全文获取类型
收费全文 | 5749篇 |
免费 | 152篇 |
国内免费 | 42篇 |
专业分类
化学 | 4278篇 |
晶体学 | 34篇 |
力学 | 158篇 |
数学 | 637篇 |
物理学 | 836篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 85篇 |
2019年 | 75篇 |
2018年 | 55篇 |
2017年 | 54篇 |
2016年 | 106篇 |
2015年 | 96篇 |
2014年 | 99篇 |
2013年 | 230篇 |
2012年 | 358篇 |
2011年 | 436篇 |
2010年 | 205篇 |
2009年 | 166篇 |
2008年 | 364篇 |
2007年 | 351篇 |
2006年 | 384篇 |
2005年 | 402篇 |
2004年 | 301篇 |
2003年 | 248篇 |
2002年 | 264篇 |
2001年 | 87篇 |
2000年 | 98篇 |
1999年 | 64篇 |
1998年 | 63篇 |
1997年 | 53篇 |
1996年 | 94篇 |
1995年 | 71篇 |
1994年 | 55篇 |
1993年 | 44篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 29篇 |
1989年 | 29篇 |
1988年 | 29篇 |
1987年 | 29篇 |
1986年 | 25篇 |
1985年 | 42篇 |
1984年 | 46篇 |
1983年 | 39篇 |
1982年 | 36篇 |
1981年 | 49篇 |
1980年 | 43篇 |
1979年 | 24篇 |
1978年 | 47篇 |
1977年 | 30篇 |
1976年 | 34篇 |
1975年 | 43篇 |
1974年 | 31篇 |
1973年 | 31篇 |
排序方式: 共有5943条查询结果,搜索用时 15 毫秒
991.
Branstetter BK Mevissen SJ Pack AA Herman LM Roberts SR Carsrud LK 《The Journal of the Acoustical Society of America》2007,121(1):626-635
A bottlenose dolphin was tested on its ability to echoically discriminate horizontal angular differences between arrays of vertically oriented air-filled PVC rods. The blindfolded dolphin was required to station in a submerged hoop 2 radial m from the stimuli and indicate if an array with two rods (S+) was to the right or the left of a single rod (S-). The angular separation between the two rods (thetaw) was held constant within each experiment while the angle between the S+ and the S-stimuli (thetab) varied to produce angular differences (deltatheta= thetab-thetaw) ranging from 0.25 to 4 degrees. In experiment I, thetaw was maintained at 2 degrees and in experiment II, thetaw was maintained at 4 degrees. Resulting 75% correct thresholds (method of constant stimuli) were 1.5 and 0.7 degrees, respectively. The two main findings of this study are: (1) decreasing the number of targets does not aid in localization, and (2) increasing the space between the rods enhances localization. Taken as a whole, the experiments suggest dolphins have a well-developed ability to resolve spatial information through sonar. 相似文献
992.
Z Bi A P Bruner J Li K N Scott Z S Liu C B Stopka H W Kim D C Wilson 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,140(1):108-119
As alternatives to the fast Fourier transform, advanced parametric methods based on the damped sinusoidal data model have been devised to better quantify the nuclear magnetic resonance (NMR) spectroscopy time-domain data. Previously, linear prediction (LP) fitting methods using Householder triangularization and singular value decomposition (SVD) techniques have been applied to the NMR spectroscopy data analysis. In this paper, we propose an alternating optimization method to quantify the time-domain NMR spectroscopy data. The proposed algorithm uses the a priori knowledge of the possible frequency intervals of the damped sinusoids to obtain more accurate parameter estimates when the NMR spectroscopy data are obtained under low signal-to-noise ratio conditions and the peaks are close together. None of the LP and SVD type of methods can use such approximate a priori knowledge. We have shown with measured NMR spectroscopy data that the proposed algorithm can be used to obtain accurate parameter estimates of frequencies, amplitudes, and damping ratios of the damped sinusoids and therefore the ultimate fit of the spectrum by using the a priori knowledge about the possible frequency intervals of the damped sinusoids. 相似文献
993.
994.
2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant. Kinetic analysis revealed unusual rate-driving force relationships. Our analysis indicated a mechanism of multi-site concerted proton–electron transfer (MS-CPET) for all of these reactions. More recently, an alternative interpretation of the kinetic data was proposed to explain the unusual rate-driving force relationships, invoking a crossover from CPET to a stepwise mechanism with an initial intramolecular proton transfer (PT) (Costentin, Savéant, Chem. Sci., 2020, 11, 1006). Here, we show that this proposed alternative pathway is untenable based on prior and new experimental assessments of the intramolecular PT equilibrium constant and rates. Measurement of the fluorenyl 9-C–H pKa, H/D exchange experiments, and kinetic modelling with COPASI eliminate the possibility of a stepwise mechanism for C–H oxidation in the fluorenyl benzoate series. Implications for asynchronous (imbalanced) MS-CPET mechanisms are discussed with respect to classical Marcus theory and the quantum-mechanical treatment of concerted proton–electron transfer.2-Fluorenyl benzoates were recently shown to undergo C–H bond oxidation through intramolecular proton transfer coupled with electron transfer to an external oxidant. 相似文献
995.
Jing Su Thibault Cheisson Alex McSkimming Conrad A. P. Goodwin Ida
M. DiMucci Thomas Albrecht-Schnzart Brian L. Scott Enrique R. Batista Andrew J. Gaunt Stosh A. Kozimor Ping Yang Eric J. Schelter 《Chemical science》2021,12(40):13343
There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(tBuNO)py]−(interchangeable hereafter with [(tBuNO)py]−), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[(tBuNO)py]4 (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, 1H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [(tBuNO)py]− ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E1/2 = −2.26 V vs. Fc+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of UIV[(tBuNO)py]4 that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [(tBuNO)py]− ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [AmIII(μ2-(tBuNO)py)((tBuNO)py)2]2 (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-(tBuNO)py]−, in good agreement with experimental observations.The coordination modes and electronic properties of a strongly coordinating hydroxylaminato ligand with Np, Pu and Am were investigated.Complexes were characterized by a range of experimental and computational techniques. 相似文献
996.
997.
998.
999.
1000.