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51.
Kovacs JM Mant CT Kwok SC Osguthorpe DJ Hodges RS 《Journal of chromatography. A》2006,1123(2):212-224
Side-chain backbone interactions (or "effects") between nearest neighbours may severely restrict the conformations accessible to a polypeptide chain and thus represent the first step in protein folding. We have quantified nearest-neighbour effects (i to i+1) in peptides through reversed-phase liquid chromatography (RP-HPLC) of model synthetic peptides, where L- and D-amino acids were substituted at the N-terminal end of the peptide sequence, adjacent to a L-Leu residue. These nearest-neighbour effects (expressed as the difference in retention times of L- and D-peptide diastereomers at pHs 2 and 7) were frequently dramatic, depending on the type of side-chain adjacent to the L-Leu residue, albeit such effects were independent of mobile phase conditions. No nearest-neighbour effects were observed when residue i is adjacent to a Gly residue. Calculation of minimum energy conformations of selected peptides supported the view that, whether a L- or D-amino acid is substituted adjacent to L-Leu, its orientation relative to this bulky Leu side-chain represents the most energetically favourable configuration. We believe that such energetically favourable, and different, configurations of L- and D-peptide diastereomers affect their respective interactions with a hydrophobic stationary phase, which are thus quantified by different RP-HPLC retention times. Side-chain hydrophilicity/hydrophobicity coefficients were generated in the presence of these nearest-neighbour effects and, despite the relative difference in such coefficients generated from peptides substituted with L- or D-amino acids, the relative difference in hydrophilicity/hydrophobicity between different amino acids in the L- or D-series is maintained. Overall, our results demonstrate that such nearest-neighbour effects can clearly restrict conformational space of an amino acid side-chain in a polypeptide chain. 相似文献
52.
Yuen MY Kui SC Low KH Kwok CC Chui SS Ma CW Zhu N Che CM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(47):14131-14141
A series of platinum(II) complexes bearing tridentate cyclometalated C^N^N (C^N^N=6-phenyl-2,2'-bipyridine and π-extended R-C^N^N=3-[6'-(naphthalen-2'-yl)pyridin-2'-yl]isoquinoline) ligands with fluorene units have been synthesised and their photophysical properties have been studied. The fluorene units are incorporated into the cyclometalated ligands by a Suzuki coupling reaction. An increase in the π-conjugation of the cyclometalated ligands confers favourable photophysical properties compared to the 6-phenyl-2,2'-bipyridine analogues. The fluorene-based platinum(II) complexes display vibronic-structured emission bands with λ(max)=558-601 nm, and high emission quantum yields up to 0.76 in degassed dichloromethane. Their emissions are tentatively assigned to excited states with mixed (3)IL/(3)MLCT parentage (IL=intraligand, MLCT=metal-to-ligand charge transfer). The crystal structures of these platinum(II) complexes reveal extensive Pt(II)···π and/or π-π interactions. The fluorene-based platinum(II) complexes are soluble in organic solvents, have high thermal stability with decomposition temperature >350 °C, and can be thermally vacuum-sublimed or solution-processed as phosphorescent dopants for the fabrication of organic light-emitting diodes (OLEDs). A monochromic OLED with 3d as dopant (2 wt%) fabricated by vacuum deposition gave a current efficiency of 14.7 cd A(-1) and maximum brightness of 27000 cd m(-2). A high current efficiency (9.2 cd A(-1)) has been achieved in a solution-processed OLED using complex 3f (5 wt%) doped in a PVK (poly(9-vinylcarbazole)) host. 相似文献
53.
The reduction of ReCl4(THF)2 in the presence of excess t-butylisocyanide by sodium amalgam produces pentakis(t-butylisocyanide)chlororhenium(I), which has been converted to the corresponding methyl and ethyl derivatives. The reaction of pentakis(trimethylphosphine)chlororhenium(I) with ButNC gives partially substituted complexes, ReCl(CNBut)2(PMe3)3 and ReCl(CNBut)3(PMe3)2. The structures of both compounds have been determined by X-ray methods. Octahedral ReCl(CNBut)2(PMe3)3 has trans isocyanide groups with one linear [C---N---C = 175(1)°] and one slightly bent [C---N---C = 159(1)°]. The Re---C bond lengths are equal within experimental error [2.004(7), 2.003(7)Å]. In the octahedral ReCl(CNBut)3(PMe3)2, for which the structure is not well defined, due to disorder, the unique isocyanide trans to chlorine is considerably bent at the nitrogen atom [C---
---C = 141(6)°] and appears to show the shortest Re---C bond length, 1.94(5) vs 2.02(5)Å for the other two isocyanides which are mutually trans. Protonation of these two isocyanide complexes with fluoroboric acid gives, respectively, the salts [ReCl(CNBut)CNHBut(PMe3)3]BF4 and [ReCl(CNBut)2CNHBut(PMe3)2]BF4, whose configurations have been determined by NMR spectroscopy. The reduction by sodium amalgam of Cr2(CO2Me)4 in tetrahydrofuran in presence of ButNC gives a high yield of Cr(CNBut)6 while similar reduction of the dimeric tungsten(II) complex of the anion (mhp) of 2-methyl-6- hydroxypyridine gives W(CNBut)6. Interaction of W2(mhp)4 in methanol-ether with ButNC gives a tungsten(I) complex W2(η-mhp)2(ButNC)4, which may be an intermediate in the reductive cleavage reaction. Interaction of cis-PtMe2(PMe3)2 with ButNC leads only to replacement of one PMe3 group to give the complex cis-PtMe2(PMe3)(CNBut). 相似文献
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56.
Chunmei Ouyang Honghai Wang Ping Shum Songnian Fu Jia Haur Wong Kan Wu Desmond Rodney Chin Siong Lim Vincent Kwok Huei Wong Kenneth Eng Kian Lee 《Optics Communications》2011,284(2):619-624
We experimentally demonstrate a passively mode-locked fiber laser employing a fiber-based semiconductor saturable absorber (SSA) operating in transmission. Polarization rotation locked vector solitons are observed in the laser. Due to the intrinsic dynamic feature of the laser, period-doubling of these vector solitons has also been observed. Furthermore, extra spectral sidebands are formed on the optical spectrum, caused by the energy exchange between the two orthogonal polarization components of the vector solitons. By careful reduction of the pump power together with fine adjustment to the cavity birefringence, period-one state can further be obtained. Additionally, the phase noise properties of the vector soliton fiber laser have also been characterized experimentally and analytically. 相似文献
57.
We show that all meromorphic solutions of the stationary reduction of the real cubic Swift‐Hohenberg equation are elliptic or degenerate elliptic. We then obtain them all explicitly by the subequation method, and one of them appears to be a new elliptic solution. 相似文献
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59.
A highly enantioselective catalyst generated by a combination of BINOL and another ligand with Ti(OiPr)4 was found to be highly effective for the alkynylzinc addition to aromatic aldehydes with enantioselectivities as high as >99%. 相似文献
60.
van Otterlo A Scalettar RT Zimanyi GT Olsson R Petrean A Kwok W Vinokur V 《Physical review letters》2000,84(11):2493-2496
We study numerically and experimentally the dynamics of driven vortex matter. Our London-Langevin simulations find that the critical current exhibits a peak both across the Bragg glass to vortex glass transition and across the melting line. The peak is accompanied by a clear crossing of the I-V curves. We report transport measurements on untwinned YBCO crystals, in complete accordance with these findings. At higher drives disorder is averaged to reduced values, and in three dimensions the vortices reorder into a "moving solid." The effect of the disorder can be well represented with a "shaking temperature" which is inversely proportional to the velocity. 相似文献