全文获取类型
收费全文 | 396篇 |
免费 | 36篇 |
国内免费 | 1篇 |
专业分类
化学 | 328篇 |
晶体学 | 8篇 |
力学 | 13篇 |
数学 | 42篇 |
物理学 | 42篇 |
出版年
2023年 | 4篇 |
2022年 | 3篇 |
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 11篇 |
2017年 | 7篇 |
2016年 | 21篇 |
2015年 | 21篇 |
2014年 | 33篇 |
2013年 | 33篇 |
2012年 | 40篇 |
2011年 | 25篇 |
2010年 | 18篇 |
2009年 | 21篇 |
2008年 | 22篇 |
2007年 | 24篇 |
2006年 | 16篇 |
2005年 | 13篇 |
2004年 | 13篇 |
2003年 | 16篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1989年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1972年 | 2篇 |
1969年 | 1篇 |
1965年 | 1篇 |
1962年 | 1篇 |
1943年 | 1篇 |
1942年 | 1篇 |
1941年 | 2篇 |
1940年 | 1篇 |
1938年 | 2篇 |
1937年 | 1篇 |
1936年 | 3篇 |
1932年 | 1篇 |
排序方式: 共有433条查询结果,搜索用时 10 毫秒
1.
JPC – Journal of Planar Chromatography – Modern TLC - Emodin and twelve phenolic acids (ellagic, gallic, protocatechuic, homoprotocatechuic, caffeic, p-hydroxybenzoic, p-coumaric,... 相似文献
2.
The purpose of this study is to investigate the influence of nickel, which is an alloying element in commonly used metallic biomaterials, on the biomaterials mineralization process. An electrochemical method was developed to quantify this metal ion in osteoblast-like cell culture medium (OST) by performing adsorptive cathodic stripping voltammetry (CSV) with dimethylglyoxime (DMG) at a mercury film microelectrode (MFM). The optimized analytical conditions and the square-wave CSV parameters for the analysis are: DMG concentration: 5.00 × 10−4 mol L−1; ammonium chloride buffer: 0.10 mol L−1 (pH 9.2); frequency: 50 Hz, amplitude 20 mV; step: 2 mV; adsorption time: 10 s, deposition potential: −0.70 V and reduction potential: −1.20 V. The limit of detection was 7.70 × 10−9 mol L−1 for an adsorption time of 10 s. The results achieved by CSV using the MFM were compared to those obtained by atomic absorption spectrometry (AAS) to ensure the reliability of the electrochemical method. The mineralization process was evaluated by biochemical and histochemical assays. 相似文献
3.
W. Ketterle P. Graßhoff H. Figger H. Walther 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1988,9(4):325-329
The emission of the ND4 Schüler band was observed after neutralization of a mass-selected ND 4 + ion beam. Thus the assignment of this band to the ammonium radical was confirmed. The lifetime of the upper state (3p 2 F 2) was determined to be 4.2 ns, which is much shorter than ab initio values of the radiative lifetime, showing that this state decays predominantly by predissociation. The Schuster band was not observed, neither after neutralization of ND 4 + , nor of ND 3 + . 相似文献
4.
Paolesse R Nardis S Venanzi M Mastroianni M Russo M Fronczek FR Vicente MG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1192-1197
5,10,15-Triphenylcorrole (1) reacts with the Vilsmeier reagent (POCl(3)/DMF) to give the corresponding 3-formyl derivative 3 as the major product. The regioselectivity of the reaction was proven by X-ray crystallography and only traces of the 2-formyl isomer were observed. A more polar product is also observed and this compound becomes the major product when an excess of DMF is used for the preparation of the Vilsmeier reagent, while the formation of the 3-formyl isomer is almost completely suppressed. X-ray crystallography allowed us to identify this compound as the fully substituted N-ethane bridged derivative 4, formed from the attack of the Vilsmeier reagent at the inner core of the macrocycle. This compound is unique among porphyrinoid macrocycles, and further confirms the peculiarity of corrole chemistry. 相似文献
5.
Krystyna Cieśla H. Rahier Grażyna Zakrzewska-Trznadel 《Journal of Thermal Analysis and Calorimetry》2004,77(1):279-293
One to three endothermal peaks atributted to melting of bulk and interfacial water were observed by DSC in the regenerated
cellulose — water system. The profiles of thermal effects depend on water content, time of conditioning, film pretreatment
and the conditions applied during the preceding freezing-thawing cycles. The occurrence might be deduced of melting-crystallisation
processes. A large amount of non-freezable strongly bounded water was also detected.
Although cellulose absorbs water quickly after immersion, the structural changes consisting on ordering of polymer fraction
occur during further conditioning due to increase in strength of water binding. Using the membranes in the separation module
at 90°C causes weakening of these bonds. Differences between interaction of particular cellulose films with water can be detected
during the first, the second and the third heating.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Formulations of the tin etiopurpurin (SnET2) have been observed to undergo a spectral change on storage in dimethylformamide solution. This results in an alteration in the action spectrum, with enhanced photodynamic activity at lower wavelengths and decreased activity at 660 nm. On the basis of structural considerations, a reduced analog of SnET2 was prepared with an absorbance maximum at 640 nm. Formation of this product, termed SnET2H(2) relieves steric strain inherent in the parent molecule. Nuclear magnetic resonance, spectral and photodynamic data are consistent with the formation of SnET2H(2) during SnET2 storage. Slight modifications in the original synthesis are also reported, resulting in improved yields of intermediate products. 相似文献
7.
This study was designed to provide more detailed information on the subcellular sites of binding of the porphycene, termed 9-capronyloxytetrakis (methoxyethyl) porphycene (CPO), with a fluorescence resonance energy transfer (FRET) technique. The proximity of CPO to two fluorescent probes was determined: nonyl acridine orange (NAO), a dye with specific affinity for the mitochondrial lipid cardiolipin, and dihexa-oxacarbocyanine iodide (DiOC6), an agent that labels the endoplasmic reticulum (ER). FRET spectra indicated energy transfer between DiOC6 and CPO but no significant transfer between NAO and CPO. These results confirm data obtained by fluorescence microscopy, suggesting a similar pattern of subcellular localization by CPO and DiOC6 but not by CPO and NAO. However, when cells containing CPO were irradiated and then loaded with NAO, FRET between the two fluorophores was observed. Hence, a relocalization of CPO can occur during irradiation. These data provide an explanation for recent studies on CPO-catalyzed photodamage to both ER and mitochondrial Bcl-2. 相似文献
8.
Pedro Domingues Francisco M. L. Amado M. Graça O. Santana-Marques A. J. Ferrer-Correia 《Journal of the American Society for Mass Spectrometry》1998,9(11):1189-1195
The product-ion spectra of the sodiated molecules of glycerol phosphatidylcholine phospholipids (GPC) were obtained, using fast-atom bombardment (FAB) as the ionization method, in a tandem mass spectrometer. The product-ion spectra of these sodiated molecules of GPCs were found to differ significantly from those of the protonated GPC molecules. This difference is due to the absence of the ion of m/z 184 (protonated-phosphocholine moiety) and to the presence of an ion resulting from the loss of trimethylamine (m=59 Da) from the polar head group, which is the dominant fragmentation. This characteristic neutral loss provides a means of identification of this class of phospholipids and of differentiation from other phospholipid classes in complex mixtures by performing a constant-neutral-loss scan of 59. 相似文献
9.
M. J. Tiera V. A. De Oliveira H. D. Burrows M. Graça Miguel M. G. Neumann 《Colloid and polymer science》1998,276(3):206-212
The binding of the cations thallium(I), calcium(II) and terbium(III) to methyl methacrylate– methacrylic acid copolymers
with different fractions of acid groups (x) has been studied in aqueous solution at various pH values using the fluorescence of covalently bonded 9-vinyl anthracene
as a probe. In all cases, the extent of binding increases as a function of the charge of the polymer with either increasing
fraction of carboxylic acids or of pH. However, differences are observed in the behavior of the three cations. With Tl(I),
quenching of the anthracene group fluorescence is observed, indicating that the thallium(I) approaches the probe and suggesting
that the alkylanthracene is probably in a relatively polar region. Binding constants have been determined from anthracene
quenching data and from studies with the fluo-rescent-probe sodium pyrenetetra-sulfonate. Good agreement is obtained between
the two methods, and values for the binding constants increase from 250 to 950 M-1 as x increases from 0.39 to 1. It is suggested that the cation is held in the polyelectrolyte domain, partly by Debye–Hückel effects
and partly by more specific interactions. Stronger binding is found with calcium(II) and terbium(III), and in this case increases
in fluorescence intensity are observed on complexation due to the anthracene group being in a more hydrophobic region, probably
as a result of conformational changes in the polymer chain. In the former case the stoichiometry of the interaction was determined
from the fluorescence data to involve two carboxylate groups bound per calcium. Association constants were found using murexide
as an indicator of free calcium to vary from 8400 to 37 000 M-1 as x increases from 0.39 to 1. It is suggested that in this case specific calcium(II)–carboxylate interactions contribute to the
binding. With terbium(III), a greater increase in the probe fluorescence intensity was observed than with calcium, and it
is suggested that the interaction with the polymer is even stronger, leading to a more pronounced conformational change in
the polymer. It is proposed that the terbium(III) interacts with six carboxylic groups on the polymer chain, with three being
coordinated and three attracted by electrostatic interactions.
Received: 10 June 1997 Accepted: 24 October 1997 相似文献
10.
Teobald Kupka Roman Wrzalik Gra
yna Pasterna Karol Pasterny 《Journal of Molecular Structure》2002,616(1-3):17-32
The results of extended MO calculations using density functional theory (DFT) approximation supported by experimental Raman, 1H and 13C NMR studies on thiophene are reported. Raman spectra of liquid thiophene were re-examined and the performance of a hybrid B3PW91 density functional was compared with the ab initio restricted Hartree–Fock (RHF) method. With the basis sets of the 6-311++G** quality, the DFT calculated bond lengths, dipole moments and harmonic vibrations were predicted in a very good agreement with available experimental data.
Additionally, the results on thiophene were extended by calculations on 3-methylthiophene and selenophene. In this case, a significant change in geometry and charge distribution in thiophene ring due to a methyl group substituent or replacement of sulphur by selene atom was observed.
A linear correlation between the predicted harmonic vibrational frequencies (scaled using SQM method) and experimental ones for thiophene, selenophene and 3-methylthiophene was shown. The theoretically calculated spectra have satisfactorily reproduced the available experimental spectra for thiophene and selenophene. 相似文献