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61.
62.
Neaţu F Li Z Richards R Toullec PY Genêt JP Dumbuya K Gottfried JM Steinrück HP Pârvulescu VI Michelet V 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(30):9412-9418
A novel class of heterogeneous gold catalysts supported on zeolite beta-NH4+ was prepared by the deposition-precipitation method. This new class of catalyst showed interesting catalytic activities for the intramolecular cycloisomerization of gamma-acetylenic carboxylic acids leading to functionalized gamma-alkylidene gamma-butyrolactones. Analysis of the supported gold species with in situ X-ray photoelectron spectroscopy (in situ XPS) suggests that cationic Au (possibly AuIII) can play an important role in such reactions. The high discrepancy in catalyst stability in favor of the Au supported on the zeolite system over bulk Au2O3 is explained by 1) the size of the particles and 2) the reversibility of the redox deactivating process (AuIII-->AuI) in the presence of oxygen for the supported system. The efficiency of this system allowed reaction under mild heterogeneous conditions. The potential for catalyst recycling was also highlighted. 相似文献
63.
Eva Marie Freiberger Dr. Julien Steffen Natalie J. Waleska-Wellnhofer Anton Harrer Felix Hemauer Valentin Schwaab Prof. Dr. Andreas Görling Prof. Dr. Hans-Peter Steinrück Prof. Dr. Christian Papp 《Chemphyschem》2023,24(22):e202300510
This study addresses a fundamental question in surface science: the adsorption of halogens on metal surfaces. Using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (XPS), temperature-programmed XPS, low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we investigated the adsorption and thermal stability of bromine on Rh(111) in detail. The adsorption of elemental bromine on Rh(111) at 170 K was followed in situ by XPS in the Br 3d region, revealing two individual, coverage-dependent species, which we assign to fcc hollow- and bridge-bound atomic bromine. In addition, we find a significant shift in binding energy upon increasing coverage due to adsorbate-adsorbate interactions. Subsequent heating shows a high thermal stability of bromine on Rh(111) up to above 1000 K, indicating strong covalent bonding. To complement the XPS data, LEED was used to study the long-range order of bromine on Rh(111): we observe a (√3×√3)R30° structure for low coverages (≤0.33 ML) and a star-shaped compression structure for higher coverages (0.33–0.43 ML). Combining LEED and DFT calculations, we were able to visualize bromine adsorption on Rh(111) in real space for varying coverages. 相似文献
64.
We establish a general identity between the Mahler measures \(\mathrm {m}(Q_k(x,y))\) and \(\mathrm {m}(P_k(x,y))\) of two polynomial families, where \(Q_k(x,y)=0\) and \(P_k(x,y)=0\) are generically hyperelliptic and elliptic curves, respectively. 相似文献
65.
Watari M Galbraith J Lang HP Sousa M Hegner M Gerber C Horton MA McKendry RA 《Journal of the American Chemical Society》2007,129(3):601-609
Free-standing cantilevers, which directly translate specific biochemical reactions into micromechanical motion, have recently attracted much attention as label-free biosensors and micro/nano robotic devices. To exploit this mechanochemical sensing technology, it is essential to develop a fundamental understanding of the origins of surface stress. Here we report a detailed study into the molecular basis of stress generation in aqueous environments focusing on the pH titration of model mercaptohexadecanoic acid self-assembled monolayers (SAMs), using in situ reference cantilevers coated with nonionizable hexadecanethiol SAMs. Semiautomated data analysis and a statistical model were developed to quantify cyclic deprotonation/protonation reactions on multiple arrays. In-plane force titrations were found to have the sensitivity to detect ionic hydrogen bond formation between protonated and nonprotonated carboxylic acid groups in the proximity of the surface pK1/2, which generated a mean tensile differential surface stress of +1.2 +/- 0.3 mN/m at pH 6.0, corresponding to 1 pN attractive force between two adjacent MHA molecules. Conversely, the magnitude of compressive differential surface stress was found to increase progressively with pH >/= 7.0, reaching a maximum of -14.5 +/- 0.5 mN/m at pH 9.0, attributed to enhanced electrostatic repulsion between deprotonated carboxylic acid groups. However, striking differences were observed in the micromechanical responses to different ionic strength and ion species present in the aqueous environment, highlighting the critical role of counter- and co-ions on surface stress. Our findings provide fundamental insights into the molecular mechanisms of in-plane mechanochemistry, which may be exploited for biosensing and nanoactuation applications. 相似文献
66.
Investigations on the activation of cellulose by mixed solutions of caustic soda and urea are reported. The structural effects of those solutions on various dissolving pulps are studied by 13C-CP/MAS-NMR spectroscopy. In a series of steeping lyes, the concentration of NaOH was varied in a range from 0% to 8% and the urea-concentration in a range from 15% to 40% at ambient temperature and −25 °C. Using solely the single NaOH or urea solutions in the concentration ranges given above, no or only minor structural changes were found. In contrast to that, the cellulose I structure was partially or completely destroyed by using the bicomponent solution with urea added to caustic soda. The structural effect of the bicomponent solutions is comparable with the effect of solely caustic soda solutions of approx. 10% to 18% NaOH. However, the 13C-CP/MAS-NMR-spectra from the bicomponent pretreated samples indicate a structure different from the usual ordered structures of sodium cellulose I or II, namely a special urea-NaOH-cellulose complex. The results show that for cellulose activation the NaOH concentration of the caustic soda can be remarkably reduced by adding urea. The improved activating effect of an optimized caustic soda solution with added urea was proved to be useful for the synthesis of cellulose carbamate. 相似文献
67.
Molecular beam cooled HCl was state selected by two-photon excitation of the V (1) summation operator(0(+)) [v=9,11-13,15], E (1) summation operator(0(+)) [v=0], and g (3) summation operator(-)(0(+)) [v=0] states through either the Q(0) or Q(1) lines of the respective (1,3) summation operator(0(+))<--<--X (1) summation operator(0(+)) transition. Similarly, HBr was excited to the V (1) summation operator(0(+)) [v=m+3, m+5-m+8], E (1) summation operator(0(+)) [v=0], and H (1) summation operator(0(+)) [v=0] states through the Q(0) or Q(1) lines. Following absorption of a third photon, protons were formed by three different mechanisms and detected using velocity map imaging. (1) H(*)(n=2) was formed in coincidence with (2)P(i) halogen atoms and subsequently ionized. For HCl, photodissociation into H(*)(n=2)+Cl((2)P(12)) was dominant over the formation of Cl((2)P(32)) and was attributed to parallel excitation of the repulsive [(2) (2)Pi4llambda] superexcited (Omega=0) states. For HBr, the Br((2)P(32))Br((2)P(12)) ratio decreases with increasing excitation energy. This indicates that both the [(3) (2)Pi(12)5llambda] and the [B (2) summation operator5llambda] superexcited (Omega=0) states contribute to the formation of H(*)(n=2). (2) For selected intermediate states HCl was found to dissociate into the H(+)+Cl(-) ion pair with over 20% relative yield. A mechanism is proposed by which a bound [A (2) summation operatornlsigma] (1) summation operator(0(+)) superexcited state acts as a gateway state to dissociation into the ion pair. (3) For all intermediate states, protons were formed by dissociation of HX(+)[v(+)] following a parallel, DeltaOmega=0, excitation. The quantum yield for the dissociation process was obtained using previously reported photoionization efficiency data and was found to peak at v(+)=6-7 for HCl and v(+)=12 for HBr. This is consistent with excitation of the repulsive A(2) summation operator(12) and (2) (2)Pi states of HCl(+), and the (3) (2)Pi state of HBr(+). Rotational alignment of the Omega=0(+) intermediate states is evident from the angular distribution of the excited H(*)(n=2) photofragments. This effect has been observed previously and was used here to verify the reliability of the measured spatial anisotropy parameters. 相似文献
68.
The title compounds (and, in part, their seleno analogs) result from the oxidation of an ylidylphosphine and also from the addition of ylides to a perthiophosphonic anhydride. They can be deprotonated and alkylated to give anions or cations, respectively. The reaction with phenacyl bromide opens a way to 1,3,2-oxathiaphosphole sulfides. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:433–437, 1998 相似文献
69.
Ralf Hilger Hans-Peter Merckens Arne Lüchow Heinz Kleindienst 《International journal of quantum chemistry》1998,66(1):25-30
Extensive Hylleraas–CI calculations for the lowest Po states of 4He were performed. The dependence of the variational energy values Eκ on the mass parameter κ given by κ=m/m is discussed. Furthermore, lower bounds to Eκ were calculated using variance minimization. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 25–30, 1998 相似文献
70.
Florian Breitsameter Hans-Peter Schrdel Alfred Schmidpeter Heinrich Nth Susana Rojas-Lima 《无机化学与普通化学杂志》1999,625(8):1293-1300
Ylide Adducts of Pentele Trichlorides Ylides Ph3PCR2 form 1 : 1 adducts with PCl3, AsCl3 und SbCl3. The addition of HCl to dichlorophosphanyl- und dichloroarsanyl ylides also leads to such adducts. In solution the PCl3 adducts 2 are dissociated into ions. For the AsCl3 adducts 3 b and 3 e of triphenylphosphonium ethylide and isopropylide X-ray analyses show distinctly different degrees of transition from a zwitterionic ψ-tbp to a cationic ψ-tetrahedral structure. The SbCl3 adduct 4 b of the triphenylphosphonium ethylide on the other hand forms a rather asymmetric dimer and approaches a square-pyramidal geometry at the antimony atom. The 2 : 1 ylide adducts 11 with PCl3 form in solution the dications (Ph3PCR2)2PCl2+. The 2 : 3 adduct 15 of triphenylphosphonium methylide to AsCl3 also has an ionic structure in the crystal. There are cations (Ph3PCH2)2AsCl2+, which pairwise join with two AsCl3 molecules to form doubly charged supracations in the presence of As2Cl82– as counterions. 相似文献