全文获取类型
收费全文 | 245篇 |
免费 | 14篇 |
国内免费 | 2篇 |
专业分类
化学 | 186篇 |
晶体学 | 5篇 |
力学 | 6篇 |
数学 | 50篇 |
物理学 | 14篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 7篇 |
2019年 | 11篇 |
2018年 | 7篇 |
2017年 | 4篇 |
2016年 | 6篇 |
2015年 | 10篇 |
2014年 | 12篇 |
2013年 | 21篇 |
2012年 | 17篇 |
2011年 | 19篇 |
2010年 | 9篇 |
2009年 | 9篇 |
2008年 | 21篇 |
2007年 | 8篇 |
2006年 | 8篇 |
2005年 | 10篇 |
2004年 | 11篇 |
2003年 | 5篇 |
2002年 | 12篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1997年 | 1篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 3篇 |
1965年 | 1篇 |
1870年 | 1篇 |
排序方式: 共有261条查询结果,搜索用时 31 毫秒
41.
Maxwell AC Franc C Pouchain L Müller-Bunz H Guiry PJ 《Organic & biomolecular chemistry》2008,6(20):3848-3853
The synthesis and resolution of electronically varied axially chiral Quinazolinaps is reported. These ligands bear different aryl groups on the donor phosphorus atom and were synthesised as part of our investigations into electronic effects within this ligand class. A diastereomerically pure palladacycle of one ligand was characterised by X-ray crystallography. The application of these Quinazolinaps to the rhodium-catalysed hydroboration of vinylarenes resulted in enantioselectivities of up to 92%. Their application to the palladium-catalysed allylic alkylation of 1,3-diphenylprop-2-enyl acetate resulted in conversions of up to 99% and enantioselectivities of up to 94%. 相似文献
42.
Nie FM Demeshko S Fuchs S Dechert S Pruschke T Meyer F 《Dalton transactions (Cambridge, England : 2003)》2008,(30):3971-3977
A pyrazolate-based binucleating ligand HL with pyridyl groups in the chelate arms has been used to synthesize the dinickel(II) complex [LNi2(N3)(MeOH)2](ClO4)2 (1) and the corresponding 1D polymeric [LNi2(mu-N3)2]n(NO3)n (2) depending on the amount of NaN3 added. X-Ray crystallography shows that structural parameters of the (LNi2(N3)) units are very similar in both compounds. This is ascribed to pi-pi stacking between the pyridyl rings that leads to rigidification of the framework and a fixed cis-orientation of the remaining coordination sites, which are filled by MeOH molecules in or by a mu1,3-bridging azide that connects the subunits in 2. Variable-temperature magnetic measurements reveal strong antiferromagnetic coupling with parameters g = 2.21 and J = -60.7 cm(-1) for compound 1. Magnetic data for the extended chain 2 have been analyzed by Quantum Monte Carlo (QMC) simulations to give g = 2.34, J1 = -55 cm(-1), and J2 = -12 cm(-1). The known J value for finally allows unambiguous assignment of J1 to the intrasubunit coupling within each bimetallic chain constituent. The alternation ratio gamma = J2/J1 signifies a singlet-dimer ground state of the new 1D polymer 2. 相似文献
43.
Frankovic V Podgornik A Krajnc NL Smrekar F Krajnc P Strancar A 《Journal of chromatography. A》2008,1207(1-2):84-93
A weak ion-exchange grafted methacrylate monolith was prepared by grafting a methacrylate monolith with glycidyl methacrylate and subsequently modifying the epoxy groups with diethylamine. The thickness of the grafted layer was determined by measuring permeability and found to be approximately 90nm. The effects of different buffer solutions on the pressure drop were examined and indicated the influence of pH on the permeability of the grafted monolith. Protein separation and binding capacity (BC) were found to be flow-unaffected up to a linear velocity of 280cm/h. A comparison of the BC for the non-grafted and grafted monolith was performed using beta-lactoglobulin, bovine serum albumin (BSA), thyroglobulin, and plasmid DNA (pDNA). It was found that the grafted monolith exhibited 2- to 3.5-fold higher capacities (as compared to non-grafted monoliths) in all cases reaching values of 105, 80, 71, and 17mg/ml, respectively. It was determined that the maximum pDNA capacity was reached using 0.1M NaCl in the loading buffer. Recovery was comparable and no degradation of the supercoiled pDNA form was detected. Protein z-factors were equal for the non-grafted and grafted monolith indicating that the same number of binding sites are available although elution from the grafted monolith occurred at higher ionic strengths. The grafted monolith exhibited lower efficiency than the non-grafted ones. However, the baseline separation of pDNA from RNA and other impurities was achieved from a real sample. 相似文献
44.
Katsiaouni S Dechert S Briñas RP Brückner C Meyer F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(16):4823-4835
A double nucleophilic substitution reaction of 3,5-bis(chloromethyl)pyrazole with pyrroles generates a novel pyrrole-pyrazole hybrid building block, the pyrazole analogue to tripyrrane. Vilsmeier-Haack formylation produces the corresponding dialdehyde, which was used in the formation of a series of nonaromatic Schiff base macrocycles. NMR and UV/Vis spectroscopy and single-crystal diffractometry were used to characterize the novel macrocycles. The solid-state structures of select free bases and protonated members of this class of macrocycles display a range of intra- and intermolecular hydrogen-bonding patterns that suggest their use in molecular-recognition systems. They also contain an acid-sensitive chromophore. Their acid-base and anion-recognition properties were ascertained; alas, only modest anion-selective spectroscopic signatures could be detected by using UV/Vis and (1)H NMR spectroscopy. The macrocycles proved resistant toward oxidation to their aromatic congeners. The pyrrole-pyrazole building blocks presented are potentially useful for the synthesis of a range of pyrazole analogues of all-pyrrole macrocycles. 相似文献
45.
A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, M?ssbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered. 相似文献
46.
Klingele J Prikhod'ko AI Leibeling G Demeshko S Dechert S Meyer F 《Dalton transactions (Cambridge, England : 2003)》2007,(20):2003-2013
The pyrazole-based diamide ligand N,N'-bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide (H(3)L) has been structurally characterised and successfully employed in the preparation of [2 x 2] grid-type complexes. Thus, the reaction of H(3)L with Cu(ClO(4))2.6H(2)O or Ni(ClO(4))2.6H(2)O in the presence of added base (NaOH) affords the tetranuclear complexes [M(4)(HL(4))].8H(2)O (1: M = Cu, 2: M = Ni). Employment of a mixture of the two metal salts under otherwise identical reaction conditions leads to the formation of the mixed-metal species [Cu(x)Ni(4-x)(HL)(4)].8H(2)O (x相似文献
47.
48.
Romana Cerc Koro?ec Barbara Lavri? Gregor Rep Franc Pohleven Peter Bukovec 《Journal of Thermal Analysis and Calorimetry》2009,98(1):189-195
Thermal treatment is one of environmental friendly wood modification processes, developed in order to improve wood’s natural
durability and dimensional stability. Beside wood species, mainly isothermal temperature of heat-treatment and process duration
affect these properties, which also correlate with the mass losses caused by the treatment. However, there is a lack of suitable
external quality control methods. In this work thermogravimetry as a potential method for determining the degree of thermal
modification is presented. Several calibration curves, representing the mass losses in a certain temperature range (the values
obtained from the TG curves) compared to weight losses caused by previous heat-treatment (known values), were established
for spruce wood samples modified at different isothermal temperatures (from 170 to 220 °C). Linear plot and good correlation
factors (R
2 = 0.95 and 0.96) were obtained for the TG mass losses from 130 to 280 °C and from 130 to 300 °C, both under nitrogen atmosphere.
The predominant cause of mass loss in this temperature region was depolymerisation and thermal decomposition of hemicelluloses
residues. Lower correlation factors were obtained under the air atmosphere and in the wider temperature range, respectively. 相似文献
49.
Perdih F Pevec A Petricek S Petric A Lah N Kogej K Demsar A 《Inorganic chemistry》2006,45(19):7915-7921
Organotitanium fluorides (C5Me4R)TiF3 (R = H, Me, Et) sublimate with formation of crystalline dimers. From solution, we obtained crystals of dimers and tetramers. The tetramer [{(C5Me5)TiF3}4] irreversibly dissociates in the solid state to dimers (DeltaH = 8.33 kcal mol(-1)). The variable-temperature (1)H and (19)F NMR spectroscopy measurements of the toluene-d(8) solution of [{(C5Me5)TiF3}2] revealed at 202 K one monomeric, two dimeric (with C2h and Cs symmetry), two tetrameric (with D2 and C2v symmetry), and two trimeric (both C2 symmetry) molecules. With the increase in temperature and dilution of the solution, the composition of the solution shifts to the smaller molecules. The thermodynamic and activation parameters for the reversible dissociation of dimers to monomers in the solution are DeltaH = 9.2 kcal mol(-1), DeltaS = 24.2 cal mol(-1) K(-1), DeltaH(double dagger) = 12.2 kcal mol(-1), DeltaS(double dagger) = 9.7 cal mol(-1) K(-1). The dissociation path with a weakly double-bridged transition-state dimer was proposed. The thermodynamic parameters for the reversible dissociation of the C2v tetramer to the dimers in solution are DeltaH = 7.9 kcal mol(-1) and DeltaS = 26.8 cal mol(-1) K(-1). From both tetramers, the D2 molecule is 0.34(5) kcal mol(-1) lower in enthalpy and 6.5(5) cal mol(-1) K(-1) lower in entropy than the C2v molecule. The structures of both trimers were proposed. The low-temperature 19F NMR spectra of the CDCl3 solution of [{(C5Me5)TiF3}2] are consistent with equilibria of a monomer, two dimers (with C2h and Cs symmetry), and a trimer. The vapor pressure osmometric molecular mass determination of CDCl3 solution of [{(C5Me5)TiF3}2] at 302 K is consistent with the equilibrium of the dimer and the monomer. 相似文献
50.
Franc Perdih 《Journal of fluorine chemistry》2006,127(10):1289-1293
[Na{Ti2(C5Me5)2F7}] (1) was prepared from sodium fluoride and [{Ti(C5Me5)F3}2] [H.W. Roesky, et al., Angew. Chem. Int. Ed. Engl. 31 (1992) 864-866]. The solid-state 1 consists of a polymeric chain of two rows of dititanate anions [Ti2(C5Me5)2F7]− connected by sodium ions in the middle of the chain. Each sodium ion is coordinated by five fluorine atoms from three [Ti2(C5Me5)2F7]− anions. The variable-temperature 19F NMR of CD3CN solution of 1 revealed interconversions of monomeric species [Na(CD3CN)n{Ti2(C5Me5)2F7}] (1solv) with different number of CD3CN ligands on the sodium ion. The addition of HMPA to the CD3CN solution of 1 allows 19F NMR observation of 1·HMPA (1a) and 1·HMPA·CD3CN (1b) in the slow exchange. The solid-state structure of [NaTi6(C5Me5)5F20(H2O)]·(THF) (2·THF) reveals the sodium ion coordinated by four fluorine atoms from the anion [Ti2(C5Me5)2F7]− and by three fluorine atoms from the cluster [Ti4(C5Me5)3F13(H2O)]. 相似文献