全文获取类型
收费全文 | 40677篇 |
免费 | 10404篇 |
国内免费 | 2370篇 |
专业分类
化学 | 44755篇 |
晶体学 | 384篇 |
力学 | 813篇 |
综合类 | 81篇 |
数学 | 2820篇 |
物理学 | 4598篇 |
出版年
2024年 | 20篇 |
2023年 | 109篇 |
2022年 | 177篇 |
2021年 | 328篇 |
2020年 | 1429篇 |
2019年 | 2760篇 |
2018年 | 1191篇 |
2017年 | 801篇 |
2016年 | 3554篇 |
2015年 | 3754篇 |
2014年 | 3694篇 |
2013年 | 4422篇 |
2012年 | 3396篇 |
2011年 | 2627篇 |
2010年 | 3195篇 |
2009年 | 3146篇 |
2008年 | 2727篇 |
2007年 | 2073篇 |
2006年 | 1783篇 |
2005年 | 1902篇 |
2004年 | 1686篇 |
2003年 | 1534篇 |
2002年 | 2300篇 |
2001年 | 1571篇 |
2000年 | 1452篇 |
1999年 | 552篇 |
1998年 | 185篇 |
1997年 | 144篇 |
1996年 | 114篇 |
1995年 | 95篇 |
1994年 | 94篇 |
1993年 | 73篇 |
1992年 | 67篇 |
1991年 | 79篇 |
1990年 | 53篇 |
1989年 | 42篇 |
1988年 | 41篇 |
1987年 | 27篇 |
1986年 | 33篇 |
1985年 | 39篇 |
1984年 | 28篇 |
1983年 | 29篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 6篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The rhodium(I) complexes trans‐[Rh(diphos)(CO)Cl] 7 (diphos=pbpb), 8 (diphos=nbpb), and 9 (diphos=cbpb) were synthesized (Scheme 4) and used as catalysts for the carbonylation of MeOH to AcOH (Scheme 1). The trans coordination imposed by the rigid C‐spacer framework of the diphos ligands pbpb, nbpb, and cbpb, demonstrated by 31P‐NMR and IR spectroscopy of 7 – 9 and unambiguously confirmed by single‐crystal X‐ray structure analysis of 7 , improved the thermal stability of the rhodium(I) system under carbonylation conditions and, hence, the catalytic performance of the complexes. For the catalytic carbonylation of MeOH, the active catalyst could be prepared in situ from the mixture of [Rh(CO)2Cl]2 and the corresponding diphos ligand pbpb, nbpb, or cbpb, giving the same results as carbonylation in the presence of the isolated complexes 7, 8 or 9 (see Table). The highest activity was observed for complex 7 (or the mixture [Rh(CO)2Cl]2/pbpb, the catalytic turnover number (TON) being 950 after 15 min (170°, 22 bar). 相似文献
992.
EPR, UV/Vis and FTIR spectroscopy as well as thermal analysis (TA/MS) were applied to study the influence of sulfate species present in the anatase support on the specific nature of VOx species in supported VOx/TiO2 catalysts. Those sulfate species modify the local structure of the supported vanadyl species and lead to the formation of two types of VO2+ sites instead of only one type being formed on sulfate‐free anatase. EPR and FTIR spectroscopic measurements revealed that a part of the VO2+ species are directly bound to the surface sulfate species. By TA/MS it was found that SO2 is released at lower temperature from VOx/TiO2 in comparison to the vanadium‐free support. The direct bonding between sulfate and VOx species stabilizes the latter on the surface of VOx/TiO2 resulting in three effects: 1) a higher V site dispersion in comparison to sulfate‐free TiO2, 2) a better resistance of surface vanadyls against diffusion into the bulk of the support and 3) a much faster reoxidation of reduced V sites than observed on sulfate‐free TiO2. 相似文献
993.
We have derived a new multipolar expansion for intermolecular potential-energy functions with applications in molecular physics, theoretical chemistry, and mathematical physics. The new formulation employs a separation of radial and angular terms with a simple index structure that leads to computational efficiency and ease of physical interpretation. For the case of the Coulomb interaction, we compare the present formulation with two conventional multipole expansions: the Cartesian tensor and the irreducible spherical tensor expansions. The new formalism leads to efficient numerical algorithms that are useful for general applications beyond intermolecular potentials. In addition to the electrostatic Coulomb interaction, we illustrate the formalism with applications to special function theory and a bipolar expansion involved in potential theory. 相似文献
994.
The chiral compounds (R)‐ and (S)‐1‐benzoyl‐2,3,5,6‐tetrahydro‐3‐methyl‐2‐(1‐methylethyl)pyrimidin‐4(1H)‐one ((R)‐ and (S)‐ 1 ), derived from (R)‐ and (S)‐asparagine, respectively, were used as convenient starting materials for the preparation of the enantiomerically pure α‐alkylated (alkyl=Me, Et, Bn) α,β‐diamino acids (R)‐ and (S)‐ 11 – 13 . The chiral lithium enolates of (R)‐ and (S)‐ 1 were first alkylated, and the resulting diasteroisomeric products 5 – 7 were aminated with ‘di(tert‐butyl) azodicarboxylate’ (DBAD), giving rise to the diastereoisomerically pure (≥98%) compounds 8 – 10 . The target compounds (R)‐ and (S)‐ 11 – 13 could then be obtained in good yields and high purities by a hydrolysis/hydrogenolysis/hydrolysis sequence. 相似文献
995.
996.
997.
998.
999.
1000.