全文获取类型
收费全文 | 1205篇 |
免费 | 41篇 |
国内免费 | 3篇 |
专业分类
化学 | 1038篇 |
晶体学 | 5篇 |
力学 | 14篇 |
数学 | 67篇 |
物理学 | 125篇 |
出版年
2023年 | 9篇 |
2022年 | 20篇 |
2021年 | 24篇 |
2020年 | 31篇 |
2019年 | 16篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 27篇 |
2015年 | 35篇 |
2014年 | 45篇 |
2013年 | 71篇 |
2012年 | 105篇 |
2011年 | 110篇 |
2010年 | 54篇 |
2009年 | 43篇 |
2008年 | 92篇 |
2007年 | 87篇 |
2006年 | 73篇 |
2005年 | 84篇 |
2004年 | 74篇 |
2003年 | 55篇 |
2002年 | 51篇 |
2001年 | 20篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1997年 | 9篇 |
1996年 | 12篇 |
1995年 | 11篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1989年 | 2篇 |
1987年 | 4篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 3篇 |
1978年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有1249条查询结果,搜索用时 15 毫秒
61.
Vitor A. Kerber Carolina S. Passos Luiz C. Klein-Júnior Jean-Charles Quirion Xavier Pannecoucke Isabelle Salliot-Maire Amélia T. Henriques 《Tetrahedron letters》2014
Three new psychollatine-derived monoterpene indole alkaloids were obtained from Psychotria umbellata Thonn.: 3,4-Dehydro-18,19-β-epoxy-psychollatine (2), N4-[1-((R)-2-hydroxypropyl)]-psychollatine (3), and N4-[1-((S)-2-hydroxypropyl)]-psychollatine (4). Their structures were determined by 1H and 13C NMR spectra, 2D correlations (COSY, HMQC, and HMBC), and mass and UV spectra. Compounds 3 and 4 were synthesized for structural confirmation and for the determination of the stereochemistry of the hydroxyl group. 相似文献
62.
Sebastian Scheidel Laurina Östreicher Isabelle Mark Ann-Christin Pöppler 《Magnetic resonance in chemistry : MRC》2022,60(6):572-582
Although solid-state nuclear magnetic resonance (NMR) is a versatile analytical tool to study polymorphs and phase transitions of pharmaceutical molecules and products, this work summarizes examples of spontaneous and unexpected (and unwanted) structural rearrangements and phase transitions (amorphous-to-crystalline and crystalline-to-crystalline) under magic angle spinning (MAS) conditions, some of them clearly being due to the pressure experienced by the samples. It is widely known that such changes can often be detected by X-ray powder diffraction (XRPD); here, the capability of solid-state NMR experiments with a special focus on 1H-13C frequency-switched Lee–Goldburg heteronuclear correlation (FSLG HETCOR)/MAS NMR experiments to detect even subtle changes on a molecular level not observable by conventional 1D NMR experiments or XRPD is presented. Furthermore, it is shown that a polymorphic impurity combined with MAS can induce a crystalline-to-crystalline phase transition. This showcases that solid-state NMR is not always noninvasive and such changes upon MAS should be considered in particular when compounds are studied over longer time spans. 相似文献
63.
Sandy Theysgeur Benoit Cudennec Barbara Deracinois Claire Perrin Isabelle Guiller Anne Lepoudre Christophe Flahaut Rozenn Ravallec 《Molecules (Basel, Switzerland)》2021,26(1)
Like their owners, dogs and cats are more and more affected by overweight and obesity-related problems and interest in functional pet foods is growing sharply. Through numerous studies, fish protein hydrolysates have proved their worth to prevent and manage obesity-related comorbidities like diabetes. In this work, a human in vitro static simulated gastrointestinal digestion model was adapted to the dog which allowed us to demonstrate the promising effects of a tilapia byproduct hydrolysate on the regulation of food intake and glucose metabolism. Promising effects on intestinal hormones secretion and dipeptidyl peptidase IV (DPP-IV) inhibitory activity were evidenced. We identify new bioactive peptides able to stimulate cholecystokinin (CCK) and glucagon-like peptide 1 (GLP-1) secretions, and to inhibit the DPP-IV activity after a transport study through a Caco-2 cell monolayer. 相似文献
64.
Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster 下载免费PDF全文
Nathan A. Seifert Dr. Daniel P. Zaleski Dr. Cristóbal Pérez Dr. Justin L. Neill Prof. Brooks H. Pate Montserrat Vallejo‐López Prof. Alberto Lesarri Dr. Emilio J. Cocinero Prof. Fernando Castaño Prof. Isabelle Kleiner 《Angewandte Chemie (International ed. in English)》2014,53(12):3210-3213
Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi‐isotopic broadband rotational spectra. This heterodimer is held together by a primary C? H???π hydrogen bond, assisted by multiple weak C? H???F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C? H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum. 相似文献
65.
Catalytic,Asymmetric Synthesis of Phosphonic γ‐(Hydroxyalkyl)butenolides with Contiguous Quaternary and Tertiary Stereogenic Centers 下载免费PDF全文
Marcus Frings Dr. Isabelle Thomé Dr. Ingo Schiffers Dr. Fangfang Pan Prof. Dr. Carsten Bolm 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(6):1691-1700
A procedure that enables high yielding access to phosphonic γ‐(hydroxyalkyl)butenolides with excellent regio‐, diastereo‐ and enantiocontrol is reported. The simultaneous construction of up to two adjacent quaternary stereogenic centers by a catalytic asymmetric vinylogous Mukaiyama aldol reaction unites biologically and medicinally relevant entities, namely α‐hydroxy phosphonates and γ‐(hydroxyalkyl)butenolides. This is achieved by utilizing a readily available chiral copper‐sulfoximine catalyst showing a broad functional group tolerance for both the electrophilic and nucleophilic reactants. A discussion about potential factors affecting the observed level of enantioselectivity, which stems from the enantiopure sulfoximine ligand, is also included. 相似文献
66.
Uncatalyzed Hydroamination of Electrophilic Organometallic Alkynes: Fundamental,Theoretical, and Applied Aspects 下载免费PDF全文
Dr. Yanlan Wang Camille Latouche Dr. Amalia Rapakousiou Colin Lopez Prof Isabelle Ledoux‐Rak Dr. Jaime Ruiz Prof. Jean‐Yves Saillard Prof. Didier Astruc 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(26):8076-8088
Simple reactions of the most used functional groups allowing two molecular fragments to link under mild, sustainable conditions are among the crucial tools of molecular chemistry with multiple applications in materials science, nanomedicine, and organic synthesis as already exemplified by peptide synthesis and “click” chemistry. We are concerned with redox organometallic compounds that can potentially be used as biosensors and redox catalysts and report an uncatalyzed reaction between primary and secondary amines with organometallic electrophilic alkynes that is free of side products and fully “green”. A strategy is first proposed to synthesize alkynyl organometallic precursors upon addition of electrophilic aromatic ligands of cationic complexes followed by endo hydride abstraction. Electrophilic alkynylated cyclopentadienyl or arene ligands of Fe, Ru, and Co complexes subsequently react with amines to yield trans‐enamines that are conjugated with the organometallic group. The difference in reactivities of the various complexes is rationalized from the two‐step reaction mechanism that was elucidated through DFT calculations. Applications are illustrated by the facile reaction of ethynylcobalticenium hexafluorophosphate with aminated silica nanoparticles. Spectroscopic, nonlinear‐optical and electrochemical data, as well as DFT and TDDFT calculations, indicate a strong push–pull conjugation in these cobalticenium– and Fe– and Ru–arene–enamine complexes due to planarity or near‐planarity between the organometallic and trans‐enamine groups involving fulvalene iminium and cyclohexadienylidene iminium mesomeric forms. 相似文献
67.
Josiane Arnaud Isabelle Coté Patrick J. Parsons Marina Patriarca Andrew Taylor Cas Weykamp 《Accreditation and quality assurance》2014,19(3):169-174
Proficiency testing schemes monitor laboratory performance and provide a stimulus for improvement in accuracy. Where several schemes operate in the same analytical sector, there are risks that assessments of performance may be in conflict. Performance assessment for the determination of trace elements such as aluminum in serum is particularly important due to the high risk of contamination and therefore erroneous results. The objectives of this work were (1) to compare several mathematical models to establish a predefined standard deviation for proficiency assessment and (2) to evaluate the influence of instrumental methods and proficiency testing scheme on the assessment of performance for serum aluminum measurements. For this purpose, three samples were sent to the participants of four proficiency testing schemes. Assigned values were calculated according to algorithm A according to ISO 13528 and standard deviation for proficiency assessment according to three methods based on individual variability, state of the art or previous proficiency testing results. The method based on individual variability produced a more stringent standard deviation compared to analytical imprecision based on the state of the art. The instrumental methods gave similar results, whereas significant differences were observed between the four proficiency testing schemes indicating that harmonization of the standard deviation for proficiency assessment fails to allow transferability from one proficiency testing scheme to another and that additional factor(s) contribute to variability in performance assessment. 相似文献
68.
69.
Caroline Ferrari Eric Olmos Frédérique Balandras Nguyen Tran Isabelle Chevalot Emmanuel Guedon Annie Marc 《Applied biochemistry and biotechnology》2014,172(2):1004-1017
The extensive use of mesenchymal stem cells (MCS) in tissue engineering and cell therapy increases the necessity to improve their expansion. Among these, porcine MCS are valuable models for tissue engineering and are classically expanded in static T-flasks. In this work, different processes of stirred cultures were evaluated and compared. First, the effect of glucose, glutamine, antioxidant, and growth factors concentrations on porcine MSC expansion were analyzed in a suitable medium by performing kinetic studies. Results showed that a lower glucose concentration (5.5 mM) enabled to increase maximal cell concentration by 40 % compared with a higher one (25 mM), while addition of 2 to 6 mM of glutamine increased maximal cell concentration by more than 25 % compared with no glutamine supplementation. Moreover, supplementation with 1 μM thioctic acid increased maximal cell concentration by 40 % compared with no supplementation. Using this adapted medium, microcarriers cultures were performed and compared with T-flasks expansion. Porcine MSC were shown to be able to proliferate on the five types of microcarriers tested. Moreover, cultures on Cytodex 1, Cytopore 2, and Cultispher G exhibited a MSC growth rate more than 40 % higher compared with expansion in T-flasks, while MSC metabolism was similar. 相似文献
70.
Dr. Sofia Curland Dr. Christoph Allolio Leah Javitt Shiri Dishon Ben-Ami Dr. Isabelle Weissbuch Dr. David Ehre Prof. Dr. Daniel Harries Prof. Dr. Meir Lahav Prof. Dr. Igor Lubomirsky 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(36):15705-15709
Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO3 and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO3−) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H3O+) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO3− ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO3− and guanidinium (Gdm+), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl− and SO42− ions of different configurations reduce the icing temperature. 相似文献