全文获取类型
收费全文 | 583篇 |
免费 | 10篇 |
国内免费 | 3篇 |
专业分类
化学 | 319篇 |
晶体学 | 26篇 |
力学 | 12篇 |
数学 | 84篇 |
物理学 | 155篇 |
出版年
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 5篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2017年 | 5篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 16篇 |
2013年 | 22篇 |
2012年 | 32篇 |
2011年 | 38篇 |
2010年 | 11篇 |
2009年 | 21篇 |
2008年 | 23篇 |
2007年 | 35篇 |
2006年 | 49篇 |
2005年 | 28篇 |
2004年 | 11篇 |
2003年 | 14篇 |
2002年 | 21篇 |
2001年 | 11篇 |
2000年 | 18篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 7篇 |
1992年 | 6篇 |
1989年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1974年 | 3篇 |
1971年 | 3篇 |
1960年 | 3篇 |
1947年 | 6篇 |
1946年 | 5篇 |
1944年 | 4篇 |
1943年 | 7篇 |
排序方式: 共有596条查询结果,搜索用时 15 毫秒
581.
Chandran Karunakaran K.R. Justin Thomas Arunachalam Shunmugasundaram Ramachandran Murugesan 《Journal of chemical crystallography》1999,29(4):413-420
Single crystals of the title Cu(II) complex, [Cu(stpy)4(ONO2)(ONO2)] [stpy = trans-4-styryl-pyridine] have been prepared and characterized by elemental and thermal analyses, IR, electronic and EPR spectral measurements, and X-ray crystal structure determination. The complex crystallizes in the monoclinic space group P21/c with unit-cell parameters, a = 12.985(2), b = 22.865(8), c = 17.024(10) Å, = 112.29(3)°, and Z = 4. The structure consists of discrete monomeric units of [Cu(stpy)4(ONO2)(ONO2)]. The equatorial positions of the Cu(II) polyhedron are occupied by nitrogen atoms of the four stpy ligands and the axial positions by the oxygens of two unidentate nitrate anions. One of these oxygens is at rather longer distance [2.609(3) Å] and may be considered to be semicoordinated. If this semibond is ignored, the coordination geometry lies closer to an idealized square pyramid than to the trigonal bipyramid geometry. IR spectra reveal nitrogen coordination from stpy and asymmetry in the monodentate oxygen binding of the two nitrate ligands. The optical reflectance band at 600 nm suggests pseudo-square-based pyramidal geometry around Cu(II). Well-resolved Cu(II) hyperfine features in the EPR spectra reveal the absence of exchange interactions between adjacent copper centers. Optical and EPR spectra of a methanolic solution of the complex indicate solvent interactions. Thermogravimetric analysis shows the complex to be stable up to 175°C. 相似文献
582.
First principles DFT calculations are used to gain insights into the molecular mechanism of Cr passivation of FeCr alloy surfaces. The systems studied represent early stages of oxidation of FeCr alloys when the oxide layers extend just a few atomic layers into the bulk. A Monte-Carlo atom-swapping algorithm was developed to efficiently explore possible atomic positions and identify the most promising structures that yield overall energy lowering. Analysis of the resulting low energy structures show that the surface oxide layer is rich in chromium while there is a reduction in chromium in the metallic phase near the alloy-oxide interface. Furthermore, there is an increased concentration of Fe near the oxide-air surface. Analysis of the molecular structure of the oxide layers found that oxidized Cr was predominantly in the Cr2O3 phase, while oxidized Fe was present as both FeO and Fe2O3. We propose that the oxidative variability of Fe facilitates O diffusion in the iron-rich phases because of the range of geometries available for accommodating the O atom. In contrast, O diffusion is less facile in Cr, which has little variability in oxidation state. 相似文献
583.
Preparation of ammonia borane in high yield and purity, methanolysis, and regeneration 总被引:3,自引:0,他引:3
Ammonia borane (AB) is emerging as a promising solid hydrogen carrier, particularly for power generation in portable devices that employ proton-exchange membrane fuel cells. A preparative-scale synthesis of AB from sodium borohydride and ammonium salts in high yields (> or =95%) and very high purity (> or =98%) has been described. The first systematic study of a transitional metal-catalyzed alcoholysis of AB, comparison of the methanolysis to the hydrolysis of AB, and regeneration of AB from ammonium tetramethoxyborate also has been described. 相似文献
584.
We experimentally generate n=0 Bessel beams via higher-order cladding mode excitation with a long period fiber grating. Our method allows >99% conversion efficiency, wide or narrow conversion bandwidth, and accurate control of the number of rings in the beam. This latter property is equivalent to tuning the beam cone angle and allows for control of width and propagation distance of the center spot. We generate Bessel-like beams from LP(0,5) to LP(0,15) cladding modes and measure their propagation-invariant characteristics as a function of mode order, which match numerical simulations and a simple geometric model. This yields a versatile tool for tuning depth of focus out of fiber tips, with potential uses in endoscopic microscopy. 相似文献
585.
Sivapalan BaskaranNarayanan Ramachandran Xiangtong HeSankar Thiruvannamalai Ho Joon LeeHyun Heo Qin ChenJohn Y. Fu 《Physics letters. A》2011,375(20):2082-2084
Recent studies have shown that giant flexoelectricity may exist in certain elastomers with bent-core molecular structures, which contradicts the previous theoretical estimation that the flexoelectric coupling is small in those materials. In this Letter, we report an analogous phenomenon, i.e., the giant direct flexoelectric effect, observed in a polyvinylidene fluoride (PVDF) film. Our experimental studies indicate that such an enhanced flexoelectric effect might be induced by the interaction between the energy couplings of the apolar and the polar molecular structures of the polymer film under elastic deformation. 相似文献
586.
587.
Dean JC Buchanan EG James WH Gutberlet A Biswas B Ramachandran PV Zwier TS 《The journal of physical chemistry. A》2011,115(30):8464-8478
Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed. 相似文献
588.
Properties of Laves phase compounds can be tailored by alloying and microstructural engineering. V-substituted cubic TiCr2 Laves phase has been studied to understand the location of V atoms in the lattice, by structural imaging and first-principle computations. Even though Ti, V and Cr appear next to each other in the periodic table, V preferentially replaces the Ti lattice producing anti-site defects. The defect formation energy for V substitution in Ti and in Cr lattice is 0.29 and 0.40 eV, respectively. V replacement in the Ti lattice generates atomic scale strain. Atomic numbers of V, Ti and Cr being very close, this phase is not quite suitable for incoherent imaging for understanding the structure and the chemistry. Instead, difference in channelling behaviour of electron waves along the Ti columns and along the Cr columns could be exploited to preferentially image the individual atom columns. Nature of the exit phase wave, phase and amplitude has been used to understand the contrast qualitatively. The intensity distribution of any particular atom column that is disturbed by the presence of foreign atom has been used to detect the position of V atoms. This method could be extended to study other Laves phases and complex intermetallic structures to understand their structure, defects and interfaces. 相似文献
589.
S. S. Krishnamurthy K. Ramachandran A. C. Sau M. N. Sudheendra Rao A. R. Vasudeva Murthy R. Keat 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):117-119
Abstract The 31P nmr spectra of 2,4- and 2,6-diamino-derivatives of octachlorocyclotetraphosphazatetraene, N4P4Cl6(NR1R2)2 (R1 = H, R2 = But; R1 = H, R2 = CH2Ph; R1 = Me, R2 = Ph), have been measured. The 2,4- and 2,6-isomers were analysed as AA'BB' and A2B2 spin systems respectively. In the 2,4-isomers the spin-spin couplings 2J(PNP) and 4J(PNPNP) were of opposite sign. 相似文献
590.
Natesan Sundaramoorthy Karthikeyan Ramachandran Gunasekar Rathore Ravindaranath 《合成通讯》2013,43(23):3429-3440
Abstract In an effort to provide a green synthetic route for heterocyclic scaffolds, new macro analogs of acyclic Mannich derivatives have been synthesized in good yields in a one-pot, efficient fashion. The acyclic symmetrical and unsymmetrical 2-[(E)-(benzylideneamino)(aryl)methyl]cyclododecanones have been synthesized by condensation of cyclododecanone, aromatic aldehyde, and ammonium acetate in ethanolic media. By infrared, NMR, and mass spectral analyses, their structures were determined. Examination of crystal structure of a p-Cl and o-Br analogs reveals the minimum energy [3333] square conformation of cyclododecanone ring. In the crystal, molecules aggregate in dimeric subunits formed by C-Cl … π and C-H..π interactions. The reported one-pot, multichain reaction of macrocycles is the first synthetic attempt of acyclic compounds in this type of Mannich reaction involving ketones containing active methylene groups. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献