全文获取类型
收费全文 | 527篇 |
免费 | 24篇 |
国内免费 | 1篇 |
专业分类
化学 | 417篇 |
晶体学 | 11篇 |
力学 | 21篇 |
数学 | 25篇 |
物理学 | 78篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 6篇 |
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 16篇 |
2016年 | 25篇 |
2015年 | 16篇 |
2014年 | 28篇 |
2013年 | 52篇 |
2012年 | 38篇 |
2011年 | 41篇 |
2010年 | 22篇 |
2009年 | 25篇 |
2008年 | 34篇 |
2007年 | 21篇 |
2006年 | 14篇 |
2005年 | 23篇 |
2004年 | 20篇 |
2003年 | 8篇 |
2002年 | 14篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有552条查询结果,搜索用时 15 毫秒
71.
X-ray absorption spectroscopy (XAS) is widely used to explore the coordination environments and structures of metal complexes in aqueous solutions and disordered phases. Although soft-XAS studies on gaseous phases, solid phases and their interfaces have shown that XAS is a versatile tool in studying the functional group composition of organic molecules, the application of XAS to studying aqueous solutions is seriously limited because of experimental difficulties. In this report, using a modified synchrotron endstation geometry, we show how soft-XAS was used to study the changes in electronic states of reactive functional groups in a bacterial macromolecule, desferrioxamine B (desB, a hydroxamate siderophore) and its structural analogue (acetohydroxamic acid (aHa)). We collected C, N, and O near edge X-ray absorption fine structure (NEXAFS) spectra of these molecules in aqueous solutions and complemented their spectral interpretation with calculated X-ray spectra of "hydrated" aHa. The experimental spectra of desB are similar to those for aHa at the C, N, and O K-edges. In addition, the electronic transitions of amide and hydroxamate functional groups in the macromolecule can be distinguished from the N spectra. Small energy differences in the pi*(C=O)NO and the transitions at the C- and N-edges of aHa and desB indicate that the substituent attached to N in desB ((CH2)n) determines the electron density in the (C=O)NO core. As the solution pH increased, the pi*(C=O)NO transition of the hydroxamate group of these two molecules exhibit energy shifts at the C-, N-, and O-edges, which are consistent with increased electron delocalization in the (C=O)NO core of aHa (and desB), predicted from the calculations. The spectra of the aHa(H2O)3- anion also provide evidence for partial N-deprotonation at pH values usually attributed to an O-acid. These results indicate that soft-XAS is well suited for studying the electronic states of different functional groups in aqueous organic macromolecules. 相似文献
72.
Dalip Singh Mehta Kanchan Saxena Satish Kumar Dubey Chandra Shakher 《Journal of luminescence》2010,130(1):96-102
We report the measurement of coherence characteristics of light-emitting diodes (LEDs). Experiments were performed using red and green color LEDs directly illuminating the Young's double slit kept in the far-zone. Fourier transform fringe analysis technique was used for the measurement of the visibility of interference fringes from which the modulus of degree of spectral coherence was determined. Low degree of spectral coherence, typically 0.4 for red and 0.2 for green LED with double-slit separation of 400 μm was observed. A variable slit was then kept in front of the LEDs and the double slit was illuminated with the light coming out of the slit. Experiments were performed with various slit sizes and the visibility of the interference fringes was observed. It was found that visibility of the interference fringes changes drastically in presence of variable slit kept in front of LEDs and a high degree of spectral coherence, typically 0.85 for red and 0.8 for green LED with double-slit separation of 400 μm and rectangular slit opening of 500 μm was observed. The experimental results are compared with the theoretical counterparts. Coherence lengths of both the LEDs were also determined and it was obtained 5.8±2 and 24±4 μm for green and red LEDs, respectively. 相似文献
73.
Pankaj Jain Satish D. Joglekar Subhadip Mitra 《The European Physical Journal C - Particles and Fields》2008,57(4):671-680
We compute the two photon exchange contributions to elastic scattering of polarized electrons from target protons. We use
a non-local field theory formalism for this calculation. The formalism maintains gauge invariance and provides a systematic
procedure for making this calculation. The results depend on one unknown parameter,
. We compute the two photon exchange correction to the ratio of electric to magnetic form factors extracted using polarization
transfer experiments. The correction is found to be small if
. However, for larger values of
, the correction can be quite significant. The correction to the polarization transfer results goes in the right direction
to explain their difference with the ratio measured by the Rosenbluth separation method. We find that the difference between
the two experimental results can be explained for a wide range of values of the parameter
. We also find that the corrections due to two photon exchange depend on the photon longitudinal polarization ε. Hence, we predict an ε dependence of the form factor ratio extracted using the polarization transfer technique. Finally, we obtain a limit on
by requiring that the non-linearity in ε dependence of the unpolarized reduced cross section is within experimental errors. 相似文献
74.
Guy Mann Gandhesiri Satish Roman Meledin Ganga B. Vamisetti Ashraf Brik 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(38):13674-13683
Chemical protein synthesis and biorthogonal modification chemistries allow production of unique proteins for a range of biological studies. Bond‐forming reactions for site‐selective protein labeling are commonly used in these endeavors. Selective bond‐cleavage reactions, however, are much less explored and still pose a great challenge. In addition, most of studies with modified proteins prepared by either total synthesis or semisynthesis have been applied mainly for in vitro experiments with very limited extension to live cells. Reported here is an approach for studying uniquely modified proteins containing a traceless cell delivery unit and palladium‐based cleavable element for chemical activation, and monitoring the effect of these proteins in live cells. This approach is demonstrated for the synthesis of a caged ubiquitin‐aldehyde, which was decaged for the inhibition of deubiquitinases in live cells. 相似文献
75.
JPC – Journal of Planar Chromatography – Modern TLC - Standardization has become mandatory for global acceptance of herbal oriental medicines which lack validated methods of analysis... 相似文献
76.
H. Surya Prakash Rao Vijjapu Satish Silambarasan Kanniyappan Priyanka Kumari 《Tetrahedron》2018,74(41):6047-6056
An efficient and single-step iodine catalyzed and metal-free synthesis of di and tri-substituted 2-methylfuran derivatives were achieved from 1-popargyl-1,2-diols. Stereospecific synthesis of starting 1,2-diols was achieved by indium mediated Barbier type propargylation on corresponding keto-alcohols or by sodium borohydride mediated reduction of 2-hydroxy-2-propargyl ketones. The furan synthesis proceeded through iodine mediated 5-exo-trig cyclization, dehydration and reductive deiodination. The method was applied to the synthesis of 2-methylfuran fused to phenanthrene, pyrene and acenaphthylene rings. 相似文献
77.
Mahanta D Madras G Radhakrishnan S Patil S 《The journal of physical chemistry. B》2008,112(33):10153-10157
A method for the removal of anionic (sulfonated) dyes from aqueous dye solutions using the chemical interaction of dye molecules with polyaniline is reported. Polyaniline (PANI) emeraldine salt was synthesized by chemical oxidation. Sulfonated dyes undergo chemical interactions with the charged backbone of PANI, leading to significant adsorption of the dyes. This phenomenon of selective adsorption of the dyes by PANI is reported for the first time and promises a green method for removal of sulfonated organics from wastewater. The experimental observations from UV-vis spectroscopy, X-ray diffraction, and conductivity measurements rule out the possibility of secondary doping of polyaniline salt by sulfonated dye molecules. A possible mechanism for the chemical interaction between the polymer and the sulfonated dye molecules is proposed. The kinetic parameters for the adsorption of sulfonated dyes on PANI are also reported. 相似文献
78.
Recent and earlier models of electrical field flow fractionation (ELFFF) have assumed that the electric field within the fluid domain is governed by Laplace's equation. This assumption results in a linear potential and a spatially constant field across the channel and is generally true for very dilute systems and relatively high effective potentials. Experimental studies show, however, that the effective potential within the channel may be less than 1% of the applied potential; this is apparently due to double layer formation and charge buildup at the poles. In such cases, local analyte concentrations can, nonetheless, be orders of magnitude higher than the bulk mean and the local potential small, both of which can lead to a nonlinear spatial distribution of the field strength. In such cases Poisson's equation must be used rather than Laplace's equation. Steady-state ELFFF simulations were performed using a Poisson's equation-based model. The domain in which Laplace's equation is valid was identified and the effects of concentration and effective field strength on device performance were explored. 相似文献
79.
Thirty-four novel hybrid lupeol derivatives (4-18) were prepared and evaluated for antimalarial activity in vitro against Plasmodium falciparum. Three compounds (13d, 16a and 17a) have shown antimalarial activity at lower dose (10 microg mL(-1)) than lupeol. 相似文献
80.
Dimple Sharma Jaibir S. Yadav Satish Kumar K.C. Singh V.K. Sharma 《Thermochimica Acta》2008,475(1-2):8-14
Molar excess volumes, VE, molar excess enthalpies, HE, and speeds of sound, u, of o-toluidine (i) + cyclohexane or n-hexane or n-heptane (j) binary mixtures have been determined over entire range of composition at 308.15 K. Speeds of sound data have been utilized to predict isentropic compressibility changes of mixing, of (i + j) mixtures. The observed VE, HE and data have been analyzed in terms of Graph theory. The analysis of VE data by Graph theory reveals that o-toluidine exists as an associated molecular entity and (i + j) mixtures contain 1:1 molecular complex. It has been observed that VE, HE and values calculated by Graph theory compare well with their corresponding experimental values. The observed data have also been analyzed in term of Flory theory. 相似文献