全文获取类型
收费全文 | 817篇 |
免费 | 36篇 |
国内免费 | 8篇 |
专业分类
化学 | 585篇 |
晶体学 | 6篇 |
力学 | 60篇 |
数学 | 97篇 |
物理学 | 113篇 |
出版年
2024年 | 1篇 |
2023年 | 7篇 |
2022年 | 13篇 |
2021年 | 18篇 |
2020年 | 54篇 |
2019年 | 42篇 |
2018年 | 40篇 |
2017年 | 37篇 |
2016年 | 50篇 |
2015年 | 48篇 |
2014年 | 49篇 |
2013年 | 89篇 |
2012年 | 66篇 |
2011年 | 70篇 |
2010年 | 43篇 |
2009年 | 53篇 |
2008年 | 33篇 |
2007年 | 22篇 |
2006年 | 22篇 |
2005年 | 21篇 |
2004年 | 17篇 |
2003年 | 10篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 6篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有861条查询结果,搜索用时 203 毫秒
831.
The combination of relative rate method with discharge flow and mass spectrometry (RR/DF/MS) technique was employed to determine the rate constant for the gas‐phase reaction of hydroxyl radicals (OH) with naphthalene at 240?340 K and a total pressure of 1–3 Torr. At 298 K, the rate constant was measured to be cm3 molecule?1 s?1, which is in good agreement with reported literature values determined using different techniques. The reaction of OH with naphthalene was found to be essentially independent of pressure in a range of 1?3 Torr at both 298 and 340 K. At 240–340 K, the rate constant of this reaction was found to be negatively dependent on temperature, with an Arrhenius expression of k1(T) cm3 molecule?1 s?1 and k1(T) cm3 molecule?1 s?1 using 1,4‐dioxane and styrene as the reference compounds, respectively. The atmospheric lifetime of naphthalene was estimated to be 9.6 h using the rate constant of naphthalene + OH determined at 277 K in the present work. 相似文献
832.
A color changeable optode for thallium(III) ions in aqueous solutions was prepared by physical inclusion of 4-(2-pyridylazo)-resorcinol into a plasticized PVC film. The increase in the absorbance of the optode at 524 nm is proportional to thallium(III) concentration. Different parameters effecting the sensitivity such as sample parameters and composition of the membrane were optimized. The response times of the prepared test-system are found to be 230, 210, and 180 s for 4.8 × 10?6, 4.8 × 10?5, and 4.8 × 10?4 M Tl(III), respectively. The analytical performance of the optode was evaluated, obtaining a linear concentration range of two decades of concentration, 3.1 × 10?6 ? 4.7 × 10?4 M Tl(III), with a limit of detection of 1.8 × 10?6 M Tl(III). Selectivity of the optode is also studied. Application of the optode to the determination of Tl(III) in some aqueous samples yields good results. 相似文献
833.
James H Campbell Tuan Hoang Masoud Jelokhani-Niaraki Matthew D Smith 《BMC biochemistry》2014,15(1):1-9
Background
We have previously shown that the P. gingivalis HmuY hemophore-like protein binds heme and scavenges heme from host hemoproteins to further deliver it to the cognate heme receptor HmuR. The aim of this study was to characterize structural features of HmuY variants in the presence and absence of heme with respect to roles of tryptophan residues in conformational stability.Results
HmuY possesses tryptophan residues at positions 51 and 73, which are conserved in HmuY homologs present in a variety of bacteria, and a tryptophan residue at position 161, which has been found only in HmuY identified in P. gingivalis strains. We expressed and purified the wildtype HmuY and its protein variants with single tryptophan residues replaced by alanine or tyrosine residues. All HmuY variants were subjected to thermal denaturation and fluorescence spectroscopy analyses. Replacement of the most buried W161 only moderately affects protein stability. The most profound effect of the lack of a large hydrophobic side chain in respect to thermal stability is observed for W73. Also replacement of the W51 exposed on the surface results in the greatest loss of protein stability and even the large aromatic side chain of a tyrosine residue has little potential to substitute this tryptophan residue. Heme binding leads to different exposure of the tryptophan residue at position 51 to the surface of the protein. Differences in structural stability of HmuY variants suggest the change of the tertiary structure of the protein upon heme binding.Conclusions
Here we demonstrate differential roles of tryptophan residues in the protein conformational stability. We also propose different conformations of apo- and holoHmuY caused by tertiary changes which allow heme binding to the protein. 相似文献834.
The binuclear complex [Pt2Me2(ppy)2(mu-dppf)], 1, in which ppy = deprotonated 2-phenylpyridyl and dppf = 1,1'-bis(diphenylphosphino)ferrocene, was synthesized by the reaction of [PtMe(SMe2)(ppy)] with 0.5 equiv of dppf at room temperature. In this reaction when 1 equiv of dppf was used, the dppf chelating complex 2, [PtMe(dppf)(ppy-kappa1C)], was obtained. The reaction of Pt(II)-Pt(II) complex 1 with excess MeI gave the Pt(IV)-Pt(IV) complex [Pt2I2Me4(ppy)2(mu-dppf)], 3. When the reaction was performed with 1 equiv of MeI, a mixture containing unreacted complex 1, a mixed-valence Pt(II)-Pt(IV) complex [PtMe(ppy)(mu-dppf)PtIMe2(ppy)], 4, and complex 3 was obtained. In a comparative study, the reaction of [PtMe(SMe2)(ppy)] with 1 equiv of monodentate phosphine PPh3 gave [PtMe(ppy)(PPh3)], A. MeI was reacted with A to give the platinum(IV) complex [PtMe2I(ppy)(PPh3)], C. All the complexes were fully characterized using multinuclear (1H, 31P, 13C, and 195Pt) NMR spectroscopy, and complex 2 was further identified by single crystal X-ray structure determination. The reaction of binuclear Pt(II)-Pt(II) complex 1 with excess MeI was monitored by low temperature 31P NMR spectroscopy and further by 1H NMR spectroscopy, and the kinetics of the reaction was studied by UV-vis spectroscopy. On the basis of the data, a mechanism has been suggested for the reaction which overall involved stepwise oxidative addition of MeI to the two Pt(II) centers. In this suggested mechanism, the reaction proceeded through a number of Pt(II)-Pt(IV) and Pt(IV)-Pt(IV) intermediates. Although MeI in each step was trans oxidatively added to one of the Pt(II) centers, further trans to cis isomerizations of Me and I groups were also identified. A comparative kinetic study of the reaction of monomeric platinum(II) complex A with MeI was also performed. The rate of reaction of MeI with complex 1 was some 3.5 times faster than that with complex A, indicating that dppf in the complex 1, as compared with PPh 3 in the complex A, has significantly enhanced the electron richness of the platinum centers. 相似文献
835.
Masoud Nasr‐Esfahani Bahador Karami Morteza Montazerozohori Karim Abdi 《Journal of heterocyclic chemistry》2008,45(4):1183-1185
Tungstate sulfuric acid catalyzes the three component condensation reaction of an aromatic aldehyde, urea and a β‐ketoester under solvent‐free conditions to afford the corresponding dihydropyrimidinones in high to excellent yields at room temperature. 相似文献
836.
Three dimeric cis-dioxomolybdenum(VI) complexes of bis-bidentate Schiff base derivatives containing aromatic nitrogen–nitrogen linkers (4,4′-diaminodiphenylmethane;
4,4′-diaminodiphenylether; 4,4′-diaminodiphenylsulfone) with 2-hydroxy-1-naphthaldehyde have been synthesised and characterized
by physico-chemical and spectroscopic methods. The catalytic activities of the complexes with respect to alkene epoxidation
using tert-butylhydroperoxide (TBHP) as oxidant have been studied. The addition of single wall nanotubes can enhance the catalytic activities
of the Mo complexes and the selectivity of epoxide formation. 相似文献
837.
Masoud Salavati-Niasari Shahrzad Abdolmohammadi 《Journal of inclusion phenomena and macrocyclic chemistry》2008,60(1-2):145-152
Ni(II) complexes of [12]aneN4: 1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone; [14]aneN4: 1,4,8,11-tetraazacyclotetradecane-2,3,9,10-tetraone; Bzo2[12]aneN4: dibenzo-1,4,7,10-tetraazacyclododecane-2,3,8,9-tetraone and Bzo2[14]aneN4: dibenzo-1,4,8,11-tetraazacyclotetradecane-2,3,9,10-tetraone have been encapsulated in the nanopores of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of [bis(diamine)nickel(II)]; [Ni(N–N)2]–NaY; in the supercages of the zeolite, and (ii) in situ condensation of the nickel(II) precursor complex with diethyloxalate. The new host-guest nanocatalyst (HGN) were characterized by several techniques: chemical analysis and spectroscopic methods (FT-IR, UV/Vis, XRD, BET, DRS) and then were used for oxidation of cyclohexene with molecular oxygen. 相似文献
838.
Hassan H. Hammud Kamal H. Bouhadir Mamdouh S. Masoud Amer M. Ghannoum Sulaf A. Assi 《Journal of solution chemistry》2008,37(7):895-917
The absorption and emission spectra of six purine derivatives: adenine (I), N(9)-hydroxyethyladenine (II), N(6)-acetyladenine (III), N(6)-isobutyryladenine (IV), guanine (V), and N(2),N(9)-diacetylguanine (VI) have been investigated. The effects of solvent and pH on the positions of λ
max (absorption) and λ
max (emission) of these compounds were determined. Correlations between the absorption wavelength (λ
max ) of these organic compounds and the solvent parameters (D,n,E) or (K,M,N) show that the peak position is affected mainly by specific- and non-specific types of interactions between the solvent and
solute. Solvent effects on the electronic absorption band shifts are indicative of the extent of charge reorganization of
the solute molecules upon electronic excitation. The Stokes shift (ν
abs−ν
em) was correlated with the orientation polarizability (Δf) and was found to depend mainly on the dielectric constant and the refractive index of the solvents. This shift reflects
the influence of the equilibrium solvent arrangement around the excited solute molecule, which rearranges inertially due to
the instantaneous charge redistribution upon radiative deactivation to the electronic ground state. A spectrofluorometric
analysis technique was applied for the quantitative analysis of the components of a ternary mixture of compounds (I–III). 相似文献
839.
Eshtiagh-Hosseini H Housaindokht MR Beyramabadi SA Beheshti S Esmaeili AA Khoshkholgh MJ Morsali A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2008,71(4):1341-1347
The new tetra dentate dianionic H2PS (N,N'-dipyridoxyl (1,3-propylenediamine)) Schiff-base ligand and its octahedral Co(III) salen complex [Co(PS)(H2O)(CH3OH)]+CH3COO(-) were synthesized, where coordinating atoms of H2PS (N,N,O(-),O(-)) occupied equatorial positions with H2O and CH3OH as axial ligands. The nature of the H2PS and its complex were determined by elemental and spectrochemical (IR, UV-vis, 1H NMR and Mass) analysis. Also, the fully optimized geometries and vibrational frequencies of them together with the 1H NMR chemical shifts of H2PS have been calculated using density functional theory (B3LYP) method. Obtained structural parameters are in good agreement with the experimental data reported for similar compounds. The calculated and experimental results confirmed the suggested structures for the ligand and complex. 相似文献
840.
Nabavizadeh SM Hoseini SJ Momeni BZ Shahabadi N Rashidi M Pakiari AH Eskandari K 《Dalton transactions (Cambridge, England : 2003)》2008,(18):2414-2421
The n-alkyl halides, RX, were oxidatively added to the platina(II)cyclopentane complexes [Pt[(CH2)4](NN)], in which NN = bpy (2,2'-bipyridyl) or phen (1,10-phenanthroline), to give the platinum(IV) complexes [PtRX[(CH2)4](NN)], R = Et and X = Br or I; R = nBu and X = I, 1-3. The same reactions with the analogous dimethyl complex [PtMe2(bpy)] gave the expected platinum(IV) complexes [PtRXMe2(bpy)], R = Et or nPr and X = Br or I; R = nBu and X = I, 4-8. Kinetics of the reactions in benzene and acetone was studied using UV-vis spectrophotometery and a common S(N)2 mechanism was suggested for each case. The platina(ii)cyclopentane complexes reacted faster than the corresponding dimethyl analogs by a factor of 2-3. This is described as being due to a lower positive charge, calculated by density functional theory (DFT), on the platinum atom of [Pt[(CH)2)4](bpy)] compared with that on the platinum atom of the dimethyl analog [PtMe2(bpy)]. The values of DeltaDeltaS(double dagger) = DeltaS(double dagger)(acetone) - DeltaS(double dagger)(benzene) were found to be either positive or negative in different reactions and this is related to the solvation of the corresponding alkyl halide. It is suggested that in these reactions of RX reagents, for a given X, the electronic effects of the R group are mainly responsible for the change in the rates of the reactions and the bulkiness of the group is far less important. 相似文献