全文获取类型
收费全文 | 77374篇 |
免费 | 15193篇 |
国内免费 | 5672篇 |
专业分类
化学 | 69347篇 |
晶体学 | 777篇 |
力学 | 3145篇 |
综合类 | 393篇 |
数学 | 6927篇 |
物理学 | 17650篇 |
出版年
2024年 | 149篇 |
2023年 | 792篇 |
2022年 | 1352篇 |
2021年 | 1567篇 |
2020年 | 2668篇 |
2019年 | 3886篇 |
2018年 | 2262篇 |
2017年 | 1837篇 |
2016年 | 5073篇 |
2015年 | 5184篇 |
2014年 | 5517篇 |
2013年 | 6918篇 |
2012年 | 6565篇 |
2011年 | 6035篇 |
2010年 | 5246篇 |
2009年 | 5225篇 |
2008年 | 5078篇 |
2007年 | 4234篇 |
2006年 | 3723篇 |
2005年 | 3433篇 |
2004年 | 2783篇 |
2003年 | 2439篇 |
2002年 | 3135篇 |
2001年 | 2240篇 |
2000年 | 2137篇 |
1999年 | 1284篇 |
1998年 | 831篇 |
1997年 | 810篇 |
1996年 | 895篇 |
1995年 | 708篇 |
1994年 | 595篇 |
1993年 | 520篇 |
1992年 | 493篇 |
1991年 | 429篇 |
1990年 | 350篇 |
1989年 | 297篇 |
1988年 | 210篇 |
1987年 | 184篇 |
1986年 | 176篇 |
1985年 | 169篇 |
1984年 | 129篇 |
1983年 | 99篇 |
1982年 | 73篇 |
1981年 | 57篇 |
1980年 | 35篇 |
1978年 | 36篇 |
1977年 | 43篇 |
1976年 | 33篇 |
1975年 | 40篇 |
1974年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Catalina Soriano‐Correa Rodolfo O. Esquivel Robin P. Sagar 《International journal of quantum chemistry》2003,94(3):165-172
A theoretical study at the Hartree–Fock and density functional theory levels is performed on sulfonamide‐type bacteriostatic compounds with the aim to provide an insight into their structure–activity relationship. The basicity of the p‐amino group is analyzed by means of the proton affinities and the protonation energies, showing that molecules presenting bacteriostatic activity are less basic, i.e., they are characterized by larger protonation energies and smaller proton affinities. The acidity of the amide group is analyzed through the deprotonation energy. The results reveal that the more acidic molecules present a larger bacteriostatic activity. This result is also confirmed from a study of bond orders. A bond order analysis of the amide group suggests that the electron attracting group in these molecules is responsible for the increase in acidity. The charge of the SO2 group is also shown to be affected by the presence of the electron attracting group and consequently related to the acidity of the molecules. A geometric analysis shows that structures in which the amino group is more coplanar with respect to the benzenic ring possess larger bacteriostatic activity. A conformational analysis of these molecules illustrates that active molecules have relatively larger torsion energy barriers. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 165–172, 2003 相似文献
952.
953.
954.
Ju-Chun Huang Chien Hong Cheng Shyang Roeng Sheen Zhi Ling Lee Chien Ming Lei Maw Kuen Wu 《中国化学会会志》1996,43(2):139-143
The synthesis of bulk Y2Ba4Cu7O15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y2Ba4Cu7O15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y2Ba4Cu7O15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described. 相似文献
955.
A novel algorithm is proposed for the fixed-node quantum Monte Carlo (FNQMC) method.In contrast to previous procedures,its "guiding function" is not optimized prior to diffusion quantum Monte Carlo (DMC) computation but synchronistically in the diffusion process The new algorithm can not only save CPU time,but also make both of the optimization and diffusion carried out according to the same sampling fashion,reaching the goal to improve each other This new optimizing procedure converges super-linearly,and thus can accelerate the particle diffusion During the diffusion process,the node of the "guiding function" changes incessantly,which is conducible to reducing the "fixed-node error" The new algorithm has been used to calculate the total energies of states X3B1 and a1A1 of CH2 as well as π-X2B1 and λ-2A1 of NH2 The singlet-triplet energy splitting (λEsT) in CH2 and π energy splitting in NH2 obtained with this present method are (45 542±1.840) and (141.644±1.589) kJ/mol,respectively The calculated 相似文献
956.
混合高序铜法立体选择性合成(±)┐EpilitsenolideC2陈祖兴李焰*黄精美黄锦霞徐章煌(湖北大学化学系武汉430062)关键词烷叉基二氢羟基甲基(3H)呋喃酮,混合高序铜,立体选择性反应1996-08-28收稿,1997-05-06修回湖北... 相似文献
957.
Jiang Hu Wei‐Dong Zhang Yun‐Heng Shen Chuan Zhang Run‐Hui Liu Xi‐Ke Xu Bin Wang 《Helvetica chimica acta》2007,90(4):720-722
Two novel alkaloids, 4,5‐dihydroxy‐1‐methyl‐3‐oxo‐2‐(trichloromethyl)‐3H‐indolium chloride ( 1 ) and 4‐(2‐methoxyethyl)‐N,N‐dimethylbenzenamine ( 2 ), were isolated from the EtOH extract of the roots of Zanthoxylum nitidum. Their structures were identified on the basis of spectroscopic and mass‐spectrometric analyses. 相似文献
958.
The hydrothermal reaction of VOSO4, As2O5, piperazine and H2O produces [H2N(CH2)4NH2]4[β‐As8V14O42(SO4)]·2HSO4 ( 1 ), which is the first arsenic‐vanadium cluster containing a spherical β‐As8V14O42 shell. The structure of this compound was characterized by single crystal X‐ray diffraction, elemental analysis, TG, and IR spectrum. Crystal data for 1 : Orthorhombic, Cmcm, a = 15.369(1) Å, b = 16.404(1) Å, c = 25.772(1) Å, V = 6497.4(9) Å3, Z = 4. 相似文献
959.
960.