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991.
992.
Knobloch B Mucha A Operschall BP Sigel H Jeżowska-Bojczuk M Kozłowski H Sigel RK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(19):5393-5403
With a view on protein–nucleic acid interactions in the presence of metal ions we studied the “simple” mixed‐ligand model systems containing histamine (Ha), the metal ions Ni2+, Cu2+, or Zn2+ (M2+), and the nucleotides adenosine 5′‐triphosphate (ATP4?) or uridine 5′‐triphosphate (UTP4?), which will both be referred to as nucleoside 5′‐triphosphate (NTP4?) . The stability constants of the ternary M(NTP)(Ha)2? complexes were determined in aqueous solution by potentiometric pH titrations. We show for both ternary‐complex types, M(ATP)(Ha)2? and M(UTP)(Ha)2?, that intramolecular stacking between the nucleobase and the imidazole residue occurs and that the stacking intensity is approximately the same for a given M2+ in both types of complexes: The formation degree of the intramolecular stacks is estimated to be 20 to 50 %. Consequently, in protein–nucleic acid interactions imidazole–nucleobase stacks may well be of relevance. Furthermore, the well‐known formation of macrochelates in binary M2+ complexes of purine nucleotides, that is, the phosphate‐coordinated M2+ interacts with N7, is confirmed for the M(ATP)2? complexes. It is concluded that upon formation of the mixed‐ligand complexes the M2+? N7 bond is broken and the energy needed for this process corresponds to the stability differences determined for the M(UTP)(Ha)2? and M(ATP)(Ha)2? complexes. It is, therefore, possible to calculate from these stability differences of the ternary complexes the formation degrees of the binary macrochelates: The closed forms amount to (65±10) %, (75±8) %, and (31±14) % for Ni(ATP)2?, Cu(ATP)2?, and Zn(ATP)2?, respectively, and these percentages agree excellently with previous results obtained by different methods, confirming thus the internal validity of the data and the arguments used in the evaluation processes. Based on the overall results it is suggested that M(ATP)2? species, when bound to an enzyme, may exist in a closed macrochelated form only, if no enzyme groups coordinate directly to the metal ion. 相似文献
993.
994.
Asymmetric aldol-Tishchenko reaction catalyzed by Yb-complexes with basic amino acid-derived ligands
Simple amino acid-derived esters have been identified as promising chiral sources for the ytterbium-catalyzed aldol-Tishchenko reaction of aromatic aldehydes with aliphatic ketones. The 1,3-anti-diols with three stereogenic centers were isolated in excellent yields, complete anti-diastereocontrol and enantioselectivities of up to 50% ee. 相似文献
995.
Urszula Domańska Kamil Paduszyński Zuzanna Żołek-Tryznowska 《The Journal of chemical thermodynamics》2011,43(2):167-171
(Liquid + liquid) phase equilibria (LLE) of binary mixtures containing hyperbranched polymer Boltorn® H2004 and n-alkanes (n-hexane, n-heptane, n-octane, and n-decane) were studied over the temperature range from about (260 up to 360) K. The polymer is partially miscible with n-alkanes and the solubility decreases with an increase of the chain length of the solvent. Corresponding LLE phase diagrams including spinodal and binodal (liquid + liquid) coexistence curves were calculated in terms of the statistical mechanics – based on the lattice-cluster theory, based only on the upper critical solution temperature, and the polymer chain architecture. The results show semi-qualitative agreement of predicted and experimental equilibrium compositions and temperatures. Boltorn® H2004 reveals complete miscibility in the liquid phase with alcohols (C1–C8), aromatic hydrocarbons (benzene, toluene, and thiophene), and ethers (methyl tetra-butyl ether, ethyl tetra-butyl ether, and tetrahydrofurane). 相似文献
996.
O. V. Reshetnyak E. P. Koval’chuk J. Błażejowski 《Russian Journal of Electrochemistry》2011,47(10):1111-1118
A short review is dedicated to the role of molecular oxygen and its active forms in generation of electrochemiluminescence
(ECL). It is shown at the example of such widely used luminogens as luminol, lucigenin, acridine esters, acridane, and indoles
that the role of O2 and its active forms, such as hydrogen peroxide, superoxide and hydroperoxide radicals in generation of ECL by organic compounds
is largely reduced to oxidation of the initial luminogen or intermediates of its partial electrochemical preoxidation/prereduction.
Such intermediates are most often particles in the doublet state (radical-cation, radical-anion, or free radicals) that form
unstable emitter precursors of peroxide (dioxetanone) type under interaction with O2, hydrogen peroxide, or other active oxygen forms. It is also shown that the product of secondary transformations of active
oxygen forms is singlet oxygen that may also be a radiation source as a result of cooperative transitions. 相似文献
997.
A simple and sensitive spectrophotometric method for the determination of L-ascorbic acid with leuco crystal violet is proposed. The determination is based on the oxidation of analyte by potassium
iodate. The colourless oxidation products were formed in the quantity equivalent to iodide ions. The iodide ions react with
the excess of iodate ions in acidic medium, to form free iodine which oxidized leuco crystal violet (LCV) to the liberated crystal violet (CV
+) dye, showing maximum absorption at 588 nm. The absorbance was measured at pH of 4.1–4.2 in 1 cm cuvettes. Beer’s law was
obeyed in the concentration range 0.5–4.0 μg/mL. The molar absorptivity of the coloured compound is 4.14 × 104 L/mol cm for L-ascorbic acid. The analytical parameters were optimized and the method was successfully applied to the determination of L-ascorbic acid in pharmaceuticals. The results were compared with those obtained by methods proposed in Polish Standard. 相似文献
998.
Maślanka A Krzek J Stolarczyk M Walczak M Głogowska A 《Journal of AOAC International》2011,94(6):1791-1799
Stability of clonazepam, diazepam, haloperidol, and doxepin was determined in acidic solutions. In addition, determination of the kinetic and thermodynamic properties of this stability was carried out. Reaction rate constants (k), half-life times (t(0.1) and t(0.5)), and activation energy (Ea) were estimated for the drugs, which differed in polarity expressed with log P values. It was observed that estimated Ea values increased from 42.13 to 125.03 kJ/mol with an increase of lipophilicity (log P) beginning from the most hydrophilic drug (clonazepam, 2.70 log P) to the most lipophilic drug (doxepin, 4.10 log P). All degradation products were studied using an HPLC/electrospray ionization-MS technique in the positive ionization mode. 相似文献
999.
Podstawka-Proniewicz E Piergies N Skołuba D Kafarski P Kim Y Proniewicz LM 《The journal of physical chemistry. A》2011,115(40):11067-11078
This study reports the Raman (FT-Raman) and absorption infrared (FT-IR) spectra, based on calculated wavenumbers and normal modes of vibrations, of the following compounds: L-Leu-D-NH-CH(Me)-PO(3)H(2) (LI), L-Leu-NH-C(Me)(2)-PO(3)H(2) (LII), L-Leu-D-NH-CH(Et)-PO(3)H(2) (LIII), L-Leu-L-NH-CH(Et)-PO(3)H(2) (LIV), L-Leu-L-NH-CH(EtOH)-PO(3)H(2) (LV), L-Leu-NH-C(Me)(Et)-PO(3)H(2) (LVI), L-Leu-L-NH-CH(PrA)-PO(3)H(2) (LVII), L-Leu-L-NH-CH(c-Pr)-PO(3)H(2) (LVIII), L-Leu-L-NH-CH(t-Bu)-PO(3)H(2) (LIX), L-Leu-L-NH-CH(BuA)-PO(3)H(2) (LX), L-Leu-L-NH-CH(c-Bu)-PO(3)H(2) (LXI), and L-Leu-L-NH-C(Adm)-PO(3)H(2) (LXII). The equilibrium geometries and vibrational wavenumbers were calculated using density functional theory (DFT) at the B3LYP, 6-311++G** level using Gaussian 03, Raint, GaussSum 0.8, and Gar2ped software. We briefly compare and analyze the experimental and calculated vibrational wavenumbers in the range 4000-400 cm(-1). In addition, the Raman wavenumbers are compared to those from the surface-enhanced Raman scattering (SERS) spectra for the phosphono analogues of l-leucine (l-Leu) adsorbed on a colloidal silver surface in an aqueous solution. The geometries of these molecules etched on the silver surface were deduced from observed changes in both the intensity and broadness of Raman bands in the spectra of the bound versus free species. For example, LVI appears to adsorb onto the colloidal silver particles mainly through the amine group and amide bond, which assists in the adsorption process, whereas LII shows strongly enhanced SERS bands due to the rocking, twisting, and stretching vibrations of the N(amid)C(sg)(Me)(2)P fragment, suggesting that this peptide's interaction with the silver surface occurs mainly via this fragment. On the other hand, the most dominant SERS bands of LIII and LIV due to the P═O bond stretches reflect P═O···Ag complex formation. 相似文献
1000.
The MP2/6-311++G(d,p) calculations were performed on several hydrogen-bonded systems. Different complexes were taken into account to analyze various types of hydrogen bonds, possessing different types of proton donors and proton acceptors as well as characterized by the broad range of the interaction energy. The Quantum Theory of Atoms in Molecules is applied. The results of the hybrid variational-perturbational approach are discussed. The unique properties of hydrogen bonds, where π-electrons act as the proton acceptor (X-H···π), are analyzed, and these interactions are compared with the other types of hydrogen bonds, mainly with C-H···Y interactions. It is shown that for X-H···π systems the ellipticity at the bond critical point of the proton···acceptor interaction is much greater than for the other types of hydrogen bonds. However, both X-H···π and C-H···Y interactions are characterized by the dominance of the dispersive energy. 相似文献