全文获取类型
收费全文 | 1799篇 |
免费 | 36篇 |
国内免费 | 10篇 |
专业分类
化学 | 1456篇 |
晶体学 | 12篇 |
力学 | 20篇 |
数学 | 94篇 |
物理学 | 263篇 |
出版年
2022年 | 10篇 |
2020年 | 18篇 |
2019年 | 16篇 |
2018年 | 20篇 |
2017年 | 9篇 |
2016年 | 22篇 |
2015年 | 19篇 |
2014年 | 27篇 |
2013年 | 96篇 |
2012年 | 84篇 |
2011年 | 79篇 |
2010年 | 37篇 |
2009年 | 41篇 |
2008年 | 81篇 |
2007年 | 100篇 |
2006年 | 107篇 |
2005年 | 107篇 |
2004年 | 90篇 |
2003年 | 84篇 |
2002年 | 96篇 |
2001年 | 35篇 |
2000年 | 22篇 |
1999年 | 16篇 |
1998年 | 23篇 |
1997年 | 14篇 |
1996年 | 58篇 |
1995年 | 19篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 23篇 |
1991年 | 21篇 |
1990年 | 24篇 |
1989年 | 17篇 |
1988年 | 24篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 31篇 |
1984年 | 36篇 |
1983年 | 17篇 |
1982年 | 33篇 |
1981年 | 21篇 |
1980年 | 29篇 |
1979年 | 26篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1976年 | 16篇 |
1975年 | 17篇 |
1974年 | 10篇 |
1971年 | 10篇 |
1969年 | 7篇 |
排序方式: 共有1845条查询结果,搜索用时 31 毫秒
41.
Takashi Kamimura Tsukio Masegi Mitsuo Sekine Tsujiaki Hata 《Tetrahedron letters》1984,25(38):4241-4244
Diisopropylethylamine was effective as a base for acylation of 2′,3′,5′-tri-O-acetyluridine with various acid chlorides. The 13C NMR spectra of the products and related compounds showed clearly that the acyl groups intoduced to the uracil moiety are attached to the N3-nitrogen. 相似文献
42.
Oderaotoshi Y Cheng W Fujitomi S Kasano Y Minakata S Komatsu M 《Organic letters》2003,5(26):5043-5046
High diastereo- and enantioselectivities were obtained for the asymmetric 1,3-dipolar cycloaddition of azomethine ylides generated from N-alkylideneglycine esters with dipolarophiles using chiral phosphine-copper complexes as catalysts. Whereas the cycloaddition of azomethine ylides catalyzed by metal salts generally afforded endo-adducts as the predominant product, the present method is the first example of an exo-selective cycloaddition. [reaction: see text] 相似文献
43.
Ishii K Takeuchi S Shimizu S Kobayashi N 《Journal of the American Chemical Society》2004,126(7):2082-2088
In this study, we have investigated the singlet oxygen ((1)Delta(g)) generation mechanism using phthalocyaninatosilicon (SiPc) covalently linked to nitroxide radicals (NRs), and we succeeded in increasing the singlet oxygen quantum yield (Phi(Delta)) by linking the NRs. This originates from both an increase in the triplet quantum yield and excited-state lifetimes long enough to utilize photochemical reactions. Because the electron exchange interactions with paramagnetic species were known to result only in very fast excited-state relaxation, leading to a decrease in photochemical reaction yields, this increase in Phi(Delta) is an unusual and precious example for increasing photochemical reaction yields by electron exchange interactions with paramagnetic species. In addition, our experiments and theoretical analyses show that the spin-selective energy transfer rate constant is not influenced by linking the NRs and can be evaluated by the product of spin-statistical factors and matrix elements between the initial and final states. 相似文献
44.
Shoji M Imai H Shiina I Kakeya H Osada H Hayashi Y 《The Journal of organic chemistry》2004,69(5):1548-1556
An oxidative dimerization reaction, involving the three successive steps of oxidation, 6 pi-electrocyclization, and Diels-Alder reaction, has been experimentally and theoretically investigated for the three 2-alkenyl-3-hydroxymethyl-2-cyclohexen-1-one derivatives epoxyquinol 3, epoxyquinone 6, and cyclohexenone 10. Of the sixteen possible modes of the oxidation/6 pi-electrocylization/Diels-Alder reaction cascade for the epoxyquinone 6, and eight for the cyclohexenone 10, only the endo-anti(epoxide)-anti(Me)-hetero and endo-anti(Me)-hetero modes are, respectively, observed, while both endo-anti(epoxide)-anti(Me)-hetero and exo-anti(epoxide)-anti(Me)-homo reaction modes occur with the epoxyquinol 3. Intermolecular hydrogen-bonding is found to be the key cause of formation of both epoxyquinols A and B with 3, although epoxyquinone 6 and cyclohexenone 10 both gave selectively only the epoxyquinol A-type product. In the dimerization of epoxyquinol 3, two monomer 2H-pyrans 5 interact with each other to afford intermediate complex 28 or 29 stabilized by hydrogen-bonding, from which Diels-Alder reaction proceeds. Theoretical calculations have also revealed the differences in the reaction profiles of epoxyquinone 6 and cyclohexenone 10. Namely, the rate-determining step of the former is the Diels-Alder reaction, while that of the latter is the 6 pi-electrocyclization. 相似文献
45.
Hitoshi?MatsukiEmail author Michio?Yamanaka Hiroshi?Kamaya Shoji?Kaneshina Issaku?Ueda 《Colloid and polymer science》2005,283(5):512-520
The dissociation equilibrium between uncharged local anesthetic lidocaine (LC) and charged local anesthetic LC (LCH+) in a surface-adsorbed film was investigated by measuring the surface tension and pH of aqueous solutions of a mixture of hydrochloric acid and LC. The surface tension values decreased slightly with increasing total molality mt at 0X20.5, where X2 is the mole fraction of LC in the mixture, while they decreased rapidly with increasing mt at 0.5<X21. It was shown from the pH measurements that almost all LC molecules were changed into LCH+ ions by protonation at 0X20.5 and both forms coexisted only at 0.5<X21. The quantities of the respective LC and LCH+ transferred from the aqueous solution to the adsorbed film, i.e., their surface densities, were calculated by applying the thermodynamic equations derived to the surface tension and pH data. A greater quantity of LC than LCH+ existed in the adsorbed film at the coexisting composition. The partitioning behavior of LC and LCH+ in the adsorbed film was characterized by three composition regions: (1) slight partitioning of low surface-active LCH+ in the region at 0X20.5, (2) preferential partitioning of LC at 0.5<X2<around 0.7, and (3) negative partitioning of LCH+ at around 0.7X21. The present results clearly indicate that uncharged local anesthetics transfer into hydrophobic environments such as cell membranes more than charged ones. 相似文献
46.
A methanol extract from Prunus mume Sieb. Et Zucc. showed a suppressive effect of the SOS-inducing activity on the mutagen 3-amino-1,4-dimethyl-SH-pyrido[4,3-b]indole(Trp-P-1) in the Salmonella typhimurium TA1535/pSK1002 umu test. The methanol extract was re-extracted with hexane, dichloromethane, ethyl acetate, butanol and water. The dichloromethane and ethyl acetate fractions showed suppressive effect. Suppressive compounds were isolated by silica gel column chromatography and identified as trimethyl citrate (1) and dimethyl citrate (2) by GC-MS, IR and 1H and 13C-NMR spectroscopy. Compounds 1 and 2 suppressed 51 and 39% of the SOS-inducing activity at a concentration of 2.0 micromol/mL. 相似文献
47.
Andrzej W. Trochimczuk Shoji AokiKazunori Yamabe Akinori Jyo 《European Polymer Journal》2002,38(5):941-945
Four porous vinylnaphthalene/divinylnaphthalene (VN/DVN) polymers having three different nominal crosslinking degrees (60, 80 and 100 wt.%) have been synthesized using the suspension polymerization method in the presence of toluene and decane.The use of various crosslinking levels and inert diluents was aimed at changing the extent of polymeric network-diluent interactions. The resultant polymers have specific surface area in the range 450-700 m2/g depending on the DVN content. Two sets of pores were detected in all resins: one with the diameter of ≈2 nm and the second one in the range of 30-40 nm. Their sorptive properties have been studied using dilute (0.5 mmol/l) solutions of phenol and its derivatives (o-chlorophenol, 2-methylphenol, o-nitrophenol, m-nitrophenol and p-nitrophenol). It has been found that sorption, at low equilibrium concentration, follows the order: o-nitrophenol > o-chlorophenol > m-nitrophenol > o-methylphenol > p-nitrophenol > phenol. Full characteristics of the porous structure of resultant polymers was obtained by nitrogen adsorption at 77 K and their surface properties analyzed using Inverse Gas Chromatography. 相似文献
48.
Ken-ichi Sato Shoji Akai Akira Yoshitomo Yoshimitsu Takai 《Tetrahedron letters》2004,45(44):8199-8201
The merits of an indirect protecting method for hydroxyl groups using allyl groups via allyloxycarbonyl groups in the synthesis of antennary β-d-mannopyranosyl disaccharides from β-d-galactopyranosyl disaccharides were studied. Regioselective allyloxycarbonylation and conversion reactions involving simultaneous double SN2 nucleophilic substitution at C-2′ and C-4′ of benzyl O-[β-d-galactopyranosyl]-(1-4)-3,6-di-O-benzyl-2-deoxy-2-N-phthalimido-β-d-glucopyranoside were examined for comparison with the direct allylation method. The required β-d-mannopyranosyl disaccharide having proper protecting groups was obtained using this indirect method in 52% yield. In contrast, the reported direct allylation method using methyl O-(β-d-galactopyranosyl) disaccharide gave the corresponding β-d-mannopyranosyl disaccharide in only 7.5% yield. 相似文献
49.
The simple removal of excess of co-extracted reagent in the solvent extraction of metal complex anions with a quaternary ammonium salt greatly improves the determination of iron(II) with pyrogallol red and zephiramine. The method with pyrogallol red is suitable for the determination of trace amounts of iron in natural waters. The apparent molar absorptivities of the iron(II) complex in chloroform are 7.5×104 and 10.3×104 1 mol-1 cm-1 at 560 and 298 nm, respectively. A large excess of reagent can be added, and the ternary complex can be completely extracted over the pH range 8.5–10. Masking agents allow most interferences to be suppressed. The method is suitable for the analysis of potable, river and sea waters. 相似文献
50.
Staudinger/aza-Wittig reaction of 6,6,6-trifluoro-5-oxohexanoyl azide with PPh(3) or PBu(3) was examined. A reactive intermediate acyl imine 1 was trapped by methanol. Without nucleophile, isomerized enamide form 3 was obtained. N-Iodobenzoylation and N-haloalkylation of 3 and following radical cyclization via the 5-exo or 6-exo mode gave benzoindolizidinone, indolizidinone, and quinolizidinone derivatives 10-14, which have a trifluoromethyl group at the bridgehead position adjacent to nitrogen. Although LiAlH(4) reduction of 10 and 11 gave a mixture of saturated benzoindolizidine 15 and amino alcohol 16, reduction with BH(3)/THF selectively gave the desired 15 and indolizidine 17 from 13. 相似文献