全文获取类型
收费全文 | 1270篇 |
免费 | 44篇 |
国内免费 | 8篇 |
专业分类
化学 | 1063篇 |
晶体学 | 25篇 |
力学 | 5篇 |
数学 | 81篇 |
物理学 | 148篇 |
出版年
2023年 | 16篇 |
2022年 | 13篇 |
2021年 | 25篇 |
2020年 | 23篇 |
2019年 | 27篇 |
2018年 | 10篇 |
2017年 | 11篇 |
2016年 | 35篇 |
2015年 | 28篇 |
2014年 | 32篇 |
2013年 | 80篇 |
2012年 | 81篇 |
2011年 | 115篇 |
2010年 | 65篇 |
2009年 | 51篇 |
2008年 | 66篇 |
2007年 | 77篇 |
2006年 | 70篇 |
2005年 | 75篇 |
2004年 | 75篇 |
2003年 | 66篇 |
2002年 | 49篇 |
2001年 | 32篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 13篇 |
1997年 | 7篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1993年 | 3篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1989年 | 10篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1968年 | 3篇 |
1935年 | 4篇 |
排序方式: 共有1322条查询结果,搜索用时 15 毫秒
61.
We have investigated and characterised possible sources of variation in measured isotope ratios in the resonance ionisation
mass spectrometry of krypton using resonant transitions at 116.48 and 558.04 nm and ionisation at 1064 nm. Variation arises
when the spectral width of the hyperfine structure of the odd isotopes is comparable to the laser bandwidth and from the alignment
of the first excited state, which depends on the relative polarisations of the laser pulses exciting the resonant transitions. 相似文献
62.
Susann Paul W. Bernd Schweizer Graham Rugg Hans Martin Senn Ryan Gilmour 《Tetrahedron》2013,69(27-28):5647-5659
Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?[NCCF]~60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN+); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes. 相似文献
63.
Paul D. Sylvester Helen E. Ryan Craig D. Smith Aaron S. Micallef Carl H. Schiesser Uta Wille 《Polymer Degradation and Stability》2013
A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering. 相似文献
64.
Helen Stoeckli‐Evans Olha Sereda Antonia Neels Sebastien Oguey Catherine Ionescu Yvan Jacquier 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(11):1057-1063
The one‐dimensional coordination polymer catena‐poly[diaqua(sulfato‐κO)copper(II)]‐μ2‐glycine‐κ2O:O′], [Cu(SO4)(C2H5NO2)(H2O)2]n, (I), was synthesized by slow evaporation under vacuum of a saturated aqueous equimolar mixture of copper(II) sulfate and glycine. On heating the same blue crystal of this complex to 435 K in an oven, its aspect changed to a very pale blue and crystal structure analysis indicated that it had transformed into the two‐dimensional coordination polymer poly[(μ2‐glycine‐κ2O:O′)(μ4‐sulfato‐κ4O:O′:O′′:O′′)copper(II)], [Cu(SO4)(C2H5NO2)]n, (II). In (I), the CuII cation has a pentacoordinate square‐pyramidal coordination environment. It is coordinated by two water molecules and two O atoms of bridging glycine carboxylate groups in the basal plane, and by a sulfate O atom in the apical position. In complex (II), the CuII cation has an octahedral coordination environment. It is coordinated by four sulfate O atoms, one of which bridges two CuII cations, and two O atoms of bridging glycine carboxylate groups. In the crystal structure of (I), the one‐dimensional polymers, extending along [001], are linked via N—H...O, O—H...O and bifurcated N—H...O,O hydrogen bonds, forming a three‐dimensional framework. In the crystal structure of (II), the two‐dimensional networks are linked via bifurcated N—H...O,O hydrogen bonds involving the sulfate O atoms, forming a three‐dimensional framework. In the crystal structures of both compounds, there are C—H...O hydrogen bonds present, which reinforce the three‐dimensional frameworks. 相似文献
65.
Helen R. Thomas Alexander J. Marsden Dr. Marc Walker Dr. Neil R. Wilson Dr. Jonathan P. Rourke 《Angewandte Chemie (International ed. in English)》2014,53(29):7613-7618
The treatment of graphene oxide (GO) with potassium thioacetate followed by an aqueous work‐up yields a new material via the ring‐opening of the epoxide groups. The new material is a thiol‐functionalized GO (GO‐SH) which is able to undergo further functionalization. Reaction with butyl bromide gives another new material, GO‐SBu, which shows significantly enhanced thermal stability compared to both GO and GO‐SH. The thiol‐functionalized GO material showed a high affinity for gold, as demonstrated by the selective deposition of a high density of gold nanoparticles. 相似文献
66.
A Dual‐Color Far‐Red to Near‐Infrared Firefly Luciferin Analogue Designed for Multiparametric Bioluminescence Imaging
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Amit P. Jathoul Dr. Helen Grounds Prof. James C. Anderson Dr. Martin A. Pule 《Angewandte Chemie (International ed. in English)》2014,53(48):13059-13063
Red‐shifted bioluminescent emitters allow improved in vivo tissue penetration and signal quantification, and have led to the development of beetle luciferin analogues that elicit red‐shifted bioluminescence with firefly luciferase (Fluc). However, unlike natural luciferin, none have been shown to emit different colors with different luciferases. We have synthesized and tested the first dual‐color, far‐red to near‐infrared (nIR) emitting analogue of beetle luciferin, which, akin to natural luciferin, exhibits pH dependent fluorescence spectra and emits bioluminescence of different colors with different engineered Fluc enzymes. Our analogue produces different far‐red to nIR emission maxima up to λmax=706 nm with different Fluc mutants. This emission is the most red‐shifted bioluminescence reported without using a resonance energy transfer acceptor. This improvement should allow tissues to be more effectively probed using multiparametric deep‐tissue bioluminescence imaging. 相似文献
67.
Dr. John A. Seed Dr. Helen R. Sharpe Harry J. Futcher Dr. Ashley J. Wooles Prof. Dr. Stephen T. Liddle 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(37):16004-16008
Treatment of [Ph3EMe][I] with [Na{N(SiMe3)2}] affords the ylides [Ph3E=CH2] (E=As, 1As ; P, 1P ). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P , confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E+−C− dipolar resonance form (sp3-C) over the E=C ene π-bonded form (sp2-C), as group 15 is descended. The uranium(IV)–cyclometallate complex [U{N(CH2CH2NSiPri3)2(CH2CH2SiPri2CH(Me)CH2)}] reacts with 1As and 1P by α-proton abstraction to give [U(TrenTIPS)(CHEPh3)] (TrenTIPS=N(CH2CH2NSiPri3)3; E=As, 2As ; P, 2P ), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U−C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U−C distance is found for 2As than 2P , consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations. 相似文献
68.
Argyro G. Fragkaki Georgia Petropoulou Ioanna Athanasiadou Polyxeni Kiousi Nassia Kioukia‐Fougia Helen Archontaki Evangelos Bakeas Yiannis S. Angelis 《Journal of separation science》2020,43(11):2154-2161
Anabolic androgenic steroids are widely abused substances in sports doping. Their detection present limitations regarding the use of soft ion sources such as electrospray or atmospheric pressure chemical ionization by liquid chromatography–tandem mass spectrometry. In the current study, a novel derivatization method was developed for the ionization enhancement of selected anabolic androgenic steroids. The proposed method aims at the introduction of an easily ionizable moiety into the steroid molecule by converting the hydroxyl groups into imidazole carbamates using 1,1′‐carbonyldiimidazole as derivatization reagent. The proposed method was applied to water and urine samples spiked with exogenous anabolic androgenic steroids in various concentration levels. Steroid imidazole carbamate derivatives have shown intensive [M+H]+ signals under electrospray ionization and common fragmentation patterns in tandem mass spectrometry mode with [M‐CO2+H]+ and [M‐ΙmCO2+H]+ as major ions with low collision energy. The obtained results showed that the majority of steroids were detectable at concentrations equal or lower to their minimum required performance level according to the World Anti‐Doping Agency technical document. The proposed method is sensitive with a preparation procedure that could be easily applied to the analysis of doping control samples. 相似文献
69.
70.
Mark Brouar Helen Chadwick Sean D. S. Gordon Cornelia G. Hei Balazs Hornung Bethan Nichols Jacek K?os Pablo G. Jambrin F. Javier Aoiz 《化学物理学报》2020,33(2):217-233
本文对NO(X)-Xe碰撞系统在碰撞能量为519 cm-1,测量了完全?-双峰分解的微分截面和碰撞引起的旋转准直力矩. 同时结合初始量子态选择,使用六极杆的非均质电场,借助量子态分辨的测量,利用(1+1'')共振增强的多光子电离和速度离子成像. 结果显示,微分截面以及偏振相关的微分截面均显示与从头算势能面上进行的量子力学散射计算[J. K?os etal. J. Chem. Phys. 137, 014312 (2012)]一致. 通过与准经典轨迹、硬壳势能的量子力学散射以及运动近端模型的比较,评估了势能对所测微分截面和碰撞引起的旋转准直力矩的影响. 相似文献