全文获取类型
收费全文 | 74623篇 |
免费 | 13278篇 |
国内免费 | 7833篇 |
专业分类
化学 | 52012篇 |
晶体学 | 685篇 |
力学 | 5009篇 |
综合类 | 376篇 |
数学 | 8671篇 |
物理学 | 28981篇 |
出版年
2024年 | 79篇 |
2023年 | 1608篇 |
2022年 | 1699篇 |
2021年 | 2495篇 |
2020年 | 3247篇 |
2019年 | 3009篇 |
2018年 | 2625篇 |
2017年 | 2431篇 |
2016年 | 3662篇 |
2015年 | 3531篇 |
2014年 | 4227篇 |
2013年 | 5484篇 |
2012年 | 6956篇 |
2011年 | 7128篇 |
2010年 | 4754篇 |
2009年 | 4544篇 |
2008年 | 4858篇 |
2007年 | 4312篇 |
2006年 | 3938篇 |
2005年 | 3311篇 |
2004年 | 2598篇 |
2003年 | 2030篇 |
2002年 | 1801篇 |
2001年 | 1575篇 |
2000年 | 1386篇 |
1999年 | 1579篇 |
1998年 | 1355篇 |
1997年 | 1311篇 |
1996年 | 1324篇 |
1995年 | 1076篇 |
1994年 | 972篇 |
1993年 | 792篇 |
1992年 | 692篇 |
1991年 | 601篇 |
1990年 | 518篇 |
1989年 | 390篇 |
1988年 | 346篇 |
1987年 | 278篇 |
1986年 | 261篇 |
1985年 | 211篇 |
1984年 | 145篇 |
1983年 | 106篇 |
1982年 | 87篇 |
1981年 | 69篇 |
1980年 | 67篇 |
1979年 | 32篇 |
1978年 | 30篇 |
1977年 | 40篇 |
1976年 | 30篇 |
1973年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
991.
Lin‐Bao Zhang Xin‐Qi Hao Zhan‐Jiang Liu Xin‐Xiang Zheng Shou‐Kun Zhang Jun‐Long Niu Mao‐Ping Song 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(34):10150-10153
A highly efficient cobalt(II)‐catalyzed alkynylation/annulation of terminal alkynes assisted by an N,O‐bidentate directing group is described. This protocol is characterized by wide substrate scope utilizing cheap cobalt catalysts, and offers a new approach to 3‐methyleneisoindolin‐1‐one, which can be converted into an oxadiazine salt in one step. Moreover, the directing group could be removed in three steps. 相似文献
992.
Yong Xia Lili Lin Fenzhen Chang Xuan Fu Xiaohua Liu Xiaoming Feng 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(46):13952-13956
A highly efficient asymmetric ring‐opening reaction of cyclopropyl ketones with a broad range of thiols, alcohols and carboxylic acids has been first realized by using a chiral N,N′‐dioxide–scandium(III) complex as catalyst. The corresponding sulfides, ethers, and esters were obtained in up to 99 % yield and 95 % ee. This is also the first example of one catalytic system working for the ring‐opening reaction of donor–acceptor cyclopropanes with three different nucleophiles, let alone in an asymmetric version. 相似文献
993.
Theoretical study on the degradation mechanism of methamidophos and chloramine phosphorus with OH radicals 下载免费PDF全文
Shengmin Sun Kun Zhang Yang Lu Ping Liu Hui Zhang 《International journal of quantum chemistry》2015,115(17):1187-1193
Theoretical investigation on the gas‐phase degradation reaction mechanism of methamidophos (MAP) and chloramine phosphorus (CHP) with OH radicals is performed. The equilibrium geometries and the harmonic vibration frequencies of the stationary points are obtained at M06‐2x/6‐31+G(d,p) level, and the higher‐level energetic information is further refined at M06‐2x/6–311++G(3df,2p) level. The rate constants for the 14 reaction channels are calculated by the improved canonical variational transition state theory with small‐curvature tunneling correction over the temperature range 200–2000 K. The three‐parameter expressions of k1(T) = 1.53 × 10?19T2.74exp(?1005.12/T), k2(T) = 1.36 × 10?20T3.02exp(?1259.56/T) are given. The total rate constants of all reaction channels of MAP with OH radicals are in good agreement with the available experimental data. Our results indicate that the H‐abstraction reactions on methyl are the major channels for the reaction of MAP and CHP with OH radicals. © 2015 Wiley Periodicals, Inc. 相似文献
994.
Subergane‐Type Sesquiterpenes from Gorgonian Coral Subergorgia suberosa with Antibacterial Activities 下载免费PDF全文
Qixi Huang Wei Cheng Hailin Long Huagang Liu Leen van Ofwegen Wenhan Lin 《Helvetica chimica acta》2015,98(9):1202-1209
A new subergane‐type sesquiterpene, named epoxysubergorgic acid ( 1 ), along with seven known sesquiterpenes, were isolated from the gorgonian coral Subergorgia suberosa. The structure of the new compound was determined by extensive spectroscopic analyses. The previously uncertain absolute configuration of the known analogues 2 – 6 was determined on the basis of CD, Mosher's method, and through chemical conversions. All compounds were evaluated for antibacterial activities. 相似文献
995.
Ren Ren Chao‐Jun Chen Xian‐Xian Liu Hui‐Ming Ge Ren‐Xiang Tan Rui‐Hua Jiao 《Helvetica chimica acta》2015,98(10):1435-1441
Three new simple trichothecenes, 15‐acetyltrichoverrol B ( 3 ), 13′‐acetyltrichoverrin B ( 5 ), and 6′‐dehydroxytrichoverrin B ( 6 ), along with five known trichothecenes trichodermadienediol B ( 1 ), trichoverrol B ( 2 ), trichoverrin B ( 4 ), and roridins A ( 7 ) and D ( 8 ), have been isolated from the liquid culture of Myrothecium roridum (strain no. QB‐1). The structures of the new compounds were established by comprehensive analysis of 1D‐ and 2D‐NMR data. All the compounds were evaluated for antifungal activity, only compounds 7 and 8 showed significant antifungal activity against the tested fungi (MIC ranged from 10 to 5 μg/ml). 相似文献
996.
Correlation between electron delocalization and structural planarization in small water rings 下载免费PDF全文
Bo Wang Li Wang Xing Dai Yang Gao Wanrun Jiang Jie Han Zhigang Wang Rui‐Qin Zhang 《International journal of quantum chemistry》2015,115(13):817-819
The discovery of the covalent‐like character of the hydrogen bonding (H‐bonding) system [Science 342 , 611(2013)] has promoted a renewal of our understanding of the electronic and geometric structures of water clusters. In this work, based on density functional theory calculations, we show that the preferential formation of a stable quasiplanar structure of (H2O)n(n = 3–6) is closely related to three kinds of delocalized molecular orbitals (MOs; denoted as MO‐I, II, and III) of water rings. These originate from the 2p lone pair electrons of oxygen (O), the 2p bond electrons of O and the 1s electrons of H and the 2s electrons of O and 1s electrons of H, respectively. To maximize the orbital overlaps of the three MOs, geometric planarization is needed. The contribution of the orbital interaction is more than 30% in all the water rings according to our energy decomposition analysis, highlighting the considerable covalent‐like characters of H‐bonds. © 2015 Wiley Periodicals, Inc. 相似文献
997.
Ya‐Zhu Zhu Yan‐Hong Li Dan Liu Ling‐Huan Gao Hong‐Mei Li Rong‐Tao Li 《Helvetica chimica acta》2015,98(8):1132-1140
Four new iridoids, incarvoids D–F ( 1 – 3 , resp.) and incarvoid B 9‐O‐β‐D ‐glucopyranoside ( 4 ), and one new monoterpenoid, argutoid B ( 5 ), along with 14 known compounds, were isolated from Rhododendron microphyton. Their structures were established by comprehensive 1D‐ and 2D‐NMR spectroscopic analysis. 相似文献
998.
999.
C? H Arylation of olefins by triarylphosphines via C? P bond cleavage has been achieved with either Pd0 or PdII catalysts. A variety of olefins and triarylphosphines are tolerated, and we inferred that both Pd0 and PdII could function directly without pre‐oxidation or pre‐reduction. 相似文献
1000.
The surface modification of Ag/Ag2CO3 with Fe(III) ions has been achieved through simply photoreduction‐impregnation method. The obtained products were characterized by means of X‐ray diffraction (XRD), scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and UV‐vis absorption spectroscopy. Under visible‐light irradiation (γ>420 nm), the Fe(III)/Ag/Ag2CO3 sample displays a higher photocatalytic activity and stability than pure Ag2CO3 and Ag/Ag2CO3 samples for the degradation of methyl orange (MO). The improved photocatalytic activity and stability of this ternary system could be ascribed to the synergetic effect between Ag nanoparticles and Fe(III) nanocluster. The metallic Ag nanoparticles cause an obviously enhanced visible‐light absorption to produce more photogenerated charges, while the Fe(III) works as an active site for the following oxygen reduction to reduce the recombination rate of photogenerated electrons and holes. 相似文献