全文获取类型
收费全文 | 8577篇 |
免费 | 176篇 |
国内免费 | 15篇 |
专业分类
化学 | 4814篇 |
晶体学 | 211篇 |
力学 | 210篇 |
数学 | 1370篇 |
物理学 | 2163篇 |
出版年
2022年 | 140篇 |
2021年 | 158篇 |
2020年 | 115篇 |
2019年 | 153篇 |
2018年 | 155篇 |
2017年 | 148篇 |
2016年 | 271篇 |
2015年 | 199篇 |
2014年 | 290篇 |
2013年 | 664篇 |
2012年 | 413篇 |
2011年 | 468篇 |
2010年 | 376篇 |
2009年 | 349篇 |
2008年 | 410篇 |
2007年 | 373篇 |
2006年 | 333篇 |
2005年 | 278篇 |
2004年 | 254篇 |
2003年 | 263篇 |
2002年 | 222篇 |
2001年 | 169篇 |
2000年 | 156篇 |
1999年 | 115篇 |
1998年 | 124篇 |
1997年 | 97篇 |
1996年 | 97篇 |
1995年 | 130篇 |
1994年 | 73篇 |
1993年 | 61篇 |
1992年 | 86篇 |
1991年 | 52篇 |
1990年 | 78篇 |
1989年 | 75篇 |
1988年 | 74篇 |
1987年 | 63篇 |
1986年 | 71篇 |
1985年 | 114篇 |
1984年 | 123篇 |
1983年 | 75篇 |
1982年 | 79篇 |
1981年 | 81篇 |
1980年 | 88篇 |
1979年 | 60篇 |
1978年 | 69篇 |
1977年 | 77篇 |
1976年 | 71篇 |
1975年 | 62篇 |
1974年 | 51篇 |
1973年 | 45篇 |
排序方式: 共有8768条查询结果,搜索用时 15 毫秒
91.
The reaction of N‐(2‐R1‐oxyphenyl)benzimidoyl chlorides with cyanamide derivatives in the presence of titanium tetrachloride as a catalyst has yielded some new 4‐amino‐8‐R1‐oxy‐2‐phenylquinazolines. pKa values have been determined for these compounds and the influence of substituents at the basicity of the parent system has been discussed. Some investigations on the methyl‐quinazolinyl ether cleavage in different medium have been done yielding the appropriate hydroxyquinazoline derivatives. In those cases when the deprotection of 4‐amino‐8‐methoxy‐2‐phenylquinazoline was carried in aqueous acidic solutions, the formation of the hydrolysis products 3,4‐dihydro‐2‐phenyl‐4‐quinazolone derivatives was observed as well. 相似文献
92.
Karpińska J Mularczyk B 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(10):2189-2194
The aim of the present work was to develop a simple and rapid method of retinol acetate, tocopherol acetate and coenzyme Q(10) determination in pharmaceuticals without involving any preparation operations like separation or masking. The values of second derivative amplitude at 212 nm for tocopherol, 351 nm for retinol and 222 nm for coenzyme were used for construction of calibration graphs. Beer's law is obeyed in the concentration range 0.5-20, 0.5-7.5 and 0.5-30 microg ml(-1) for retinol acetate, tocopherol acetate and coenzyme, respectively. The elaborated procedures were successfully applied to the simultaneous determination of studied compounds in their binary synthetic mixtures and in commercial preparations with high reliability and repeatability. Spectral properties of retinol acetate allows to determine its contents in ternary mixture which includes Vitamin E and coenzyme Q(10). 相似文献
93.
J. Epsztajn J. Z. Brzeziński K. Czech 《Monatshefte für Chemie / Chemical Monthly》1993,124(5):549-558
Summary The preparation of the pyridazinones10a,10b,11a,11b, and12a,12b from the ketoamides7,8, and9 and hydrazine hydrate is described. It was found that from ketoamides8b and9b in addition to the expected pyridopyridazinones11b and12b also aminopyridopyridazines14 and15 were formed and that ketoamide7b gave exclusively aminopyridopyridazine13. The pyridopyridazinones10b,11b, and12b were alkylated with alkyl iodides.
Anwendungen von Organolithium und verwandten Reagenzien in organischen Synthesen, Teil XII. Synthese von Phenyl- und Pyridylpyridopyridazinonen und ihren Derivaten
Zusammenfassung Die Darstellung der Pyridazinone10a,10b,11a,11b und12a,12b aus Ketoamiden7,8 und9 und Hydrazinhydrat wird beschrieben. Es wurde festgestellt, daß aus Ketoamiden8b und9b außer den erwarteten Pyridopyridazinonen11b und12b auch Aminopyridopyridazine14 und15 enstanden und daß das Ketoamid7b ausschließlich ins Aminopyridopyridazin13 überführt wurde. Die Pyridopyridazinone10b,11b und12b wurden mit Alkyljodiden alkyliert.相似文献
94.
The thermodynamic quantities Kn?1 n, ΔG0n?1, n and ΔS0n?1, n for the gas phase equilibrium reactions RNH+3(RNH2)n?1 + RNH2 = RNH+3(RNH2)n, where n ? 3 and R indicates an alkyl group (CH3, C2H5, n-C3H7 and iso-C3H7), have been determined. 相似文献
95.
Przychodzeń P Lewiński K Bałanda M Pełka R Rams M Wasiutyński T Guyard-Duhayon C Sieklucka B 《Inorganic chemistry》2004,43(9):2967-2974
The syntheses, X-ray structures, and magnetic behaviors of two new cyano-bridged assemblies, the molecular [Mn(III)(salen)H2O]3[W(V)(CN)8].H2O (1) and one-dimensional [Mn(salen)(H2O)2]2[[Mn(salen)(H2O)][Mn(salen)]2[Mo(CN)(8)]].0.5ClO4.0.5OH.4.5H2O (2), are presented. Compound 1 crystallizes in the monoclinic system, has space group P2(1)/c, and has unit cell constants a = 13.7210(2) A, b = 20.6840(4) A, c = 20.6370(2) A, and Z = 4. Compound 2 crystallizes in the triclinic system, has space group P, and has unit cell dimensions a = 18.428(4) A, b = 18.521(3) A, c = 18.567(4) A, and Z = 2. The structure of 1 consists of the asymmetric V-shaped Mn-NC-W-NC-Mn-O(phenolate)-Mn molecules, where W(V) coordinates with [Mn(salen)H2O] and singly phenolate-bridged [Mn(salen)H2O]2 moieties through the neighboring cyano bridges. The [W(V)(CN)8]3- ion displays distorted square-antiprism geometry. The structure of 2 consists of the cyano-bridged [Mn3(III)Mo(IV)]n- repeating units linked by double phenolate bridges into one-dimensional zigzag chains. The Mn(III) centers are bound to Mo(IV) of square-antiprism geometry through the neighboring cyano bridges. The magnetic studies of 1 reveal the antiferromagnetic intramolecular interactions through the CN and phenolate bridges and the relatively weak intermolecular interactions. Compound 1 becomes antiferromagnetically ordered below TN = 4.6 K. The presence of the magnetic anisotropy is documented with the MH measurements carried out for both polycrystalline and single-crystal samples. At T = 1.9 K, the spin-flop transition is observed in the field of 18 kOe applied parallel to the bc plane, which is the easy plane of magnetization. Field dependence of magnetization of 1 shows field-induced metamagnetic behavior from the antiferromagnetic ground state of ST = 3/2 to the state of ST = 5/2. The magnetic properties of 2 indicate a weak antiferromagnetic interaction between Mn(III) centers in double-phenolate-bridged [Mn(III)(salen)]2 dinuclear subunits and a very weak ferromagnetic interaction between them through the diamagnetic [Mo(IV)(CN)8]4- spacer. 相似文献
96.
A master equation is used to study transitions between the stable limit cycle and stable focus in the two-variable bistable system. The distribution function of the mean first passage time between these attractors and the relative dispersion of the mean first return time from the stable focus to itself as a function of the intensity of fluctuations are calculated and discussed. A coherence resonance is observed for the return time from the focus to itself. 相似文献
97.
Denifl S Ptasińska S Sonnweber B Scheier P Liu D Hagelberg F Mack J Scott LT Märk TD 《The Journal of chemical physics》2005,123(10):104308
Electron attachment to the polyaromatic hydrocarbons coronene and corannulene is studied in the electron energy range of about 0-14 eV using a high-resolution crossed electron-neutral beam setup. The major anions observed are the parent anions peaking at about 0 eV with cross sections of 3.8 x 10(-20) and 1 x 10(-19) m(2), respectively. The only fragment anions formed in coronene and corannulene are the dehydrogenated coronene and corannulene anions. Other anions observed in the negative mass spectra at about 0 eV can be ascribed to impurities of the sample. High-level quantum-mechanical studies are carried out for the determination of electron affinities, hydrogen binding energies, and structures of both molecules. The behavior of coronene and corannulene upon electron attachment is compared with that of other polyaromatic hydrocarbons studied previously. 相似文献
98.
Jakub Wojciechowski Henryk Krawczyk Marcin
liwiski Karolina Kafarska Wojciech M. Wolf 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(5):o280-o282
The title compound, C14H20O2, adopts a conformation in which the δ‐valerolactone and cyclohexane rings are almost coplanar with one another. The γ‐methyl substituent occupies an axial position with respect to the cyclohexane ring. The δ‐valerolactone moiety adopts an envelope arrangement, while the cyclohexane ring exists in a chair conformation. 相似文献
99.
Calorimetry has been used in the investigations of calcium aluminate materials produced as a binder for aluminate-corundum
composites of high refractoriness. The kinetics and of hydration process was thus characterized and the optimum compositions
of initial binders and cement-corundum refractory filler blends could be selected for further tests. The acceleration of heat
evolution - the shortening of so-called induction period and relatively high heat output in the presence of corundum was observed.
It means the acceleration of hydration process, that is early crystallisation of hydration products and subsequent further
dissolution of initial anhydrous aluminate phases. In the presence of fine grained corundum particles these phenomena should
be attributed to the nucleating effect of fine corundum particles.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
100.
Teresa Borowia Marek Mączyński Marek Pietraszkiewicz Oksana Pietraszkiewicz 《Journal of inclusion phenomena and macrocyclic chemistry》1999,35(1-2):131-138
The crystal structure of the molecular complex of C-undecylcalix[4]resorcinarene with dioxane has been determined by X-ray analysis. The asymmetric unit contains one host and four guest molecules. The calix[4]resorcinarene moiety adopts a bowl conformation with C4v
symmetry. Four undecyl chains are axially oriented. Calix molecules are packed in a bowl-to-bowl fashion with alternating hydrophilic and hydrophobic layers. One of the hydrophilic dioxane molecules is located at the rim of the calix moiety and is hydrogen bonded to the other one. There is no interaction to attract, or direct the dioxane molecule into the interior of the cavity. There is an exo complex formed. The dioxane molecules – located in the hydrophobic part – are highly disordered. 相似文献