全文获取类型
收费全文 | 1260篇 |
免费 | 75篇 |
国内免费 | 4篇 |
专业分类
化学 | 1081篇 |
晶体学 | 15篇 |
力学 | 11篇 |
数学 | 58篇 |
物理学 | 174篇 |
出版年
2023年 | 9篇 |
2022年 | 4篇 |
2021年 | 20篇 |
2020年 | 29篇 |
2019年 | 35篇 |
2018年 | 23篇 |
2017年 | 16篇 |
2016年 | 37篇 |
2015年 | 39篇 |
2014年 | 45篇 |
2013年 | 45篇 |
2012年 | 105篇 |
2011年 | 106篇 |
2010年 | 64篇 |
2009年 | 51篇 |
2008年 | 63篇 |
2007年 | 92篇 |
2006年 | 89篇 |
2005年 | 72篇 |
2004年 | 54篇 |
2003年 | 69篇 |
2002年 | 62篇 |
2001年 | 18篇 |
2000年 | 14篇 |
1999年 | 19篇 |
1998年 | 7篇 |
1997年 | 12篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1971年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有1339条查询结果,搜索用时 234 毫秒
41.
42.
Haru Ogawa Yoshinari Taketugu Taiji Imoto Yōichi Taniguchi Hidefumi Kato 《Tetrahedron letters》1979,20(36):3457-3460
Rearrangement of 2,4-di--pentadienals was described as a consequence of the successive alkyl substitutions on the C-2 and C-4 positions. 1H-FT nmr spectroscopy suggested that the reaction involves ketene as an intermediate. 相似文献
43.
Fumio Kiyono Hideo Tajima Keiichi Ogasawara Akihiro Yamasaki 《Fluid Phase Equilibria》2005,230(1-2):90-98
A method for determining quadruple points of a two-component system containing a simple hydrate phase is proposed. This method utilizes the quasi-static change of the system along three-phase equilibrium lines and was proved to be able to determine the quadruple points as accurately as the conventional method. By using this method, even though some preparation is necessary, a quadruple point can be determined in just a single experimental run. The behavior of the system near the quadruple points was also examined experimentally, for both the quasi-static and the irreversible change cases. At the quadruple points, the temperature and pressure of the system were kept constant for a while, as at the triple point of water. In both cases, the representative point of the state of the system passed through the quadruple point on a p–T diagram. 相似文献
44.
Reaction of 3-acetyl-4,6-dimethyl-2-(1H)pyridone ( 9a ) with phosphoryl chloride gives 2-chloro-3-ethynyl-4,6-dimethylpyridine ( 10a ). 3-Acetyl-4-hydroxy-6-methyl-2(1H)pyridone (14a) and 3-acetyl-2,6-dimethyl-4-(1H)-pyridone (21) undergoes similar reaction to give the corresponding ethynyl (16 and 23) and chlorovinyl (15 and 22) pyridines, respectively. The chlorination of 3-acetylpyridine and pyrimidine derivatives is further described. 相似文献
45.
46.
Kittaka S Iwashita T Serizawa A Kranishi M Takahara S Kuroda Y Mori T Yamaguchi T 《The journal of physical chemistry. B》2005,109(49):23162-23169
The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C[triple bond]N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol(-1) compared to the bulk value of 12.7 kJ mol(-1). The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values. 相似文献
47.
Tetsuo Yamasaki Yuji Yoshihara Yoshinari Okamoto Tadashi Okawara Mitsuru Furukawa 《Journal of heterocyclic chemistry》1992,29(5):1313-1316
The reaction of 5-hydrazinopyridazin-3(2H)-ones 1 with α-keto diester 2 in acetic acid afforded the corresponding 4,6-dihydropyridazino[4,5-c]pyridazin-5(1H)-ones 3 and pyrrolo[2,3-d)pyridazin-4(5H)-ones 4 . Compounds 3 were also obtained from 4-bromo-5-hydrazinopyridazin-3(2H)-ones 8 and 2 under milder conditions. 5-Bromo-4-hydrazinopyridazin-3(2H)-one 9 , the regioisomer of 8b , also reacted readily with 2a to give 4,7-dihydropyridazino[4,5-c]pyridazin-8(1H)-one 10b , the regioisomer of 3b . 相似文献
48.
Yoshinari K 《Rapid communications in mass spectrometry : RCM》2000,14(4):215-223
A numerical simulation method has been developed for the analysis of trapping ions injected into an ion trap mass spectrometer. This method was applied to clarify the effects of the following parameters on trapping efficiencies: (1) initial phase of the radio frequency (RF) drive voltage, (2) ion injection energy, and (3) RF peak voltage while injecting ions. The following conclusions were obtained by theoretical and simulation approaches. 1. The second and third dominant oscillations contribute significantly to the trapping mechanism of the injected ions, even for low q values. 2. A formula relating the operating parameters, which gives the maximum trapping efficiency, is derived. 3. Based on the above-mentioned formula, an advanced injection method is proposed, in which the RF peak voltage is decreased while injecting ions. The ability of this method to solve the problem of unequal sensitivity among different ion species is indicated by numerical simulation. Copyright 2000 John Wiley & Sons, Ltd. 相似文献
49.
Novel phosphonylation of the active Me group of pyridine derivatives is described.Reaction of 2 (and 4) -methylpyridine (3 and 5) with PCl5 in POCl3 gave 2 (and 4) -trichloromethylpyridine (4 and 6). Similarly, reaction of 4 (and 6) methyl-3-nitropyridine (7 and 9) afforded the corresponding trichloromethyl derivative (8 and 10). However, the similar reaction of 2-methyl-3-nitropyridine (11) gave dichloro-(3-nitro-2-pyridyl)-methylphosphonic dichloride (12).Mechanisms of the formation of these products are discussed. 相似文献
50.
Fluorous reverse-phase silica gel (FRPSG)-supported Lewis acids which have fluorous ligands acted as effective catalysts of Baeyer-Villiger and Diels-Alder reactions in water. Direct esterification of carboxylic acid with alcohol in organic media was also catalyzed. The FRPSG-supported Lewis acids could be recycled by simple filtration after the reaction. 相似文献