全文获取类型
收费全文 | 3023篇 |
免费 | 78篇 |
国内免费 | 6篇 |
专业分类
化学 | 2102篇 |
晶体学 | 41篇 |
力学 | 91篇 |
数学 | 179篇 |
物理学 | 694篇 |
出版年
2024年 | 14篇 |
2023年 | 23篇 |
2022年 | 33篇 |
2021年 | 62篇 |
2020年 | 70篇 |
2019年 | 78篇 |
2018年 | 69篇 |
2017年 | 73篇 |
2016年 | 92篇 |
2015年 | 76篇 |
2014年 | 98篇 |
2013年 | 255篇 |
2012年 | 220篇 |
2011年 | 223篇 |
2010年 | 133篇 |
2009年 | 132篇 |
2008年 | 194篇 |
2007年 | 122篇 |
2006年 | 131篇 |
2005年 | 114篇 |
2004年 | 86篇 |
2003年 | 61篇 |
2002年 | 68篇 |
2001年 | 42篇 |
2000年 | 38篇 |
1999年 | 32篇 |
1998年 | 18篇 |
1997年 | 23篇 |
1996年 | 27篇 |
1995年 | 32篇 |
1994年 | 23篇 |
1993年 | 30篇 |
1992年 | 18篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 15篇 |
1988年 | 19篇 |
1987年 | 16篇 |
1986年 | 15篇 |
1985年 | 22篇 |
1984年 | 26篇 |
1983年 | 27篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 23篇 |
1979年 | 28篇 |
1978年 | 26篇 |
1977年 | 33篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有3107条查询结果,搜索用时 15 毫秒
81.
A novel design for initiating radical-based chemistry in a catalytic fashion is described. The design of the concept is based on the phenylselenyl group transfer reaction from alkyl phenyl selenides by utilizing PhSeSiR(3) (1) as a catalytic reagent. The reaction is initiated by the homolytic cleavage of -C-Se- bond of an alkyl phenyl selenide by the in situ generated alkylsilyl radical (R(3)Si(*)), obtained by the mesolysis of PhSeSiR(3)](*)(-)( )()(1(*)(-)). The oxidative dimerization of counteranion PhSe(-) to PhSeSePh functions as radical terminator. The generation of 1(*)(-) is achieved by the photoinduced electron transfer (PET) promoted reductive activation of 1 through a photosystem comprising of a visible-light (410 nm)-absorbing electron rich DMA as an electron donor and ascorbic acid as a co-oxidant (Figure 1). The optimum mole ratio between the catalyst 1 and alkyl phenyl selenides for successful reaction is established to be 1:10. The generality of the concept is demonstrated by carrying out variety of radical reactions such as cyclization (10, 15-18), intermolecular addition (25), and tandem annulations (32). 相似文献
82.
Ramesh C. Kapoor Raj N. Mehrotra Shared K. Vajpai Purnima Chaudhary 《Transition Metal Chemistry》1991,16(1):65-70
Summary The kinetics of the OsVIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in [Fe(CN)
6
3–
] and first-order dependence in [OsO4]. The order with respect to glycols is less than unity, whereas the rate dependence on [OH–] is a combination of two rate constants; one independent of and the other first-order in [OH–]. These observations are commensurate with a mechanism in which two complexes, [OsO4(H2O)G]– and [OsO4(OH)G]2–, are formed either from [OsO4(H2O)(OH)]– or [OsO4(OH)2]2– and the glycol GH, or by [OsO4(H2O)2] and [OsO4(H2O)(OH)]– and the glycolate ion, G–, which is in equilibrium with the glycol GH through the reaction between GH and OH–. Hence there is an ambiguity about the true path for the formation of the two OsVIII-glycol complexes. A reversal in the reactivity order of glycols in the two rate-determining steps, despite the common attack of OH– ion on the two species of OsVIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k11) to positive (related to k2). 相似文献
83.
Ritu Pandey Sanjay Chauhan Gauri S. Singhal 《Journal of photochemistry and photobiology. B, Biology》1997,40(3):228-232
The effect of UVB irradiation on the phycobilisomes (PBSs) of Synechococcus sp. PCC 7942 cells was studied. The sucrose density-gradient-isolated PBSs from in vivo UVB-treated (280–320 nm) cells showed a strong decrease in β-phycocyanin (βPC) and -phycocyanin (PC) polypeptides. In addition to a decrease in the linker polypeptides LCM 75 (linker connecting the core to the thylakoid membranes), LR 33 (linker in the rod structure), LRC 31.5 (linker connecting the rod to the core) and LRC 29. In vitro UVB treatment of gradient-isolated intact PBSs for 1 h had no effect on any of the constituent polypeptides, and only after 2 h was a degradation of LCM 75 and LR 33 and a decrease in βPC evident. Further investigation of phycobiliproteins (4 h of UVB irradiation) using polyclonal antibody directed against purified whole PBSs revealed that, in vivo, there was a gradual decline in the levels of LCM 75, LR 33, LRC 31.5, LRC 29, βPC and PC. 相似文献
84.
PHOTOCHEMICAL INACTIVATION OF SINGLE-STRANDED VIRAL DNA IN THE PRESENCE OF UROCANIC ACID* 总被引:3,自引:0,他引:3
Irwin Tessman† H. Morrison † C. Bernasconi † G. Pandey † L. Ekanayake† † 《Photochemistry and photobiology》1983,38(1):29-35
Abstract— Urocanic acid (UA) has previously been shown to react photochemically in vitro with N,N-dimethylthymine. In this study, mixtures of UA and phage G4 single-stranded DNA have been irradiated with UV light (λ≥ 254 nm) and the DNA assayed for infectivity. At the concentrations of UA employed (typically 5.4 × 10-3 M ) there is extensive absorption of the incident light by the UA. The DNA is inactivated at rates greater than that predicted from the calculated shielding by UA, indicating that photosensitization is occurring. Photosensitization is also indicated by the fact that at high UA concentrations the inactivation rate does not decrease to zero but approaches a residual value. Furthermore, the ability to photoreactivate DNA that has been photolyzed in the presence of UA is much reduced relative to that observed upon photolysis of the DNA alone. UA is therefore responsible for the production of UV-induced DNA lesions, which are resistant to photoreactivation.
A general analysis of the effects of photosensitization on the kinetics of UV inactivation is presented in an appendix. 相似文献
A general analysis of the effects of photosensitization on the kinetics of UV inactivation is presented in an appendix. 相似文献
85.
The force constants, Coriolis coupling constants and mean amplitudes of vibration at 0, 298.16 and 500 K for GaF63?FeF63? have been reported for the first time employing recent vibrational data. The results are discussed in the light of available information. 相似文献
86.
Bhagwan R. Pandey Sushil Kumar Shiva P. Singh Surendra S. Parmar 《Journal of heterocyclic chemistry》1980,17(5):1119-1120
Several 4-(arylaminothiocarbonyl)-1-(1-o-methoxyphenylcarbamido)-ethylpiperazines were synthesized and evaluated for their anticonvulsant activity against pentylenetetrazol-induced seizures in mice. The ability of substituted piperazines to inhibit in vitro respiratory activity of rat brain homogenates was also determined to study their structure-activity relationship. 相似文献
87.
A solid state copper(II) ion sensor is reported based on the application of electropolymerized undoped (neutral) polycarbazole
(PCz) and polyindole (PIn) modified electrodes. The new sensor shows high selectivity to Cu2+ ions with a detection limit of 10–5 M. PCz and PIn are formed respectively by the anodic oxidation of 50 mM carbazole and 5 mM indole monomers in dichloromethane
containing 0.1 M tetrabutylammonium perchlorate on a platinum electrode using a single compartment cell. Potentiostatic polymerization
of both the monomers are carried out at 1.3 V and 1.0 V vs. Ag/AgCl, respectively. Perchlorate ions were electrochemically
removed from the polymer films by applying – 0.2 V vs. Ag/AgCl. Polymer-coated electrodes are incubated in 1 M KCl solution
for 8 h followed by incubation in distilled water for 2 h before using as a metal ion sensor. The undoped PCz and PIn electrodes
were found to be highly selective and sensitive for Cu2+ ions with little selectivity for Pb2+ and negligible response towards Ag+, Hg2+, Cu+, Ni2+, Co2+, Fe2+, Fe3+ or Zn2+. Potentiometric responses for Cu2+ ions are recorded for both the sensor electrodes together with a double-junction Ag/AgCl reference electrode. Calibration
curves for Cu2+ are reported for both ion sensors. The polymer-modified electrodes were found to be stable for several weeks.
Electronic Publication 相似文献
88.
Propargyloxycarbonyl chloride, 1, has been used to protect the hydroxyl and amino functionalities of amino alcohols and aminophenols in one pot using triethylamine or pyridine as a base. The increased reactivity of benzyltriethylammonium tetrathiomolybdate, 2, toward propargyl carbonates over propargyl carbamates is studied in detail and has been exploited further to develop an orthogonal protection strategy for the hydroxyl and amino functionalities. For example, 2-amino-1-butanol, 6a, was treated with 1 to get the N,O-diPoc compound 7a in 90% yield, which when treated with 1.1 equiv of 2 at room temperature removes the Poc group attached to oxygen while leaving the one attached to nitrogen intact to yield compound 8a in 85% yield. This particular observation offers a new protecting strategy where an amine and an alcohol group can be protected simultaneously in one pot, and in a later synthetic step, if the alcohol group has to be deprotected selectively, it can be achieved with 1 equiv of 2. 相似文献
89.
Maya Ramesh 《Journal of organometallic chemistry》2004,689(8):1425-1430
The molecular box [CpCo(CN)3]4[Cp*Ru]4 (Co4Ru4) reacts readily with a variety of monocations to form M⊂Co4Ru4+ (M=K+, Cs+, Rb+). Ion competition experiments, monitored by ESI-MS, show that the molecular box binds the smaller K+ more rapidly than Cs+, but that thermodynamically Co4Ru4 prefers the larger ion. The rates of ion-insertion for K+ and Cs+ into Co4Ru4 were found to qualitatively follow second order kinetics with K+, 300 M−1 s−1 and Cs+, 36 M−1 s−1. The ratio kK/kCs qualitatively matched the ESI-MS results from ion competition experiments. The rates of ion-insertion into Co4Ru4 were found to depend on the counter anions. In particular, RbBF4 reacted with Co4Ru4 more slowly than did RbOTf. The slower rates allowed us to establish second order kinetics. 1H NMR studies reveal that the Cp signal for Co4Ru4 is very sensitive to the presence of entering ions, e.g., Rb+, whereas the corresponding Cp signal for Rb⊂Co4Ru4+ was insensitive to the presence of Rb+. The molecular structures of [Co4Ru4] · 6MeCN, [K⊂Co4Ru4]BF4 · 7MeCN, [Cs⊂Co4Ru4]BF4 · 6MeCN and [Tl⊂Co4Ru4]BF4 · 6MeCN, determined by X-ray diffraction, showed that although the compounds crystallized in the same space group I23, a correlation exists between the Ru-N/Co-C bond distances and the size of the interstitial ion. 相似文献
90.
A novel series of complexes of the type [M(TML)X2]; where TML is a tetradentate macrocyclic ligand; M = Co(II), Ni(II), Cu(II) or Zn(II); X = Cl−, CH3COO− or NO
3
−
have been synthesized by template condensation of benzil and thiocarbohydrazide in the presence of divalent metal salts in
methanolic medium. The complexes have been characterized with the help of elemental analyses, conductance measurements, molecular
weight determination, magnetic measurements, electronic, NMR, infrared and far infrared spectral studies. Electronic spectra
along with magnetic moments suggest the six coordinate octahedral geometry for these complexes. The low value of molar conductance
indicates them to be non-electrolytes. The biological activities of metal complexes have been tested in vitro against a number of pathogenic bacteria to assess their inhibiting potential. 相似文献