全文获取类型
收费全文 | 3708篇 |
免费 | 88篇 |
国内免费 | 17篇 |
专业分类
化学 | 2288篇 |
晶体学 | 17篇 |
力学 | 197篇 |
数学 | 760篇 |
物理学 | 551篇 |
出版年
2023年 | 21篇 |
2022年 | 53篇 |
2021年 | 102篇 |
2020年 | 75篇 |
2019年 | 53篇 |
2018年 | 67篇 |
2017年 | 59篇 |
2016年 | 112篇 |
2015年 | 107篇 |
2014年 | 83篇 |
2013年 | 201篇 |
2012年 | 210篇 |
2011年 | 256篇 |
2010年 | 146篇 |
2009年 | 126篇 |
2008年 | 207篇 |
2007年 | 224篇 |
2006年 | 168篇 |
2005年 | 162篇 |
2004年 | 155篇 |
2003年 | 126篇 |
2002年 | 102篇 |
2001年 | 48篇 |
2000年 | 32篇 |
1999年 | 40篇 |
1998年 | 37篇 |
1997年 | 35篇 |
1996年 | 37篇 |
1995年 | 31篇 |
1994年 | 31篇 |
1993年 | 48篇 |
1992年 | 19篇 |
1991年 | 28篇 |
1990年 | 29篇 |
1989年 | 18篇 |
1988年 | 26篇 |
1987年 | 28篇 |
1986年 | 24篇 |
1985年 | 47篇 |
1984年 | 46篇 |
1983年 | 26篇 |
1982年 | 26篇 |
1981年 | 31篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 27篇 |
1977年 | 24篇 |
1976年 | 27篇 |
1975年 | 14篇 |
1974年 | 24篇 |
排序方式: 共有3813条查询结果,搜索用时 0 毫秒
71.
N. Bjerre A. O. Mitrushenkov P. Palmieri P. Rosmus 《Theoretical chemistry accounts》1998,100(1-4):51-59
The spin-orbit and the spin-spin coupling constants of the 4Πg state of the He2
− ion, of the parent a3Σu
+, and of the b3Πg states of He2 have been evaluated by a multireference configuration interaction method. The theoretical spin-spin splittings of the a3Σu
+ state and the R-dependent spin-spin function are found to be in excellent agreement with experiment, with deviations in the range of a few
MHz. The theoretical spin-orbit constants and splittings of the b3Πg state are larger than the experimental values by about 370 MHz. The spin-orbit coupling constant of the 4Πg state of He2
− is␣estimated to be three times smaller than in the b3Πg state, but one of the intramultiplet off-diagonal spin-spin interactions is predicted to give a large contribution to the
fine structure of the metastable ion. The theoretical fine structure constants for the He2
− ion are expected to␣aid future spectroscopic investigations of the fine structure splittings of the negative ion.
Received: 14 April 1998 / Accepted: 27 July 1998 / Published online: 19 October 1998 相似文献
72.
Mario Di Braccio Giorgio Roma Gian Carlo Grossi Giovanni Ciarallo 《Journal of heterocyclic chemistry》1992,29(1):25-31
The reaction of 2-[(N-acyl, N-alkyl or phenyl)amino]-4H-pyrido[1,2-a]pyrimidin-4-ones 8a-g with the N,N-dimethylformamide/phosphorus oxychloride Vilsmeier reagent 1 (95°, 90 minutes) afforded 1-alkyl or phenyl-2H-dipyrido[1,2-a:2′,3′-d]pyrimidine-2,5(1H)?diones, 3-alkyl substituted or not, 10a-g . The starting compounds 8 were prepared by treating 2-amino-4H-pyrido[1,2-a]pyrimidin-4-ones N-alkyl substituted 7a,b or N-phenyl substituted 4 with excess anhydrides (130°, 7 hours) when the 2-(alkylamino) derivatives 7 were used in the reaction, compounds 8 were obtained along with very small amounts of 3-acyl-2-(alkylamino)-4H-pyrido[1,2-a]pyrimidin-4-ones 9 . 相似文献
73.
Giovanni Matarazzo 《Rendiconti del Circolo Matematico di Palermo》1991,40(2):316-324
We prove the uniqueness of weak solutions of initial boundary value problems Each functiona i is required to be sufficiently smooth and must satisfy the following conditions: \(e) \sum\limits_1^n {ij} \partial _{\eta _j \eta _h }^2 a_i (x, \ldots , \eta 1, \ldots , \eta _n )\xi _i \xi _j \leqslant 0, h = 1, \ldots , n,\) for some positive constantsK 0, α, some non negative constantsK i , some positive functionsH(t)∈L 1(0,T) and for all ξ≡(ξ i ), η≡(η i )∈R n 相似文献
74.
In this paper some new results on positive \(\partial \bar \partial - closed\) currents are applied to modifications \(f:\bar M \to M\) . The main result in this topic is that every smooth proper modification of a compact Kähler manifoldM is balanced. Moreover, under suitable hypotheses on the map, the Kähler degrees of \(\bar M\) corresponds to homological properties of the exceptional set of the modification. More examples ofp-Kähler manifolds are discussed in the last section of the paper. 相似文献
75.
Marco Arfelli Giovanni Cossu Giulia Mattogno Carla Ferragina Maria Antonietta Massucci 《Journal of inclusion phenomena and macrocyclic chemistry》1990,9(2):161-170
X-ray photoelectron spectroscopy provides evidence that when [Cu(phen)2]2+(phen = 1,10-phenanthroline) is diffused between the layers of -zirconium phosphate, the complex species does not remain intact after intercalation, but some Cu-N bonds are broken, replaced with Cu-O bonds and the released nitrogen atoms can now interact with the PO3-OH groups of the host. XPS also provides evidence for coordination of the Cu2+ ions when they are diffused by ion exchange in the phenanthroline--zirconium phosphate intercalation compound. Although Cu2+ and phen are in a 1 : 1 molar ratio in the interlayer region of the host, so that a 1 : 1 coordination could be expected between the two species, the characteristic peaks of the uncoordinated phenanthroline, even though at a low intensity, are still present. The differences between the two Cu(II)-intercalation compounds are discussed. 相似文献
76.
Giovanni Appendino Giancarlo Cravotto Gian Mario Nano Giovanni Palmisano Rita Annunziata 《Helvetica chimica acta》1993,76(3):1194-1202
The reaction of 4-hydroxycoumarin and certain aliphatic aldehydes affords 1:1 or complex 2:2 adducts besides (or in place) of the expected 2:1 bis(coumarin) adducts. Reaction with heptanal, cyclohexanecarbaldehyde, and pivalaldehyde are reported as representative. The structure of the reaction products was established by spectroscopical techniques, including X-ray analysis, and their formation was mechanistically rationalized. Some of the 1:1 adducts are synthetically useful for the preparation of 3-alkyl-4-hydroxycoumarins. 相似文献
77.
We investigate by numerical simulations the dynamics of alignment of linear molecules in resonant pulsed laser fields and its dependence on pulse length, field strength, and molecular parameters. We propose an analytical short-time approximation for the time-dependent wave packets. We provide a theoretical basis for the occurrence of saturation in the rotational pumping. We present a formula to predict the time at which the maximum alignment occurs. We discuss the magnitude of the laser-induced alignment and we relate it to a theoretical upper limit. 相似文献
78.
2,6-Diacetylpyridine bis(benzoylhydrazone) (H2DPBH) is proposed as a ligand for the extraction of uranium(VI). Complete extraction from aqueous solutions into dichloromethane is achieved with a ligand/metal mole ratio of < for 10?5?10?4 M uranyl ion. Potentiometric measurements indicate that the extracted species is UO2 (DPBH). Uranium can be determined in the extract by spectrophotometric measurements at 420 nm and by differentail pulse polarography (Ep = ?0.67 V) with tetraethylammonium bromide as supporting electrolyte. For both methods, the detection limit is about 2 × 10?6 M in the extract. 相似文献
79.
Giovanni Alessandrini 《Annali di Matematica Pura ed Applicata》1981,127(1):365-381
Sunto Si considera l'equazione (*) auxx+2buxy+cuyy+(d/y)uy=f dove a, b, c, d L, d > 0, f L2. Supponendo che (*) sia uniformemente eliittica e che l'oscillazione del quoziente d/a verifichi una resirizione vicino all'asse della x, si provano stime a priori e un teorema di esistenza per soluzioni W2,2 di un problema di Dirichlet relativo all'equazione (*).
Work supported by the Istituto di Analisi Globale e Applicazioni. 相似文献
Work supported by the Istituto di Analisi Globale e Applicazioni. 相似文献
80.
Rosace G Giuffrida G Saitta M Guglielmo G Campagna S Lanza S 《Inorganic chemistry》1996,35(23):6816-6822
Tight contact ion pairs of general formula {Pt(H(2)-R(2)-dto)(2)(2+),(X(-))(2)} have been prepared, and their absorption spectra and luminescence properties (at room temperature in dichloromethane fluid solution and at 77 K in butyronitrile rigid matrix) have been studied (dto = dithiooxamide; R = methyl, X = Cl (1); R = butyl, X = Cl (2); R = benzyl, X = Cl (3); R = cyclohexyl, X = Cl (4); R = cyclohexyl, X = Br (5); R = cyclohexyl, X = I (6)). The absorption spectra of all the compounds are dominated by moderately strong Pt(dpi)/S(p) to dithiooxamide (pi) charge transfer (Pt/S --> dto CT) bands in the visible region (epsilon in the 10(4)-10(5) M(-)(1) cm(-)(1) range). Absorption features are also present at higher energies, due to pi-pi transitions centered in the dto ligands (ligand centered, LC). All the compounds exhibit a unstructured luminescence band in fluid solution at room temperature, with the maximum centered in the 700-730 nm range. The luminescence bands are blue-shifted about 4000 cm(-)(1) on passing to the rigid matrix at 77 K. Luminescence lifetimes are on the 10(-)(8)-10(-)(7) s time scale at room temperature and 1 order of magnitude longer at 77 K. Luminescence is assigned to triplet Pt/S --> dto CT excited states in all cases. Compounds 3-6 also exhibit a second higher-energy luminescence band at room temperature, centered at about 610 nm, attributed to a LC excited state. Charge transfer interactions between halides and dto ligands destabilize dto-centered orbitals, affecting the energy of Pt/S --> dto CT transitions and states. The X counterions and X --> dto CT levels are proposed to play a role in promoting excited state conversion between LC and Pt/S --> dto CT levels. The R substituents on the nitrogen atoms of the dto ligands influence the absorption and photophysical properties of the compounds, by affecting proximity of the ion pairs. The possibility to functionalize the R substituents may open the way to interface these luminescent compounds with desired substrates and to construct supramolecular assemblies. 相似文献