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61.
Bailey PJ Dick CM Fabre S Parsons S Yellowlees LJ 《Dalton transactions (Cambridge, England : 2003)》2006,(13):1602-1610
Treatment of dimethylmagnesium with the alpha-diimine ligands Ar'N=C(R)C(R)=NAr' [R = naphth-1,8-diyl (1), H (2), CH3 (3); Ar' = 2,6-diisopropylphenyl] in diethyl ether provides the neutral methyl-bridged dimeric complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET) to the coordinated diimine and elimination of a methyl radical. These biradical species have been characterised by EPR spectroscopy and, for the ligand , X-ray crystallography. In the presence of THF the reaction of ligand proceeds to the diamagnetic [(ene-1,2-diamide)Mg(THF)3] complex in which the diimine ligand has been doubly reduced to an ene-diamide by two successive SET processes. Comparison of the structural data for the free ligand with that obtained for the alpha-diimine radical anion and ene-diamide complexes shows the expected increases in C-N, and decreases in C-C, bond lengths within the N-C-C-N unit consistent with the progressive reduction of the ligand. In the case of ligand , reaction at low temperature provides the complex [Mg(mu2-Me){Ar'NC(Me)2C(Me)NAr'}]2 in which methyl transfer to a ligand imine carbon atom has occurred. This species has also been structurally characterised. This contrasts with the formation of the radical species at room temperature, and indicates the involvement of an intermediate in which the radical products of the SET process are held in close proximity by the solvent cage. Two competing processes of methyl radical escape and methyl transfer to the ligand account for the formation of the observed products at different temperatures. 相似文献
62.
Knowledge of the dynamical changes in molecular configurations in various amino acid structures over a wide range of time scales is important since such changes may influence the structural transformations and the diverse biological functionalities of proteins. Using the temperature dependence of the rotating-frame NMR spin-lattice relaxation times T(1rho) of protons as a probe, we have investigated the low-frequency (approximately 60-100 kHz) dynamics in the crystal structures of L-, D-, and DL- alanine (C(12)H(28)O(8)N(4)) polymorphs. The proton relaxation times T(1rho) were obtained from (13)C <-- (1)H and (15)N <-- (1)H cross-polarization magic-angle-spinning NMR experiments over a temperature range of 192-342 K. The data reveal that the time scales of these low-frequency dynamical processes are distinctly different from the localized, high-frequency rotational motion of methyl and amine groups. The strongly asymmetric T(1rho) versus temperature curves and the subtle dynamical differences between the DL-alanine and the L- and d-enantiomorphs indicate that these low-frequency processes are cooperative in nature and are sensitive to molecular packing. 相似文献
63.
The influence of roof shape on sound propagation in a densely build-up city centre is evaluated. Road traffic noise propagation from a street canyon to an adjacent, non-directly exposed building façade is numerically studied by the finite-difference time-domain method. A large number of common and less common roof shapes were analyzed in an idealized city canyon configuration. Roof shape can be responsible for differences in road traffic noise shielding exceeding 10 dBA, averaged over the shielded façade. Therefore, roof shape can be considered as an important means to limit sound pressure levels at a quiet side; various researchers indicated that the presence of silent zones and façades may limit city noise annoyance. With increasing vehicle speed, the choice of roof shape becomes more important. The roof top height was shown to be a bad predictor of shielding efficiency in the equal-building-volume approach followed in this study. 相似文献
64.
Let F be the class of all 1-periodic real functions with absolutely convergent Fourier series expansion and let (xn)n ≧ 0 be the van der Corput sequence. In this paper results on the boundedness of
are given. We give a criterion on the convergence rate of the Fourier coefficients of f such that the above sum is bounded independently of N. Further we show that our result is also best possible.
The first author is supported by the Australian Research Council under its Center of Excellence Program. The second author
is supported by the Austrian Research Foundation (FWF), Project S9609 that is part of the Austrian National Research Network
“Analytic Combinatorics and Probabilistic Number Theory”.
Received: 14 December 2005 相似文献
65.
66.
Farkhanda Shaheen Amin Badshah Christine Gieck Dick de Vos 《Journal of organometallic chemistry》2008,693(6):1117-1126
The new four-coordinated mononuclear palladium(II) complexes 1-9 with chelating heterocyclic thiolates and tertiary phosphines with general formula [Pd(L)nCl(R′R2P)] (L = Pym2SH (pyrimidine-2-thiolate), Pur6SH (purine-6-thiolate), Py2SH (pyridine-2-thiolate), R3P = PPh3, P(o-tolyl)3, PPh2Cl), n = 1, 2) have been synthesized by the direct reaction of [PdCl2(R′R2P)2] with polyfunctional heterocyclic thiolates which display a wide variety of coordinations. These compounds were characterized by elemental analysis, FT-IR and multinuclear (1H, 13C and 31P) NMR. The X-ray diffraction study of non-ionic compound 5 showed that the thiolate acts as unidentate and that the chelating (-N,S) ligand adopts a slightly distorted square planar geometry around the palladium atom. In vitro the anti-inflammatory inhibition of compounds 1-9 was 10-15% greater than that of the standard drug Declofenac. Compounds 1 and 4 showed mostly a moderate to low cytotoxicity against seven human tumor cell lines whereas compound 3 was somewhat more active. 相似文献
67.
Alice Di Girolamo Michele Pedrotti Alex Koot Francel Verstappen Adèle van Houwelingen Celina Vossen Harro Bouwmeester Dick de Ridder Jules Beekwilder 《Journal of mass spectrometry : JMS》2023,58(6):e4951
In this work, we introduce the application of proton transfer reaction mass spectrometry (PTR-MS) for the selection of improved terpene synthase mutants. In comparison with gas chromatography mass spectrometry (GC-MS)-based methods, PTR-MS could offer advantages by reduction of sample preparation steps and analysis time. The method we propose here allows for minimal sample preparation and analysis time and provides a promising platform for the high throughput screening (HTS) of large enzyme mutant libraries. To investigate the feasibility of a PTR-MS-based screening method, we employed a small library of Callitropsis nootkatensis valencene synthase (CnVS) mutants. Bacterial cultures expressing enzyme mutants were subjected to different growth formats, and headspace terpenes concentrations measured by PTR-Qi-ToF-MS were compared with GC-MS, to rank the activity of the enzyme mutants. For all cultivation formats, including 96 deep well plates, PTR-Qi-ToF-MS resulted in the same ranking of the enzyme variants, compared with the canonical format using 100 mL flasks and GC-MS analysis. This study provides a first basis for the application of rapid PTR-Qi-ToF-MS detection, in combination with multi-well formats, in HTS screening methods for the selection of highly productive terpene synthases. 相似文献
68.
Haak RM Berthiol F Jerphagnon T Gayet AJ Tarabiono C Postema CP Ritleng V Pfeffer M Janssen DB Minnaard AJ Feringa BL de Vries JG 《Journal of the American Chemical Society》2008,130(41):13508-13509
The direct chemo-enzymatic DKR of racemic beta-haloalcohols is reported, yielding the corresponding optically active epoxides in a single step. The mutant haloalcohol dehalogenase HheC Cys153Ser Trp249Phe is used for the asymmetric ring closure, whereas racemization of the remaining enantiomer of the haloalcohol is achieved using the new iridacycle 3, one of the most effective racemization catalysts to date for beta-haloalcohols. 相似文献
69.
Inductively coupled plasma mass spectrometry (ICP-MS) is a suitable tool for multi-element analysis at low concentration levels. Rare earth element (REE) determinations in standard reference materials and small volumes of molten ice core samples from Antarctica have been performed with an ICP-time of flight-MS (ICP-TOF-MS) system. Recovery rates for REE in e.g. SPS-SW1 amounted to ∼103%, and the relative standard deviations were 3.4% for replicate analysis at REE concentrations in the lower ng L−1 range. Analyses of REE concentrations in Antarctic ice core samples showed that the ICP-TOF-MS technique meets the demands of restricted sample mass. The data obtained are in good agreement with ICP-Quadrupole-MS (ICP-Q-MS) and ICP-Sector Field-MS (ICP-SF-MS) results. The ICP-TOF-MS system determines accurately and precisely REE concentrations exceeding 5 ng L−1 while between 0.5 and 5 ng L−1 accuracy and precision are element dependent. 相似文献
70.