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461.
Näslund LA Lüning J Ufuktepe Y Ogasawara H Wernet P Bergmann U Pettersson LG Nilsson A 《The journal of physical chemistry. B》2005,109(28):13835-13839
Recent studies, based on X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS), have shown that the hydrogen bond network in liquid water consists mainly of water molecules with only two strong hydrogen bonds. Since this result is controversial, it is important to demonstrate the reliability of the experimental data, which is the purpose of this paper. Here we compare X-ray absorption spectra of liquid water recorded with five very different techniques sensitive to the local environment of the absorbing molecule. Overall, the spectra obtained with photon detection show a very close similarity and even the observable minor differences can be understood. The comparison demonstrates that XAS and XRS can indeed be applied reliably to study the local bonding of the water molecule and thus to reveal the hydrogen bond situation in bulk water. 相似文献
462.
Enantioselective synthesis of (?)-mesembrine(1) has been achieved using (D)-mannitol with the intention of developing the enantioselective synthesis of the Amaryllidaceae alkaloids. 相似文献
463.
Makoto Ishida Haruhiko Tamaoki Hideki Masuda Hirofumi Okabayashi Charmian J. O'Connor 《Colloid and polymer science》2002,280(12):1157-1161
N-Butyloxycarbonyl(BOC)-L-glutamic acid oligomeric benzyl esters with exact residue numbers (BOCNpZ, Np=4, 6 and 8) have been synthesized by a stepwise procedure in a liquid phase. The SAXS intensity spectra of the BOCNpZ-benzene systems have been analyzed on the basis of the rod-like aggregate model, in which the #-sheet monomers are one-dimensionally stacked antiparallel to each other. The extracted parameters (the number-averaged aggregation number, the monomer-monomer bond energies corresponding to hydrogen bonding energies, and the number-averaged molecular weights) for these aggregates have been compared with those for the aggregates formed by N-acetyl-L-glutamic acid oligomeric benzyl esters (ANpZ, Np=4, 6 and 8) in benzene (PCCP, 2001, 3, 3140-3149). The results indicate that it is more difficult to form aggregates in the BOCNpZ systems than in the ANpZ systems. This difference is due to the bulky BOC group, which hinders the formation of aggregates. 相似文献
464.
Shoji Ando Haruhiko Aoyagi Michinori Waki Tetsuo Kato Nobuo Izumiya Kouji Okamoto Michio Kondo 《Tetrahedron letters》1982,23(21):2195-2198
Gramicidin S (GS) analogs, (D-Ser(Bzl)4,4′]-GS (), [D-Ser4,4′]-GS (), and [L-Orn(Boc)2,2′,D-Ser4,4′]-GS () were synthesized and showed high antibacterial activity. One residue of D-Ser in was converted asymmetrically to D-Ala via α,β-dehydroalanine. 相似文献
465.
Okuda T Kidoaki S Ohsaki M Koyama Y Yoshikawa K Niidome T Aoyagi H 《Organic & biomolecular chemistry》2003,1(8):1270-1273
Dendritic poly(L-lysine) of the 6th generation shows high transfection efficiency into several cultivated cells with low cytotoxicity. In order to understand the mechanism of complex formation with plasmid DNA, the complex was observed using atomic force microscopy. After mixing for 15 min, 1-2 microns assemblies of complexes composed of several small particles (50-200 nm) were observed. At the same time, individual small complexes of 50 to 500 nm were observed on a mica surface. After incubation for 2 h, only the large complexes were found on the mica surface. As a result of further dynamic light scattering analysis and measurement of the transfection efficiency at different time points, the transfection efficiency of KG6 was found to increase with increasing size of the DNA-complexes. This result indicates that large complexes of more than 1 micron are major species that contribute to transfection in vitro. 相似文献
466.
Chi C Tanaka R Okuda Y Ikota N Yamamoto H Urano S Ozawa T Anzai K 《Chemical & pharmaceutical bulletin》2005,53(11):1411-1415
To find efficient methods to evaluate oxidative stress in mouse skin caused by X-ray irradiation, several markers and methodologies were examined. Hairless mice were irradiated with 50 Gy X-rays and skin homogenates or skin strips were prepared. Lipid peroxidation was measured using the skin homogenate as the level of thiobarbituric acid reactive substances. The level of lipid peroxidation increased with time after irradiation and was twice that of the control at 78 h. ESR spectra of skin strips showed a clear signal for the ascorbyl radical, which increased with time after irradiation in a manner similar to that of lipid peroxidation. To measure levels of glutathione (GSH) and its oxidized forms (GSSG) simultaneously, two HPLC methods, sample derivatization with 1-fluoro-2,4-dinitrobenzene and detection with a UV detector (method A) and no derivatization and detection with an electrochemical detector (method B), were compared and the latter was found to be better. No significant change was observed within 24 h after irradiation in the levels of GSH and GSSG measured by method B. The GSH/GSSG ratio may be a less sensitive parameter for the evaluation of acute oxidative stress caused by X-ray irradiation in the skin. Monitoring the ascorbyl radical seems to be a good way to evaluate oxidative stress in skin in vivo. 相似文献
467.
The (19)F NMR shieldings for 53 kinds of perfluoro compounds were calculated by the B3LYP-GIAO method using the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311++G(d,p), 6-311G(2d,2p), 6-311++G(2d,2p), 6-311++G(2df,2p), 6-311++G(3d,2p), and 6-311++G(3df,2p) basis sets. The diffuse functions markedly reduce the difference between the calculated and experimental chemical shifts. The calculations using the 6-31++G(d,p) basis set give the chemical shifts within 10 ppm deviations from experimental values except for the fluorine nuclei attached to an oxygen atom, a four- and a six-coordinated sulfur atom, and FC(CF(3))(2) attached to a sulfur atom. 相似文献
468.
Total synthesis of (-)-gambierol 总被引:1,自引:0,他引:1
Fuwa H Kainuma N Tachibana K Sasaki M 《Journal of the American Chemical Society》2002,124(50):14983-14992
The first total synthesis of (-)-gambierol (1), a marine polycyclic ether toxin, has been achieved. Key features of the successful synthesis include (1) a convergent union of the ABC and EFGH ring fragments (5 and 6, respectively) via our developed B-alkyl Suzuki-Miyaura cross-coupling strategy leading to the octacyclic polyether core 4 and (2) a late-stage introduction of the sensitive triene side chain by use of Pd(PPh(3))(4)/CuCl/LiCl-promoted Stille coupling. The ABC ring fragment 5 was synthesized in a linear manner (B --> AB --> ABC), wherein the A ring was formed by intramolecular hetero-Michael reaction and the C ring was constructed via 6-endo cyclization of hydroxy epoxide 7. An improved synthetic entry to the EFGH ring fragment 6 is also described, in which SmI(2)-induced reductive cyclization methodology was applied to the stereoselective construction of the F and H rings, leading to 6 with remarkable overall efficiency. Stereoselective hydroboration of 5 and subsequent Suzuki-Miyaura coupling with 6 provided endocyclic enol ether 45 in high yield, which was then converted to octacyclic polyether core 4. Careful choice of the global deprotection stage was a key element for the successful total synthesis. Functionalization of the H ring and global desilylation gave (Z)-vinyl bromide 2. Finally, cross-coupling of 2 with (Z)-vinyl stannane 3 under Corey's Pd(PPh(3))(4)/CuCl/LiCl-promoted Stille conditions completed the total synthesis of (-)-gambierol (1). 相似文献
469.
[chemical reaction: see text]. A general route of converting alkenyl ketones to functionalized allenes was developed. Substituted 1,3-dien-2-yl triflates, which were prepared from the alkenyl ketones via silyl dienol ethers, were excellent substrates for the palladium-catalyzed reaction with soft nucleophiles giving the multisubstituted allenes in high yields. Comparison between the dienyl triflates and analogous bromodienes in the Pd-catalyzed reaction was studied as well. 相似文献
470.
Masakazu Nishida Haruhiko Fukaya Yoshio Hayakawa Taizo Ono Kotaro Fujii Hidehiro Uekusa 《Helvetica chimica acta》2006,89(11):2671-2685
The reagent Me3Si(C6F5) was used for the preparation of a series of perfluorinated, pentafluorophenyl‐substituted 3,6‐dihydro‐2H‐1,4‐oxazines ( 2 – 8 ), which, otherwise, would be very difficult to synthesize. Multiple pentafluorophenylation occurred not only on the heterocyclic ring of the starting compound 1 (Scheme), but also in para position of the introduced C6F5 substituent(s) leading to compounds with one to three nonafluorobiphenyl (C12F9) substituents. While the tris(pentafluorophenyl)‐substituted compound 3 could be isolated as the sole product by stoichiometric control of the reagent, the higher‐substituted compounds 5 – 8 could only be obtained as mixtures. The structures of the oligo(perfluoroaryl) compounds were confirmed by 19F‐ and 13C‐NMR, MS, and/or X‐ray crystallography. DFT simulations of the 19F‐ and 13C‐NMR chemical shifts were performed at the B3LYP‐GIAO/6‐31++G(d,p) level for geometries optimized by the B3LYP/6‐31G(d) level, a technique that proved to be very useful to accomplish full NMR assignment of these complex products. 相似文献