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951.
It has been reported that crystalline 7-methoxy-4-methylcoumarin undergoes a [2 + 2] photodimerization reaction to give two different products; the major product is thesyn head-head dimer, whereas the minor product is thesyn head-tail dimer. In this paper, high-resolution solid-state13C NMR and solution-state1H and13C NMR techniques have been applied to investigate various structural issues relating to this solid-state photodimerization reaction. In particular, spectra have been recorded at various stages during the progress of the photodimerization reaction with the aim of charting the structural changes in the system as a function of reaction progress, with particular interest in assessing the potential of this technique for investigating the structural reasons for the production of the minor product. Conformational and dynamic properties of the product obtained in this solid-state photodimerization reaction have also been assessed. 相似文献
952.
Ronald F. Borne Catherine L. Gatchell Cheryl L. Klein John Harris Portia Ellis 《Journal of heterocyclic chemistry》1990,27(2):375-384
Several isonipecotinamide derivatives which represent reverse amide derivatives of the potent analgetic agent fentanyl were prepared and evaluated for analgetic activity. The synthetic approaches utilized and stereochemical assignments are discussed. The most potent compound, 3 , displayed much weaker analgetic activity than fentanyl itself. 相似文献
953.
Fred C. Anson 《先进技术聚合物》1995,6(3):112-114
Various electroactive reactants that undergo multipleelectron transfer reactions at electrodes are divided into two broad classes. The first class exhibits a series of single electron steps at different electrode potentials. The second class exhibits multiple-electron transfer in a single voltammetric step. Examples from each class are offered and the reasons for their electrochemical behavior are described. 相似文献
954.
Russell P. Newton Terence J. Walton Salem A. Basaif Andrea M. Jenkins A. Gareth Brenton Dipankar Ghosh Frank M. Harris 《Journal of mass spectrometry : JMS》1989,24(8):679-688
The syntheses of the dibutyryl derivatives of the 3′,5′-cyclic monophosphates of adenosine, guanosine and cytidine are described. The fast atom bombardment mass spectra of these compounds are discussed, together with the mass-analysed ion kinetic energy spectra of their protonated molecular ions and of diagnostic fragments. A protocol for the identification of the derivatives is reported which includes criteria for confirming retention of the cyclic phosphodiester moiety, substitution of both heterocyclic base and ribose ring, and butyrylation of the 2′-O-position. The origins of significant fragments in the spectra are discussed. 相似文献
955.
The reaction of 1,3-disubstituted thioureas with formaldehyde under acidic conditions with removal of water yielded 3,5-disubstituted tetrahydro-4H-1,3,5-oxadiazine-4-thiones. When hydrogen sulfide was bubbled through the reaction mixture, the corresponding tetrahydro-4H-1,3,5-thiadiazine-4-thiones were formed. Similarly, starting from 1,3-disubstituted ureas, a number of tetrahydro-4H-1,3,5-thiadiazine-4-ones were prepared. The latter compounds were also oxidized to the corresponding sulfoxides and sulfones. 相似文献
956.
957.
The coherent nature of molecular excited dimers in their triplet states is unequivocally established by a series of low temperature optically detected magnetic resonance experiments in zero field. The results clearly establish that a minimum coherence time for the dimer ψ(+) state in 1,2,4,5-tetrachlorobenzene (≈ 1.7 K) is longer than 5 × 10?7 s while the exchange time in these systems is approximately 16 × 10?12 s. This implies that the coherence persists for a time approximately 105 times that associated with the stochastic limit. The coherence time could, however, be even longer. Moreover, these new results clearly demonstrate that there is a direct and quantitative relationship between coherent properties of dimers and the coherent properties of triplet Frenkel excitons. 相似文献
958.
Syntheses of 8-aminoimidazo[4′,5′:5,6]pyrido[2,3-d]pyrimiciines: Linear tricyclic analogs of adenine
The syntheses of 8-aminoimidazo[4′,5′:5,6]pyrido[2,3-d]pyrimidines (7), stretched-out versions of the naturally occuring nucleoside base adenine, are reported. Their preparation involves conversion of purine into 5-arninoimidazo[4,5-b]pyrimidine-6-carbonitrile ( 1 ) by reaction with malononitrile, followed by construction of the pyrimidine ring in two steps via the ethoxymethylene derivative 3 . 8-Azapurine can be converted to 8-amino-1,2,3-triazolo[4′,5′:5,6]pyrido[2,3-d]pyrimidines 8 in a similar fashion. 相似文献
959.
The ligands (L) bis (2-pyridyl) methane (BPM) and 6-methyl-bis (2-pyridyl)methane (MBPM) form the three complexes CuL2+, CuL, and Cu2L2H with Cu2+. Stability constants are log K1 = 6.23 ± 0.06, log K2 = 4.83 ± 0.01, and log K (Cu2L2H + 2H2+ ? 2 CuL2+) = ?10.99 ± 0.03 for BPM and 4.56 ± 0.02, 2.64 ± 0.02, and ?11.17 ± 0.03 for MBPM, respectively. In the presence of catalytic amounts of Cu2+, the ligands are oxygenated to the corresponding ketones at room temperature and neutral pH. With BPM and 2,4,6-trimethylpyridine (TMP) as the substrate and the buffer base, respectively, the kinetics of the oxygenation can be described by the rate law with k1 = (5.9 ± 0.2) · 10?13 mol l?1 s?1, k2 = (4.0 ± 0.6) · 10?4 mol?1 ls?1, k3 = (1.1 ± 0.1) · 10?12 mol l?1 s?1, and k4 = (9 ± 2) · 10?14 mol l?1 s?1. 相似文献
960.
Dorrestein PC Zhai H Taylor SV McLafferty FW Begley TP 《Journal of the American Chemical Society》2004,126(10):3091-3096
Thiazole synthase (ThiG) catalyzes an Amadori-type rearrangement of 1-deoxy-d-xylulose-5-phosphate (DXP) via an imine intermediate. In support of this, we have demonstrated enzyme-catalyzed exchange of the C2 carbonyl of DXP. Borohydride reduction of the enzyme DXP imine followed by top-down mass spectrometric analysis localized the imine to lysine 96. On the basis of these observations, a new mechanism for the biosynthesis of the thiazole phosphate moiety of thiamin pyrophosphate in Bacillus subtilis is proposed. This mechanism involves the generation of a ketone at C3 of DXP by an Amadori-type rearrangement of the imine followed by nucleophillic addition of the sulfur carrier protein (ThiS-thiocarboxylate) to this carbonyl group. 相似文献