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531.
Three novel hybrid polyketide-terpenoid metabolites were isolated from a Penicillium minioluteum strain. Their structures were determined by NMR spectroscopic analyses and X-ray crystallography. The proposed biosynthetic pathway including a unique retro-Claisen migration of methyl carbonate correlates the three compounds with berkeleydione and berkeleytrione. 相似文献
532.
A. L. Ahmad B. Koohestani S. Bhatia B. S. Ooi 《Journal of Sol-Gel Science and Technology》2010,56(3):327-332
Vanadium oxide nanotubes were synthesized using V2O5 powder as the precursor and hexadecylamine as the structure-directing template using a sol-gel reaction method followed by
a one-step hydrothermal treatment. The effect of ultrasonics on the formation of nanotubes is reported. The structure and
morphology of the nanotubes were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron
microscopy. The inner and outer diameters of the nanotubes varied from 20 to 40 nm and 80 to 100 nm, respectively. The nanotubes
measured several micrometers in length. 相似文献
533.
Tee Lee Theng Zaharuddin Ahmad Che Abd. Rahim Mohamed 《Journal of Radioanalytical and Nuclear Chemistry》2003,256(1):115-120
Sediment cores were obtained from Teluk Brunei, Sipitang, Teluk Kimanis, Kota Kinabalu and Kuala Penyu at the coastal water of Sabah and analyzed for 210Pb and 210Po to estimate the sedimentation rates. The calculated sedimentation rates of 210Pb and 210Po varied from 0.003 to 0.049 cm/y and 0.74 to 8.77 cm/y, respectively. The highest sedimentation rates were determined for 210Pb and 210Po at stations located at Sipitang (mean: 0.027 cm/y) and Teluk Kimanis (mean: 5.53 cm/y), respectively. The sedimentation rate estimated for 210Pb is not fully reliable because the activity of 226Ra was higher than that of 210Pb and bioturbation was active at the sampling stations. 相似文献
534.
Elimination reactions of (4'-ClC6H4)2CHCO2C6H3-2-X-4-NO2 promoted by R2NH-R2NH2+ in 70 mol% MeCN (aq.) have been studied kinetically. The reactions are second-order and exhibit Br?nsted beta = 0.44-0.86 and /beta(lg)/ = 0.41-0.71. The Br?nsted beta decreased with a poorer leaving group and /beta(lg)/ increased with a weaker base. The results are consistent with an E2 mechanism. When X=H, the reaction proceeded by the concurrent E2 and Elcb mechanism. 相似文献
535.
536.
Takashi Ooi Makoto Takahashi Keiji Maruoka 《Angewandte Chemie (International ed. in English)》1998,37(6):835-837
Two aluminum centers aligned in the same direction capture carbonyl groups in such a way that efficient alkyl transfer becomes possible from aluminum to the carbon atom. This occurs via a favorable cyclic six-membered transition state ( 1 ). Carbonyl compounds can now be alkylated with otherwise less reactive alkylmetal species. 相似文献
537.
Highly enantioselective conjugate addition of nitroalkanes to alkylidenemalonates has been accomplished for the first time by the utilization of efficient phase-transfer catalysis of N-spiro C2-symmetric chiral quaternary ammonium bromide 1. For instance, simple mixing of nitropropane (2, R1 = Et), diisopropyl benzylidienemalonate (3, R2 = Ph), Cs2CO3 (1 equiv), and (S,S)-1 (1 mol %) in toluene at 0 degrees C for 2.5 h gave rise to the desired conjugate addition product 4 (R1 = Et, R2 = Ph) quantitatively (anti/syn = 86:14) with 97% ee (anti isomer). The applicability of this procedure has been demonstrated with other representative alkylidenemalonates and nitroalkanes. Since 4 can be readily transformed into the corresponding gamma-amino acid hydrochloride 5 without loss of diastereo- and enantioselectivity, the present method provides a new and practical access to various optically active gamma-amino acid derivatives. 相似文献
538.
The purpose of this study was to develop a reversed-phase high-performance liquid chromatographic method (RP-HPLC) for separating each positional isomer from low- to high-molecular-weight mono-PEGylated octreotides prepared by polyethylene glycol (PEG) derivatives with various molecular weights (2, 5, or 20 kDa). In the gradient elution using acetonitrile and 10 mM phosphate buffer at pH 7.0 on a Phenomenex Gemini C-18 column (250 mm × 4.6 mm id, 5 μm), each positional isomer of the mono-PEGylated octreotides was completely resolved with good resolution (PEG-2K: 7.6, PEG-5K: 6.6, and PEG-20K: 3.1). The optimal RP-HPLC condition also resolved the degradation products of mono-PEG-octreotide isomers in thermal stability studies at 55 °C and enzymatic stability studies with trypsin. In conclusion, the developed RP-HPLC method will be valuable for studying the effect of PEGylation site and the attached PEG size on the physicochemical and pharmacological properties of PEGylated octreotides. 相似文献
539.
An oligonucleotide of triazole‐linked RNA (TLRNA) was synthesized by performing consecutive copper‐catalyzed azide‐alkyne cycloaddition reactions for elongation. The reaction conditions that had been optimized for the synthesis of 3‐mer TLRNA were found to be inappropriate for longer oligonucleotides, and the conditions were reoptimized for the solid‐phase synthesis of an 11‐mer TLRNA oligonucleotide. Duplex formation of the 11‐mer TLRNA oligonucleotide was examined with the complementary oligonucleotide of natural RNA to reveal the effects of the 2′‐OH groups on the duplex stability. 相似文献
540.
Daisuke Uraguchi Yuto Tsuchiya Tsuyoshi Ohtani Takafumi Enomoto Shigeyuki Masaoka Daisuke Yokogawa Takashi Ooi 《Angewandte Chemie (International ed. in English)》2020,59(9):3665-3670
Unlike carbonyl compounds, it has long been common understanding that excited imines show virtually no photoreactivity, and hence their properties and potential utility in chemical science remain largely unexplored. Now, a strategy is presented for eliciting latent photoreactivity of imines based on the introduction of a donor–acceptor (D‐A) structure to extend the lifetime of their photoexcited states. A series of spectroscopic analyses and density functional theory calculations reveal unique photophysical properties of the D‐A‐type imines. Furthermore, the reactivity of the D‐A‐type imines is demonstrated by using them as a photoredox catalyst for atom‐transfer radical addition. These findings illuminate a previously neglected chemical space in the field of photochemistry, which will be exploited by taking advantage of the inherent structural modularity of imines. 相似文献