全文获取类型
收费全文 | 2527篇 |
免费 | 127篇 |
国内免费 | 34篇 |
专业分类
化学 | 1420篇 |
晶体学 | 18篇 |
力学 | 78篇 |
综合类 | 1篇 |
数学 | 152篇 |
物理学 | 502篇 |
无线电 | 517篇 |
出版年
2023年 | 25篇 |
2022年 | 14篇 |
2021年 | 46篇 |
2020年 | 40篇 |
2019年 | 64篇 |
2018年 | 46篇 |
2017年 | 32篇 |
2016年 | 61篇 |
2015年 | 53篇 |
2014年 | 85篇 |
2013年 | 117篇 |
2012年 | 174篇 |
2011年 | 198篇 |
2010年 | 102篇 |
2009年 | 124篇 |
2008年 | 150篇 |
2007年 | 139篇 |
2006年 | 134篇 |
2005年 | 137篇 |
2004年 | 99篇 |
2003年 | 76篇 |
2002年 | 74篇 |
2001年 | 59篇 |
2000年 | 51篇 |
1999年 | 38篇 |
1998年 | 21篇 |
1997年 | 34篇 |
1996年 | 50篇 |
1995年 | 35篇 |
1994年 | 42篇 |
1993年 | 41篇 |
1992年 | 32篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 28篇 |
1988年 | 25篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 25篇 |
1984年 | 20篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 7篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1972年 | 3篇 |
排序方式: 共有2688条查询结果,搜索用时 250 毫秒
111.
Ming-Tsz Chen Yu-Yang Chen Guan-Lin Li Kai-Wen Chou Ching-Han Hu Guo-Cyuan Huang Chi-Tien Chen 《应用有机金属化学》2020,34(4):e5464
This paper describes the effect of solvent-induced synthetic routes of aluminium pendant oxazoline-amido-phenolate complexes. Treatment of ligand precursor L with AlMe3 in a 1:1 ratio in diethyl ether yielded the four-coordinated complex (LAlMe)2. Reaction of ligand precursor L with AlMe3 in a 1:2 ratio in hexane generated the four-coordinated complex L(AlMe2)2. A novel transformation mode occurred from L(AlMe2)2 to (LAlMe)2 when using diethyl ether or tetrahydrofuran as solvent. A density functional theory computational study also supports a plausible mechanism. All results were supported by spectroscopic data and in agreement with single-crystal X-ray diffraction structural analysis. 相似文献
112.
Carlos Camacho‐Camacho Irma Rojas‐Oviedo Ariadna Garza‐Ortiz Rubén Alfredo Toscano Luis Sánchez‐Sánchez Jorge Cardenas Hugo López‐Muñoz 《应用有机金属化学》2016,30(4):199-207
The synthesis in one‐pot reactions and structural characterization of six new tri‐n‐butyltin(IV) derivatives of Schiff bases are reported. The compounds are derived from a condensation reaction between l ‐alanine, l ‐valine, l ‐isoleucine, l ‐methionine, l ‐phenylalanine or l ‐tryptophan and 3,5‐di‐tert‐butyl‐2‐hydroxybenzaldehyde. Characterization was completed using elemental analysis, infrared spectroscopy, mass spectrometry, one‐ and two‐dimensional solution NMR (1H, 13C and 119Sn) as well as solid‐state 119Sn NMR. In addition, the crystal structures of three of the compounds were confirmed using single‐crystal X‐ray diffraction. Although five‐coordinated and polymeric in the solid state, the tin compounds are four‐coordinated and monomeric in solution. The coordination environment around the triorganotin units comprises three carbon atoms and two oxygen atoms from two ligands in a trigonal bipyramidal geometry. The anti‐proliferative effect of these compounds on the cervical carcinoma cell lines HeLa, CaSki and ViBo was screened in vitro, the compounds showing cytotoxic activity against all three strains and null or low cytotoxic activity (necrotic) as well. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
113.
114.
Martin Pomerantz Whe-Narn Chou 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract A series of N-phenyl-P,P,P-triarylphospha-λ5-azenes (1) as well as their l5N labeled analogs was synthesized. The 13C, 31P, and 15N NMR spectra of this series and those of two other series of related compounds, namely triarylphosphines (2) and triarylphosphine oxides (3), were measured and are reported. Many satisfactory correlations using the mono-substituent parameter (MSP) and the Taft dual-substituent parameter (DSP) treatments with the 13C substituent chemical shifts (SCS), 31P SCS, 15N SCS and the one bond P-N, P-C and C-N coupling constants were observed and will be discussed. Thus, for example, the 31P and 15N chemical shifts in 1 correlated with [sgrave]?with negative slopes while the 31P chemical shifts in 3 correlated with those in 1 with a slope of 2.0. The 13C chemical shifts in 1 correlated excellently with the corresponding ones in 3 with slopes very close to unity. The substituent effects on the chemical shifts of the various nuclei were shown to be mainly due to changes in the charge distribution on those nuclei. In 1 the one bond P-N and P-C coupling constants correlated with [sgrave]p and [sgrave]R respectively. The one bond P-C coupling constants of 1 correlated quite well with those of 3 with a slope of 0.93 while the corresponding correlation of 1 with 2 was quite poor. Taft DSP treatment of 1JPCin 1 and 3 were quite similar, ρI and ρR were both negative and ρR was much larger than ρI. Series 2 showed behavior which was different from that shown by 1 and 3 but similar to that shown by other systems with a lone electron pair on the atom bound to the phenyl ring. The substituent effects on the one bond P-N, P-C and C-N coupling constants will be discussed in terms of bonding and hybridization changes between the directly bonded nuclei. 相似文献
115.
Hua-Tsung Chen Chien-Li Chou Wei-Chin Tsai Suh-Yin Lee Bao-Shuh P. Lin 《Journal of Visual Communication and Image Representation》2012,23(5):767-781
With the dramatic growth of fandom population, a considerable amount of research efforts have been devoted to baseball video processing. However, little work focuses on the detailed follow-ups of ball hitting events. This paper proposes a HMM-based ball hitting event exploration system for broadcast baseball video. Utilizing the strictly-defined layout of the baseball field, the proposed system first detects the game-specific spatial patterns in the field, such as the field lines, the bases, the pitch mound, etc. Then, the play region—the currently camera-focused region of the baseball field is identified for frame type classification. Since the temporal patterns of presenting the game progress follow a prototypical order, we consider the classified frame types as observation symbols and recognize ball hitting events using HMM. Experiments conducted on broadcast baseball video show encouraging results in frame type classification and ball hitting event recognition. Three practical applications, including highlight clip extraction by user-designated query, storyboard construction, and similar event retrieval, are introduced to address the applicability of our system. 相似文献
116.
117.
118.
119.
120.