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11.
Crucial role of three-center hydrogen bonding in a challenging chiral molecular recognition 总被引:1,自引:0,他引:1
Kim SG Kim KH Kim YK Shin SK Ahn KH 《Journal of the American Chemical Society》2003,125(45):13819-13824
The enantio-discrimination of beta-chiral primary ammonium ions is achieved by a rational approach that utilizes three-center (bifurcated) hydrogen bonding. The extraction experiments on various selected guests reveal that the bifurcated H-bonding plays a crucial role for the chiral discrimination. The X-ray data obtained for an inclusion complex substantiate such interactions. Using the bifurcated H-bonding, the chiral molecular recognition with our C(3)-symmetric tripodal oxazoline receptors is extended generally toward ammonium ions of alpha-, beta-, and alpha,beta-chiral amines. Simple molecular models, evoking the bifurcated H-bonding, explain the chiral discrimination modes. 相似文献
12.
Pyun DK Kim BJ Jung HJ Kim JH Lee JS Lee WK Lee CH 《Chemical & pharmaceutical bulletin》2002,50(3):415-418
A new series of 1beta-methylcarbapenems 1a-d bearing 5-methyl-4-mercaptopyrrolidinone rings has been prepared and evaluated for in vitro antibacterial activity and pharmacokinetic parameters. Most compounds showed excellent antibacterial activity and high stability to dehydropeptidase-1. We have synthesized optically active 5-methyl-4-hydroxypyrrolidinones from enantiomerically pure aziridine esters. 相似文献
13.
Preparation of Silver Nanoparticles through Alcohol Reduction with Organoalkoxysilanes 总被引:1,自引:0,他引:1
Hoe Jin Hah Sang Man Koo Sang Hoon Lee 《Journal of Sol-Gel Science and Technology》2003,26(1-3):467-471
Silver nanoparticles of narrow size distribution were prepared through the chemical reduction in an alcohol solution with several organoalkoxysilanes. In this system, organoalkoxysilanes served as a stabilizer, protecting silver nanoparticles from aggregation. The changes in size and morphology of colloidal silver nanoparticles were investigated with the addition of organoalkoxysilanes such as 3-aminopropyltriethoxysilane (APS), methyltriethoxysilane (MTS), phenyltrimethoxysilane (PTS), vinyltriethoxysilane (VTS), and 3-glycidoxypropyltrimethoxysilane (GPS) as stabilizers. The organic functional groups of organoalkoxysilanes interact with silver ions and clusters, which stabilize silver nanoparticles in the system. The silver nanoparticles obtained were characterized with transmission electron microscopy (TEM), UV-vis spectroscopy, etc. 相似文献
14.
Shultz DA Vostrikova KE Bodnar SH Koo HJ Whangbo MH Kirk ML Depperman EC Kampf JW 《Journal of the American Chemical Society》2003,125(6):1607-1617
We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes. 相似文献
15.
Joseph C. P. Koo Janice S. W. Lam Gregory A. Chass Ladislaus L. Torday Andras Varro Julius Gy. Papp 《Journal of Molecular Structure》2003,620(2-3):231-255
The sidechain conformational potential energy hypersurfaces (PEHS) for the γL, βL, αL, and αD backbone conformations of N-acetyl-
-aspartate-N′-methylamide were generated. Of the 81 possible conformers initially expected for the aspartate residue, only seven were found after geometric optimizations at the B3LYP/6-31G(d) level of theory. No stable conformers could be located in the δL, L, γD, δD, and D backbone conformations. The ‘adiabatic’ deprotonation energies for the endo and exo forms of N-acetyl-
-aspartic acid-N′-methylamide were calculated by comparing their optimized relative energies against those found for the seven stable conformers of N-acetyl-
-aspartate-N′-methylamide. Sidechain conformational PEHSs were also generated for the estimation of ‘vertical’ deprotonation energies for both endo and exo forms of N-acetyl-
-aspartic acid-N′-methylamide. All backbone–sidechain (N–H−O–C) and backbone–backbone (N–HO=C) hydrogen bond interactions were analyzed. A total of two backbone–backbone and four backbone–sidechain interactions were found for N-acetyl-
-aspartate-N′-methylamide. The deprotonated sidechain of N-acetyl-
-aspartate-N′-methylamide may allow the aspartyl residue to form strong hydrogen bond interactions (since it is negatively charged) which may be significant in such processes as protein–ligand recognition and ligand binding. As a primary example, the molecular geometry of the aspartyl residue may be important in peptide folding, such as that in the RGD tripeptide. 相似文献
16.
The magnetic properties of Cu(2)(dca)(4)(2,5-me(2)pyz) have been reexamined. The extended structure of Cu(2)(dca)(4)(2,5-me(2)pyz) can be viewed in terms of Cu(2)(2,5-me(2)pyz)(4+) dimer units interconnected via mu(1,5)-dca ligands. The bulk magnetic susceptibility chi(T) and the isothermal M(H) of Cu(2)(dca)(4)(2,5-me(2)pyz) are shown to be well described by an isolated dimer model. This finding was confirmed by carrying out a spin dimer analysis based on tight-binding calculations, which shows that the 2,5-me(2)pyz ligand provides a substantial spin exchange interaction between the Cu(2+) ions while the dca ligands do not. 相似文献
17.
18.
Symmetrically hindered methylphenols 1 react smoothly with NBS to form transient intermediates, p-benzoquinone methides (BM), which can be further processed to give hydroxybenzaldehydes in the presence of DMSO. This reaction is initiated by the formation of the phenoxy radical, followed by disproportionation to afford BM. None of the side-chain-brominated product is observed. The existence of BM is supported by the following observations: the formation of BM in solution can be monitored by GC and GC-MS; the electrophilic methine part participates in electrophilic aromatic substitution with anisoles to give hydroxybenzylated products 15; and the double bond character of the exocyclic methine plays a role in [4 + 2] cycloaddition with diene to afford Diels-Alder adducts. In contrast, unsymmetrically hindered or simple methylphenol (p-cresol) with NBS gives the nuclear brominated products, as usual. The energies of symmetrically hindered BMs, unsymmetrically hindered BM, and simple BM were calculated using density functional theories. Relative stabilization energies calculated at the B3LYP/6-31G//B3LYP/6-31G level by an isodesmic equation are enhanced 3-6 kcal/mol for symmetrically hindered BMs. 相似文献
19.
The magnetic structures of the Cu(2)O(3) spin lattices present in Cu(4)O(3) and Ag(2)Cu(2)O(3) were analyzed by studying their spin exchange interactions on the basis of spin dimer analysis. Calculations of spin exchange parameters were calibrated by studying LiCuVO(4) whose intrachain and interchain antiferromagnetic spin exchange parameters are known experimentally. The magnetic phase transition of Cu(4)O(3) at 42.3 K doubles the unit cell along each crystallographic direction. The spin arrangements of the Cu(2)O(3) lattice consistent with this experimental observation are different from conventional antiferromagnetic ordering. Our analysis indicates that spin fluctuation should occur in Cu(4)O(3), low-dimensional magnetism should be more important than magnetic frustration in Cu(4)O(3), and Ag(2)Cu(2)O(3) and Cu(4)O(3) should have similar structural and magnetic properties. 相似文献
20.
Hydrolyses of phosphorus halides, (RO)(2)POX where R = H or Me and X = F or Cl, in the gas phase and in the reaction field have been investigated theoretically with ab initio and the density functional theory (DFT). The free energy of activation in the reaction field was also estimated using the Onsager method with a correction of entropy change and basis set superposition error (BSSE). The reaction of (MeO)(2)POF proceeds through a path with bifunctional catalysis regardless of the medium, but the reaction of (MeO)(2)POCl proceeds through bifunctional and general base catalysis in the gas phase and in water, respectively. The estimated free energy barrier of 23 kcal/mol for the hydrolysis of (MeO)(2)POF is in good agreement with the experimental values of 24 kcal/mol, and relative barrier of 3 kcal/mol to the (MeO)(2)POCl is also in good agreement with the experimental values of 5 kcal/mol of diisopropyl phosphorus halides ((Pr(i)O)(2)POX, X = F and Cl). 相似文献