全文获取类型
收费全文 | 48377篇 |
免费 | 8811篇 |
国内免费 | 6560篇 |
专业分类
化学 | 34905篇 |
晶体学 | 704篇 |
力学 | 3108篇 |
综合类 | 437篇 |
数学 | 5040篇 |
物理学 | 19554篇 |
出版年
2024年 | 166篇 |
2023年 | 1068篇 |
2022年 | 1900篇 |
2021年 | 1988篇 |
2020年 | 2160篇 |
2019年 | 2052篇 |
2018年 | 1776篇 |
2017年 | 1693篇 |
2016年 | 2496篇 |
2015年 | 2506篇 |
2014年 | 3009篇 |
2013年 | 3672篇 |
2012年 | 4589篇 |
2011年 | 4600篇 |
2010年 | 3253篇 |
2009年 | 3017篇 |
2008年 | 3267篇 |
2007年 | 2805篇 |
2006年 | 2703篇 |
2005年 | 2207篇 |
2004年 | 1673篇 |
2003年 | 1368篇 |
2002年 | 1278篇 |
2001年 | 1015篇 |
2000年 | 880篇 |
1999年 | 948篇 |
1998年 | 706篇 |
1997年 | 662篇 |
1996年 | 701篇 |
1995年 | 587篇 |
1994年 | 545篇 |
1993年 | 412篇 |
1992年 | 382篇 |
1991年 | 321篇 |
1990年 | 266篇 |
1989年 | 235篇 |
1988年 | 194篇 |
1987年 | 121篇 |
1986年 | 142篇 |
1985年 | 104篇 |
1984年 | 58篇 |
1983年 | 47篇 |
1982年 | 38篇 |
1981年 | 29篇 |
1980年 | 17篇 |
1975年 | 9篇 |
1972年 | 7篇 |
1965年 | 6篇 |
1964年 | 5篇 |
1957年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Lei Chen Hui-Qing Yang Cheng-Yu Jin Zhao-Xu Chen 《International journal of quantum chemistry》2020,120(22):e26366
We studied the ring opening of propylene oxide (PO) by salen-M coordinated OH− group [M = Al(III), Sc(III), Cr(III), Mn(III), Fe(III), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Ru(III) and Rh(III)]. The results show that the ring-opening energy barriers for M(II) complexes are much lower than those with M(III) complexes in the gas phase, and the barriers correlate linearly with the negative charges on the OH− group and the Fukui function condensed on the OH− group. The nucleophilicity ordering in the gas phase can be rationalized by the ratio of formal positive charges/radius of M cations. Solvent effect greatly increases the barriers of M(II) complexes but slightly changes the results of M(III) ones, making the barriers similar. Analysis indicates that the reaction heats are linearly proportional to the reverse reaction barriers. The relationships established here can be used to estimate the ring-opening barriers and to screen epoxide ring-opening catalysts. 相似文献
22.
Annals of the Institute of Statistical Mathematics - To obtain M-estimators of a response variable when the data are missing at random, we can construct three bias-corrected nonparametric... 相似文献
23.
24.
25.
Qin Yin Shou-Guo Wang Xiao-Wei Liang De-Wei Gao Jun Zheng Shu-Li You 《Chemical science》2015,6(7):4179-4183
An organocatalytic asymmetric chlorinative dearomatization of naphthols was realized for the first time, providing chiral naphthalenones with a Cl-containing all-substituted stereocenter in excellent yields and enantioselectivity (up to 97% yield and 96% ee). The reaction features mild reaction conditions, good tolerance of diverse functional groups and simple reaction operation. 相似文献
26.
27.
Richiro Ushimaru Zhang Chen Houyuan Zhao Po-hsun Fan Prof. Dr. Hung-wen Liu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(9):3586-3590
Albomycin δ2 is a sulfur-containing sideromycin natural product that shows potent antibacterial activity against clinically important pathogens. The l -serine-thioheptose dipeptide partial structure, known as SB-217452, has been found to be the active seryl-tRNA synthetase inhibitor component of albomycin δ2. Herein, it is demonstrated that AbmF catalyzes condensation between the 6′-amino-4′-thionucleoside with the d -ribo configuration and seryl-adenylate supplied by the serine adenylation activity of AbmK. Formation of the dipeptide is followed by C3′-epimerization to produce SB-217452 with the d -xylo configuration, which is catalyzed by the radical S-adenosyl-l -methionine enzyme AbmJ. Gene deletion suggests that AbmC is involved in peptide assembly linking SB-217452 with the siderophore moiety. This study establishes how the albomycin biosynthetic machinery generates its antimicrobial component SB-217452. 相似文献
28.
Jie Lu Ju-Kang Wu Yao Jiang Dr. Peng Tan Lin Zhang Yu Lei Prof. Xiao-Qin Liu Prof. Lin-Bing Sun 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(16):6490-6496
Hierarchically porous metal–organic frameworks (HP-MOFs) are promising in various applications. Most reported HP-MOFs are prepared based on the generation of mesopores in microporous frameworks, and the formed mesopores are connected by microporous channels, limiting the accessibility of mesopores for bulky molecules. A hierarchical structure is formed by constructing microporous MOFs in uninterrupted mesoporous tunnels. Using the confined space in as-prepared mesoporous silica, highly dispersed metal precursors for MOFs are coated on the internal surface of mesoporous tunnels. Ligand vapor-induced crystallization is employed to enable quantitative formation of MOFs in situ, in which sublimated ligands diffuse into mesoporous tunnels and react with metal precursors. The obtained hierarchically porous composites exhibit record-high adsorption capacity for the bulky molecule trypsin. The thermal and storage stability of trypsin is improved upon immobilization on the composites. 相似文献
29.
Panpan Li Quanbin Liang Eugene Yau-Hin Hong Chin-Yiu Chan Yat-Hin Cheng Ming-Yi Leung Mei-Yee Chan Kam-Hung Low Hongbin Wu Vivian Wing-Wah Yam 《Chemical science》2020,11(42):11601
A class of acceptor–donor–acceptor chromophoric small-molecule non-fullerene acceptors, 1–4, with difluoroboron(iii) β-diketonate (BF2bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF2bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from −3.65 to −3.72 eV) and relatively high electron mobility (7.49 × 10−4 cm2 V−1 s−1), these BF2bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF2bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.Boron(iii) β-diketonates have been demonstrated to serve as multi-functional materials in NFA-based OPVs and organic resistive memories. 相似文献
30.
Mass concentration and isotopic values δ13C and 14C are presented for the water-insoluble refractory carbon(WIRC) component of total suspended particulates(TSP),collected weekly during 2003,as well as from October 2005 to May 2006 at the WMO-GAW Mt.Waliguan(WLG) site.The overall average WIRC mass concentration was(1183±120)ng/m3(n = 79),while seasonal averages were 2081 ±1707(spring),454±205(summer),650±411(autumn),and 1019±703(winter) ng/m3.Seasonal variations in WIRC mass concentrations were consistent with black carbon measurements from an aethalometer,although WIRC concentrations were typically higher,especially in winter and spring.The δ13C PDB value(-25.3 ± 0.8)%.determined for WIRC suggests that its sources are C3 biomass or fossil fuel combustion.No seasonal change in δ13C PDB was evident.The average percent Modern Carbon(pMC) for 14C in WIRC for winter and spring was(67.2 ± 7.7)%(n = 29).Lower pMC values were associated with air masses transported from the area east of WLG,while higher pMC values were associated with air masses from the Tibetan Plateau,southwest of WLG.Elevated pMC values with abnormally high mass concentrations of TSP and WIRC were measured during a dust storm event. 相似文献