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101.
Fan Li Michela Baccini Fabrizia Mealli Elizabeth R. Zell Constantine E. Frangakis Donald B. Rubin 《Journal of computational and graphical statistics》2013,22(3):877-892
Multiple imputation (MI) has become a standard statistical technique for dealing with missing values. The CDC Anthrax Vaccine Research Program (AVRP) dataset created new challenges for MI due to the large number of variables of different types and the limited sample size. A common method for imputing missing data in such complex studies is to specify, for each of J variables with missing values, a univariate conditional distribution given all other variables, and then to draw imputations by iterating over the J conditional distributions. Such fully conditional imputation strategies have the theoretical drawback that the conditional distributions may be incompatible. When the missingness pattern is monotone, a theoretically valid approach is to specify, for each variable with missing values, a conditional distribution given the variables with fewer or the same number of missing values and sequentially draw from these distributions. In this article, we propose the “multiple imputation by ordered monotone blocks” approach, which combines these two basic approaches by decomposing any missingness pattern into a collection of smaller “constructed” monotone missingness patterns, and iterating. We apply this strategy to impute the missing data in the AVRP interim data. Supplemental materials, including all source code and a synthetic example dataset, are available online. 相似文献
102.
Braga D Giaffreda SL Grepioni F Pettersen A Maini L Curzi M Polito M 《Dalton transactions (Cambridge, England : 2003)》2006,(10):1249-1263
This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to "make crystals", which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding. 相似文献
103.
Yue W Lv A Gao J Jiang W Hao L Li C Li Y Polander LE Barlow S Hu W Di Motta S Negri F Marder SR Wang Z 《Journal of the American Chemical Society》2012,134(13):5770-5773
Hybrid rylene arrays have been prepared via a combination of Stille coupling and C-H transformation. The ability to extend the π system along the equatorial axis of rylenes not only leads to broadened light absorption but also increases the electron affinity, which can facilitate electron injection and transport with ambient stability. 相似文献
104.
105.
Negri F di Donato E Tommasini M Castiglioni C Zerbi G Müllen K 《The Journal of chemical physics》2004,120(24):11889-11900
Polycyclic aromatic hydrocarbons (PAHs) are employed to model the Raman features that are generally associated with sp(2) nanostructures in carbon materials or with disorder and defects in graphitic materials. To this end molecular parameters (geometry changes upon electronic excitation, vibrational normal modes, and displacement parameters) are computed with semiempirical quantum-chemical methods for a series of PAHs ranging from 6 to 384 carbon atoms, and Raman intensities are evaluated according to Albrecht's formalism restricted to the A term. The computed preresonance and resonance Raman intensities are compared with available experimental data for hexa-peri-hexabenzocoronene and for pyrene. For the latter compound, simulations carried out at semiempirical and ab initio levels of theory are shown to be of comparable quality. Finally, the collection of displacement parameters computed for the sample of conjugated molecules is used to model the effect of disorder and defects in the Raman response of a carbon material containing sp(2) islands. It is shown that the computed D-band frequency dispersion, with respect to excitation wavelength, reproduces closely the experimental data measured for sp(2) hybridized carbon materials. 相似文献
106.
Cinzia Cepek Andrea Goldoni Silvio Modesti Fabrizia Negri Giorgio Orlandi Francesco Zerbetto 《Chemical physics letters》1996,250(5-6):537-543
The electron energy loss (EEL) spectrum of the triplet exciton of the (111) surface of solid C60 has been recorded at 105 K. Its rich vibronic structure is suggestive of a high degree of molecular distortion that occurs upon excitation. The static Jahn-Teller distortion of the isolated molecule, along the adiabatic potential energy surface of T1, is determined by quantum-chemical calculations and the relaxed molecular structure is found to belong to the D5d point group symmetry. The vibrational force field is evaluated at the distorted structure and used to simulate the EEL intensities of phonon-assisted transitions in the strong coupling regime. 相似文献
107.
Fumagalli A Ulivieri P Costa M Crispu O Della Pergola R Fabrizi de Biani F Laschi F Zanello P Macchi P Sironi A 《Inorganic chemistry》2004,43(6):2125-2131
The redox aptitude of the dinitrido anion [Co10N2(CO)19]4- has been tested from both chemical and electrochemical points of view, together with its reactivity toward CO that induces disproportionation. In any case, through a remarkable overlapping of intermediate steps, the new anion [Co11N2(CO)21]3- (4) is eventually obtained. A detailed study of the pathways to 4 allowed the identification of three labile intermediates by their characteristic IR spectra as well as their electrochemical and paramagnetic properties. The unprecedented structure of trianion 4 has been studied in details in two different crystalline salts. 相似文献
108.
Giacomo Bondietti Gianfranco Suardi Renzo Ros Raymond Roulet Fabrizia Grepioni Dario Braga 《Helvetica chimica acta》1993,76(8):2913-2925
The synthesis of [Ir2Rh2(CO)12] ( 1 ) by the literature method gives a mixture 1 /[IrRh3(CO)12] which cannot be separated using chromatography. The reaction of [Ir(CO)4]? with 1 mol-equiv. of [Rh(CO)2(THF)2]+ in THF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unlike those of its derivative [Ir2Rh2(CO)5(μ2-CO)3(norbornadiene)2] which were analysed using X-ray diffraction. The ground-state geometry of 1 in solution has three edge-bridging CO's on the basal IrRh2 face of the metal tetrahedron. Time averaging of CO's takes place above 230 K. The CO site exchange of lowest activation energy is due to one synchronous change of basal face, as shown by 2D- and VT-13C-NMR. Substitution of CO by X? in 1 takes place at a Rh-atom giving [Ir2Rh2(CO)8(μ2-CO)3X]? (X = Br, I). Substitution by bidentate ligands gives [Ir2Rh2(CO)7(μ2-CO)3(η4-L)] (L = norbornadiene, cycloocta-1,5-diene) where the ligand L is chelating a Rh-atom of the basal IrRh2 face. Carbonyl substitution by tridentate ligands gives [Ir2Rh2(CO)6(μ2-CO)3(μ3-L)] (L = 1,3,5-trithiane, tripod) with L capping the triangular basal face of the metal tetrahedron. Carbonyl scrambling is also observed in these substituted derivatives of 1 and is mainly due to the rotation of three terminal CO's about a local C3 axis on the apical Ir-atom. 相似文献
109.
Braga D Polito M Giaffreda SL Grepioni F 《Dalton transactions (Cambridge, England : 2003)》2005,(16):2766-2773
The synthesis and structural characterization of the hexafluorophosphate salts of the substituted bis-amido molecular complexes [Co(III)(eta5-C5H4CONHC4H3N2)2]+ (1), [Co(III)(eta5-C5H4CONHCH2C5H4N)2]+ (2), [Co(III)(eta5-C5H4CON(C5H4N)2)2]+ (3), and of the amido-carboxyl complexes [Co(III)(eta5-C5H4CON(C5H4N)2)(eta5-C5H4COOH)]+ (4), and [Co(III)(eta5-C5H4CONHC2N3(C5H4N)2)(eta5-C5H4COOH)]+ (5) are reported. The pyridyl and pyrazine substituted amido ligands on the sandwich cores have been chosen because they allow both coordination to metal centres and participation in hydrogen bonding. The hydrogen bonding interactions established by the family of complexes in the solid state has been investigated. The utilization of complex 5 for the preparation of the complex of complexes[Cd(NO3)2{Co(III)(eta5-C5H4CONHC2N3(C5H4N)(C5H4NH))(eta5-C5H4COOH)}2]6+ (6) is reported as a first example of the potential of the substituted mono-and bis-amides as ligands. The isolation and structural characterization of the carbonyl chloride cation [Co(III)(eta5-C5H4COCl)2]+ (7) as its tetrachloro cobaltate anion salt is also described. 相似文献