全文获取类型
收费全文 | 172755篇 |
免费 | 2200篇 |
国内免费 | 869篇 |
专业分类
化学 | 93208篇 |
晶体学 | 2494篇 |
力学 | 7670篇 |
综合类 | 22篇 |
数学 | 17871篇 |
物理学 | 54559篇 |
出版年
2021年 | 1049篇 |
2020年 | 1157篇 |
2019年 | 1174篇 |
2018年 | 1198篇 |
2017年 | 1201篇 |
2016年 | 2381篇 |
2015年 | 1981篇 |
2014年 | 2564篇 |
2013年 | 7804篇 |
2012年 | 6388篇 |
2011年 | 8069篇 |
2010年 | 4954篇 |
2009年 | 5105篇 |
2008年 | 7403篇 |
2007年 | 7381篇 |
2006年 | 7088篇 |
2005年 | 6613篇 |
2004年 | 5930篇 |
2003年 | 5096篇 |
2002年 | 5009篇 |
2001年 | 6072篇 |
2000年 | 4616篇 |
1999年 | 3519篇 |
1998年 | 2654篇 |
1997年 | 2651篇 |
1996年 | 2600篇 |
1995年 | 2299篇 |
1994年 | 2185篇 |
1993年 | 2051篇 |
1992年 | 2371篇 |
1991年 | 2330篇 |
1990年 | 2047篇 |
1989年 | 2013篇 |
1988年 | 2035篇 |
1987年 | 1971篇 |
1986年 | 1877篇 |
1985年 | 2784篇 |
1984年 | 2759篇 |
1983年 | 2217篇 |
1982年 | 2394篇 |
1981年 | 2284篇 |
1980年 | 2229篇 |
1979年 | 2175篇 |
1978年 | 2246篇 |
1977年 | 2204篇 |
1976年 | 2133篇 |
1975年 | 2122篇 |
1974年 | 2023篇 |
1973年 | 2108篇 |
1972年 | 1199篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Daniel C. Grohs 《Tetrahedron letters》2005,46(25):4373-4376
We have recently described the synthesis of diazabicyclo[4.X.0]alkanes and their use as ligands for the prostate specific membrane antigene (PSMA). The key step of our synthetic route toward these diazabicycloalkanes is an oxidative cleavage of a bicyclic diol moiety followed by the attack of a nitrogen nucleophile to the resulting intermediate bisaldehyde. We herein describe the mechanism of this ring closure and its stereochemical consequences. In addition, we report a convenient method for trapping intermediate bisaldehydes by Wittig reagents. This trapping allows the synthesis of 3,5-disubstituted proline derivatives, which are shown to be versatile precursors for functionalized diazabicycloalkane dipeptide mimetics. 相似文献
992.
Summary Thermal field-flow fractionation separates polymers with high selectivity according to their Soret coefficient,S
τ, hence, according to their molar mass, and therefore consitutes an efficeint physicochemical tool for the determination of
the Soret coefficient of a given polymer in the carrier liquid from its retention time. However, the polymer concentration
in the sample influences the retention time and, hence, the value ofS
τ derived from it. An experimental study of the influence of sample concentration on retention,S
τ, and peak shape was performed for the polystyrene-decalin system over a relatively large temperature domain and for various
molar masses.
It is found that the retention time and the value ofS
τ increase with increasing sample concentration, the more so as the cold wall temperature is lower. This appears to be in contradiction
with the general non-equilibrium thermodynamic expression derived for polymer-solvent systems with positive second virial
coefficients, such as the present system over the temperature range investigated. There seems to be a temperature for which
the dependence ofS
τ on sample concentration vanishes. This temperature is about 375 K for the polystyrene-decalin system. As the sample concentration
increases, the peak barycentre and the standard deviation increases. As the peaks are fronting, the skewness is negative and
becomes more negative as the sample concentration increases. The peak skewness appears to be a good indicator of the onset
of sample concentration effects. The threshold concentration, for which these effects begin to become significant, decreases
with increasing molar mass. 相似文献
993.
A simple, sensitive, selective, fast and inexpensive assay for the determination of diquat is proposed. The method is based on the reduction of the herbicide to a strongly fluorescent monocation radical with sodium dithionite. The initial rate of this reaction is directly proportional to the diquat concentration. The stopped-flow mixing technique was used because the kinetic data can be obtained in only 7 s, meaning that the method can be automated. The calibration graph is linear over the range 5-500 ng ml-1 and the precision (RSD) is close to 1.2%. The applicability of the method was demonstrated by determining the herbicide in different kinds of samples. 相似文献
994.
Solid state radioluminescent lighting 总被引:1,自引:0,他引:1
C. L. Renschler J. T. Gill R. J. Walko C. S. Ashley T. J. Shepodd S. T. Reed G. M. Malone L. E. Leonard R. E. Ellefson R. L. Clough 《Radiation Physics and Chemistry》1994,44(6):629-644
We have demonstrated novel types of tritium-powered, solid-phase radioluminescent (RL) light sources. These lights include: (1) all-organic formulations comprised of polystyrene and fluorescent organic dyes; (2) polydimethylsiloxane-based systems incorporating inorganic phosphors; and (3) inorganic aerogel-based systems doped with phosphors. Three principal design consideration for these new RL lights are described. These include: (1) incorporation of tritium into the solid matrix; (2) molecular engineering to maximize light-output efficiency; and (3) strategies for increasing longevity. The advanced RL lights, which are of interest due to potential advantages in efficiency, brightness and safety, are being developed for emergency lighting uses and for battery and lighting applications in remote locations. 相似文献
995.
T. P. Elango V. Ramakrishnan S. Vancheesan J. C. Kuriacose 《Journal of Chemical Sciences》1984,93(1):47-52
Rate constants for the reaction of carbonate radical with aniline and some parasubstituted anilines have been determined by
the flash photolysis technique. Using σ+ para values the rate constants at pH 8.5 correlate very well with the Hammett equation yielding ρ= − 1. The carbonate radical oxidises aniline giving the anilino radical. The products so formed have been identified through
studies under conditions of continuous irradiation. 相似文献
996.
M C Woodle P L Bustamante K E Zebrowski-Morrison P A Loach 《Photochemistry and photobiology》1984,40(4):525-531
Abstract Improvements in instrumentation and methodology have allowed us to collect data of high signal to noise and reliability on the kinetics of recovery of both light-induced absorbance changes and ESR signals at 95 K. The results obtained by the two methods are identical and can not be fit with a single exponential curve. The decay kinetics can be fit well with three exponential components which represent 85, 9 and 6% of the total change with rate constants of 29 s−1, 69 s−1 and 2.3 s−1, respectively. An interesting effect by molecular oxygen on the relaxation time of the donor cation radical was found by ESR measurements at low temperatures and higher microwave power. This interaction with oxygen could be blocked by addition of small amounts (e.g. 0.05%) of organic solvents such as ethanol. A variety of systems were examined including R. rubrum whole cells and chromatophores prepared from R. rubrum and Rps. sphaeroides. R. rubrum chromatophore samples were examined at high and low light intensities, at pH values from 6 to 10, in the presence and absence of air and after equilibration in D2O media. In all cases, the same decay kinetics were observed. It seems possible that the observed complex decay may be a characteristic of phototraps of all photosynthetic material and reflect fundamental structural and functional features yet to be uncovered. 相似文献
997.
We investigate the role that local motions and slow cooperative fluctuations have on the relaxation of the intrapair dipolar order in the nematic 5CB. With this purpose we present a theoretical and experimental systematic study which allow us to quantify the contribution from each type of molecular fluctuation to the intrapair dipolar order relaxation time, T(1D). The experimental work includes measurements of Zeeman and intrapair dipolar order relaxation times (T(1Z) and T(1D)) as a function of temperature at conventional NMR frequencies, in three complementary samples: normal and chain deuterated 4-n-pentyl-4(')-cyanobiphenyl (5CB and 5CB(d11)) and a mixture of normal 5CB and fully deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB(d19)), 50% in weight. Additionally we perform T(1Z) field-cycling Larmor frequency-dependent measurements to obtain the spectral density of the cooperative fluctuations. The obtained results are as follows. (a) The cooperative molecular fluctuations have a strong relative weight in the relaxation of the intrapair dipolar order state, even at Larmor frequencies in the range of conventional NMR. (b) Alkyl chain rotations are an important relaxation mechanism of the intrapair dipolar order at megahertz frequencies. (c) Intermolecular fluctuations mediated by translational self-diffusion of the molecules is not an efficient mechanism of relaxation of the intrapair dipolar order. 相似文献
998.
C. Vassalo 《Optical and Quantum Electronics》1984,16(5):427-433
Weakly-guiding, anisotropic step-index fibres are considered in the case where the permittivity tensor is everywhere diagonalized in the axes of the fibre. A method is shown which allows one to derive a mode and its dispersion equation with arbitrary accuracy in terms of the powers of the weak-guiding parameter. The method is applied to the derivation of the leakage loss of they-polarized LP01 mode when
x>y. 相似文献
999.
Ab initio and density functional calculations have been performed to gain a better understanding of the epoxide ring-opening reaction catalyzed by epoxide hydrolase. The S(N)2 reaction of acetate with 1S,2S-trans-2-methylstyrene oxide to provide the corresponding diol acetate ester was studied with and without general-acid catalysis. MP2 and DFT (B3LYP) calculations predict, for the noncatalyzed reaction, a central barrier of approximately 20-21 kcal/mol separating the reactants from products depending on which carbon center in the epoxide is undergoing attack. From these gas-phase reactions the immediate alkoxide products are not energetically far below their associated transition states such that the reaction is predicted to be endothermic. Inclusion of aqueous solvation effects via a polarizable continuum model predicts the activation barrier to increase by almost 10 kcal/mol due to the solvation of the acetate ion nucleophile. The activation barrier for the epoxide ring-opening reaction is reduced to approximately 10 kcal/mol when phenol, as the general-acid catalyst, is included in the gas-phase calculations. This is due to the immediate product being the neutral ester rather than the corresponding alkoxide. The transition state in the general-acid-catalyzed reaction is earlier than that for the noncatalyzed reaction and the reaction is highly exothermic. Molecular mechanics calculations of 1S,2S-trans-2-methylstyrene oxide in the active site of murine epoxide hydrolase show two possible binding conformations. Both conformers have the epoxide oxygen forming hydrogen bonds with the acidic hydrogens of the catalytic tyrosines (Tyr381 and Tyr465). These two conformations likely lead to different products since the nucleophile (Asp333-CO(2)(-)) is positioned to react with either carbon center in the epoxide. 相似文献
1000.
MoSe2 and WSe2 nanotubes are obtained by the reduction of the corresponding triselenides in hydrogen or by the decomposition of the ammonium selenometallates in a hydrogen atmosphere. 相似文献