全文获取类型
收费全文 | 960篇 |
免费 | 48篇 |
专业分类
化学 | 800篇 |
晶体学 | 15篇 |
力学 | 17篇 |
数学 | 68篇 |
物理学 | 108篇 |
出版年
2024年 | 4篇 |
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 22篇 |
2020年 | 29篇 |
2019年 | 26篇 |
2018年 | 16篇 |
2017年 | 25篇 |
2016年 | 32篇 |
2015年 | 31篇 |
2014年 | 42篇 |
2013年 | 66篇 |
2012年 | 67篇 |
2011年 | 59篇 |
2010年 | 40篇 |
2009年 | 54篇 |
2008年 | 63篇 |
2007年 | 49篇 |
2006年 | 65篇 |
2005年 | 46篇 |
2004年 | 42篇 |
2003年 | 37篇 |
2002年 | 20篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1969年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1008条查询结果,搜索用时 31 毫秒
991.
G. Srinivas RaoB. Venkateswara Rao 《Tetrahedron letters》2011,52(46):6076-6079
The highly stereoselective Grignard addition on chiralimine 8 and opening of chiral epoxide 9 with Grignard reagent have been used as key steps in the stereoselective synthesis of cytotoxic anhydrophytosphingosine pachastrissamine (jaspine B) and N,O,O,O-tetra-acetyl d-lyxo-phytosphingosine. 相似文献
992.
Sahu B Sacui I Rapireddy S Zanotti KJ Bahal R Armitage BA Ly DH 《The Journal of organic chemistry》2011,76(14):5614-5627
Developed in the early 1990s, peptide nucleic acid (PNA) has emerged as a promising class of nucleic acid mimic because of its strong binding affinity and sequence selectivity toward DNA and RNA and resistance to enzymatic degradation by proteases and nucleases; however, the main drawbacks, as compared to other classes of oligonucleotides, are water solubility and biocompatibility. Herein we show that installation of a relatively small, hydrophilic (R)-diethylene glycol ("miniPEG", R-MP) unit at the γ-backbone transforms a randomly folded PNA into a right-handed helix. Synthesis of optically pure (R-MP)γPNA monomers is described, which can be accomplished in a few simple steps from a commercially available and relatively cheap Boc-l-serine. Once synthesized, (R-MP)γPNA oligomers are preorganized into a right-handed helix, hybridize to DNA and RNA with greater affinity and sequence selectivity, and are more water soluble and less aggregating than the parental PNA oligomers. The results presented herein have important implications for the future design and application of PNA in biology, biotechnology, and medicine, as well as in other disciplines, including drug discovery and molecular engineering. 相似文献
993.
Gowravaram Sabitha M. Bhikshapathi Sambit Nayak R. Srinivas J. S. Yadav 《Journal of heterocyclic chemistry》2011,48(2):267-271
A one‐pot, three‐component synthesis of 2‐amino‐2‐chromenes is described at ambient temperature by the reaction of an aldehyde and malononitrile or ethyl cyanoacetate with α‐naphthol or β‐naphthol in ethanol in presence of a catalytic amount of Triton B. J. Heterocyclic Chem., (2011). 相似文献
994.
Srinivas B Shubhamangala B Lalitha K Reddy PA Kumari VD Subrahmanyam M De BR 《Photochemistry and photobiology》2011,87(5):995-1001
TiO(2) and different Cu wt% loaded TiO(2) (TC(0.5-5.0)), 10 wt% TC(2.0) supported on molecular sieve 5A (10 wt% TC(2.0)/MS) were prepared by impregnation and solid-state dispersion methods. The photocatalysts prepared were characterized using XRD, SEM, and UV-Vis DRS, TEM, XPS spectroscopy techniques. Photocatalytic reduction of CO(2) in water and alkaline solution are investigated in a batch reactor. The yield of oxalic acid increased notably when TC was supported on molecular sieve. The Cu-TiO(2) supported on molecular sieve catalyst promotes the charge separation that leads to an increase in the selective formation of oxalic acid in addition to methanol, acetic acid and traces of methane. The product formation is due to the high adsorption of CO(2), water and product shape selectivity of the composite photocatalyst. The maximum yield of oxalic acid was found to be 65.6 μg h(-1) g(-1) per cat using 0.2 N NaOH containing solution over 10 wt% TC(2.0)/MS photocatalyst. The difference in the photocatalytic activity is related to its physicochemical properties. 相似文献
995.
We present a novel protocol for the efficient synthesis of fused benzimidazolo piperazines starting from a four-component Ugi-Smiles reaction and a subsequent three-step cascade involving an acid-catalyzed cyclization, an intramolecular reductive cyclization, and an oxidation. 相似文献
996.
The 3,5-di-O-benzoyl n-pentenyl orthoesters of the four pentofuranoses have been prepared. The first key intermediate in each case is the methyl pentofuranoside(s), and a user-friendly procedure for the preparation of each, based on the Callam-Lowary precedent, is described, whereby formation of the crucial α/β anomeric mixture is optimized. The mixture is used directly to prepare the corresponding perbenzoylated pentofuranosyl bromide(s) and then the title compounds. 相似文献
997.
Ramesh M Raju B Srinivas R Sureshbabu VV Vishwanatha TM Hemantha HP 《Rapid communications in mass spectrometry : RCM》2011,25(14):1949-1958
A series of positional isomeric pairs of Fmoc-protected dipeptides, Fmoc-Gly-Xxx-OY/Fmoc-Xxx-Gly-OY (Xxx=Ala, Val, Leu, Phe) and Fmoc-Ala-Xxx-OY/Fmoc-Xxx-Ala-OY (Xxx=Leu, Phe) (Fmoc=[(9-fluorenylmethyl)oxy]carbonyl) and Y=CH(3)/H), have been characterized and differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-IT-MS(n)). In contrast to the behavior of reported unprotected dipeptide isomers which mainly produce y(1)(+) and/or a(1)(+) ions, the protonated Fmoc-Xxx-Gly-OY, Fmoc-Ala-Xxx-OY and Fmoc-Xxx-Ala-OY yield significant b(1)(+) ions. These ions are formed, presumably with stable protonated aziridinone structures. However, the peptides with Gly- at the N-terminus do not form b(1)(+) ions. The [M+H](+) ions of all the peptides undergo a McLafferty-type rearrangement followed by loss of CO(2) to form [M+H-Fmoc+H](+). The MS(3) collision-induced dissociation (CID) of these ions helps distinguish the pairs of isomeric dipeptides studied in this work. Further, negative ion MS(3) CID has also been found to be useful for differentiating these isomeric peptide acids. The MS(3) of [M-H-Fmoc+H](-) of isomeric peptide acids produce c(1)(-), z(1)(-) and y(1)(-) ions. Thus the present study of Fmoc-protected peptides provides additional information on mass spectral characterization of the dipeptides and distinguishes the positional isomers. 相似文献
998.
Raju B Ramesh M Srinivas R Chandrasekhar S Kiranmai N Sarma VU 《Journal of the American Society for Mass Spectrometry》2011,22(4):703-717
A new class of positional isomeric pairs of -Boc protected oligopeptides comprised of alternating nucleoside derived β-amino
acid (β-Nda-) and L-amino acid residues (alanine, valine, and phenylalanine) have been differentiated by both positive and
negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-MS
n
). The protonated dipeptide positional isomers with β-Nda- at the N-terminus lose CH3OH, NH3, and C2H4O2, whereas these processes are absent for the peptides with L-amino acids at the N-terminus. Instead, the presence of L-amino
acids at the N-terminus results in characteristic retro-Mannich reaction involving elimination of imine. A good correlation
has been observed between the conformational structure of the peptides and the abundance of yn+ and bn+ ions in MS
n
spectra. In the case of tetrapeptide isomers that are reported to form helical structures in solution phase, no yn+ and bn+ ions are observed when the corresponding amide -NH- participates in the helical structures. In contrast, significant yn+ and bn+ ions are formed when the amide -NH- is not involved in the H-bonding. In the case of tetra- and hexapeptides, it is observed
that abundant bn+ ions are formed, presumably with stable oxazolone structures when the C-terminus of the bn+ ions possessed L-amino acid and the β-Nda- at the C-terminus appears to prevent the cyclization process leading to the absence
of corresponding bn+ ions. 相似文献
999.
V. Srinivas S.K. Barik Bhaskarjyoti Bodo Debjani Karmakar T.V. Chandrasekhar Rao 《Journal of magnetism and magnetic materials》2008
Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural, magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical measurements. 相似文献
1000.
Nelson F Eng Srinivas Garlapati Volker Gerdts Lorne A Babiuk George K Mutwiri 《Journal of immune based therapies and vaccines》2010,8(1):4