全文获取类型
收费全文 | 286篇 |
免费 | 8篇 |
专业分类
化学 | 247篇 |
晶体学 | 1篇 |
力学 | 1篇 |
数学 | 5篇 |
物理学 | 40篇 |
出版年
2023年 | 3篇 |
2022年 | 1篇 |
2021年 | 6篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2016年 | 5篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 12篇 |
2012年 | 24篇 |
2011年 | 15篇 |
2010年 | 12篇 |
2009年 | 5篇 |
2008年 | 17篇 |
2007年 | 23篇 |
2006年 | 25篇 |
2005年 | 17篇 |
2004年 | 18篇 |
2003年 | 8篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 7篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有294条查询结果,搜索用时 31 毫秒
41.
Single crystals of Ba2NaNb5O15 (BNN) singly doped with Sm3+, Eu3+, Tb3+ or Dy3+ have been grown by means of the flux growth method. Their visible emission and excitation spectra and the decay profiles of the luminescence have been measured at room temperature. All spectral features are significantly inhomogeneously broadened in consequence of the structural disorder of the host and of the doping mechanisms. The analysis of the observed spectra allows formulating an hypothesis about the site occupancy of the active ions in the BNN lattice. 相似文献
42.
Site selectivity in the reactions of the hexanuclear platinum cluster [Pt6(mu-PtBu2)4(CO)6][CF3SO3]2
Bonaccorsi C de Biani FF Leoni P Marchetti F Marchetti L Zanello P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(3):847-856
The previously reported hexanuclear cluster [Pt(6)(mu-PtBu(2))(4)(CO)(6)](2+)[Y](2) (1-Y(2): Y=CF(3)SO(3) (-)) contains a central Pt(4) tetrahedron bridged at each of the opposite edges by another platinum atom; in turn, four phosphido ligands bridge the four Pt-Pt bonds not involved in the tetrahedron, and, finally, one carbonyl ligand is terminally bonded to each metal centre. Interestingly, the two outer carbonyls are more easily substituted or attacked by nucleophiles than the inner four, which are bonded to the tetrahedron vertices. In fact, the reaction of 1-Y(2) with 1 equiv of [nBu(4)N]Cl or with an excess of halide salts gives the monochloride [Pt(6)(mu-PtBu(2))(4)(CO)(5)Cl](+)[Y], 2-Y, or the neutral dihalide derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)X(2)] (3: X=Cl; 4: X=Br; 5: X=I). Moreover, the useful unsymmetrically substituted [Pt(6)(mu-PtBu(2))(4)(CO)(4)ICl] (6) was obtained by reacting equimolar amounts of 2 and [nBu(4)N]I, and the dicationic derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)L(2)](2+)[Y](2) (7-Y(2): L=(13)CO; 8-Y(2): L=CNtBu; 9-Y(2): L=PMe(3)) were obtained by reaction of an excess of the ligand L with 1-Y(2). Weaker nitrogen ligands were introduced by dissolving the dichloride 3 in acetonitrile or pyridyne in the presence of TlPF(6) to afford [Pt(6)(mu-PtBu(2))(4) (CO)(4)L(2)](2+)[Z](2) (Z=PF(6) (-), 10-Z(2): L=MeCN; 11-Z(2): L=Py). The "apical" carbonyls in 1-Y(2) are also prone to nucleophilic addition (Nu(-): H(-), MeO(-)) affording the acyl derivatives [Pt(6)(mu-PtBu(2))(4)(CO)(4)(CONu)(2)] (12: Nu=H; 13: Nu=OMe). Complex 12 is slowly converted into the dihydride [Pt(6)(mu-PtBu(2))(4)(CO)(4)H(2)] (14), which was more cleanly prepared by reacting 3 with NaBH(4). In a unique case we observed a reaction involving also the inner carbonyls of complex 1, that is, in the reaction with a large excess of the isocyanides R-NC, which form the corresponding persubstituted derivatives [Pt(6)(mu-tPBu(2))(4)(CN-R)(6)](2+)[Y](2), (15-Y(2): R=tBu; 16-Y(2) (2-): R=-C(6)H(4)-4-C triple bond CH). All complexes were characterized by microanalysis, IR and multinuclear NMR spectroscopy. The crystal and molecular structures of complexes 3, 5, 6 and 9-Y(2) are also reported. From the redox viewpoint, all complexes display two reversible one-electron reduction steps, the location of which depends both upon the electronic effects of the substituents, and the overall charge of the original complex. 相似文献
43.
Braga D Palladino G Polito M Rubini K Grepioni F Chierotti MR Gobetto R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(32):10149-10159
Three crystal forms of the co-crystal 4,4'-bipy/pimelic acid (bipy: bipyridine), [NH(4)C(5)-C(5)H(4)N][HOOC(CH(2))(5)COOH], have been prepared and their relationship investigated by single-crystal X-ray diffraction, variable-temperature X-ray powder diffraction, differential scanning calorimetry and solid-state NMR spectroscopy. Both X-ray and NMR spectroscopic results indicate that no proton transfer takes place, that is, the three crystal forms are true co-crystals of neutral molecules. Forms I and II both convert into Form III at high temperature, Forms II and III being the thermodynamically stable forms at room and high temperature, respectively. 相似文献
44.
De Fazio D Aquilanti V Cavalli S Aguilar A Lucas JM 《The Journal of chemical physics》2008,129(6):064303
In this paper, we present the results of a theoretical investigation on the dynamics of the title reaction at collision energies below 1.2 kcal/mol using rigorous quantum reactive scattering calculations. Vibrationally resolved integral and differential cross sections, as well as product rotational distributions, have been calculated using two electronically adiabatic potential energy surfaces, developed by us on the basis of semiempirical modifications of the entrance channel. In particular, we focus our attention on the role of the exothermicity and of the exit channel region of the interaction on the experimental observables. From the comparison between the theoretical results, insight about the main mechanisms governing the reaction is extracted, especially regarding the bimodal structure of the HF(v = 2) nascent rotational state distributions. A good overall agreement with molecular beam scattering experiments has been obtained. 相似文献
45.
Zhiqian?Yang Monica?Argenziano Paolina?Salamone Elisa?Pirro Andrea?E.?Sprio Federica?Di Scipio Maria?E.?Carere Elena?Quaglino Federica?Cavallo Roberta?Cavalli Giovanni?N.?BertaEmail author 《Journal of inclusion phenomena and macrocyclic chemistry》2016,86(3-4):263-271
Trans-Resveratrol (RV) is a natural polyphenol characterized by interesting pleiotropic potentials and health benefits, but its administration is hampered by a unsatisfactory pharmacokinetics. Various approaches have been identified to circumvent it: among them, 2-hydroxypropyl-β-cyclodextrins (HPβCD) are valuable strategy. Here, we compare the employment of HPβCD based formulation with a resveratrol nanosupension (obtained by diluting a RV ethanol solution with PBS, added of 0.05 % hydroxyethylcellulose) to improve RV bioavailability after oral administration to mice. The inclusion of RV in HPβCD was confirmed by differential scanning calorimetry, Fourier transformed infrared spectroscopy, and phase solubility study. The two formulations were orally administered to BALB-c mice. RV concentrations in plasma and tissues were detected at different time (0–120 min) by HPLC method. HPβCD complexation mediate a approximately fourfold increment in plasma RV Cmax and approximately twofold augment of RV AUC0-120 in comparison with RV nanosuspension. Similar increased concentrations were observed in heart, liver, kidney and gut. In particular, HPβCD mediated a 5.5-folds increase of resveratrol concentration in the intestine, in comparison to the nanosuspension. In conclusion, based on our results, HPβCD complexation is a promising approach to increase the oral bioavailability of RV. Moreover, the achievement of high concentrations in gut suggested a potential employment of oral RV-HPβCD as anti-inflammatory/chemopreventive agent in this tissue. 相似文献
46.
The acoustic structure of diplophonia was investigated spectrographically and in terms of perturbation measures, while the perception of diplophonia among other voice qualities was confirmed auditorily by trained listeners. Recordings of pathological voices were presented to listeners for systematic evaluation, and narrowband spectrography was used to quantify the subharmonics within each sample. Subharmonics strongly correlated with the perception of diplophonia, but occurrence was mostly intermittent, and structure was highly variable between samples. Uniquely among voice qualities identified perceptually, diplophonia was found to correlate positively with the number of subharmonics (irrespective of percentage of occurrence) and with perturbation parameters measured separately. Exceptions to these group results indicated that diplophonia was perceived sometimes in the absence of subharmonic structure, and subharmonic structure was observed without a commensurate perception of diplophonia. In light of these data, a less deterministic relation between diplophonia and subharmonic structure is proposed. 相似文献
47.
E. Cavalli E. Zannoni A. Belletti V. Carozzo A. Toncelli M. Tonelli M. Bettinelli 《Applied physics. B, Lasers and optics》1999,68(4):677-681
3 Sc2Ge3O12 (CaSGG) single crystals with the garnet structure have been grown by means of the flux growth technique. The doping with
Nd3+ and Mg2+ (as charge compensator) yields samples suitable for optical spectroscopy experiments. The absorption and emission properties
have been measured at temperatures ranging from 10 to 298 K. The emission spectra give evidence of the presence of non-equivalent
Nd3+ sites. The decay time of the 1.06-μm emission band has been measured as a function of temperature and incident power. The
intensities of the 298 K absorption transitions have been analyzed by means of the Judd–Ofelt theory. The radiative lifetimes,
the branching ratios (β), and the spontaneous emission probabilities have been evaluated for the 4F3/2 excited state using the calculated intensity parameters. The stimulated-emission cross sections and the branching ratios
have been estimated from the experimental data for the most important laser transitions indicating that this crystal can be
considered an interesting material for solid-state laser applications.
Received: 2 June 1998 / Revised version: 28 October 1998 / Published online: 24 February 1999 相似文献
48.
49.
Dario Braga Lucia Maini Fabrizia Grepioni 《Angewandte Chemie (International ed. in English)》1998,37(16):2240-2242
Cooperative strong negatively charged and neutral O–H⋅⋅⋅O as well as weak charge-assisted C–Hδ+⋅⋅⋅Oδ− hydrogen-bonding interactions have been utilized for the crystal engineering of organometallic FeII/CoIII and FeII/CrI sandwich complexes. A section of the structure of [(η6-C6H6)2Cr]+{[(η5-C5H4COOH)(η5-C5H4COO)Fe][(η5-C5H4COOH)2Fe]0.5}− is shown in the picture. 相似文献
50.
Jacopo Sgrignani Claudia Bonaccini Giovanni Grazioso Matteo Chioccioli Andrea Cavalli Paola Gratteri 《Journal of computational chemistry》2009,30(15):2443-2454
A combined quantum mechanical (QM)‐polarized docking and molecular dynamics approach to study the binding mode and to predict the binding affinity of ligands acting at the α4β2‐nAChR is presented. The results obtained in this study indicate that the quantum mechanical/molecular mechanics docking protocol well describes the charge‐driven interactions occurring in the binding of nicotinic agonists, and it is able to represent the polarization effects on the ligand exerted by the surrounding atoms of the receptor at the binding site. This makes it possible to properly score agonists of α4β2‐nAChR and to reproduce the experimental binding affinity data with good accuracy, within a mean error of 2.2 kcal/mol. Moreover, applying the QM‐polarized docking to an ensemble of nAChR conformations obtained from MD simulations enabled us to accurately capture nAChR‐ligand induced‐fit effects. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 相似文献