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31.
Teresa Borowiak Maciej Kubicki Grzegorz Dutkiewicz Marek Pietraszkiewicz Oksana Pietraszkiewicz 《Journal of inclusion phenomena and macrocyclic chemistry》2005,51(3-4):181-189
Two newly identified supramolecular structures arise from self-assembly of the macrocyclic 1,5,9,18,22,26- hexaaza[11.11]-p-cyclophane salts with o-nitrophenol (C28H50N6)4+·4(C6H4NO2O)− (1) and with HCl (C28H52N6)6+·6Cl-·4H2O (2). In both cases two-dimensional supramolecular sheets are formed. 相似文献
32.
Roland Boese Dieter Bläser Michael Nussbaumer Tadeusz Marek Krygowski 《Structural chemistry》1992,3(5):363-368
The crystal and molecular structure of nitrobenzene was determined at 103 K by X-ray diffraction, yieldingR=0.034 and a highly precise geometry of the molecule (esds of bond lengths 0.001 Å and bond angles 0.1°). The bond angles in the ring agree very well with additive scheme based on angular substituent parameters. X-X electron density maps support the view that the C-N bond does not exhibit-character, and, hence, the nitro group interacts with the ring mostly by inductive effects. 相似文献
33.
Marek Trojanowicz Joanna Szpunar-Łobińska Zdzislaw Michalski 《Mikrochimica acta》1991,103(3-4):159-169
An original computer-controlled solid-state photometer has been developed and its potential for simultaneous multicomponent flow-injection analysis of binary and ternary mixtures is demonstrated. The device is simple in construction. Its principle of operation is based on rapid sequential measurements of the absorbance of the complexes formed by the analytes with chromogenic reagents at the wavelengths corresponding to the emission maxima of three light emitting diodes (563, 580 and 638 nm). 相似文献
34.
Richardson NA Wesolowski SS Schaefer HF 《Journal of the American Chemical Society》2002,124(34):10163-10170
The adiabatic electron affinity (AEA) for the Watson-Crick guanine-cytosine (GC) DNA base pair is predicted using a range of density functional methods with double- and triple-zeta plus polarization plus diffuse (DZP++ and TZ2P++) basis sets in an effort to bracket the true electron affinity. The methods used have been calibrated against a comprehensive tabulation of experimental electron affinities (Chem.Rev. 2002, 102, 231). Optimized structures for GC and the GC anion are compared to the neutral and anionic forms of the individual bases as well as Rich's 1976 X-ray structure for sodium guanylyl-3',5'-cytidine nonahydrate, GpC.9H(2)O. Structural distortions and natural population (NPA) charge distributions of the GC anion indicate that the unpaired electron is localized primarily on the cytosine moiety. Unlike treatments using second-order perturbation theory (MP2), density functional theory consistently predicts a substantial positive adiabatic electron affinity for the GC pair (e.g., TZ2P++/B3LYP: +0.48 eV). The stabilization of C(-) via three hydrogen bonds to guanine is sufficient to facilitate adiabatic binding of an electron to GC and is also consistent with the positive experimental electron affinities obtained by photoelectron spectroscopy of cytosine anions incrementally microsolvated with water molecules. The pairing (dissociation) energy for GC(-) (35.6 kcal/mol) is determined with inclusion of electron correlation and shows the anion to have greater thermodynamic stability; the pairing energy for neutral GC (TZ2P++/B3LYP 23.9 kcal/mol) compares favorably to previous MP2/6-31G (23.4 kcal/mol) results and a debated experiment (21.0 kcal/mol). 相似文献
35.
Lucjan Chmielarz Piotr Kustrowski Marek Michalik Barbara Dudek Magdalena Czajka Roman Dziembaj 《Reaction Kinetics and Catalysis Letters》2007,91(2):369-378
Natural phlogophite, pre-treated with acids and intercalated with alumina pillars, was used as catalytic support. Transition
metals (Fe, Cu) were deposited on the surface of the modified clay materials by an ion-exchange method. The obtained samples
were characterized with respect to structure (XRD), texture (BET), composition (EPMA) and chemical nature of the deposited
transition metals species (UV-vis-DRS). The phlogophite based materials have been found to be active and selective catalysts
of the DeNOx process. The Fe-containing samples were catalytically active at lower temperatures than the clays modified with
copper. A competitive ammonia oxidation by oxygen decreased the effectiveness of the DeNOx process in the high temperature
range. 相似文献
36.
A simple intra- and inter-molecular potential for water molecules engaged in hydrogen bonding has been used in a one-dimensional approximation for a theoretical interpretation of the experimental correlation OH/OD versus OH in solid hydrates. The variation in the equilibrium OH(D) distance has also been considered. 相似文献
37.
Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state. 相似文献
38.
Trávníček Zdeněk Machala Viktor Szüčová Lucie Maloň Michal Marek Jaromír 《Transition Metal Chemistry》2004,29(4):352-357
A series of new [NiX(S2P{O-c-Hex}2)(PPh3)](X = Cl–, Br–, I– and NCS–)(1)–(4) and [Ni(NCS)(S2P{OR}2)(PPh3)][R =n-Pr (5), i-Pr (6)] complexes has been synthesized and characterized by elemental analyses, f.i.r., i.r., u.v.–vis., 1H-, 13C{1H}- and 31P{1H}-n.m.r. spectra, magnetochemical and conductivity measurements. A single crystal X-ray analysis of [Ni(NCS)(S2P{O-n-Pr}2)(PPh3)](5) reveals the molecular structure of the complex and confirms a square-planar geometry around the central atom of nickel with the NCS anion coordinated via the nitrogen atom. 相似文献
39.
Neugebauer J Louwerse MJ Belanzoni P Wesolowski TA Baerends EJ 《The Journal of chemical physics》2005,123(11):114101
In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle. 相似文献
40.
Wagner-Czauderna Elżbieta Boroń-Cegiełkowska Agata Orłowska Elżbieta Kalinowski Marek K. 《Transition Metal Chemistry》2004,29(1):61-65
The visible absorption spectra of CoCl2 in binary mixtures of acetone, dmf, dma and dmso with H2O show the existence of configuration equilibria between tetrahedral (T) and octahedral (O) species. Linear correlations between the values of log K (K = [O]/[T]) and the mole fraction of H2O,
w, have been observed in all the systems under study; their slopes decrease linearly with an increase of the donor number, DN, of the organic component of the mixture. The formation of octahedral species is enthalpy favoured but entropy disfavoured. It was found that both H and S values change linearly with
w and the slopes of these relationships depend clearly on the DN values. 相似文献