全文获取类型
收费全文 | 60292篇 |
免费 | 8629篇 |
国内免费 | 1544篇 |
专业分类
化学 | 56038篇 |
晶体学 | 561篇 |
力学 | 1354篇 |
综合类 | 1篇 |
数学 | 4086篇 |
物理学 | 8425篇 |
出版年
2023年 | 156篇 |
2022年 | 423篇 |
2021年 | 630篇 |
2020年 | 1603篇 |
2019年 | 2900篇 |
2018年 | 1291篇 |
2017年 | 923篇 |
2016年 | 3881篇 |
2015年 | 3942篇 |
2014年 | 3997篇 |
2013年 | 5271篇 |
2012年 | 4660篇 |
2011年 | 4070篇 |
2010年 | 3911篇 |
2009年 | 3749篇 |
2008年 | 3895篇 |
2007年 | 3182篇 |
2006年 | 2807篇 |
2005年 | 2902篇 |
2004年 | 2529篇 |
2003年 | 2191篇 |
2002年 | 2797篇 |
2001年 | 1942篇 |
2000年 | 1756篇 |
1999年 | 662篇 |
1998年 | 280篇 |
1997年 | 313篇 |
1996年 | 337篇 |
1995年 | 260篇 |
1994年 | 275篇 |
1993年 | 279篇 |
1992年 | 265篇 |
1991年 | 205篇 |
1990年 | 154篇 |
1989年 | 140篇 |
1988年 | 141篇 |
1987年 | 119篇 |
1986年 | 96篇 |
1985年 | 172篇 |
1984年 | 116篇 |
1983年 | 101篇 |
1982年 | 123篇 |
1981年 | 88篇 |
1980年 | 81篇 |
1978年 | 80篇 |
1977年 | 85篇 |
1976年 | 94篇 |
1975年 | 101篇 |
1974年 | 79篇 |
1973年 | 102篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Treatment of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one ( 1 ) with picryl fluoride (PkF) in 1-methyl-2-pyrrolidinone (NMP) gave a mixture of a monopicryl and a dipicryl derivative of 1 in a ratio of 2 :1 , respectively, regardless of the initial concentrations of 1 and PkF. The products were identified as 5-nitro-2-picryl-2,4-dihydro-3H-1,2,4-triazol-3-one ( 2 ) by X-ray crystallography and 5-nitro-2,4-dipicryl-2,4-dihydro-3H-1,2,4-triazol-3-one ( 4 ) by 15N labeling experiments. 相似文献
992.
Jorge Bravo Jesús Castro Soledad García‐Fontn Elvira M. Lamas Pilar Rodríguez‐Seoane 《无机化学与普通化学杂志》2003,629(2):297-302
The manganese carbonyl complex [MnBr(CO)3 L ] ( 1 ), where L = Ph2POCH2CH2OPPh2, was prepared by reacting [MnBr(CO)5] with the bidentate ligand 1, 2‐Bis(diphenylphosphinite)ethane. From this compound and the appropriate phosphite, phosphinite or phosphonite ligands were synthesized the complexes [MnBr(CO)2 LL ′], where L ′ = P(OMe)3 ( 2 ) or P(OEt)3 ( 3 ) and [MnBr(CO)3 L ′2], where L ′ =PPh(OEt)2 ( 4 ) or PPh2(OEt) ( 5 ). The obtained compounds have been characterized by elemental analysis, mass spectrometry, IR and NMR (1H, 13C and 31P) spectroscopies and X‐ray diffractometry for the complexes 1 , 4 and 5 . 相似文献
993.
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
994.
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
995.
996.
997.
998.
999.
Two‐dimensional (2D) organic nanomaterials are attracting increasing research interest and expected to be the ideal candidate for future‐ proofed flexible electronics and biotechnologies. Owing to the complex molecular structures and multiple intermolecular interactions in organic systems, deeper understanding of rational molecular design and assembly principles is urgently required. In this review, a collection of molecular packing mode in the 2D organic nanomaterials via supramolecular assembly is presented, so as to help explicit the relationship among molecular structures, supramolecular interactions and molecular packing motifs in 2D assembly systems. We also provide a rational and accessible schematic model to demonstrate several typical kinds of molecular packing motifs for the prediction of the 2D morphology. 相似文献
1000.
Theoretical ab initio calculations using the HF and B3LYP methods have been carried out to investigate the conformational differences of three cyclic rings, dibenzo-p-dioxin (DPD), thianthrene (THT), and selenanthrene (SET). The physical origin for the conformational preference of each molecule has been studied using the natural bond orbital (NBO) analysis. The NBO results indicate that DPD exists in a planar form due to strong electron delocalization caused by the specific orbital interaction, around the X atom. On the other hand, THT and SET exist as puckered forms with high inversion barriers due to less effective electron delocalization. The NBO analysis also shows that the conformational stabilization in DPD is caused by a more effective overlap of the orbitals, compared with the overlap of the orbitals in THT. 相似文献