全文获取类型
收费全文 | 2427篇 |
免费 | 70篇 |
国内免费 | 18篇 |
专业分类
化学 | 1675篇 |
晶体学 | 11篇 |
力学 | 59篇 |
数学 | 432篇 |
物理学 | 338篇 |
出版年
2023年 | 22篇 |
2022年 | 30篇 |
2021年 | 40篇 |
2020年 | 60篇 |
2019年 | 52篇 |
2018年 | 17篇 |
2017年 | 27篇 |
2016年 | 72篇 |
2015年 | 70篇 |
2014年 | 84篇 |
2013年 | 114篇 |
2012年 | 131篇 |
2011年 | 176篇 |
2010年 | 93篇 |
2009年 | 117篇 |
2008年 | 162篇 |
2007年 | 140篇 |
2006年 | 141篇 |
2005年 | 107篇 |
2004年 | 113篇 |
2003年 | 81篇 |
2002年 | 84篇 |
2001年 | 36篇 |
2000年 | 38篇 |
1999年 | 32篇 |
1998年 | 29篇 |
1997年 | 24篇 |
1996年 | 24篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 16篇 |
1992年 | 17篇 |
1991年 | 16篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 11篇 |
1987年 | 24篇 |
1985年 | 12篇 |
1984年 | 20篇 |
1983年 | 20篇 |
1982年 | 11篇 |
1981年 | 18篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1972年 | 9篇 |
排序方式: 共有2515条查询结果,搜索用时 0 毫秒
131.
Kiryutin AS Morozova OB Kuhn LT Yurkovskaya AV Hore PJ 《The journal of physical chemistry. B》2007,111(38):11221-11227
Relative values of the 1H and 13C isotropic hyperfine couplings in the cationic oxidized tryptophan radical TrpH*+ in aqueous solution are determined. The data are obtained from the photo-CIDNP (chemically induced dynamic nuclear polarization) enhancements observed in the microsecond time-resolved NMR spectra of the diamagnetic products of photochemical reactions in which TrpH*+ is a transient intermediate. The method is validated using the tyrosyl neutral radical Tyr*, whose 1H and 13C hyperfine couplings have previously been determined by electron paramagnetic resonance spectroscopy. Good agreement is found with hyperfine coupling constants for TrpH*+ calculated using density functional theory methods but only if water molecules are explicitly included in the calculation. 相似文献
132.
Catalytic Asymmetric Oxidative γ‐Coupling of α,β‐Unsaturated Aldehydes with Air as the Terminal Oxidant 下载免费PDF全文
Line Næsborg Vasco Corti Lars Astrup Leth Dr. Pernille H. Poulsen Prof. Karl Anker Jørgensen 《Angewandte Chemie (International ed. in English)》2018,57(6):1606-1610
A novel concept for catalytic asymmetric coupling reactions is presented. Merging organocatalysis with single‐electron oxidation by using a catalytic amount of a copper(II) salt and air as the terminal oxidant, we have developed a highly stereoselective carbon–carbon oxidative coupling reaction of α,β‐unsaturated aldehydes. The concept relies on the generation of a dienamine intermediate, which is oxidized to an open‐shell activated species that undergoes highly selective γ‐homo‐ and γ‐heterocoupling reactions. In the majority of examples presented, only a single stereoisomer was formed. 相似文献
133.
Effect of SIMS ionization probability on depth resolution for organic/inorganic interfaces 下载免费PDF全文
Nicholas J. Popczun Lars Breuer Andreas Wucher Nicholas Winograd 《Surface and interface analysis : SIA》2017,49(10):933-939
Secondary ion mass spectrometry (SIMS) relies on the fact that surface particles ejected from a solid surface are ionized under ion bombardment. By comparing the signal of molecular secondary ions desorbed from an organic film with that of the corresponding sputtered neutral precursor molecules, we investigate the variation of the molecular ionization probability when depth profiling through the film to the substrate interface. As a result, we find notable variations of the ionization probability both at the original surface and in the interface region, leading to a strong distortion of the measured SIMS depth profile. The experiments show that the effect can act in two ways, leading either to an apparent broadening or to an artificial sharpening of the observed film‐substrate transition. As a consequence, we conclude that care must be taken when assessing interface location, width, or depth resolution from a molecular SIMS depth profile. 相似文献
134.
Andersson LK Caspersson M Baltzer L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(16):3687-3697
Five 42-residue polypeptides have been designed to fold into hairpin helix-loop-helix motifs that dimerize to form four-helix bundles, and to serve as protein scaffolds for the elucidation at the molecular level of the principles that control and fine-tune lysine and ornithine reactivities in a protein context. Site-selective control of Lys and Orn reactivity provides a mechanism for addressing directly individual residues and is a prerequisite for the site-selective functionalization of folded proteins. Several lysine and one ornithine residues were introduced on the surface and in the hydrophobic core of the folded motif. The reactivity of each residue was determined by measuring the degree of acylation of the trypsin cleaved fragments by HPLC and mass spectrometry. The most reactive residues were Orn34 and Lys19, both of which were located in d positions in the heptad repeat, and therefore in hydrophobic environments. Upon reaction of the helix-loop-helix dimer KA-I with one equivalent of mono-p-nitrophenyl fumarate, Orn34 was acylated approximately three times more efficiently than Lys19, whereas Lys10 (b position), Lys15 (g position), and Lys33 (c position) remained unmodified. In the sequence KA-I-A(15) Lys15 was replaced by an alanine residue and the selectivity of Orn34 over Lys19 increased to approximately a factor of six, probably because Lys15 had the capacity to reduce the pK(a) value of Lys19 and 85 % of site-selectively monoacylated product was obtained. The pH dependence of the acylation reaction was determined and showed that the pK(a) of the reactive residues were 9.3, more than a pK(a) unit below the magnitude of the corresponding residue in a solvent exposed position. Introducing Lys and Orn residues into a or d positions of the heptad repeat therefore serves as a mechanism of depressing their pK(a) to increase their reactivity site selectively. Extensive NMR and CD spectroscopic analyses showed that the sequences fold according to prediction. 相似文献
135.
136.
137.
138.
We consider finite element operators defined on ``rough' functions in a bounded polyhedron in . Insisting on preserving positivity in the approximations, we discover an intriguing and basic difference between approximating functions which vanish on the boundary of and approximating general functions which do not. We give impossibility results for approximation of general functions to more than first order accuracy at extreme points of . We also give impossibility results about invariance of positive operators on finite element functions. This is in striking contrast to the well-studied case without positivity.
139.
Clark SM Leaist DG Konermann L 《Rapid communications in mass spectrometry : RCM》2002,16(15):1454-1462
This work describes a novel approach for monitoring analyte diffusion in solution that is based on electrospray ionization mass spectrometry (ESI-MS). A mass spectrometer at the end of a laminar flow tube is used to measure the Taylor dispersion of an initially sharp boundary between two solutions of different analyte concentration. This boundary is dispersed by the laminar flow profile in the tube. However, this effect is diminished by analyte diffusion that continuously changes the radial position, and hence the flow velocity of individual analyte molecules. The steepness of the resulting dispersion profile therefore increases with increasing diffusion coefficient of the analyte. A theoretical framework is developed to adapt the equations governing the dispersion process to the case of mass spectrometric detection. This novel technique is applied to determine the diffusion coefficients of choline and cytochrome c. The measured diffusion coefficients, (11.9 +/- 1.0) x 10(-10) m(2) s(-1) and (1.35 +/- 0.08) x 10(-10) m(2) s(-1), respectively, are in agreement with the results of control experiments where the Taylor dispersion of these two analytes was monitored optically. Due to the inherent selectivity and sensitivity of ESI-MS, it appears that the approach described in this work could become a valuable alternative to existing methods for studying diffusion processes, especially for experiments on multicomponent systems. 相似文献
140.