全文获取类型
收费全文 | 1726篇 |
免费 | 38篇 |
国内免费 | 8篇 |
专业分类
化学 | 1012篇 |
晶体学 | 8篇 |
力学 | 80篇 |
数学 | 306篇 |
物理学 | 366篇 |
出版年
2019年 | 13篇 |
2018年 | 18篇 |
2017年 | 11篇 |
2016年 | 30篇 |
2015年 | 33篇 |
2014年 | 34篇 |
2013年 | 84篇 |
2012年 | 69篇 |
2011年 | 62篇 |
2010年 | 37篇 |
2009年 | 38篇 |
2008年 | 68篇 |
2007年 | 61篇 |
2006年 | 74篇 |
2005年 | 61篇 |
2004年 | 57篇 |
2003年 | 53篇 |
2002年 | 65篇 |
2001年 | 22篇 |
2000年 | 25篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 22篇 |
1995年 | 24篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 26篇 |
1991年 | 23篇 |
1990年 | 35篇 |
1989年 | 12篇 |
1988年 | 26篇 |
1987年 | 23篇 |
1986年 | 17篇 |
1985年 | 40篇 |
1984年 | 29篇 |
1983年 | 32篇 |
1982年 | 26篇 |
1981年 | 26篇 |
1980年 | 32篇 |
1979年 | 30篇 |
1978年 | 31篇 |
1977年 | 34篇 |
1976年 | 43篇 |
1975年 | 20篇 |
1974年 | 17篇 |
1973年 | 13篇 |
1972年 | 15篇 |
1971年 | 9篇 |
1966年 | 9篇 |
排序方式: 共有1772条查询结果,搜索用时 0 毫秒
61.
Bakhmutova EV Bakhmutov VI Belkova NV Besora M Epstein LM Lledós A Nikonov GI Shubina ES Tomàs J Vorontsov EV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(3):661-671
The interaction of [NbCp(2)H(3)] with fluorinated alcohols to give dihydrogen-bonded complexes was studied by a combination of IR, NMR and DFT methods. IR spectra were examined in the range from 200-295 K, affording a clear picture of dihydrogen-bond formation when [NbCp(2)H(3)]/HOR(f) mixtures (HOR(f) = hexafluoroisopropanol (HFIP) or perfluoro-tert-butanol (PFTB)) were quickly cooled to 200 K. Through examination of the OH region, the dihydrogen-bond energetics were determined to be 4.5+/-0.3 kcal mol(-1) for TFE (TFE = trifluoroethanol) and 5.7+/-0.3 kcal mol(-1) for HFIP. (1)H NMR studies of solutions of [NbCp(2)H(2)(B)H(A)] and HFIP in [D(8)]toluene revealed high-field shifts of the hydrides H(A) and H(B), characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T(1) relaxation time measurements show preferential coordination of the alcohol to the central hydride H(A), but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T(1) data are in good accord with the results of DFT calculations. DFT calculations for the interaction of [NbCp(2)H(3)] with a series of non-fluorinated (MeOH, CH(3)COOH) and fluorinated (CF(3)OH, TFE, HFIP, PFTB and CF(3)COOH) proton donors of different strengths showed dihydrogen-bond formation, with binding energies ranging from -5.7 to -12.3 kcal mol(-1), depending on the proton donor strength. Coordination of proton donors occurs both to the central and to the lateral hydrides of [NbCp(2)H(3)], the former interaction being of bifurcated type and energetically slightly more favourable. In the case of the strong acid H(3)O(+), the proton transfer occurs without any barrier, and no dihydrogen-bonded intermediates are found. Proton transfer to [NbCp(2)H(3)] gives bis(dihydrogen) [NbCp(2)(eta(2)-H(2))(2)](+) and dihydride(dihydrogen) complexes [NbCp(2)(H)(2)(eta(2)-H(2))](+) (with lateral hydrides and central dihydrogen), the former product being slightly more stable. When two molecules of TFA were included in the calculations, in addition to the dihydrogen-bonded adduct, an ionic pair formed by the cationic bis(dihydrogen) complex [NbCp(2)(eta(2)-H(2))(2)](+) and the homoconjugated anion pair (CF(3)COO...H...OOCCF(3))(-) was found as a minimum. It is very likely that these ionic pairs may be intermediates in the H/D exchange between the hydride ligands and the OD group observed with the more acidic alcohols in the NMR studies. 相似文献
62.
Anthrone has been found to react with N-methyl maleimide in the presence of catalytic amounts of various chiral β-amino alcohols. The optically active cycloadduct 3a has been obtained in excellent yield. Several features of the reaction have been studied. 相似文献
63.
64.
Michèle Postel Félix Tomi Henri Li Kam Wah Laurence Mordenti Patricia Guillaume 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4):453-457
Abstract Fe(NO)2XL and Fe(NO)X2L nitrosyl iron complexes, X = Cl, I, and L = HMPA, dppe, PPh3, activate molecular O2 to yield nitrato complexes. The phosphorous ligand is decisive for the oxidative power of these nitrates: with HMPA or dppe, oxygen transfer occurs only to phosphines. On going from HMPA to PPh3 the single nitrato complex obtained, Fe(NO)2X(OPPh3)2 selectively epoxidizes cyclohexene, and this is the first example of oxygen transfer from a nitrato ligand to an olefin. 相似文献
65.
Henri J.-M. Dou Parina Hassanaly Jacky Kister Jacques Metzger 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):355-358
Abstract Alkylation via phase transfer catalysis of several ambident anions of the |N–C–S|? type leads exclusively to S-substitution. Yields obtained are better or equal to those given by conventional methods and experimental work-up is very much simplified compared to the latter. 相似文献
66.
Ilya Kuselman Elena Kardash Emil Bashkansky Francesca Pennecchi Stephen L. R. Ellison Karen Ginsbury Malka Epstein Aleš Fajgelj Yury Karpov 《Accreditation and quality assurance》2013,18(6):459-467
A new technique for quantification of human errors in chemical analysis using expert judgments is described. This technique is based on the house-of-security approach developed recently in the field of safety and security for prevention of terrorist and criminal attacks against an organization. The following relative quantification parameters (expressed in %) are proposed in the technique: (a) likelihood score of human error in a chemical analytical measurement/testing method, (b) severity score of human error for reliability of the test results, (c) importance score of a component of a laboratory quality system, and (d) effectiveness score of the quality system as a whole in preventing/blocking human error. As an example, 34 scenarios of human error in pH measurement of groundwater are discussed and quantified. 相似文献
67.
68.
We introduce a method for change detection under nonuniform changes of intensity using an improved least-squares method. A locally adaptive normalizing window is correlated with the two images, and a morphological postprocessing is then applied to isolate objects that have been added or removed from the scene. We use a modification of the least-squares solution to get rid of clutter caused by intensity changes that do not satisfy the model assumed for the least-squares solution. 相似文献
69.
Generalizations and Properties of the Principal Eigenvalue of Elliptic Operators in Unbounded Domains 下载免费PDF全文
Using three different notions of the generalized principal eigenvalue of linear second‐order elliptic operators in unbounded domains, we derive necessary and sufficient conditions for the validity of the maximum principle, as well as for the existence of positive eigenfunctions for the Dirichlet problem. Relations between these principal eigenvalues, their simplicity, and several other properties are further discussed. © 2015 Wiley Periodicals, Inc. 相似文献
70.