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181.
Stevenson CD Heinle LJ Davis JP Reiter RC 《Journal of the American Chemical Society》2002,124(11):2704-2708
Electron paramagnetic resonance (EPR) studies have revealed that the steric interaction between the methyl hydrogens on a tert-butoxy substituent and the cyclooctatetraene (COT) ring system sterically induces a puckering of the eight-membered ring in the anion radical of tert-butoxy-COT. The induced nonplanarity of the COT ring system causes a large attenuation of the EPR coupling constants. Since the C-D bond length is slightly shorter than is the C-H bond length, replacement of the tert-butyl group with a tert-butyl-d(9) group results in less steric interaction and measurably larger electron proton coupling constants. The oscillation between the two close to planar alternating bond length (ABL) D(2d) conformers of the COT moiety was found to be extremely rapid (k > 10(12) s(-1)) and quantum mechanical tunneling is proposed to be involved. 相似文献
182.
Cheryl D Stevenson Lance J Heinle Richard C Reiter 《The Journal of organic chemistry》2002,67(1):119-124
In contrast to the thermolysis of p-benzoquinone, which does not decompose until the temperature is over 800 degrees C, and then primarily yields vinylacetylene, the corresponding anion radical, precipitated from liquid ammonia [Na(+)(NH(3))C(6)H(4)O(2)(*-)], decomposes at 380 degrees C and fragments primarily into phenol, hydroquinone, ammonia, methane, carbon monoxide, hydrogen, and minor amounts of other simple compounds. When the benzoquinone is replaced with perdeuteriobenzoquinone, deuterium and hydrogen are randomly scrambled into the products, and both ND(3) and CH(4) are formed. When the hot pyrolysis container is completely sealed, preventing the escape of volatile materials, p-aminophenol, as opposed to phenol, is the major liquid product. 相似文献
183.
Our preliminary results towards the synthesis of hyperbranched polyamidoamine (PAMAM) polymers as ‘dendrimer equivalents’ is described. An aromatic AB2 bis-amino acid monomer was polymerised at 165 °C (under vacuum) and the crude mixture purified by dialysis. Analysis by GPC and MALDI-TOF mass spectroscopy showed that the purified hyperbranched polymers possessed an Mn of 2000 and a PD of 3.2. 相似文献
184.
Catalytic sites can be placed at the core, at interior positions or at the periphery of a dendrimer. There are many examples of the use of peripherally functionalized dendrimers in catalysis and this subject has been thoroughly reviewed in the recent literature. This review is concerned only with dendrimer based catalysis involving catalytic sites at the core of a dendrimer and within the interior voids. In covering the significant achievements in this area, we have concentrated on examples that highlight key features with respect to positive and/or negative catalytic activity. 相似文献
185.
Matsubara K Kelly KL Sakai N Tatsuma T 《Physical chemistry chemical physics : PCCP》2008,10(16):2263-2269
Ag nanoparticles photocatalytically deposited on single-crystalline TiO(2)(111) and (100) surfaces exhibit visible light-induced topographic and spectral changes corresponding to the light wavelength. A certain amount of adsorbed water on the surface is essential for the changes. The photoinduced behaviors strongly depend on the amount of the adsorbed water and on interparticle spacing, indicating that ionic conductivity on the TiO(2) surface is a key factor. The results obtained here are rationally explained in terms of a photoelectrochemical mechanism in which photoanodic dissolution of Ag nanoparticles to Ag(+) and cathodic re-deposition of Ag play essential roles. 相似文献
186.
Brian Jefferies Gerald W. Johnson Lance Nielsen 《Mathematical Physics, Analysis and Geometry》2007,10(1):65-80
The spectral theorem for commuting self-adjoint operators along with the associated functional (or operational) calculus is
among the most useful and beautiful results of analysis. It is well known that forming a functional calculus for noncommuting
self-adjoint operators is far more problematic. The central result of this paper establishes a rich functional calculus for
any finite number of noncommuting (i.e. not necessarily commuting) bounded, self-adjoint operators A
1,..., A
n
and associated continuous Borel probability measures μ
1, ⋯, μ
n
on [0,1]. Fix A
1,..., A
n
. Then each choice of an n-tuple of measures determines one of Feynman’s operational calculi acting on a certain Banach algebra of analytic functions even
when A
1, ..., A
n
are just bounded linear operators on a Banach space. The Hilbert space setting along with self-adjointness allows us to extend
the operational calculi well beyond the analytic functions. Using results and ideas drawn largely from the proof of our main
theorem, we also establish a family of Trotter product type formulas suitable for Feynman’s operational calculi.
相似文献
187.
Dembowski Mateusz Margiotta Cheriece Smythe Nathan C. Flanagan Dylan C. Meininger Daniel James Michael R. Hudston Lisa A. Boswell Melissa Lance Camilla A. Rendon Randy J. Goff George S. May Iain 《Journal of Radioanalytical and Nuclear Chemistry》2022,331(5):2295-2302
Journal of Radioanalytical and Nuclear Chemistry - Rapid and efficient isolation of individual rare earth element (REE) radioisotopes from complex mixtures is necessary to support the fields of... 相似文献
188.
Angel PM Lim JM Wells L Bergmann C Orlando R 《Rapid communications in mass spectrometry : RCM》2007,21(5):674-682
A common procedure for identifying N-linked glycosylation sites involves tryptic digestion of the glycoprotein, followed by the conversion of glycosylated asparagine residues into (18)O-labeled aspartic acids by PNGase F digestion in (18)O water. The 3 Da mass tag created by this process is readily observable by liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis, and is often used to identify the sites of N-linked glycosylation. While using this procedure, we noticed that 60% of the asparagines identified as being glycosylated were not part of the consensus sequence required for N-linked glycosylation, and thus were not biologically possible. Investigation into the source of this unacceptably high false positive rate demonstrated that even after reversed-phase cleanup and heat denaturation, the trypsin used for proteolysis was still active and led to the incorporation of (18)O into the C-termini of the peptides during the deglycosylation step. The resulting mass shift accounted for most of the false positive sites, as the database search algorithm confused it with an (18)O-labeled Asp residue near the C-terminus of a peptide. This problem can be overcome by eliminating trypsin from the solution prior to performing the deglycosylation process, by resuspending the peptides in natural abundance water following deglycosylation, or by allowing (18)O incorporation into the C-terminus as a variable modification during the database search. These methods have been demonstrated on a model protein, and are applicable to the analyses of glycoproteins that are digested with trypsin or another serine protease prior to enzymatic release of the carbohydrate side chains. This study should alert investigators in the field to this potential and unexpected pitfall and provide strategies to overcome this phenomenon. 相似文献
189.
Dr. Svetlana N. Samovich Dr. Karolina Mikulska-Ruminska Dr. Haider H. Dar Dr. Yulia Y. Tyurina Dr. Vladimir A. Tyurin Austin B. Souryavong Dr. Alexander A. Kapralov Dr. Andrew A. Amoscato Dr. Ofer Beharier Dr. S. Ananth Karumanchi Dr. Claudette M. St Croix Dr. Xin Yang Dr. Theodore R. Holman Dr. Andrew P. VanDemark Dr. Yoel Sadovsky Dr. Rama K. Mallampalli Dr. Sally E. Wenzel Dr. Wei Gu Dr. Yuri L. Bunimovich Dr. Ivet Bahar Dr. Valerian E. Kagan Dr. Hülya Bayir 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(9):e202314710
The vast majority of membrane phospholipids (PLs) include two asymmetrically positioned fatty acyls: oxidizable polyunsaturated fatty acids (PUFA) attached predominantly at the sn2 position, and non-oxidizable saturated/monounsaturated acids (SFA/MUFA) localized at the sn1 position. The peroxidation of PUFA-PLs, particularly sn2-arachidonoyl(AA)- and sn2-adrenoyl(AdA)-containing phosphatidylethanolamines (PE), has been associated with the execution of ferroptosis, a program of regulated cell death. There is a minor subpopulation (≈1–2 mol %) of doubly PUFA-acylated phospholipids (di-PUFA-PLs) whose role in ferroptosis remains enigmatic. Here we report that 15-lipoxygenase (15LOX) exhibits unexpectedly high pro-ferroptotic peroxidation activity towards di-PUFA-PEs. We revealed that peroxidation of several molecular species of di-PUFA-PEs occurred early in ferroptosis. Ferrostatin-1, a typical ferroptosis inhibitor, effectively prevented peroxidation of di-PUFA-PEs. Furthermore, co-incubation of cells with di-AA-PE and 15LOX produced PUFA-PE peroxidation and induced ferroptotic death. The decreased contents of di-PUFA-PEs in ACSL4 KO A375 cells was associated with lower levels of di-PUFA-PE peroxidation and enhanced resistance to ferroptosis. Thus, di-PUFA-PE species are newly identified phospholipid peroxidation substrates and regulators of ferroptosis, representing a promising therapeutic target for many diseases related to ferroptotic death. 相似文献
190.
Recent molecular techniques have made it feasible to simulate evolutionary processes and apply in vitro selection to evolve enzymes with novel properties that may have potential benefits for medical and industrial applications. The characterization of such mutants has also provided new insights into how molecular structure determines enzyme function. 相似文献