全文获取类型
收费全文 | 8079篇 |
免费 | 1378篇 |
国内免费 | 965篇 |
专业分类
化学 | 5797篇 |
晶体学 | 120篇 |
力学 | 454篇 |
综合类 | 53篇 |
数学 | 1136篇 |
物理学 | 2862篇 |
出版年
2024年 | 25篇 |
2023年 | 176篇 |
2022年 | 324篇 |
2021年 | 284篇 |
2020年 | 306篇 |
2019年 | 300篇 |
2018年 | 256篇 |
2017年 | 268篇 |
2016年 | 429篇 |
2015年 | 416篇 |
2014年 | 552篇 |
2013年 | 596篇 |
2012年 | 766篇 |
2011年 | 776篇 |
2010年 | 503篇 |
2009年 | 456篇 |
2008年 | 512篇 |
2007年 | 404篇 |
2006年 | 409篇 |
2005年 | 374篇 |
2004年 | 327篇 |
2003年 | 241篇 |
2002年 | 224篇 |
2001年 | 186篇 |
2000年 | 126篇 |
1999年 | 187篇 |
1998年 | 125篇 |
1997年 | 97篇 |
1996年 | 113篇 |
1995年 | 114篇 |
1994年 | 95篇 |
1993年 | 58篇 |
1992年 | 59篇 |
1991年 | 61篇 |
1990年 | 68篇 |
1989年 | 46篇 |
1988年 | 29篇 |
1987年 | 22篇 |
1986年 | 30篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1979年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1928年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Utilizing fountain codes to control the peak-to-average power ratio (PAPR) is a classic scheme in Orthogonal Frequency Division Multiplexing (OFDM) wireless communication systems. However, because the robust soliton distribution (RSD) produces large-degree values, the decoding performance is severely reduced. In this paper, we design statistical degree distribution (SD) under a scenario that utilizes fountain codes to control the PAPR. The probability of the PAPR produced is combined with RSD to design PRSD, which enhances the smaller degree value produced. Subsequently, a particle swarm optimization (PSO) algorithm is used to search the optimal degree value between the binary exponential distribution (BED) and PRSD distribution according to the minimum average degree principle. Simulation results demonstrate that the proposed method outperforms other relevant degree distributions in the same controlled PAPR threshold, and the average degree value and decoding efficiency are remarkably improved. 相似文献
992.
铋及其化合物具有相对价廉、低毒性、低放射性等特点,已应用于医药、催化、化妆品和电子技术等领域。本文回顾了近10多年来有机铋化学的研究状况,主要从以下4个方面进行综述:(1)新型有机铋化合物的合成途径与结构特征;(2)有机铋化合物作为交联偶合、氧化、芳基化和其它反应的试剂;(3)作为催化剂;(4)作为生物医药用于溃疡、肿瘤和放射治疗等。文中着重于从分子水平关联新型有机铋化合物的结构与其化学和生物活性之间关系。此外,还介绍了有机铋化学研究领域的不足和今后的发展趋势。 相似文献
993.
Shijie Ye Dongjie Yin Xiaoyan Sun Qinyi Chen Ting Min Hongxun Wang Limei Wang 《Molecules (Basel, Switzerland)》2022,27(23)
Trapa bispinosa Roxb. is an economical crop for medicine and food. Its roots, stems, leaves, and pulp have medicinal applications, and its shell is rich in active ingredients and is considered to have a high medicinal value. One of the main functional components of the Trapa bispinosa Roxb. shell is 1-galloyl-beta-D-glucose (βG), which can be used in medical treatment and is also an essential substrate for synthesizing the anticancer drug beta-penta-o-Galloyl-glucosen (PGG). Furthermore, gallate 1-beta-glucosyltransferase (EC 2.4.1.136) has been found to catalyze gallic acid (GA) and uridine diphosphate glucose (UDPG) to synthesize βG. In our previous study, significant differences in βG content were observed in different tissues of Trapa bispinosa Roxb. In this study, Trapa bispinosa Roxb. was used to clone 1500 bp of the UGGT gene, which was named TbUGGT, to encode 499 amino acids. According to the specificity of the endogenous expression of foreign genes in Escherichia coli, the adaptation codon of the cloned original genes was optimized for improved expression. Bioinformatic and phylogenetic tree analyses revealed the high homology of TbUGGT with squalene synthases from other plants. The TbUGGT gene was constructed into a PET-28a expression vector and then transferred into Escherichia coli Transsetta (DE3) for expression. The recombinant protein had a molecular weight of 55 kDa and was detected using SDS-PAGE. The proteins were purified using multiple fermentation cultures to simulate the intracellular environment, and a substrate was added for in vitro reaction. After the enzymatic reaction, the levels of βG in the product were analyzed using HPLC and LC-MS, indicating the catalytic activity of TbUGGT. The cloning and functional analysis of TbUGGT may lay the foundation for further study on the complete synthesis of βG in E. coli. 相似文献
994.
Yawei Jia Pengfei Li Kanglei Liu Chenglong Li Meiyan Liu Jiaqi Di Nan Wang Xiaodong Yin Niu Zhang Pangkuan Chen 《Chemical science》2022,13(39):11672
π-Conjugated macrocycles involving electron-deficient boron species have received increasing attention due to their intriguing tunable optoelectronic properties. However, most of the reported B(sp2)-doped macrocycles are difficult to modify due to the synthetic challenge, which limits their further applications. Motivated by the research of non-strained hexameric bora- and aza-cyclophanes, we describe a new class of analogues MC-BN5 and MC-ABN5 that contain charge-reversed triarylborane (Ar3B) units and oligomeric triarylamines (Ar3N) in the cyclics. As predicted by DFT computations, the unique orientation of the donor–acceptor systems leads to an increased dipole moment compared with highly symmetric macrocycles (M1, M2 and M3), which was experimentally represented by a significant solvatochromic effect with large Stokes shifts up to 12 318 cm−1. Such a ring-structured design also allows the easy peripheral modification of aza-boracyclophanes with tetraphenylethenyl (TPE) groups, giving rise to a change in the luminescence mechanism from aggregation-caused quenching (ACQ) in MC-BN5 to aggregation-induced emission (AIE) in MC-ABN5. The open-shell characteristics have been chemically enabled and were characterized by UV-Vis-NIR spectroscopy and electron paramagnetic resonance (EPR) for MC-BN5. The present study not only showed new electronic properties, but also could expand the research of B/N doped macrocycles into the future scope of supramolecular chemistry, as demonstrated in the accessible functionalization of ring systems.We have achieved the synthesis of new B/N doped macrocycles by stitching oligoarylamine pentamers using arylborane acceptor segments. They exhibit open-shell radical species and AIE under ambient conditions. 相似文献
995.
996.
To reveal the nature of CO2 reduction to formate with high efficiency by in situ hydrogen produced from hydrothermal reactions with iron, DFT calculations were used. A reaction pathway was proposed in which the formate was produced through the key intermediate species, namely iron hydride, produced in situ in the process of hydrogen gas production. In the in situ hydrogenation of CO2, the charge of H in the iron hydride was −0.135, and the Fe–H bond distance was approximately 1.537 Å. A C-H bond was formed as a transition state during the attack of Hδ− on Cδ+. Finally, a HCOO species was formed. The distance of the C-H bond was 1.107 Å. The calculated free energy barrier was 16.43 kcal/mol. This study may provide new insight into CO2 reduction to formate in hydrothermal reactions with metal. 相似文献
997.
Guo-Ying Chen Zheng-Ming Qian Shi-Jun Yin Xi Zhou Feng-Qing Yang 《Molecules (Basel, Switzerland)》2022,27(21)
In this study, a simple colorimetric method was established to detect copper ion (Cu2+), sulfathiazole (ST), and glucose based on the acetylcholinesterase (AChE)-like activity of zeolitic imidazolate framework-8 (ZIF-8). The AChE-like activity of ZIF-8 can hydrolyze acetylthiocholine chloride (ATCh) to thiocholine (TCh), which will further react with 5,5′-dithiobis (2-nitrobenzoic acid) (DTNB) to generate 2-nitro-5-thiobenzoic acid (TNB) that has a maximum absorption peak at 405 nm. The effects of different reaction conditions (buffer pH, the volume of ZIF-8, reaction temperature and time, and ATCh concentration) were investigated. Under the optimized conditions, the value of the Michaelis-Menten constant (Km) is measured to be 0.83 mM, which shows a high affinity toward the substrate (ATCh). Meanwhile, the ZIF-8 has good storage stability, which can maintain more than 80.0% of its initial activity after 30 days of storage at room temperature, and the relative standard deviation (RSD) of batch-to-batch (n = 3) is 5.1%. The linear dependences are obtained based on the AChE-like activity of ZIF-8 for the detection of Cu2+, ST, and glucose in the ranges of 0.021–1.34 and 5.38–689.66 µM, 43.10–517.24 µM, and 0.0054–1.40 mM, respectively. The limit of detections (LODs) are calculated to be 20.00 nM, 9.25 µM, and 5.24 µM, respectively. Moreover, the sample spiked recoveries of Cu2+ in lake water, ST in milk, and glucose in strawberry samples were measured, and the results are in the range of 98.4–115.4% with the RSD (n = 3) lower than 3.3%. In addition, the method shows high selectivity in the real sample analysis. 相似文献
998.
Pan Du Yuhao Yin Dai Shi Kexin Mao Qianyuan Yu Jiyang Zhao 《Molecules (Basel, Switzerland)》2022,27(17)
This study investigates the mechanism of metal-free pyridine phosphination with P(OEt)3, PPh3, and PAr2CF3 using density functional theory calculations. The results show that the reaction mechanism and rate-determining step vary depending on the phosphine and additive used. For example, phosphination of pyridine with P(OEt)3 occurs in five stages, and ethyl abstraction is the rate-determining step. Meanwhile, 2-Ph-pyridine phosphination with PPh3 is a four-step reaction with proton abstraction as the rate-limiting step. Energy decomposition analysis of the transition states reveals that steric hindrance in the phosphine molecule plays a key role in the site-selective formation of the phosphonium salt. The mechanism of 2-Ph-pyridine phosphination with PAr2CF3 is similar to that with PPh3, and analyses of the effects of substituents show that electron-withdrawing groups decreased the nucleophilicity of the phosphine, whereas aryl electron-donating groups increased it. Finally, TfO− plays an important role in the C–H fluoroalkylation of pyridine, as it brings weak interactions. 相似文献
999.
Jiaqi He Yu Yin Yingjie Shao Wenkai Zhang Yanling Lin Xiuping Qian Qizhi Ren 《Molecules (Basel, Switzerland)》2022,27(18)
Porphyrin derivatives are popular photodynamic therapy (PDT) agents; however, their typical insolubility in water has made it challenging to separate cells of organisms in a liquid water environment. Herein, a novel water-soluble 5,10,15,20-tetrakis(4-methoxyphenyl-3-sulfonatophenyl) porphyrin (TMPPS) was synthesized with 95% yield by modifying the traditional sulfonation route. The reaction of TMPPS with AgNO3 afforded AgTMPPS an unusual Ag(II) oxidation state (97% yield). The free base and Ag(II) complex were characterized by matrix-assisted laser desorption ionization-mass spectroscopy, and 1H nuclear magnetic resonance, Fourier-transform infrared, UV-vis, fluorescence, and X-ray photolectron spectroscopies. Upon 460 nm laser irradiation, AgTMPPS generated a large amount of 1O2, whereas no ⦁OH was detected. Antibacterial experiments on methicillin-resistant Staphylococcus aureus (MRSA) revealed that the combined action of AgⅡ ions and PDT could endow AgTMPPS with a 100% bactericidal ratio for highly concentrated MRSA (108 CFU/mL) at a very low dosage (4 μM) under laser irradiation at 360 J/cm2. Another PDT response was demonstrated by photocatalytically oxidizing 1,4-dihydronicotinamide adenine dinucleotide to NAD+ with AgTMPPS. The structural features of the TMPPS and AgTMPPS molecules were investigated by density functional theory quantum chemical calculations to demonstrate the efficient chemical and photodynamical effects of AgTMPPS for non-invasive antibacterial therapy. 相似文献
1000.