全文获取类型
收费全文 | 716篇 |
免费 | 12篇 |
国内免费 | 2篇 |
专业分类
化学 | 537篇 |
晶体学 | 1篇 |
力学 | 21篇 |
数学 | 115篇 |
物理学 | 56篇 |
出版年
2022年 | 20篇 |
2021年 | 22篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 19篇 |
2017年 | 6篇 |
2016年 | 21篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 42篇 |
2012年 | 40篇 |
2011年 | 47篇 |
2010年 | 41篇 |
2009年 | 28篇 |
2008年 | 46篇 |
2007年 | 40篇 |
2006年 | 33篇 |
2005年 | 35篇 |
2004年 | 35篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 6篇 |
2000年 | 15篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1967年 | 2篇 |
1935年 | 1篇 |
排序方式: 共有730条查询结果,搜索用时 15 毫秒
41.
This survey critically discusses high-concentration-capacity (HCC) headspace (HS) techniques applied to sample the volatile fraction of matrices of interest in the flavors and fragrance fields. In particular, the advantages, limits, and fields of application of HS solid-phase microextraction (SPME), high-capacity HS sorptive extraction (HSSE) and HS solid-phase dynamic extraction (SPDE) are evaluated. These techniques are discussed in view of the peculiar characteristic of HCC-HS techniques, from the standpoint that these techniques are a bridge between static (S-HS) and dynamic HS (D-HS) because they are as simple, fast, easy to automate, and reliable as S-HS, yet afford analyte concentration factors comparable to those of D-HS. Moreover, the different degree of their development is a consequence of the different times in which they were introduced into the market, because the potential of HS-SPME is now well known, having been introduced approximately 12 years ago, but that of HSSE has still to be fully explored, and HS-SPDE still has to be investigated because it is very recent. 相似文献
42.
Ambrosi G Dapporto P Formica M Fusi V Giorgi L Guerri A Micheloni M Paoli P Pontellini R Rossi P 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3468-3474
The synthesis and characterization of two new macrocyclic ligands, the bis-macrocyclic compound 2,6-bis(1,4,13-triaza-7,10-dioxacyclopentadec-1-ylmethyl)phenol (L) and 38-methoxy-1,4,13,16,19,28-hexaaza-7,10,22,25-tetraoxatricyclo[14.14.7.1(32,36)]octatriconta-32,34,Delta(36,38)-triene (L1) are reported. Equilibrium studies of basicity and coordination properties toward metal ions such as Cu(II), Zn(II), Cd(II) and Pb(II) were performed for ligand by potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I= 0.15 mol dm(-3)). L behaves as a hexaprotic base (logK(1)= 10.93, logK(2)= 9.70, logK(3)= 8.79, logK(4)= 8.05, logK(5)= 6.83, logK(6)= 2.55). All metal ions form stable mono- and dinuclear complexes: logK(MLH(-1))= 25.61 for Cu(II), 15.37 for Zn(II), 12.58 for Cd(II) and 13.79 for Pb(II); logK(M(2)LH(-1))= 31.61 for Cu(II), 23.38 for Zn(II), 24.49 for Cd(II) and 23.68 for Pb(II). All these dinuclear species show a great tendency to add the OH(-) group: the equilibrium constant for the addition reaction was found to be logK(M(2)LH(-1)OH)= 4.77 for Cu(II), 5.66 for Zn(II), 2.8 for Cd(II) and 3.18 for Pb(II). In the case of Ni(II), kinetic inertness prevents the possibility of solution studies. The dinuclear solid adducts [Ni(2)H(-1)L(N(3))(3)].EtOH and [Cu(2)H(-1)L(N(3))](ClO(4))(2) were characterized by X-ray analysis. 相似文献
43.
Bazzicalupi C Bencini A Berni E Bianchi A Fornasari P Giorgi C Valtancoli B 《Inorganic chemistry》2004,43(20):6255-6265
Zn(II) binding by the dipyridine-containing macrocycles L1-L3 has been analyzed by means of potentiometric measurements in aqueous solutions. These ligands contain one (L1, L2) or two (L3) 2,2'-dipyridine units as an integral part of a polyamine macrocyclic framework having different dimensions and numbers of nitrogen donors. Depending on the number of donors, L1-L3 can form stable mono- and/or dinuclear Zn(II) complexes in a wide pH range. Facile deprotonation of Zn(II)-coordinated water molecules gives mono- and dihydroxo-complexes from neutral to alkaline pH values. The ability of these complexes as nucleophilic agents in hydrolytic processes has been tested by using bis(p-nitrophenyl) phosphate (BNPP) as a substrate. In the dinuclear complexes the two metals play a cooperative role in BNPP cleavage. In the case of the L2 dinuclear complex [Zn(2)L2(OH)(2)](2+), the two metals act cooperatively through a hydrolytic process involving a bridging interaction of the substrate with the two Zn(II) ions and a simultaneous nucleophilic attack of a Zn-OH function at phosphorus; in the case of the dizinc complex with the largest macrocycle L3, only the monohydroxo complex [Zn(2)L3(OH)](3+) promotes BNPP hydrolysis. BNPP interacts with a single metal, while the hydroxide anion may operate a nucleophilic attack. Both complexes display high rate enhancements in BNPP cleavage with respect to previously reported dizinc complexes, due to hydrophobic and pi-stacking interactions between the nitrophenyl groups of BNPP and the dipyridine units of the complexes. 相似文献
44.
Paolo Matteini Giovanni Agati Patrizia Pinelli Andrea Goti 《Monatshefte für Chemie / Chemical Monthly》2011,45(2):885-893
Abstract
The interaction of the model flavonoid glycoside rutin with the flavonoid reagent diphenylborinic acid 2-aminoethyl ester (DPBA) was investigated using a combination of HPLC–DAD–ESI–MS analysis, UV–visible spectroscopy, and semiempirical calculations. Mass spectra and spectroscopic data made it possible to describe the complexation pathway as addition of diphenylboron groups to the two available 3′,4′-o-diphenolic and the 5-hydroxy-4-keto coordinating sites of rutin. Semiempirical calculations were carried out to obtain the conformation of the most stable DPBA/rutin adducts. The results showed that a number of complexation dynamics can occur as a function of the characteristics of the medium (type and pH of the solvent) and of the amount of DPBA. This work suggests the possibility of substantially improving existing procedures for recognition of flavonoid compounds by choice of suitable experimental conditions. 相似文献45.
Francesca Tessore Dr. Elena Cariati Prof. Franco Cariati Prof. Dominique Roberto Prof. Renato Ugo Prof. Patrizia Mussini Prof. Cristiano Zuccaccia Dr. Alceo Macchioni Prof. 《Chemphyschem》2010,11(2):495-507
A series of 4‐X‐1‐methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)‐CH?CHC6H5; (E)‐CH?CHC6H4‐4′‐C(CH3)3; (E)‐CH?CHC6H4‐4′‐N(CH3)2; (E)‐CH?CHC6H4‐4′‐N(C4H9)2; (E,E)‐(CH?CH)2C6H4‐4′‐N(CH3)2) with various organic (CF3SO3?, p‐CH3C6H4SO3?), inorganic (I?, ClO4?, SCN?, [Hg2I6]2?) and organometallic (cis‐[Ir(CO)2I2]?) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second‐order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second‐order NLO response, investigated by the electric‐field‐induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10?3 M , and their dipole moments were determined. Generally, below 5×10?4 M , ion pairs start to dissociate into ions with parallel increase of the second‐order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10?3 M , some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second‐order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self‐aggregate. 相似文献
46.
47.
48.
Francesco De Vincentiis Giorgio Bencivenni Dr. Fabio Pesciaioli Andrea Mazzanti Dr. Giuseppe Bartoli Prof. Patrizia Galzerano Dr. Paolo Melchiorre Prof. Dr. 《化学:亚洲杂志》2010,5(7):1652-1656
The first catalytic method for the asymmetric aziridination of cyclic enones is described. The presented organocatalytic strategy is based on the use of an easily available organocatalyst that is able to convert a wide range of cyclic enones into the desired aziridines with very high enantiomeric purity and good chemical yield. Such a method may very well open up new opportunities to stereoselectively prepare complex chiral molecules that possess an indane moiety, a framework that is found in a large number of bioactive and pharmaceutically important molecules 相似文献
49.
50.
M. Assunta Girasolo Patrizia Portanova Giuseppe Ruisi 《Journal of organometallic chemistry》2010,695(4):609-3579
Novel diorganotin(IV) derivatives of l-Arginine (HArg), Nα-(tert-Butoxycarbonyl)-l-Arginine (Boc-Arg-OH) and l-Ala-l-Arg (H2Ala-Arg), H2NC(NH)NH(CH2)3CH(NHR′)CO2H, where R′ = H in HArg, R′ = C(O)OC(CH3)3 in Boc-Arg-OH, R′ = H2NCH(CH3)CO in H2Ala-Arg and triorganotin(IV) derivatives of Boc-Arg-OH have been synthesized and structurally characterized. The complexes were investigated by FT-IR and 119Sn Mössbauer in the solid state and by 1H, 13C, 119Sn and 1H-1H COSY NMR spectroscopy, in solution. The spectroscopic characterization leading to the proposed molecular structures was accomplished on the basis of these experiments. l-Arginine appears to behave as a chelating ligand through carboxylate and -NH2 groups in Me2Sn(Arg)2, while in Nα-t-Boc-l-Arginine complex, the Nα-protected amino group being exempted from coordination, only the carboxylate groups are effectors of bonding to the organometallic moieties. FT-IR spectra give a clear indication that guanidino groups in all the complexes are not involved in coordination, since ν(CN-H) frequency of the terminal guanidino group is fairly constant and unshifted relative to the free ligand. The biological activity of organotin(IV)-complexes was also investigated by use of human HT29 colorectal carcinoma cells. The cytotoxic activity of the compounds was determined by the MTT quantitative colorimetric assay, capable of detecting viable cells in comparison with that exerted by cisplatin. A marked cytotoxic activity for nearly all complexes, is evident being higher than that exerted by cisplatin, while no significant improvement of activity was observed for Me2Sn(Arg)2 and Me2Sn(Ala-Arg), which was confirmed by IC50 values. Then, we assessed whether the cytotoxicity induced by organotin(IV) complexes was associated with the induction of apoptosis. Light microscopy analysis, performed to study the morphological changes induced in HT29 cells, confirmed the results obtained with MTT test. No significant morphological alterations were observed in HT29 cells after treatment with Me2Sn(Ala-Arg) and Me2Sn(l-Arg)2. Cells treated with nBu2Sn(Boc-Arg)2, nBu2Sn(Ala-Arg), nBu3Sn(Boc-Arg) and Me3Sn(Boc-Arg), appeared rounded, isolated and detached from culture substrate, indicating the commitment to apoptotic cell death. 相似文献