全文获取类型
收费全文 | 3977篇 |
免费 | 201篇 |
国内免费 | 117篇 |
专业分类
化学 | 2254篇 |
晶体学 | 54篇 |
力学 | 166篇 |
综合类 | 6篇 |
数学 | 710篇 |
物理学 | 645篇 |
综合类 | 460篇 |
出版年
2024年 | 9篇 |
2023年 | 59篇 |
2022年 | 97篇 |
2021年 | 119篇 |
2020年 | 141篇 |
2019年 | 116篇 |
2018年 | 115篇 |
2017年 | 91篇 |
2016年 | 151篇 |
2015年 | 128篇 |
2014年 | 157篇 |
2013年 | 301篇 |
2012年 | 268篇 |
2011年 | 302篇 |
2010年 | 191篇 |
2009年 | 181篇 |
2008年 | 252篇 |
2007年 | 232篇 |
2006年 | 204篇 |
2005年 | 207篇 |
2004年 | 150篇 |
2003年 | 142篇 |
2002年 | 142篇 |
2001年 | 89篇 |
2000年 | 53篇 |
1999年 | 61篇 |
1998年 | 36篇 |
1997年 | 35篇 |
1996年 | 26篇 |
1995年 | 26篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1977年 | 4篇 |
1975年 | 4篇 |
1973年 | 4篇 |
1948年 | 3篇 |
排序方式: 共有4295条查询结果,搜索用时 10 毫秒
991.
(C6F5)3B Catalyzed Chemoselective and ortho‐Selective Substitution of Phenols with α‐Aryl α‐Diazoesters 下载免费PDF全文
Zhunzhun Yu Yongfeng Li Jiameng Shi Ben Ma Prof. Dr. Lu Liu Prof. Dr. Junliang Zhang 《Angewandte Chemie (International ed. in English)》2016,55(47):14807-14811
The development of an efficient method for the site‐selective substitution of unprotected phenols has long been considered as an attractive but challenging task. Herein, we describe a highly chemo‐ and ortho‐selective substitution reaction of phenols with α‐aryl α‐diazoacetates with commercially available (C6F5)3B as the catalyst. This reaction proceeds under simple and mild conditions with high efficiency, it features a wide substrate scope and can be easily scaled up. 相似文献
992.
Michael M. Pollard Prof. Dr. Matthijs K. J. ter Wiel Dr. Richard A. van Delden Dr. Javier Vicario Dr. Nagatoshi Koumura Dr. Coenraad R. van den Brom Dr. Auke Meetsma Dr. Ben L. Feringa Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(36):11610-11622
We report the synthesis of unidirectional light‐driven rotary molecular motors based on chiral overcrowded alkenes and their immobilisation on the surface of gold nanoparticles through two anchors. Using a combination of 1H and 13C NMR, UV/Vis and CD spectroscopy, we show that these motors preserve their photochemical and thermal behaviour after they have been attached to gold nanoparticles. Furthermore, we describe the synthesis of 2H‐ and 13C‐labelled derivatives that were used to verify the unidirectionality of the rotary cycle of these motors both in solution and while grafted to gold nanoparticles. Taken together, these data support the conclusion that these motors maintain their unidirectional rotary cycle when grafted to the surface of small (ca. 2 nm) gold nanoparticles. Thus, continuous irradiation of the system under appropriate conditions leads to unidirectional rotation of the upper half of the molecules relative to the entire nanoparticle. 相似文献
993.
Tabbene O Karkouch I Slimene IB Elfeddy N Cosette P Mangoni ML Jouenne T Limam F 《Applied biochemistry and biotechnology》2011,164(1):34-44
When cultured in minimal growth medium, the B38 strain of Bacillus subtilis did not exhibit any antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) clinical isolate. Coculturing B38 strain with viable MRSA cells weakly increased antibacterial activity production
(20 AU/ml). Addition of dead MRSA cells in a B38 culture, increased by 8-fold the B. subtilis strain antibacterial activity reaching 160 AU/ml against MRSA strain. This antibacterial activity recovered from cell-free
supernatants was stimulated by an autoinducing compound which is sensitive to the action of proteinase K suggesting a proteinaceous
nature. This compound was heat-stable till 80 °C and showed a molecular mass around 20 kDa as determined by SDS-PAGE. These
results suggest that the production of antibacterial compounds by B38 strain is dependent on the amount of the autoinducing
compound. 相似文献
994.
Lubbe AS Ruangsupapichat N Caroli G Feringa BL 《The Journal of organic chemistry》2011,76(21):8599-8610
A study is presented on the control of rotary motion of an appending rotor unit in a light-driven molecular motor. Two new light driven molecular motors were synthesized that contain aryl groups connected to the stereogenic centers. The aryl groups behave as bidirectional free rotors in three of the four isomers of the 360° rotation cycle, but rotation of the rotors is hindered in the fourth isomer. Kinetic studies of both motor and rotor functions of the two new compounds are given, using (1)H NMR, 2D-EXSY NMR, and UV-vis spectroscopy. In addition, we present the development of a new method for introducing a range of aryl substituents at the α-carbon of precursors for molecular motors. The present study shows how the molecular system can be photochemically switched between a state of free rotor rotation and a state of hindered rotation and reveals the dynamics of coupled rotary systems. 相似文献
995.
Reduced mixing generates oscillations and chaos in the oceanic deep chlorophyll maximum 总被引:2,自引:0,他引:2
Deep chlorophyll maxima (DCMs) are widespread in large parts of the world's oceans. These deep layers of high chlorophyll concentration reflect a compromise of phytoplankton growth exposed to two opposing resource gradients: light supplied from above and nutrients supplied from below. It is often argued that DCMs are stable features. Here we show, however, that reduced vertical mixing can generate oscillations and chaos in phytoplankton biomass and species composition of DCMs. These fluctuations are caused by a difference in the timescales of two processes: (1) rapid export of sinking plankton, withdrawing nutrients from the euphotic zone and (2) a slow upward flux of nutrients fuelling new phytoplankton production. Climate models predict that global warming will reduce vertical mixing in the oceans. Our model indicates that reduced mixing will generate more variability in DCMs, thereby enhancing variability in oceanic primary production and in carbon export into the ocean interior. 相似文献
996.
L. Beji L. Sfaxi B. Ismaïl A. Missaoui F. Hassen H. Maaref H. Ben Ouada 《Physica E: Low-dimensional Systems and Nanostructures》2005,25(4):636-642
A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I1 and I2, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I1 and I2 PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I1 PL band exhibits a red shift while the I2 PL band exhibits a blue shift with increasing injection levels. The I2 PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I2 PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states. 相似文献
997.
建立了顶空-固相微萃取(HS-SPME)结合全二维气相色谱-串联四极杆飞行时间高分辨质谱(GC×GC-Q-TOF MS)与化学计量学相结合的方法鉴别回收和原生的聚对苯二甲酸乙二醇酯(PET)。首先,对回收和原生PET中的挥发性有机物(VOCs)进行非靶向分析。然后根据正交偏最小二乘判别分析(OPLSDA)和T检验筛选出对鉴别贡献度高的标记物质,并基于所有物质、标记物质和高标准标记物质建立主成分分析(PCA)和线性判别分析(LDA)模型。结果表明,PCA模型很好地展示了回收PET组和原生PET组之间的差异,基于高标准标记物的LDA模型训练集和验证集的鉴别准确率分别达到100%和97.1%。该方法具有可行性、高稳定性和可预测性,能够达到鉴别回收PET材料的要求。此外,共检测和鉴定了468种挥发性有机物,其中31种挥发性有机物为对鉴别起重要作用的高标准标记物质。这些高标准标记物质的可能来源是食品、药品、化妆品、农药、塑料、工业和未知来源。 相似文献
998.
The pyrophosphate K2NiP2O7 has been synthesized by the classic ceramic method and characterized by X-ray diffraction, solid-state 31P magic angle spinning (MAS) NMR, and IR and electrical impedance spectroscopy. The solid-state 31P MAS NMR, performed at 121.49 MHz, shows two isotropic resonances at ?17.66 and ?19.94 ppm, revealing the existence of two phosphorus environments in the structure. The electrical conductivity and dielectric properties have been investigated in the frequency and the temperature range of 200 Hz–1 MHz and 603–728 K, respectively. The frequency dependence of the conductivity is interpreted using the augmented Jonscher relation. The close values of activation energies obtained from the analysis of hopping frequency and dc conductivity imply that the transport is through ion hopping mechanism. The charge carrier concentration in the investigated sample has been evaluated using the Almond–West formalism and shown to be independent of temperature. Thermodynamic parameters such as the free energy of activation ΔF, the enthalpy ΔH, and the change in entropy ΔS have been calculated. 相似文献
999.
Pineschi M Del Moro F Gini F Minnaard AJ Feringa BL 《Chemical communications (Cambridge, England)》2004,(10):1244-1245
For the first time, an excellent enantioselectivity has been obtained in the conjugate addition of hard organometallic reagents to alpha,beta-unsaturated lactams bearing appropriate protecting-activating groups on the nitrogen. 相似文献
1000.
Ben Mouaz A Lindheimer M Montet JC Zajac J Lagerge S 《Colloids and surfaces. B, Biointerfaces》2001,20(2):119-127
Bile salts play a central role in the promotion of cytotoxicity or cytoprotection. In this study, we examined the interaction of different bile salts with egg lecithin vesicles using 31P NMR spectroscopy. The effects of taurochenodeoxycholate (TCDC or 3alpha,7alpha,-dihydroxy-5beta-cholanoyl taurine, of tauroursodeoxycholate (TUDC) or 3alpha,7beta,-dihydroxy-5beta-cholanoyl taurine) and of taurobetamuricholate (TbetaMC or 3alpha,6beta,7beta,-trihydroxy-5beta-cholanoyl taurine), at various bile salt/lecithin ratios, were evaluated. From the percent 31P present in vesicles, the micellar capacity of bile salts to dissolve lecithin was determined. TCDC was incorporated into vesicles for bile salt/lecithin molar ratios lower than 0.62 while for TUDC and TbetaMC, the critical ratios were 0.94 and 1.1, respectively. The 31P chemical shift change was markedly larger with TCDC than that found with TUDC and TbetaMC. In order to specify the low interactions observed between hydrophilic bile salts and lecithin, we determined the intermixed micellar/vesicular bile salt concentrations (IMVC) of bile salt/lecithin solutions using rapid ultrafiltration-centrifugation for TUDC and lecithin solubility measurements for TUDC, TbetaMC and TCDC. The low IMVC obtained indicate that even hydrophilic bile salts were bound mostly to the mixed aggregates. In conclusion, the low disturbance in the arrangement of lecithin induced by TUDC and TbetaMC appears to be due to the interfacial location of these bile salts. TCDC (7alpha OH) penetrates more deeply in the membrane than the 7beta hydroxylated bile salts that may partly explain the distinct damaging effects of these bile salts. 相似文献