全文获取类型
收费全文 | 1282篇 |
免费 | 57篇 |
专业分类
化学 | 834篇 |
晶体学 | 25篇 |
力学 | 46篇 |
数学 | 91篇 |
物理学 | 343篇 |
出版年
2023年 | 28篇 |
2022年 | 31篇 |
2021年 | 33篇 |
2020年 | 28篇 |
2019年 | 36篇 |
2018年 | 33篇 |
2017年 | 37篇 |
2016年 | 69篇 |
2015年 | 48篇 |
2014年 | 44篇 |
2013年 | 100篇 |
2012年 | 96篇 |
2011年 | 118篇 |
2010年 | 60篇 |
2009年 | 53篇 |
2008年 | 57篇 |
2007年 | 66篇 |
2006年 | 52篇 |
2005年 | 39篇 |
2004年 | 30篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 15篇 |
1998年 | 11篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 14篇 |
1982年 | 5篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1971年 | 7篇 |
1970年 | 6篇 |
排序方式: 共有1339条查询结果,搜索用时 15 毫秒
991.
Permanganate, the versatile oxidizing agent, has limited solubility in non-polar media, which limits its use for oxidation of organic substrate in the non-aqueous media. But the combination of PTCs with permanganate makes it mild, selective and lipopathic. These lipopathic oxidants are useful for many important organic transformations. In this communication, we have presented the synthesis and utility of some selected lipopathic permanganate oxidants, which oxidize many functional groups, including alcohol, aldehyde, ether, alkane, alkene, alkyne, and oxime and have use in many synthetic organic procedures with chemo-, regio- and stereoselectivity. 相似文献
992.
Ajoy Saha Narendra A. Gajbhiye Biraj B. Basak Ponnuchamy Manivel 《International journal of environmental analytical chemistry》2018,98(7):622-643
Since the discovery of aflatoxins (AFs) in the 1960s, much research has focused on detecting the toxins in contaminated food and feedstuffs. But the quality determination in medicinal plant matrices with respect to AFs is scare. Hence, a simple, accurate and sensitive high-performance liquid chromatography coupled with tandem mass spectrometry (LC–MS/MS) method was developed for simultaneous determination of AFs AFB1, AFB2, AFG1 and AFG2 in two Indian popular medicinal herbs i.e. senna (Cassia angustifolia) and kalmegh (Andrographis paniculata). AFs have been extracted from herb matrix using a QuEChERS (quick, easy, cheap, effective, rugged and safe)-based extraction procedure followed by applying primary secondary amine and C18 for further clean-up step and then were quantified under the multiple reaction monitoring together with positive ionisation modes. Matrix-matched calibration was used for quantification in order to reduce the matrix effect. Validation of the method was carried out in herbs by recovery experiments. Recoveries of the spiked samples were in the range of 61.9–111.5% with an inter-day and intraday relative standard deviation lower than 20.0%. Limits of detection and quantification ranged from 0.41 to 0.95 ng mL?1 and 1.2 to 3.8 μg kg?1, respectively. The expanded uncertainty of the method was <21% for all the toxins in both the herbs. Finally, the proposed method was successfully applied to determine AF residues in real field samples of senna and kalmegh obtained from different locations in India. 相似文献
993.
Subhan Md. Abdus Saha Pallab Chandra Sarker Prosenjit Al-Mamun Mohammad 《Research on Chemical Intermediates》2018,44(10):6311-6326
Research on Chemical Intermediates - A trimetallic, MoO3·CuO·ZnO nanocomposite was synthesized by co-precipitation method. The nanocomposite was characterized by X-ray... 相似文献
994.
Structural Chemistry - Matrix metalloproteinase-2 (MMP-2) is established as one of the most important metalloenzymes for targeting cancer. However, homologous MMP-9 of the gelatinase family is... 相似文献
995.
Flavones are present in a variety of medicines and natural products and are important structural motif due to their unique mode of physiological action. Hence the structural importance of flavone moiety has elicited a great deal of interest in the field of organic synthesis and chemical biology to develop some new and improved synthesis of this molecular skeleton. Herein, we have described an up to date overview on the recent advances in the diverse synthetic methodologies of flavones. The review covers the basic conceptual and practical catalytic synthesis like carbonylative annulation, cyclodehydration, Suzuki Miyaura coupling, Heck coupling, green methodologies, metal catalyzed reactions, organocatalytic transformations, microwave irradiation, etc. which are significant for constructing flavone skeleton. This review will satisfy the expectations of readers who are interested in the development of the field and looking for an update. It will stimulate researchers to develop new and creative synthetic access to this heterocyclic system, which will be instrumental in the advancement of flavone chemistry. 相似文献
996.
997.
A convenient route for the synthesis of benzohydropentalene and benzohydroazulenes and is described for entry into a number of tri- and tetracyclic terpenoids. 相似文献
998.
Roy DK Saha A Mukherjee AK 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(9):2017-2022
Cloxacillin sodium has been shown to form charge transfer (CT) complexes of 1:1 stoichiometry with a number of electron acceptors in 50% (v/v) aqueous ethanol medium. From the trends in the CT absorption bands, the vertical ionization potential of the drug molecule (cloxacillin sodium) has been estimated to be 7.89 eV. The enthalpies and entropies of formation of two such complexes have been determined by estimating the formation constants spectrophotometrically at five different temperatures. The oscillator strengths and transition dipole moments of these complexes have been determined. It has further been noted that the reduction of o-chloranil by aqueous ethanol is completely inhibited by cloxacillin sodium, a phenomenon that makes the present study of formation equilibrium possible. 相似文献
999.
Subit K. Saha Sneh K. Dogra 《Journal of photochemistry and photobiology. A, Chemistry》1997,110(3):1895-266
Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Berg's equation indicate different geometries of the amines in the S0 and S1 states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S1 state in comparison to that in the S0 state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S0 and S1 states. Results so obtained are compared and discussed. 相似文献
1000.
The minimum-energy structures on the torsional potential-energy surface of 1,3-butadiene have been studied quantum mechanically using a range of models including ab initio Hartree-Fock and second-order M?ller-Plesset theories, outer valence Green's function, and density-functional theory with a hybrid functional and statistical average orbital potential model in order to understand the binding-energy (ionization energy) spectra and orbital cross sections observed by experiments. The unique full geometry optimization process locates the s-trans-1,3-butadiene as the global minimum structure and the s-gauche-1,3-butadiene as the local minimum structure. The latter possesses the dihedral angle of the central carbon bond of 32.81 degrees in agreement with the range of 30 degrees-41 degrees obtained by other theoretical models. Ionization energies in the outer valence space of the conformer pair have been obtained using Hartree-Fock, outer valence Green's function, and density-functional (statistical average orbital potentials) models, respectively. The Hartree-Fock results indicate that electron correlation (and orbital relaxation) effects become more significant towards the inner shell. The spectroscopic pole strengths calculated in the Green's function model are in the range of 0.85-0.91, suggesting that the independent particle picture is a good approximation in the present study. The binding energies from the density-functional (statisticaly averaged orbital potential) model are in good agreement with photoelectron spectroscopy, and the simulated Dyson orbitals in momentum space approximated by the density-functional orbitals using plane-wave impulse approximation agree well with those from experimental electron momentum spectroscopy. The coexistence of the conformer pair under the experimental conditions is supported by the approximated experimental binding-energy spectra due to the split conformer orbital energies, as well as the orbital momentum distributions of the mixed conformer pair observed in the orbital cross sections of electron momentum spectroscopy. 相似文献