全文获取类型
收费全文 | 2997篇 |
免费 | 132篇 |
国内免费 | 9篇 |
专业分类
化学 | 2161篇 |
晶体学 | 3篇 |
力学 | 52篇 |
数学 | 435篇 |
物理学 | 487篇 |
出版年
2023年 | 27篇 |
2022年 | 23篇 |
2021年 | 53篇 |
2020年 | 59篇 |
2019年 | 75篇 |
2018年 | 38篇 |
2017年 | 32篇 |
2016年 | 94篇 |
2015年 | 116篇 |
2014年 | 108篇 |
2013年 | 175篇 |
2012年 | 230篇 |
2011年 | 221篇 |
2010年 | 127篇 |
2009年 | 118篇 |
2008年 | 152篇 |
2007年 | 165篇 |
2006年 | 176篇 |
2005年 | 153篇 |
2004年 | 169篇 |
2003年 | 122篇 |
2002年 | 97篇 |
2001年 | 56篇 |
2000年 | 32篇 |
1999年 | 37篇 |
1998年 | 43篇 |
1997年 | 39篇 |
1996年 | 28篇 |
1995年 | 38篇 |
1994年 | 40篇 |
1993年 | 21篇 |
1992年 | 30篇 |
1991年 | 23篇 |
1990年 | 14篇 |
1989年 | 20篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 25篇 |
1978年 | 14篇 |
1977年 | 14篇 |
1976年 | 8篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1971年 | 6篇 |
1964年 | 4篇 |
排序方式: 共有3138条查询结果,搜索用时 15 毫秒
111.
Two corresponding-states forms have been developed for direct correlation function integrals in liquids to represent pressure effects on the volume of ionic liquids over wide ranges of temperature and pressure. The correlations can be analytically integrated from a chosen reference density to provide a full equation of state for ionic liquids over reduced densities from 1.5 to more than 3.6. One approach is empirical with 3 parameters, the other is a 2-parameter theoretical form which is directly connected to a method for predicting gas solubilities in ionic liquids. Parameters for both methods have been obtained for 28 different ionic liquid systems by reduction of carefully validated PρT data. The results show excellent agreement with the experimental data for nearly all substances over all ranges of conditions. Group contributions to parameters are also explored, leading to an entirely predictive method for ambient pressure densities and densities of compressed ionic liquids. Extensive comparisons are made with other techniques. 相似文献
112.
Jens Beckmann Dainis Dakternieks Andrew Duthie Naomi A. Lewcenko Cassandra Mitchell Markus Schürmann 《无机化学与普通化学杂志》2005,631(10):1856-1862
The reaction of (p‐MeOC6H4)2TeO with two equivalents of HO3SCF3 and HO2PPh2 provided the tetraorganoditelluroxanes (F3CSO3)(p‐MeOC6H4)2TeOTe(p‐MeOC6H4)2(O3SCF3) ( 1 ) and (Ph2PO2)(p‐MeOC6H4)2TeOTe(p‐MeOC6H4)2(O2PPh2)·2 Ph2PO2H ( 2 ) in good yields. Compounds 1 and 2 were characterized by solution and solid‐state 31P and 125Te NMR spectroscopy, IR spectroscopy, electrospray mass spectrometry, conductivity measurements and single crystal X‐ray diffraction. In solution, compound 1 undergoes an electrolytic dissociation and reversibly reacts with traces of water to give the mononuclear cation [(p‐MeOC6H4)2TeOH]+ and triflate anions. Theoretical aspects of the protonation and hydration of model telluroxanes R2TeO (R = H, Me, Ph) were investigated by preliminary DFT calculations and compared to the corresponding selenoxanes R2SeO. The tellurium dihydroxides R2Te(OH)2 seem to be more stable than the hydrogen‐bonded complexes R2TeO·H2O. 相似文献
113.
Markus R. Meyer Jens Wilhelm Frank T. Peters Hans H. Maurer 《Analytical and bioanalytical chemistry》2010,397(3):1225-1233
In recent years, a new class of designer drugs has appeared on the drugs of abuse market in many countries, namely, the so-called
beta-keto (bk) designer drugs such as mephedrone (bk-4-methylmethamphetamine), butylone (bk-MBDB), and methylone (bk-MDMA).
The aim of the present study was to identify the metabolites of mephedrone in rat and human urine using GC-MS techniques and
to include mephedrone, butylone, and methylone within the authors’ systematic toxicological analysis (STA) procedure. Six
phase I metabolites of mephedrone were detected in rat urine and seven in human urine suggesting the following metabolic steps:
N-demethylation to the primary amine, reduction of the keto moiety to the respective alcohol, and oxidation of the tolyl moiety
to the corresponding alcohols and carboxylic acid. The STA procedure allowed the detection of mephedrone, butylone, methylone,
and their metabolites in urine of rats treated with doses corresponding to those reported for abuse of amphetamines. Besides
macro-based data evaluation, an automated evaluation using the automated mass spectral deconvolution and identification system
was performed. Mephedrone and butylone could be detected also in human urine samples submitted for drug testing. Assuming
similar kinetics in humans, the described STA procedure should be suitable for proof of an intake of the bk-designer drugs
in human urine. 相似文献
114.
Dr. Krishna Kanti Dey Frances Ying Pong Dr. Jens Breffke Dr. Ryan Pavlick Dr. Emmanuel Hatzakis Dr. Carlos Pacheco Dr. Ayusman Sen 《Angewandte Chemie (International ed. in English)》2016,55(3):1113-1117
While momentum transfer from active particles to their immediate surroundings has been studied for both synthetic and biological micron‐scale systems, a similar phenomenon was presumed unlikely to exist at smaller length scales due to the dominance of viscosity in the ultralow Reynolds number regime. Using diffusion NMR spectroscopy, we studied the motion of two passive tracers—tetramethylsilane and benzene—dissolved in an organic solution of active Grubbs catalyst. Significant enhancements in diffusion were observed for both the tracers and the catalyst as a function of reaction rate. A similar behavior was also observed for the enzyme urease in aqueous solution. Surprisingly, momentum transfer at the molecular scale closely resembles that reported for microscale systems and appears to be independent of swimming mechanism. Our work provides new insight into the role of active particles on advection and mixing at the Ångström scale. 相似文献
115.
Michael Limbach Markus Lweneck JürgV. Schreiber Jens Frackenpohl Dieter Seebach Andreas Billich 《Helvetica chimica acta》2006,89(7):1427-1441
β‐Peptides and, to a certain extent, also mixed α,β‐peptides, are resistant to degradation by a variety of proteolytic enzymes that rapidly degrade natural α‐peptides. This is one of many characteristics that make β‐peptides an attractive class of compounds for drug‐discovery studies. On the other hand, modern organometallic reactions such as the Suzuki–Miyaura cross‐coupling have become standard tools in industry laboratories to derivatize side chains of α‐peptidic compounds to build up libraries of unnatural peptides. Combining both features, we prepared (4‐bromo)‐β3‐homophenylalanine derivatives 3 – 5 and 12 as precursors for Suzuki–Miyaura couplings. From these bromo compounds, we synthesized biaryl‐substituted β‐homoamino acids 6 , and analogs 13 and 15 of the anti‐AIDS drug Saquinavir. 相似文献
116.
The molecular and vibrational structure of the title compound (C4-HSL) was studied by experimental and theoretical methods. The infrared (IR) absorption spectrum was measured in the solid state and in CCl4 suspension. The observed absorption bands were compared with transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT) calculations. Two stable molecular conformations were predicted, corresponding to an endo- and an exo-conformer with similar energies. Intermolecular amide–amide hydrogen bonding in the crystal state was approximated by a simple cluster model, leading to excellent agreement with the observed solid state IR spectrum. Due to the low solubility of C4-HSL in common solvents for IR spectroscopy, such as CS2 and CCl4, a liquid solution spectrum of pure, monomeric C4-HSL was not obtained. However, absorbance peaks observed in oversaturated CCl4 solution could be assigned to distinct contributions from suspended micro-crystalline aggregates and dissolved monomeric species. The key vibrational bands of the monomeric form of C4-HSL are reported here for the first time: 3425 cm?1 [ν(N–H)], 1784 cm?1 [ν(CO), lactone], 1688 cm?1 [amide I], and 1494 cm?1 [amide II] (CCl4). 相似文献
117.
Markus Follmann Nils Griebenow Michael G. Hahn Ingo Hartung Franz‐Josef Mais Joachim Mittendorf Martina Schfer Hartmut Schirok Johannes‐Peter Stasch Friederike Stoll Alexander Straub Peter Jeschke Ralf Nauen Michael Edmund Beck Hans‐Wilhelm Engels Hans‐Georg Pirkl Reinhard Albers Rolf W. Albach Jens Krause Andreas Hoffmann Holger Casselmann Jeff Dormish 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2013,125(36):9503-9503
118.
119.
Jens A. Pedersen 《Molecular physics》2013,111(4):1031-1035
A set of equations for assigning hyperfine coupling constants of substituted 1,4-benzosemiquinones are presented. The equations, based on additivity rules, yield assignments for a range of substituents with a correlation between predicted and experimental constants to better than five per cent. The investigation shows that t-butyl groups in semiquinones rotate freely against prior assertions. 相似文献