全文获取类型
收费全文 | 38880篇 |
免费 | 3078篇 |
国内免费 | 2422篇 |
出版年
2024年 | 62篇 |
2023年 | 441篇 |
2022年 | 1196篇 |
2021年 | 2040篇 |
2020年 | 1276篇 |
2019年 | 1591篇 |
2018年 | 1571篇 |
2017年 | 1154篇 |
2016年 | 1615篇 |
2015年 | 2360篇 |
2014年 | 2768篇 |
2013年 | 3128篇 |
2012年 | 3488篇 |
2011年 | 3136篇 |
2010年 | 1910篇 |
2009年 | 1595篇 |
2008年 | 1946篇 |
2007年 | 1685篇 |
2006年 | 1510篇 |
2005年 | 1236篇 |
2004年 | 1043篇 |
2003年 | 889篇 |
2002年 | 811篇 |
2001年 | 700篇 |
2000年 | 654篇 |
1999年 | 613篇 |
1998年 | 349篇 |
1997年 | 369篇 |
1996年 | 364篇 |
1995年 | 338篇 |
1994年 | 314篇 |
1993年 | 227篇 |
1992年 | 365篇 |
1991年 | 261篇 |
1990年 | 227篇 |
1989年 | 211篇 |
1988年 | 139篇 |
1987年 | 149篇 |
1986年 | 104篇 |
1985年 | 105篇 |
1984年 | 62篇 |
1983年 | 51篇 |
1982年 | 43篇 |
1981年 | 27篇 |
1980年 | 33篇 |
1979年 | 35篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1975年 | 18篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Yang Bai Zhong-Jun Zhou Jia-Jun Wang Ying Li Di Wu Wei Chen Zhi-Ru Li Chia-Chung Sun 《Journal of molecular modeling》2013,19(9):3983-3991
How to generate a non-zero first hyperpolarizability for a centrosymmetric molecule is a challenging question. In this paper, an external (pump) electric field is used to make a centrosymmetric benzene molecule generate a non-zero value of the electric field induced first hyperpolarizability (β F ). This comes from the centrosymmetry breaking of electron cloud. Two interesting rules are exhibited. (1) β F is anisotropic for different directional fields (F i, i?=?X, Y, Z). (2) The field dependence of β F is a non-monotonic function, and an optimum external electric field causes the maximum value of β F . The largest first hyperpolarizability β F reaches the considerable level of 3.9?×?105 a.u. under F Y?=?330?×?10?4 a.u. for benzene. The external electric field effects on non-centrosymmetric edge-modified graphene ribbon H2N-(3,3)ZGNR-NO2 was also studied in this work. The first hyperpolarizability reaches as much as 2.1?×?107 a.u. under F X?=?600?×?10?4 a.u. for H2N-(3,3)ZGNR-NO2. We show that the external electric field can not only create a non-zero first hyperpolarizability for centrosymmetric molecule, but also remarkably enhance the first hyperpolarizability for a non-centrosymmetric molecule. 相似文献
992.
In the present paper, a new type of Lewis acid–base complex BX3???Li@Calix[4]pyrrole (X = H and F) was designed and assembled based on electride molecule Li@calix[4]pyrrole (as a Lewis base) and the electron deficient molecule BX3 (as a Lewis acid) by employing quantum mechanical calculation. The new Lewis acid–base complex offers an interesting push-excess electron-pull (P-e-P) framework to enhance the stability and nonlinear optical (NLO) response. To measure the nonlinear optical response, static first hyperpolarizabilities (β 0) are exhibited. Significantly, point-face assembled Lewis acid–base complex BF3???Li@Calix[4]pyrrole (II) has considerable first hyperpolarizabilities (β 0) value (1.4?×?106 a.u.), which is about 117 times larger than reported 11,721 a.u. of electride Li@Calix[4]pyrrole. Further investigations show that, in BX3???Li@Calix[4]pyrrole with P-e-P framework, a strong charge-transfer transition from the ground state to the excited state contributes to the enhancement of first hyperpolarizability. Theory calculation of enthalpies of reaction (ΔrH0) at 298 K demonstrates that it is feasible to synthetize the complexes BX3???Li@Calix[4]pyrrole. In addition, compared with Li@Calix[4]pyrrole, the vertical ionization potential (VIP) and HOMO–LUMO gap of BX3???Li@Calix[4]pyrrole have obviously increased, due to the introduction of the Lewis acid molecule BX3. The novel Lewis acid–base NLO complex possesses not only a large nonlinear optical response but also higher stability. Figure
A novel Lewis acid–base complex is first proposed by the combination of usual Lewis acid and an electride. It offers an interesting push-excess electron-pull framework to enhance the stability and nonlinear optical response. 相似文献
993.
Min Cen Jian Fen Fan Dong Yan Liu Xue Zeng Song Jian Liu Wei Qun Zhou He Ming Xiao 《Journal of molecular modeling》2013,19(2):601-611
Molecular dynamic (MD) simulations have been performed to study the behaviors of ten kinds of cyclo-hexa-peptides (CHPs) composed of amino acids with the diverse hydrophilic/hydrophobic side chains at the water/cyclohexane interface. All the CHPs take the “horse-saddle” conformations at the interface and the hydrophilicity/hydrophobicity of the side chains influences the backbones’ structural deformations. The orientations and distributions of the CHPs at the interface and the differences of interaction energies (ΔΔE) between the CHPs and the two liquid phases have been determined. RDF analysis shows that the H-bonds were formed between the OC atoms of the CHPs’ backbones and Hw atoms of water molecules. N atoms of the CHPs’ backbones formed the H-bonds or van der Waals interactions with the water solvent. It was found that there is a parallel relationship between ΔΔE and the lateral diffusion coefficients (D xy ) of the CHPs at the interface. The movements of water molecules close to the interface are confined to some extent, indicating that the dynamics of the CHPs and interfacial water molecules are strongly coupled. Figure
Scheme of the ten kinds of CHPs formed by even alternating D- and L- amino acids with the different hydrophilic/hydrophobic side chains. The letters in the parentheses stand for the abbreviations of the composed amino acids in the CHPs 相似文献
994.
995.
Xiao‐Bin Li Fei Xie Shan‐Shan Liu Ying Li Jin‐Chuan Zhou Yong‐Qing Liu Hui‐Qing Yuan Hong‐Xiang Lou 《化学与生物多样性》2013,10(7):1193-1201
Bioactivity‐guided fractionation of the cytotoxic extract of Aspergillus niger, an endophytic fungus from the Chinese liverwort Heteroscyphus tener (Steph .) Schiffn ., afforded five new naphtho‐γ‐pyrones, rubrofusarin‐6‐O‐α‐D ‐ribofuranoside ( 1 ), (R)‐10‐(3‐succinimidyl)‐TMC‐256A1 ( 2 ), asperpyrone E ( 3 ), isoaurasperone A ( 4 ), and isoaurasperone F ( 5 ), as well as four known ones, dianhydroaurasperone C ( 6 ), aurasperone D ( 7 ), asperpyrone D ( 8 ), and asperpyrone A ( 9 ), together with a cytotoxic cyclic pentapeptide, malformin A1 ( 10 ). Their structures were determined by extensive spectroscopic analysis. The absolute configurations of dimeric naphtho‐γ‐pyrones 3 – 9 were also determined by analysis of their respective CD spectra. 相似文献
996.
997.
Jianmei Luo Meiling Chi Hongyu Wang Huanhuan He Minghua Zhou 《Bioprocess and biosystems engineering》2013,36(12):1889-1896
A convenient and promising alternative to surface modification of carbon mesh anode was fulfilled by electrochemical oxidation in the electrolyte of nitric acid or ammonium nitrate at ambient temperature. It was confirmed that such an anode modification method was low cost and effective not only in improving the efficiency of power generation in microbial fuel cells (MFCs) for synthetic wastewater treatment, but also helping to reduce the period for MFCs start-up. The MFCs with anode modification in electrolyte of nitric acid performed the best, achieving a Coulombic efficiency enhancement of 71 %. As characterized, the electrochemical modification resulted in the decrease of the anode potential and internal resistance but the increase of current response and nitrogen-containing and oxygen-containing functional groups on the carbon surface, which might contribute to the enhancement on the performances of MFCs. 相似文献
998.
999.
1000.
Jie Zhou Changtao Wang Zeyu Zhao Yanqin Wang Jiayu He Xing Tao Xiangang Luo 《Plasmonics (Norwell, Mass.)》2013,8(2):1073-1078
Ag tip–insulator–metal structure with bottom–up light illumination is proposed and theoretically analyzed. It shows that there is a strong plasmonic coupling between Ag tip and metallic surface. Different from oblique light illumination, this novel design possesses unique advantages of symmetrical hot spot profile and enlarged depth of focus at a sub-10-nm spatial resolution. Influences of tip size, insulator, and metallic layer thickness are studied. It is found that the metallic layer thickness greatly affects the plasmonic hot spot quality. Meanwhile, the thickness of photoresist plays a major role in controlling light spot size, indicating that much higher resolution can be achieved for the Ag tips with large curvature radius. 相似文献