全文获取类型
收费全文 | 573篇 |
免费 | 84篇 |
国内免费 | 123篇 |
专业分类
化学 | 675篇 |
晶体学 | 7篇 |
力学 | 14篇 |
综合类 | 4篇 |
物理学 | 80篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 25篇 |
2021年 | 36篇 |
2020年 | 46篇 |
2019年 | 45篇 |
2018年 | 29篇 |
2017年 | 48篇 |
2016年 | 41篇 |
2015年 | 28篇 |
2014年 | 42篇 |
2013年 | 61篇 |
2012年 | 32篇 |
2011年 | 25篇 |
2010年 | 19篇 |
2009年 | 30篇 |
2008年 | 26篇 |
2007年 | 20篇 |
2006年 | 28篇 |
2005年 | 27篇 |
2004年 | 28篇 |
2003年 | 26篇 |
2002年 | 7篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1980年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有780条查询结果,搜索用时 15 毫秒
61.
The ultrafast dynamics of CS2 in the 1B2(1Σu+) state was studied by photoelectron imaging with a time resolution of 22 fs. The photoelectron signal intensity exhibited clear vibrational quantum beats due to wave packet motion. The signal intensity decayed with a lifetime of about 400 fs. This decay was preceded by a lag of around 30 fs, which was considered to correspond to the time for a vibrational wave packet to propagate from the Franck–Condon region to the region where predissociation occurred. The photoelectron angular distribution remained constant when the pump–probe delay time was varied. Consequently, variation of the electronic character caused by the vibrational wave packet motion was not identified within the accuracy of our measurements. 相似文献
62.
Issa Yavari Mina Haghdadi Rahebeh Amiri 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2015-2023
Ab initio calculations at HF/6-31+G? level of theory for geometry optimization, and MP2/6-31+G?//HF/6-31+G? and B3LYP/6-31+G?//HF/6-31+G? levels for a single-point total energy calculation, are reported for the chair and twist conformations of 1,2-dithiane (1), 3,3,6,6-tetramethyl-1,2-dithiane (2), 1,2,4,5-tetrathiane (3), and 3,3,6,6-tetramethyl-1,2,4,5-tetrathiane (4). The C2 symmetric chair conformations of 1 and 2 are calculated to be 21.9 and 8.6 kJ mol?1 more stable than the corresponding twist forms. The calculated energy barriers for chair-to-twist processes in 1 and 2 are 56.3 and 72.8 kJ mol?1, respectively. The C2h symmetric chair conformation of 3 is 10.7 kJ mol?1 more stable than the twist form. Interconversion of these forms takes place via a C2 symmetric transition state, which is 67.5 kJ mol?1 less stable than 3-Chair. The D2 symmetric twist-boat conformation of 4 is calculated to be 4.0 kJ mol?1 more stable than the C2h symmetric chair form. The calculated strain energy for twist to chair process is 61.1 kJ mol?1. 相似文献
63.
Yang-Gen Hu Wen-Qing Wang Ming-Wu Ding 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):857-864
2-Alkylthiobenzofuro[3,2-d]pyrimidinones 5 and 6 were synthesized by S-alkylation of 2,3-dihydro-2-thioxobenzofuro[3,2-d]pyrimidin-4(1H)-ones 4, which were obtained via aza-Wittig reaction of iminophosphorane 2 with CS2 and further reaction of the product with various amines. Compounds 5 and 6 exhibited fungicidal activity. For example, compound 6a, which has a small N-substituted methyl group, showed the best inhibitive activity (91%) against Dothiorella gregaria at 50 mg/L. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
64.
Esmael Rostami Masume Zangooei Marziyh Ghaedi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):394-399
Abstract New aza thia crowns were prepared from the reaction of 2,2′-diaminodiphenyl disulfide and diacids or diacid chlorides. Diacids are malonic acid, adipic acid, sebacic acid, and 2,2′-thiodiacetic acid and diacid dichlorides are malonyl chloride, adipoyl chloride, sebacoyl chloride, and 2,2′-thiodiacetyl chloride. [Supplementary materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfer, and Silicon and the Related Elements for the following free supplemental files: Additional figures.] 相似文献
65.
Cheng-Xia Tan Li-Yan Pan Guo-Fu Zhang Yong-Shu Li 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):16-21
Abstract A convenient and facile catalytic oxidation of thiols to the corresponding disulfides is described using CoSalen as the catalyst and air as the oxidizing agent. This new approach provides an efficient method for the preparation of symmetrical disulfides in high yields and under mild conditions. [Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental resource: Spectroscopic Identification of Products 3a–3l.] 相似文献
66.
Devdutt Chaturvedi Nisha Mishra Amit K. Chaturvdi Virendra Mishra 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):550-558
A quick and efficient, one-pot synthesis of dithiocarbazates was accomplished in high yields by the reaction of various alcoholic tosylates of primary, secondary, and tertiary alcohols, with substituted hydrazines using an Amberlite IRA 400 (basic resin)/CS 2 system. The reaction conditions are mild with simpler work-up procedures than previously reported methods. 相似文献
67.
Chih‐Hsien Wang Chia‐Chi Huang Long‐Liu Lin Wenlung Chen 《Journal of Raman spectroscopy : JRS》2016,47(8):940-947
Disulfide bond is relevant to many protein folding/unfolding functions and conformational diseases. To elucidate the effects of disulfide bonds on protein folding, unfolding, and misfolding, we performed Fourier transform–Raman measurements on serial chemical‐induced denaturations of bovine serum albumin (BSA). By directly monitoring Raman stretching at S–S (~507 cm−1), S–H (~2566 cm−1), amide I (1655 cm−1 for α‐helix; 1667 cm−1 for β‐sheet structure), and amide III (>1300 cm−1 for α‐helix; 1246 cm−1 for β‐sheet structure), the status of disulfide bonds and secondary structure of BSA at different states were elucidated. Both disulfide bonds and secondary structure (mostly in α‐helix) of BSA appeared relatively stable even when the protein was unfolded by urea solution. However, disulfide bonds were completely reduced and protein secondary structure changed from α‐helix to a relatively β‐sheet dominant when the protein was modified by the mixed solution of urea and dithiothreitol (urea/DTT). Adhering to these structural changes, the protein proceeded to different degrees of polymerization. BSA would aggregate into a high molecular mass (over 700 kDa) of protein ensemble when it was exposed to the mixed urea/DTT solution. An irreversible change in S–S/S–H conversion and secondary structure was responsible for protein misfolding. We demonstrate here that Fourier transform–Raman directly probe S–S/S–H conversion and secondary structural change of BSA at different states, and these results clearly indicate that disulfide bonds and secondary structure of BSA serve as concrete frameworks to stabilize protein structure. As the frameworks collapse, the protein undergoes an irreversible structural change and results in protein misfolding. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
68.
N. Narendra 《Tetrahedron letters》2010,51(47):6169-6173
A simple protocol for the synthesis of a new class of trithiocarbonate-linked peptidomimetics and neoglycosylated amino acids is described. N-Protected amino alkyl thiols were treated with CS2 in the presence of triethylamine (TEA) to generate trithiocarbonate salt, which upon reaction with appropriate halides afforded dipeptidomimetics in good yields. Further, the procedure was also extended for the synthesis of N,N′-orthogonally protected trithiocarbonate-linked dipeptidomimetics. 相似文献
69.
《高分子科学杂志,A辑:纯化学与应用化学》2013,50(11):1119-1134
Abstract The degree of randomization, q, of structural units in melt blends of the polysulfide homopolymers A(PS1) and B(PS2), wherein the disulfide equivalents D A/D B = 1, were studied by electron ionization mass spectrometry. Over the temperature range of 207–219°C, the relaxation process, due to the dominant disulfide–disulfide interchange reactions, is postulated to follow an associative reaction mechanism. These intermolecular disulfide–disulfide interactions promote a transient enhancement of the sulfur rank in the activated complex resulting in formation of the randomized co‐polymer AB. The mass spectrometric experimental design enabled measurement of concentrations of reactants A(PS1) and B(PS2), as well as the randomized copolymer AB, by monitoring the abundance of dimer units a2, b2, and ab, respectively as a function of time. The degree of randomization, q, was observed in the absence of catalysts or solvents, notwithstanding the solvent/solute and solute/solvent characteristics of the polymer melt blend. The mechanism of this randomization process, was rationalized on the basis of the properties of sulfur, aided by the observation of macrocyclic monomeric and dimeric units during the retro‐polymerization reactions under the EI/MS conditions employed. The model polysulfide polymers A(PS1) and B(P52), used in this study were synthesized from bis(2‐chloroethyl)ether and bis(2‐chloro ethoxy)methane, respectively. 相似文献
70.
Dongyao Li Chunmiao Ma Prof. Junfeng Xiang Kai Zhang Ling Yang Prof. Quan Gan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(45):11663-11669
A strategy to reversibly switch the parallel/antiparallel helical conformation of aromatic double helices through the formation/breakage of a disulfide bond is presented. Single-crystal X-ray structures, NMR, and circular dichroism spectroscopy demonstrate that the double helices with terminal thiol groups favor an antiparallel helical arrangement both in the solid state and in solution, while the P/M bias of helicity induced by chiral segments from another extremity of the sequence is weak in this structural motif. The antiparallel helices can be rearranged to parallel helices through the disulfide connection of the sequences. This change enhances the bias of helical handedness and results in absolute chirality control of the double helices. The handedness-mediated process can be governed by the oxidation-reduction cycle, thereby switching the structural arrangement and the enhancement of chiral bias. In addition, we find that the sequences can dimerize into an intermolecular double helix with the disulfide connection. And the helical handedness is also fully controlled due to the head-to-head structural motif. 相似文献