全文获取类型
收费全文 | 1874篇 |
免费 | 64篇 |
国内免费 | 7篇 |
专业分类
化学 | 1197篇 |
晶体学 | 15篇 |
力学 | 63篇 |
数学 | 231篇 |
物理学 | 439篇 |
出版年
2023年 | 11篇 |
2022年 | 24篇 |
2021年 | 58篇 |
2020年 | 38篇 |
2019年 | 40篇 |
2018年 | 46篇 |
2017年 | 34篇 |
2016年 | 53篇 |
2015年 | 66篇 |
2014年 | 63篇 |
2013年 | 134篇 |
2012年 | 136篇 |
2011年 | 136篇 |
2010年 | 82篇 |
2009年 | 93篇 |
2008年 | 120篇 |
2007年 | 103篇 |
2006年 | 91篇 |
2005年 | 90篇 |
2004年 | 80篇 |
2003年 | 56篇 |
2002年 | 62篇 |
2001年 | 32篇 |
2000年 | 19篇 |
1999年 | 29篇 |
1998年 | 18篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 23篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1944年 | 2篇 |
排序方式: 共有1945条查询结果,搜索用时 15 毫秒
921.
Free NH 3,3-diarylacrylamides are cyclized to substituted 2-quinolones in the presence of CuI, PPh(3), and KO-t-Bu in o-xylene at 100 °C. The reaction proceeds through a C-H functionalization/C-N bond formation process. With unsymmetrical 3,3-diarylacrylamides, high selectivity is observed using substrates where the aromatic ring trans to the amide group bears o-methyl, -chloro, or -bromo substituents. 相似文献
922.
Andrea Vasas Edvárd Sulyok Ana Martins Dóra Rédei Peter Forgo Zoltán Kele István Zupkó Joseph Molnár Gyula Pinke Judit Hohmann 《Tetrahedron》2012,68(4):1280-1285
Two new and one known cyclomyrsinane-type diterpenes (1–3) were isolated from the methanol extract of the whole, undried plant of Euphorbia falcata, native to Hungary. The structures and relative configuration of the compounds were established on the basis of extensive spectroscopic analysis, including HRESIMS and one- and two-dimensional NMR techniques. These compounds are hexa- and heptaesters of diterpene polyols, esterified with acetic, propanoic, isobutanoic, 2-methylbutanoic, and benzoic acids. This type of diterpenes was previously detected only in three Euphorbia species (Euphorbia teheranica, Euphorbia seguieriana, and Euphorbia prolifera). The compounds, together with premyrsinanes isolated previously from this plant were evaluated for their antiproliferative activity against HeLa, Ishikawa, and MCF7 mouse lymphoma cells, and for their capacity to modulate resistance to doxorubicin in L5178 mouse lymphoma cells that over-express the MDR1 efflux pump. Compounds did not show substantial antiproliferative activity against any of the tested cell lines. However all compounds act in a mild to a very strong synergism with doxorubicin against the MDR mouse lymphoma cell line. 相似文献
923.
Emerson A. Guarda Mara R.B. Marzari Clarissa P. Frizzo Patrícia M. Guarda Nilo Zanatta Helio G. Bonacorso Marcos A.P. Martins 《Tetrahedron letters》2012,53(2):170-172
Synthesis of 4-alkoxy-1,1-dichloro-3-alken-2-ones [CHCl2C(O)C(R2)C(R1)-OR, where R, R1, R2 = Et, H, H; Me, Me, H; Et, H, Me; Me, –(CH2)2–; Me, –(CH2)3–; Et, Et, H; Et, Bu, H; Et, i-Pr, H; Et, i-Bu, H; Me, Ph, H; Me, thien-2-yl, H] from acylation of enol ethers and acetals with dichloroacetyl chloride, in ionic liquid ([BMIM][BF4] or [BMIM][PF6]) is reported. The synthesis of alkenones [R3–C(O)C(R2)C(R1)-OR], where R/R1/R2/R3 = Et/H/H/Ph, t-Bu/H/H/Ph, Me/-(CH2)4/Ph, Me/-(CH2)4/Me] from the reaction of enol ethers with benzoyl chloride or acetyl chloride, in ionic liquid [BMIM][BF4], is also reported. Last products are described for the first time. 相似文献
924.
Amanda S. SantanaDiego B. Carvalho Nadla S. CasemiroGabriela R. Hurtado Luiz H. VianaNájla M. Kassab Sandro L. BarbosaFrancisco A. Marques Palimécio G. Guerrero Jr.Adriano C.M. Baroni 《Tetrahedron letters》2012,53(43):5733-5738
Hydrothiolation of symmetrical and unsymmetrical buta-1,3-diynes with sodium organylthiolate anions in reflux, generated in situ by reacting C4H9SH with NaOH, afforded (Z)-organylthioenynes in low to good yields (25-80%). By using tetrabutylammonium hydroxide (TBAOH) as base instead of NaOH, the hydrothiolation of buta-1,3-diynes was more rapid and efficient, providing (Z)-organylthioenynes in good to excellent yields (70-95%). 相似文献
925.
Angelo Domenico Quartarolo Sandro Giuseppe Chiodo Nino Russo 《Journal of computational chemistry》2012,33(11):1091-1100
The conformational structure and electronic spectra properties of a series of bay substituted perylenediimides (PDI) derivatives have been investigated by means of density functional theory (DFT) and time‐dependent DFT. The B3LYP and PBE0 hybrid exchange‐correlation functionals were applied in conjunction with the double‐ζ quality SVP basis set. These compounds are interesting for organic materials science and as photosensitizers in cancer phototherapy (PDT), because of their intense absorption in the visible region. Results show that the substitution at the bay position of the PDI parent molecule with N‐alkyl groups shifts the absorption maxima towards the red part of the visible spectrum (around 650–700 nm) as required for the applications in PDT. The main PDT action mechanisms have been investigated by computing of electron affinities, ionization potentials, triplet energies and spin‐orbit matrix elements between singlet and triplet excited states. © 2012 Wiley Periodicals, Inc. 相似文献
926.
927.
Barandun LJ Immekus F Kohler PC Tonazzi S Wagner B Wendelspiess S Ritschel T Heine A Kansy M Klebe G Diederich F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(30):9246-9257
The foodborne illness shigellosis is caused by Shigella bacteria that secrete the highly cytotoxic Shiga toxin, which is also formed by the closely related enterohemorrhagic Escherichia coli (EHEC). It has been shown that tRNA-guanine transglycosylase (TGT) is essential for the pathogenicity of Shigella flexneri. Herein, the molecular recognition properties of a guanine binding pocket in Zymomonas mobilis TGT are investigated with a series of lin-benzohypoxanthine- and lin-benzoguanine-based inhibitors that bear substituents to occupy either the ribose-33 or the ribose-34 pocket. The three inhibitor scaffolds differ by the substituent at C(6) being H, NH(2), or NH-alkyl. These differences lead to major changes in the inhibition constants, pK(a) values, and binding modes. Compared to the lin-benzoguanines, with an exocyclic NH(2) at C(6), the lin-benzohypoxanthines without an exocyclic NH(2) group have a weaker affinity as several ionic protein-ligand hydrogen bonds are lost. X-ray cocrystal structure analysis reveals that a new water cluster is imported into the space vacated by the lacking NH(2) group and by a conformational shift of the side chain of catalytic Asp102. In the presence of an N-alkyl group at C(6) in lin-benzoguanine ligands, this water cluster is largely maintained but replacement of one of the water molecules in the cluster leads to a substantial loss in binding affinity. This study provides new insight into the role of water clusters at enzyme active sites and their challenging substitution by ligand parts, a topic of general interest in contemporary structure-based drug design. 相似文献
928.
V. Gokulakrishnan S. Parthiban E. Elangovan K. Jeganathan D. Kanjilal K. Asokan R. Martins E. Fortunato K. Ramamurthi 《Radiation Physics and Chemistry》2012,81(6):589-593
Molybdenum (0.5 at%) doped indium oxide thin films deposited by spray pyrolysis technique were irradiated by 100 MeV O7+ ions with different fluences of 5×1011, 1×1012 and 1×1013 ions/cm2. Intensity of (222) peak of the pristine film was decreased with increase in the ion fluence. Films irradiated with the maximum ion fluence of 1×1013 ions/cm2 showed a fraction of amorphous nature. The surface microstructures on the surface of the film showed that increase in ion fluence decreases the grain size. Mobility of the pristine molybdenum doped indium oxide films was decreased from ~122 to 48 cm2/V s with increasing ion fluence. Among the irradiated films the film irradiated with the ion fluence of 5×1011 ions/cm2 showed relatively low resistivity of 6.7×10?4 Ω cm with the mobility of 75 cm2/V s. The average transmittance of the as-deposited IMO film is decreased from 89% to 81% due to irradiation with the fluence of 5×1011 ions/cm2. 相似文献
929.
Cristiane M. Becker Teo A. Dick Jonathan T. Ramos Fernando Wypych Sandro C. Amico 《Macromolecular Symposia》2012,319(1):129-135
Summary: Layered double hydroxides (LDH) are chemical compounds that can be added to polymeric resins to confer fire retardant characteristics. The focus of this work is to study the incorporation of LDH intercalated with dodecylsulfate anions into epoxy resins. The mechanical properties were investigated using tensile, flexural and impact mechanical tests. The flame-retardant properties were assessed using horizontal (UL 94 HB) and vertical burning (UL 94 V) tests. The highest tensile strength was obtained for the composite with 3 wt% of LDH, whereas the highest flexural strength was found by incorporating 1 wt% of LDH. All samples containing LDH showed self-extinguishing behavior in the vertical test and lower burning rate than pristine epoxy. 相似文献
930.