全文获取类型
收费全文 | 540篇 |
免费 | 15篇 |
专业分类
化学 | 433篇 |
力学 | 9篇 |
数学 | 47篇 |
物理学 | 66篇 |
出版年
2023年 | 5篇 |
2022年 | 22篇 |
2021年 | 35篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 19篇 |
2017年 | 13篇 |
2016年 | 13篇 |
2015年 | 23篇 |
2014年 | 31篇 |
2013年 | 59篇 |
2012年 | 35篇 |
2011年 | 58篇 |
2010年 | 36篇 |
2009年 | 24篇 |
2008年 | 35篇 |
2007年 | 22篇 |
2006年 | 24篇 |
2005年 | 17篇 |
2004年 | 14篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 1篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有555条查询结果,搜索用时 15 毫秒
481.
Selective sulfoxidation of BEDT-TTF (bis(ethylenedithio)-tetrathiafulvalene) with enantiopure (camphoryl-sulfonyl)oxaziridine derivatives provided the inner monosulfoxide, as demonstrated using single crystal X-ray analysis, with an enantiomeric excess of 44% (up to 74% after recrystallization). 相似文献
482.
Cojocariu AM Mutin PH Dumitriu E Fajula F Vioux A Hulea V 《Chemical communications (Cambridge, England)》2008,(42):5357-5359
A SiO(2)-TiO(2) mesoporous xerogel prepared in one-step by a non-hydrolytic route shows excellent performance in the mild oxidation of sulfides, sulfoxides and thiophenes with aqueous solutions of H(2)O(2). 相似文献
483.
Tarpan M Sagstuen E Pauwels E Vrielinck H Waroquier M Callens F 《The journal of physical chemistry. A》2008,112(17):3898-3905
Primary free radical formations in fructose single crystals X-irradiated at 10 K were investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three principal crystallographic planes and a fourth skewed plane allowed the unambiguous determination of five proton hyperfine coupling tensors. From the EIE studies, these hyperfine interactions were assigned to three different radicals, labeled T1, T1* and T2. For the T1 and T1* radicals, the close similarity in hyperfine coupling tensors suggests that they are due to the same type of radical stabilized in two slightly different geometrical conformations. Periodic density functional theory calculations were used to aid the identification of the structure of the radiation-induced radicals. For the T1/T1* radicals a C3 centered hydroxyalkyl radical model formed by a net H abstraction is proposed. The T2 radical is proposed to be a C5 centered hydroxyalkyl radical, formed by a net hydrogen abstraction. For both radicals, a very good agreement between calculated and experimental hyperfine coupling tensors was obtained. 相似文献
484.
Ionescu S Popovici D Balaban AT Hillebrand M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,66(4-5):1165-1170
The absorption and emission spectra in cyclohexane and methanol of the title phenoxathiinyl-phenyloxazole derivatives are presented and discussed. Comparing to the unsubstituted diphenyloxazole (PPO), the experimental results show a bathochromic shift of the emission band, a significant dependence of the maximum on the solvent polarity and a drastic decrease of the fluorescence quantum yield. Semiempirical MO calculations (AM1) in both the ground and excited states support the experimental findings. A charge transfer from the phenoxathiin fragment to the oxazole ring is predicted in the excited state explaining the solvatochromism of the compounds. The values for the singlet-triplet gap, 3500-5000 cm-1 point to an enhanced probability of intersystem crossing (ISC) non-radiative deactivation processes, in agreement with the low fluorescence quantum yields. 相似文献
485.
The chemoenzymatic preparation of novel enantiopure phenylfuran-based γ-amino alcohols with N-Boc-protection starting from the corresponding aldehydes is described. Enantiopurity (ee 98–99%) is introduced using Thermomyces lanuginosus lipase as the IMMTLL-T1-1500 preparation with β-hydroxy nitriles in an acylation/alcoholysis sequence in tert-butylmethyl ether. 相似文献
486.
Mihaela Avadanei Mioara Drobota Iuliana Stoica Elena Rusu Virgil Barboiu 《Journal of polymer science. Part A, Polymer chemistry》2010,48(23):5456-5467
Surface of biaxially oriented poly(ethylene terephthalate) films was chemically modified by exposure to ethylenediamine (EDA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA) for different treatment times. Variable angle attenuated total reflection Fourier transform infrared (ATR‐FTIR) spectroscopy was used in conjunction with weight loss measurements, scanning electron microscopy (SEM), and atomic force microscopy to establish the surface modification and to draw the depth profile of the newly created species, with emphasis on amide group. A clear differentiation was found between the effects of the three amines studied: EDA produces the highest amidation degree but, because of its deep penetration into the film, leads to delamination of rather thick layers, TETA reacts at and near surface and develops surface cracks without delamination, and TEPA is the mildest reactant, generating amide groups on the surface without visible deterioration of the sample. It was proved that the amide II absorption band became weaker with increasing analyzed depth, with a pronounced heterogeneity near the surface. SEM micrographs showed the development of cracks onto the surface at longer aminolysis time, which allowed a better understanding of ATR‐FTIR observations. Assuming an exponential decay for the depth profile spectrally obtained, the surface concentration of amide groups and the decay constant were determined for the amines and reaction times used. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010 相似文献
487.
Phenoxathiinsulphone derivatives–cyclodextrin interactions: induced chirality and TDDFT calculations
Romică Sandu Cristina Tablet Mihaela Hillebrand 《Journal of inclusion phenomena and macrocyclic chemistry》2013,77(1-4):183-193
The inclusion process of phenoxathiin-10,10-dioxide and 2-CH2Br-phenoxathiin-10,10-dioxide in α-, β-, γ- and 2-hydroxypropyl-γ-cyclodextrins was studied by circular dichroism spectroscopy. The dependence of the induced circular dichroism signal on the host concentration was analyzed in terms of a nonlinear model yielding the stoichiometry and the association constants of the complexes. Time dependent density functional theory (TDDFT) calculations were used to rationalize the experimental data considering two aspects. Firstly, to support on theoretical grounds the experimentally observed achirality of the studied compounds that present two structural elements to confer chirality: the butterfly motion of the roof-shaped heteroring and the rotation of the CH2Br group. In this last process, some favorable position of the bromine atom could influence the overall chirality. Secondly, the TDDFT calculations of the polarizations of the electronic transitions in correlation with the signs of the induced dichroic bands were used to establish the axial or equatorial way the guest is included in the host cavity. 相似文献
488.
Gabriela Sãcãrescu Nicolae Voiculescu Mihai Marcu Liviu Sãcãrescu Rodinel Ardeleanu Mihaela Simionescu 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(3):509-516
Abstract Using the well known Wurtz coupling technique, polyhydrosilanes copolymers with diphenyl - and methyl(phenyl) - sequences, were synthesised. This work describes the possibility to maintain stable Si-H group and to obtain in good yields, soluble, high molecular, functional polysilanes. IR, 1-NMR, 13C, UV spectral data, quantitative elemental Si and H (from Si-H) analyses and indirect molecular weight investigation sustain the molecular structure of the copolysilanes. 相似文献
489.
Narcisa Vrinceanu Diana Tanasa Claudia Mihaela Hristodor Florin Brinza Eveline Popovici Daniel Gherca Aurel Pui Diana Coman Andreea Carsmariu Ionut Bistricianu Gianina Broasca 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1107-1119
The research is aimed at synthesis and characterization of nanoscaled zinc oxide particles and their application on linen fibrous supports, for thermal properties. To impart thermal activity to the fibrous nanocomposites, nanoparticles as well as fibrous nanocomposites were produced in different hydrothermal conditions of temperature (90 °C). To characterize the nanoparticles composition, their shape, size, and crystallinity, investigations technique, such as Fourier transformed infrared spectroscopy, scanning electron microscopy, and X-ray powder diffractometry were used. Differential scanning calorimetry analysis profiles were also revealed. The thermal treatment of linen fabrics with nanosized ZnO does not modify significantly their thermal stability. 相似文献
490.
Roman M Cannizzo C Pinault T Isare B Andrioletti B van der Schoot P Bouteiller L 《Journal of the American Chemical Society》2010,132(47):16818-16824
Gathering precise knowledge on weak supramolecular interactions is difficult yet is of utmost importance for numerous scientific fields, including catalysis, crystal engineering, ligand binding, and protein folding. We report on a combined theoretical and experimental approach showing that it is possible to vastly improve the sensitivity of current methods to probe weak supramolecular interactions in solution. The concept consists of using a supramolecular platform involving a highly cooperative configurational transition, the perturbation of which (by the modification of the molecular building blocks) can be monitored in a temperature scanning experiment. We tested this concept with a particular bisurea platform, and our first results show that it is possible to detect the presence of interaction differences as low as 60 J/mol, which may be due to steric repulsion between vinyl and alkyl groups or may be the result of solvation effects. 相似文献