. Charnock Dr. Daniele Castagnolo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(46):10422-10426
The enantioselective synthesis of α-thiocarboxylic acids by biocatalytic dynamic kinetic resolution (DKR) of nitrile precursors exploiting nitrilase enzymes is described. A panel of 35 nitrilase biocatalysts were screened and enzymes Nit27 and Nit34 were found to catalyse the DKR of racemic α-thionitriles under mild conditions, affording the corresponding carboxylic acids with high conversions and good-to-excellent ee. The ammonia produced in situ during the biocatalytic transformation favours the racemization of the nitrile enantiomers and, in turn, the DKR without the need of any external additive base. 相似文献
943.
944.
Amylopectin tris(phenylcarbamate) has been evaluated as a chiral stationary phase for HPLC; the influence on its cptical resolving capabilities of mobile phase composition and nature of the alcohol used as modifier has been studied. Separation and resolution of twelve arylalcohol racemates were examined. In most instances, the stationary phase exhibited high optical resolving capacity. 相似文献
945.
Masaru Mitsuda 《Tetrahedron letters》2006,47(46):8073-8077
Kinetic resolution of N-benzoylated vic-amino alcohols was achieved by benzoylation in the presence of copper triflate and (R,R)-Ph-BOX as catalysts. The observed enantioselectivity was moderate to high. The method was applied to a kinetic resolution of racemic prolinol and piperidinemethanol derivatives as well as an asymmetric desymmetrization of 2-amino-1,3-diol derivatives. 相似文献
946.
As the enantiomers of 1-phenylethanol are valuable intermediates in several industries, the lipase catalyzed kinetic resolution of (R,S) -1-phenylethanol is a relevant research topic. In this study, the goal was to determine the optimum reaction parameters to produce enantiomerically pure 1-phenylethanol by lipase (Novozyme 435) catalyzed kinetic resolution using response surface methodology (RSM). Reactions were performed with 40–400 mM (R,S)-1-phenylethanol, 120–1200 mM vinyl acetate and 2–22 mg/mL biocatalyst concentrations (BC L ), at 20–60 °C and with a stirring rate of 50–400 rpm for 5–120 min. The samples were analyzed using high performance liquid chromatography (HPLC) with a Chiralcel OB column. Optimum reaction parameters to reach 100% enantiomeric excess for the substrate ( ee s ) were determined as follows: substrate concentration (C s ): 240 mM, BC L : 11 mg/mL, at 42 °C with a reaction time of 75 min. Model validation was performed using these conditions and ee s was calculated as 100%, which indicates the predicted model was efficient and accurate. When compared to the literature, it was observed that the reaction time decreased significantly. This is an important result considering the industrial scale perspective. 相似文献
947.
T. Nagai K. Sakai T. Asaka A. Yamazaki Y. Matsui 《Journal of solid state chemistry》2006,179(6):1898-1903
The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI2-type CoO2 sheet and a rock-salt-type Sr2O2 double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [MmA2Om+2]qCoO2 with the formula of [Sr2O2]qCoO2, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO2 sheet and the Sr2O2 block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P21/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory. 相似文献
948.
J. Sivaguru Catherine Hooper Marissa R. Solomon Waldemar Adam Yoshihisa Inoue Nicholas J. Turro 《Tetrahedron》2006,62(46):10647-10659
The stereoselectivity in the reactions of the E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for singlet oxygen (1O2), ozone (O3), and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in a variety of solvents as a function of temperature. The oxidative cleavage of the alkenyl functionality by 1O2 and O3 releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the electronic nature of the oxidant. A high stereoselectivity, substantially dependent on solvent and temperature, is displayed for the reactions with 1O2, whereas the ground-state reactants O3 and PTAD are rather unaffected by solvent and temperature variations. The present comparative analysis clearly substantiates our hypothesis of stereoselective vibrational quenching of the attacking 1O2, whereas O3 and PTAD are only subject to steric impositions. The electronically excited 1O2 is sensitive to all three stereochemically relevant structural characteristics embodied in the chiral enecarbamates, namely the R/S configuration at the C4 position of the oxazolidinone chiral auxiliary, the Z/E geometry of the ‘alkene’ functionality, and R/S configuration at the C3′ position of the enecarbamate side chain. 相似文献
949.
950.
Córdova A Sundén H Xu Y Ibrahem I Zou W Engqvist M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(21):5446-5451
The origins of biological homochirality have intrigued researchers since Pasteur's discovery of the optical activity of biomolecules. Herein, we propose and demonstrate a novel alternative for the evolution of homochirality that is not based on autocatalysis and forges a direct relationship between the chirality of sugars and amino acids. This process provides a mechanism in which a racemic mixture of an amino acid can catalyze the formation of an optically active organic molecule in the presence of a sugar product of low enantiomeric excess. 相似文献