全文获取类型
收费全文 | 45902篇 |
免费 | 15689篇 |
国内免费 | 56篇 |
专业分类
化学 | 55609篇 |
晶体学 | 49篇 |
力学 | 2044篇 |
数学 | 2748篇 |
物理学 | 1197篇 |
出版年
2024年 | 373篇 |
2023年 | 4085篇 |
2022年 | 1457篇 |
2021年 | 2483篇 |
2020年 | 4624篇 |
2019年 | 2309篇 |
2018年 | 2277篇 |
2017年 | 602篇 |
2016年 | 5581篇 |
2015年 | 5534篇 |
2014年 | 4956篇 |
2013年 | 5152篇 |
2012年 | 3207篇 |
2011年 | 1062篇 |
2010年 | 3414篇 |
2009年 | 3370篇 |
2008年 | 1047篇 |
2007年 | 764篇 |
2006年 | 124篇 |
2005年 | 88篇 |
1997年 | 86篇 |
1996年 | 84篇 |
1995年 | 143篇 |
1994年 | 90篇 |
1993年 | 218篇 |
1992年 | 109篇 |
1988年 | 125篇 |
1987年 | 105篇 |
1985年 | 105篇 |
1984年 | 114篇 |
1983年 | 107篇 |
1982年 | 136篇 |
1981年 | 158篇 |
1980年 | 196篇 |
1979年 | 189篇 |
1978年 | 194篇 |
1977年 | 310篇 |
1976年 | 363篇 |
1975年 | 461篇 |
1974年 | 474篇 |
1973年 | 286篇 |
1972年 | 370篇 |
1971年 | 355篇 |
1970年 | 541篇 |
1969年 | 413篇 |
1968年 | 456篇 |
1967年 | 114篇 |
1966年 | 89篇 |
1965年 | 83篇 |
1963年 | 112篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Francesca Porta Sergio Cenini Sergio Giordano Maddalena Pizzotti 《Journal of organometallic chemistry》1978,150(2):261-271
The complexes M(CO)2(PPh3)3 (I, M = Fe; II, M = Ru) readily react with H2 at room temperature and atmospheric pressure to give cis-M(H)2(CO)2(PPh3)2 (III, M = Fe;IV,M = Ru). I reacts with O2 to give an unstable compound in solution, in a type of reaction known to occur with II which leads to cis-Ru(O2)(CO)2(PPh3)2(V). Even compound IV reacts with O2 to give V with displacement of H2; this reaction has been shown to be reversible and this is the first case where the displacement of H2 by O2 and that of O2 by H2 at a metal center has been observed. III and IV are reduced to M(CO)3(PPh3)2 by CO with displacement of H2; Ru(CO)3- (PPh3)2 is also formed by treatment of IV with CO2, but under higher pressure. Compounds II and IV react with CH2CHCN to give Ru(CH2CHCN)(CO)2- (PPh3)2(VI) which reacts with H2 to reform the hydride IV.cis-Ru(H)2(CO)2(PPh3)2(IV) has been studied as catalyst in the hydrogenation and isomerization of a series of monoenes and dienes. The catalysts are poisoned by the presence of free triphenylphosphine. On the other hand the ready exchange of H2 and O2 on the “Ru(CO)2(PPh3)2” moiety makes IV a catalyst not irreversibly poisoned by the presence of air. It has been found that even Ru(CO)2(PPh3)3(II) acts as a catalyst for the isomerization of hex-1-ene at room temperature under an inert atmosphere. 相似文献
162.
Iterative extended Hükel calculations for all valence electrons and iterative PPP calculations in the variable electronegativity formalism for -electrons were performed on benzene, pyridine, fluorobenzene, and the pyrylium ion. The charge distributions for all compounds were found more uniform and plausible with the iteration procedures than without. Polarization effects from the-electrons were found to be of importance for the -electrons. The lone-pair picture of the highest occupied MO in pyridine is preserved in the iterative extended Hückel method, and two lone-pairs were obtained on the fluorine atom of fluorobenzene. The results indicate that this atom is not hybridized.
Sponsored in part by King Gustaf VI Adolfs 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, and in part by the Swedish National Research Council. 相似文献
Zusammenfassung Benzol, Pyridin, Fluorbenzol und Pyriliumion wurden mit einer iterativen EH-Methode und einer iterativen PPP-VE-Methode berechnet. Es zeigte sich, daß Ladungsverteilungen bei Benutzung des Iterationsverfahrens besser beschrieben werden. Polarisationseffekte der-Elektronen auf die -Elektronen stellen sich als wichtig heraus. Auch in der IEH-Methode bleibt das oberste besetzte MO des Pyridins ein einsames Elektronenpaar. Am Fluoratom im Fluorbenzol werden zwei einsame-Elektronenpaare erhalten. Die Resultate führen zu der Annahme, daß dieses Atom nicht hybridisiert ist.
Resumé Des calculs par les méthodes itératives de Hückel étendu pour tous les électrons de valence et de PPP avec électronégativité variable pour les électrons ont été effectués pour le benzène, la pyridine, le fluorobenzène et l'ion pyrylium. Les distributions de charge pour tons ces composés sont plus uniformes et plus plausibles avec les procédés itératis que sans. Les électrons ont un effet de polarisation important sur les électrons . La plus haute orbitale occupée dans la pyridine reste identifiable comme la paire libre dans la méthode de Hückel étendue, et l'on obtient deux paires libres sur l'atome de fluor du fluorobenzène. Cet atome n'est pas hybride.
Sponsored in part by King Gustaf VI Adolfs 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, and in part by the Swedish National Research Council. 相似文献
163.
Contributions to the Chemistry of Transition Metal Alkyl Compounds. XLIII. Tetrakis(alkoxycarbonylmethyl)titanium Compounds Organotitanium(IV) compounds of the type (ROCOCH2)4Ti (R ? C2H5, i-C3H7, t-C4H9, C6H5, C6H5CH2) were obtained by reactions of ROCOCH2Li derivatives with TiCl4 at low temperature. The compounds which decompose only above 90°C were characterized by the hydrolysis products, anaerobic reactions with iodine, and the i.r. spectra. The bond conditions are discussed. 相似文献
164.
Empirical solvent parameters of silica surfaces are determined by means of solvatochromic dyes, e.g., Cu(tmen) (acac)+ ClO
4
–
/B(C6H5)
4
–
, Fe(phen)2(CN)2, and Michlers Ketone dissolved in 1,2-dichloroethane or cyclohexane. The values obtained by UV/VIS-measurements are compared with both the polarity scales of Kamlet and Taft and with Gutmann's donor and acceptor numbers. Kamlet and Taft's -parameter (the hydrogen-bond donation capacity of a solvent) is determined independently by means of salicylideneaniline. The * of silica is determined using 5-N,N-dimethylamino-5-nitro-2,2-bithiophene and the complex of tetracyanoethylene with Michlers Ketone, respectively. Further on the reliabilities of the obtained Kamlet-Taft parameters , , and * are checked by comparism with experimental and calculated values of theE
T(30)-parameter of silica by means of multi-parameter regression analysis. The surface properties of silica are influenced mainly by HBD-properties (75%) and dipolarity/polarizability * (20%), but hardly by the HBA-properties (<5%). UV/VIS-measurements were carried out in a special equipment with glass fiber optics, A drawing of the apparatus is given. 相似文献
165.
166.
At sufficiently high concentrations of methanol and of alkali salts in aqueous silica dispersions, the negative -potentials of the particles are decreased. This effect is more pronounced with lithium than with other alkali cations, and it is independent on the anion. The NMR spectra indicate lithium adsorption on silica from mixed solvents, but not in the absence of alcohol.Supported by a contract with the XMX Corporation, Burlington, Massachusetts, USA 相似文献
167.
Prof. Dr. Hans Sachs 《Journal of Geometry》1992,43(1-2):148-165
According to H. BRAUNER a flag space is a three-dimensional affine space with an absolute {,f,F}, wheref is a line in the plane of infinity andF a point onf. A cyclide is a surface of order 4 that intersects only atf. A STROMMER-cyclide is a cyclide generated by translation of a circle in an isotropic plane. In this paper we proof that all STROMMER-cyclides can be generated by translation of an isotropic circle along a divergent Newton-parabola or a conic in a full isotropic plane. We give normal-forms and CAD-pictures of these surfaces. Finaly some geometrical results about STROMMER-cyclides are developed.
Herrn o.Prof. Dr. Gy. Strommer zum 70. Geburtstag gewidmet. 相似文献
Herrn o.Prof. Dr. Gy. Strommer zum 70. Geburtstag gewidmet. 相似文献
168.
H. U. Burger Dipl.-Math. Professor Dr. U. Müller-Funk Professor Dr. H. Witting 《Mathematical Methods of Operations Research》1992,36(2):163-184
Summary There is an abundancy of problems in which no parametric model realistically describes the situation and in which, accordingly, we have to resort to nonparametric methods. As the numerical problems connected with nonparametric tests are becoming less and less important, rank tests, permutation tests and the like are becoming more and more part of the standard armatory of applied statisticians. The lack of tabulated critical values, for instance, should no longer be a serious objection against the use of permutation tests in practice; cf. Edgington (1987).The rationale underlying permutation and rank tests has been outlined in quite a number of text books and papers; cf. Fraser (1957), Lehmann (1959), Hájek-Sidák (1967) or Witting (1970). Roughly speaking, permutation tests are constructibel if the data can be condensed by means of a sufficient and complete statistic allowing for the proper kind of conditioning. Rank tests arise if the underlying problem is invariant with respect to (w.r.t.) a large group of transformations which leads to a maximal invariant statistic consisting of (signed) ranks.Most practical nonparametric problems, however, are too complex to be tractable by just one of those approaches. Many of them, however, can be handled by a combination of both techniques. In this paper we outline the logic underlying that combined reduction method and apply it to construct locally most powerful tests. Moreover, we discuss what we label Hoeffding's transfer problem, i.e. the uniformity aspect of locally most powerful tests with respect to the starting point at the boundary. We are concentrating on the discussion of the nonparametric two-sample location and scale problem. Further important problems are mentioned in Section III.This is a written account of an invited lecture delivered by the third author on occasion of the 14th Symposium über Operations Research, Ulm, September 6–8, 1989. 相似文献
169.
170.
IntegersN for which 2 (n has weak uniform distribution (modN) are determined. 相似文献