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351.
Pérez C Suardíaz R Ortiz PJ Crespo-Otero R Bonetto GM Gavín JA García de la Vega JM San Fabián J Contreras RH 《Magnetic resonance in chemistry : MRC》2008,46(9):846-850
A remarkable difference for (2)J(C(2)-H(f)) coupling constant in syn and anti conformers of 5-X-furan-2-carboxaldehydes (X = CH(3), Ph, NO(2), Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi-contact term transmission, a rather unusual dual-coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the J(H-H) in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the pi(C=C) electronic system. The homoallylic coupling pathway can be labeled as sigma*(C-H) <-- pi(C=C) --> sigma*(C-H). In the present case, this additional coupling pathway, using an analogous notation, can be labeled as sigma*(C(2)-C(C)) <-- LP(1)(O(1))...LP(2)(O(C)) --> sigma*(C(C)-H(f)) (sigma*(C(2)-C(C))) where O(1) and O(C) stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap. 相似文献
352.
Contreras JA Murray JA Tolley SE Oliphant JL Tolley HD Lammert SA Lee ED Later DW Lee ML 《Journal of the American Society for Mass Spectrometry》2008,19(10):1425-1434
A novel gas chromatograph-mass spectrometer (GC-MS) based on a miniature toroidal ion trap mass analyzer (TMS) and a low thermal
mass GC is described. The TMS system has an effective mass/charge (m/z) range of 50–442 with mass resolution at full-width half-maximum (FWHM) of 0.55 at m/z 91 and 0.80 at m/z 222. A solid-phase microextraction (SPME) fiber mounted in a simple syringe-style holder is used for sample collection and
introduction into a specially designed low thermal mass GC injection port. This portable GC-TMS system weighs <13 kg (28 lb),
including batteries and helium carrier gas cartridge, and is totally self-contained within dimensions of 47×36×18 cm (18.5×14×7in.).
System start-up takes about 3 min and sample analysis with library matching typically takes about 5 min, including time for
column cool-down. Peak power consumption during sample analysis is about 80 W. Battery power and helium supply cartridges
allow 50 and 100 consecutive analyses, respectively. Both can be easily replaced. An on-board library of target analytes is
used to provide detection and identification of chemical compounds based on their characteristic retention times and mass
spectra. The GC-TMS can detect 200 pg of methyl salicylate on-column. n-Butylbenzene and naphthalene can be detected at a concentration of 100 ppt in water from solid-phase microextraction (SPME)
analysis of the headspace. The GC-TMS system has been designed to easily make measurements in a variety of complex and harsh
environments. 相似文献
353.
In this paper we show that any static and spherically symmetric anisotropic solution of the Einstein field equations can be thought as a system sourced by certain deformed isotropic system in the context of Minimal Geometric Deformation-decoupling approach. To be more precise, we developed a mechanism to obtain an isotropic solution from any anisotropic solution of the Einstein field equations. As an example, we implement the method to obtain the sources of a simple static anisotropic and spherically symmetric traversable wormhole. 相似文献
354.
Margarita Tlahuextl Antonio R. Tapia‐Benavides Angelina Flores‐Parra Rosalinda Contreras Hugo Tlahuext Elso M. Cruz 《Heteroatom Chemistry》2005,16(6):513-519
3,3′‐[2,2′‐Oxy‐bis‐(4S‐methyl, 5R‐phenyl‐1,3,2‐oxazaborolidine)]ethylene ( 4a ) and 3,3′‐[2, 2′‐oxy‐(4S‐methyl‐5R‐phenyl‐1,3,2‐oxazaborolidine)‐ (1,3,2‐benzoxazaborolidine)]ethylene ( 4b ) were synthesized by the reaction of N,N′‐bis‐[(1R,2S)‐norephedrine]oxalyl ( 3a ) or N,N′‐[((1R,2S)‐norephedrine, o‐hydroxyphenylamine]oxalyl ( 3b ) with BH3‐THF. The molecular structure of these compounds was established by NMR and infrared spectroscopy. The molecular geometry for 4 was studied by means of theoretical methods, resulting in structures that were in total agreement with those obtained by spectroscopy data and X‐ray diffraction. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:513–519, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20151 相似文献
355.
Adriana Esparza-RuizAdrián Peña-Hueso Edgar MijangosGuadalupe Osorio-Monreal Heinrich NöthAngelina Flores-Parra Rosalinda Contreras 《Polyhedron》2011,30(12):2090-2098
The structural and spectroscopic characterization of coordination compounds of four aromatic amines derived from benzimidazole, 2-aminobenzimidazole (L1), 1-(S-methylcarbodithioate)-2-aminobenzimidazole (L2), 2-(2-aminophenyl)-1H-benzimidazole (L3) and 6,6-dimethyl-5H-benzimidazolyl[1,2-c]quinazoline (L4) are reported. Cobalt(II) [Co(L1)2(CH3COO)2] (1) and nickel(II) [Ni(L1)2(CH3COO)2] (2) acetate coordination compounds of L1 are discussed. The synthesis and the X-ray crystal structure of the new 1-(S-methylcarbodithioate)-2-aminobenzimidazole (L2) is informed, together with its cobalt(II) [Co(L2)2Cl2] (3), [Co(L2)2Br2] (4) and zinc(II) [Co(L2)2Cl2] (5), [Zn(L2)2Br2] (6) coordination compounds. In these compounds the imidazolic nitrogen is coordinated to the metal center, while the ArNH2 and the S-methylcarbodithioate groups do not participate as coordination sites. A co-crystal of L1 and L2 is analyzed. Structural analyses of the coordination compounds of L3 showed that this ligand behaves as a bidentate ligand through the aniline and the imidazole groups forming six membered rings in the cobalt(II) [Co(L3)Cl2] (7) and zinc(II) [Zn(L3)Cl2] (8) compounds, as well as the nickel(II) nitrate [Ni(L3)2(H2O)2](NO3)2 (9). The quinazoline L4 was produced by insertion of one acetone molecule and water elimination in L3, its X-ray crystal diffraction analysis, as well as that of its zinc(II) coordination compound [Zn(L4)2Cl2] (10), are discussed. 相似文献
356.
The expression of the vector field generator of a Ricci collineation for diagonal, spherically symmetric and non-degenerate Ricci tensors is obtained. The resulting expressions show that the time and radial first derivatives of the components of the Ricci tensor can be used to classify the collineation, leading to 64 families. Some examples illustrate how to obtain the collineation vector. 相似文献
357.
358.
C. G. Giribet M. C. Ruiz de Azúa S. B. Gmez E. L. Botek R. H. Contreras W. Adcock E. W. Della A. R. Krstic I. J. Lochert 《Journal of computational chemistry》1998,19(2):181-188
In the present work, the relationship between the large substituent effects on 3J(C1H) in 1-X-3-M-bicyclo[1.1.1]pentanes, I , and the polarizability of the bridgehead C3(SINGLE BOND)Mα bond is investigated. The existence of such a relationship is suggested by the finding that the effect of an electronegative substituent X on 3J(C1Mα) couplings in I (M=H) is due to a distortion of the C3(SINGLE BOND)H bond toward the C1 center, which enhances the Fermi contact interaction. If such distortion originates in an electrostatic effect, then in other members of this series it can be expected that the substituent effects on 3J(C1Mα) couplings should depend strongly on the C3(SINGLE BOND)Mα bond polarizability. Two approaches are followed. First, the ab initio CLOPPA-IPPP method is applied to study the C3(SINGLE BOND)Mα bond contribution to the molecular static polarizability tensor in I (M=H, F, CH3). Such bond polarizabilities are found to follow the same trend as calculated as well as experimentally determined substituent effects on 3J(C1Mα) couplings, which were measured as part of this work in I [X=H, Cl; M=F, CH3 and X=OCH3; M=Sn(CH3)3]. Second, 3J(C1Mα) couplings (M=H, CH3) are calculated at an ab initio level for X=H, F, and they are compared with those obtained in the parent compound (X=H) if the calculation is carried out in the presence of an inhomogeneous electric field. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 181–188, 1998 相似文献
359.
Carlos Camacho-Camacho Hugo Tlahuext Heinrich Nth Rosalinda Contreras 《Heteroatom Chemistry》1998,9(3):321-326
Two new dibenzobicyclic penta- andhexacoordinated tin compounds ( 1–2 ) were prepared, derived from bis(2-hydroxy-3,5-di-tert-butyl-phenol)imine and dichloro-dimethylstannane and trichloro-n-butylstannane, respectively. Each compound is an example of two different coordination forms of the ligand. The crystalline complexes have been characterized by magnetic moment, infrared and mass spectrometry, elemental analysis, and X-ray diffraction studies. Compound 1 crystallized in the monoclinic system, space group P21/n; it contains a five-coordinated trigonal bipyramidal tin atom with methyl groups in equatorial positions and a planar dibenzobicyclic ring. It is a paramagnetic compound (μeff = 1.59 μB) due to the radical dianion nature of the ligand. Compound 2 crystallized also in the monoclinic system, space group P21/c; it contains a six-coordinated tin atom with an octahedral geometry; the dibenzobicyclic ring is planar, and the chlorine atoms are in trans positions. It is diamagnetic with a delocalized monoanion structure of the ligand. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:321–326, 1998 相似文献
360.
Roberto Alexander-Katz Aída De La Rosa Fernando Rojas Claudia Contreras Judith Cardoso 《Macromolecular Symposia》1994,84(1):245-254
Non-isotropic scattering patterns showing a minimum for small and almost transparent core-shell particles are discussed. It is shown that for any particle size, close to transparency, a core-shell particle behaves optically differently from a non-structured particle due to the presence of a minimum in the scattering pattern. Transparency criteria are discussed in the light of the non-isotropic character of the scattered light. Direct experimental evidence of the existence of this minimum is presented and compared with the theoretical predictions of the scattering by colloids having a distribution of particle composition. 相似文献