全文获取类型
收费全文 | 8447篇 |
免费 | 1264篇 |
国内免费 | 797篇 |
专业分类
化学 | 5886篇 |
晶体学 | 137篇 |
力学 | 490篇 |
综合类 | 73篇 |
数学 | 813篇 |
物理学 | 3109篇 |
出版年
2024年 | 37篇 |
2023年 | 193篇 |
2022年 | 323篇 |
2021年 | 317篇 |
2020年 | 386篇 |
2019年 | 357篇 |
2018年 | 314篇 |
2017年 | 267篇 |
2016年 | 425篇 |
2015年 | 413篇 |
2014年 | 552篇 |
2013年 | 626篇 |
2012年 | 693篇 |
2011年 | 720篇 |
2010年 | 472篇 |
2009年 | 385篇 |
2008年 | 511篇 |
2007年 | 439篇 |
2006年 | 384篇 |
2005年 | 327篇 |
2004年 | 224篇 |
2003年 | 220篇 |
2002年 | 169篇 |
2001年 | 128篇 |
2000年 | 162篇 |
1999年 | 157篇 |
1998年 | 135篇 |
1997年 | 135篇 |
1996年 | 157篇 |
1995年 | 113篇 |
1994年 | 113篇 |
1993年 | 92篇 |
1992年 | 79篇 |
1991年 | 70篇 |
1990年 | 72篇 |
1989年 | 48篇 |
1988年 | 49篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 26篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1970年 | 12篇 |
1937年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Jing-Yuan Chang Yen-Jin Pan Pei-Yu Huang Yi-Ting Sun Chen-Hsu Yu Zhi-Jun Ning Shou-Ling Huang Shing-Jong Huang Richard P. Cheng 《Molecules (Basel, Switzerland)》2022,27(13)
The β-sheet is one of the common protein secondary structures, and the aberrant aggregation of β-sheets is implicated in various neurodegenerative diseases. Cross-strand interactions are an important determinant of β-sheet stability. Accordingly, both diagonal and lateral cross-strand interactions have been studied. Surprisingly, diagonal cross-strand ion-pairing interactions have yet to be investigated. Herein, we present a systematic study on the effects of charged amino acid side-chain length on a diagonal ion-pairing interaction between carboxylate- and ammonium-containing residues in a β-hairpin. To this end, 2D-NMR was used to investigate the conformation of the peptides. The fraction folded population and the folding free energy were derived from the chemical shift data. The fraction folded population for these peptides with potential diagonal ion pairs was mostly lower compared to the corresponding peptide with a potential lateral ion pair. The diagonal ion-pairing interaction energy was derived using double mutant cycle analysis. The Asp2-Dab9 (Asp: one methylene; Dab: two methylenes) interaction was the most stabilizing (−0.79 ± 0.14 kcal/mol), most likely representing an optimal balance between the entropic penalty to enable the ion-pairing interaction and the number of side-chain conformations that can accommodate the interaction. These results should be useful for designing β-sheet containing molecular entities for various applications. 相似文献
152.
An N-heterocyclic carbene(NHC)-catalyzed enantioselective Mannich reaction of the remote γ-carbon of cyclopropylcarbaldehydes is disclosed for the first time. Diastereo-and enantiomerically enriched multicyclic δ-lactam compound is afforded as the main product from 8 possible stereo-specific isomers through dynamic kinetic asymmetric transformation(DYKAT) processes. Multiple chiral functional molecules can be afforded from the lactam products through simple protocols with retentions of the optical purities. 相似文献
153.
154.
Active control of a flexible cantilever plate with multiple time delays is investigated using the discrete optimal control method. A controller with multiple time delays is presented. In this controller, time delay effect is incorporated in the mathematical model of the dynamic system throughout the control design and no approximations and assumptions are made in the controller derivation, so the system stability is easily guaranteed. Furthermore, this controller is available for both small time delays and large time delays. The feasibility and efficiency of the proposed controller are verified through numerical simulations in the end of this paper. 相似文献
155.
Ming Xia Prof. Miriding Mutailipu Fuming Li Prof. Zhihua Yang Prof. Shilie Pan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(38):9753-9757
Herein, a new congruently melting mixed-anion compound Cs4B4O3F10 has been characterized as the first fluorooxoborate with [BF4] involving heteroanionic units. Compound Cs4B4O3F10 possesses two highly fluorinated anionic clusters and therefore its formula can be expressed as Cs3(B3O3F6) ⋅ Cs(BF4). The influence of [BF4] units on micro-symmetry and structural evolution was discussed based on the parent compound. More importantly, Cs4B4O3F10 shows the lowest melting point among all the available borates and thus sets a new record for such system. This work is of great significance to enrich and tailor the structure of borates using perfluorinated [BF4] units. 相似文献
156.
157.
Chromatographia - An automated online column extraction coupled with high-performance liquid chromatography (HPLC) and equipped with a double-trap column system was developed to determine valproic... 相似文献
158.
Dayang Yu Guoliang Mao Huaixun Cai Shuhua Wang Jiandang Liu Pengju Pan Yongzhong Bao 《Journal of polymer science. Part A, Polymer chemistry》2021,59(9):754-763
2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene (PDD-TFE) copolymer is a good candidate to prepare gas separation membranes with excellent permeability due to its free volume characteristics. However, the influence of PDD-TFE copolymer structure on its free volume characteristics is less studied. In this paper, PDD-TFE copolymers with different compositions and molecular weights were synthesized, and their free volume characteristics were analyzed by positron annihilation lifetime spectroscopy and a molecular dynamics simulation. It indicated that the molar fraction of PDD in copolymers had a significant effect on free volume characteristics, while the molecular weight of copolymers exerted a slight influence on free volume when the molecular weight exceeded a critical region (intrinsic viscosity [η] > 68 ml g−1). PDD-TFE copolymers with greater PDD molar fractions (i.e., 72% and 84%) showed bimodal distributions in positron lifetime and free volume size distributions, while PDD-TFE copolymers with lower PDD molar fractions (i.e., 27% and 35%) exhibited a single peak. The long-lifetime parameter τ3 was assigned to micro-cavities formed by [-(TFE)y-PDD-] segments and τ4 was attributed to micro-cavities formed by [-(PDD)x-TFE-] segments. The cis and trans transitions of PDD led to a local multilayer spiral structure with a 2.6–4.3 Å layer spacing, which would also increase the free volume of copolymers. 相似文献
159.
Shixiong Chen Xiaotian Zhao Ming Jin Wanqiu Huang Guodong Ye Haiyan Pan Decheng Wan 《Journal of polymer science. Part A, Polymer chemistry》2021,59(17):1899-1911
Four 1,5-diphenyl-3-aromatic heterocyclyl-2-pyrazoline-based sulfonium salt photoacid generators (PAGs) with different aromatic heterocycles substituted on C3 atom and dimethyl sulfonium group on C5 atom were synthesized. These PAGs were highly photosensitive in the 365–425 nm light-emitting diode region, and the intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high ΦH+. The heterocycles as well as their substituted positions significantly influenced the energy of the S2 orbital, which was determined by the electrochemical and absorption properties of the PAGs. The raising of the S2 orbital energy enlarged the energy gap of S0–S2 and S1–S2, resulting in blue shift of the absorption spectra and increase in the quantum yield of photoacid generation (ΦH+), respectively. When the energy of excited electrons was higher than that of the S2 orbital, the transition from S0 to S2 (π–π*) occurred before the C-S cleavage on S1 and the PAGs showed high ΦH+ values (0.52–0.72). The transition from S0 to S1 (π–σ*) occurred when the energy of electrons is lower than that of the S2 orbital, and the PAGs showed low ΦH+ value. The photopolymerization kinetics demonstrated that these PAGs were highly efficient cationic photoinitiators. 相似文献
160.