全文获取类型
收费全文 | 514篇 |
免费 | 18篇 |
国内免费 | 1篇 |
专业分类
化学 | 456篇 |
晶体学 | 1篇 |
数学 | 47篇 |
物理学 | 29篇 |
出版年
2023年 | 6篇 |
2022年 | 24篇 |
2021年 | 26篇 |
2020年 | 10篇 |
2019年 | 20篇 |
2018年 | 10篇 |
2017年 | 9篇 |
2016年 | 24篇 |
2015年 | 19篇 |
2014年 | 18篇 |
2013年 | 33篇 |
2012年 | 26篇 |
2011年 | 25篇 |
2010年 | 22篇 |
2009年 | 20篇 |
2008年 | 34篇 |
2007年 | 26篇 |
2006年 | 25篇 |
2005年 | 30篇 |
2004年 | 21篇 |
2003年 | 30篇 |
2002年 | 15篇 |
2001年 | 9篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有533条查询结果,搜索用时 15 毫秒
111.
Summary We prove a ?virtual connectedness? theorem for plane curves with nodes, cusps and ordinary triple points. Irreducible curves
are so obtained from virtually connected arrangements of lines.
Riassunto Viene dimostrato un teorema di ?virtuale connessione? per curve piane con nodi, cuspidi e punti tripli ordinari. Tale teorema viene poi applicato alla costruzione di curve irriducibili a partire da configurazioni di rette virtualmente connesse.相似文献
112.
The syntheses of dinuclear calcium perchlorate and/or nitrate complexes by template and direct methods, employing macrocyclic ligands with 18, 20, 22, and 26 membered rings are reported. The presence of pendant arms provide with coordinative NxOy donor atoms in the smaller macrocycles, the high number of donor atoms between 7 and 10, and the dinuclear composition obtained in all the systems examined, point out that in the formed solid complexes both Ca2+ ions could be located inside of the macrocycle cavities. Transmetallation reaction of a lanthanide(III) complex, [L5Sm](ClO4)3·9H2O, with Ca(ClO4)2·xH2O leads the formation of the new dinuclear orange [L5Ca2](ClO4)4·3H2O complex, manifesting the versatility of this macrocyclic cavity. All complexes have been characterized by microanalysis, IR, UV‐vis, 1H NMR spectroscopy, FAB mass spectrometry, FAAS spectroscopy, and conductivity measurements. 相似文献
113.
Agnieszka Małgorzata Pudlarz Ewa Czechowska Katarzyna Ranoszek-Soliwoda Emilia Tomaszewska Grzegorz Celichowski Jarosław Grobelny Janusz Szemraj 《Applied biochemistry and biotechnology》2018,185(3):717-735
Human catalase cDNA was cloned into a pEX-C-His vector. Purified recombinant catalase was immobilized on nanoparticles. Gold and silver nanoparticles were synthesized in a variety of sizes by chemical reduction; no agglomerates or aggregates were observed in any of the colloids during dynamic light scattering or scanning transmission electron microscopy analysis. After immobilization on gold nanoparticles, recombinant catalase activity was found to be lower than that of the same amount of enzyme in aqueous solution. However, after 10 days of storage at room temperature, the activity of catalase immobilized on gold nanoparticles (AuNPs) of 13 and 20 nm and coverage of 133% was 68 and 83% greater than catalase in aqueous solution, respectively. During 10 days of experiment, percentage activity of catalase immobilized on those gold nanoparticles was higher in comparison to CAT in aqueous solution. Catalase immobilized on silver nanoparticles did not lose activity as significantly as catalase immobilized on AuNPs. Those results confirm the ability to produce recombinant human enzymes in a bacterial expression system and its potential use while immobilized on silver or gold nanoparticles. 相似文献
114.
Zurowski Radoslaw Gluszek Malgorzata Antosik Agnieszka Pietrzak Emilia Rokicki Gabriel Szafran Mikolaj 《Journal of Thermal Analysis and Calorimetry》2018,133(1):453-463
Journal of Thermal Analysis and Calorimetry - To study the effect of ionic liquids (ILs) of the microstructure on the surface of the coal, four ILs ([Emim][BF4], [Bmim][BF4], [Bmim][NO3], and... 相似文献
115.
We consider smooth non-degenerate surfaces in ℙ4, and prove that there is a finite number of such surfaces which are:
A complete list is given in both cases. 相似文献
(a) | sectionally non-special, i.e.h1(O C(1))=0, where C is a general hyperplane section of S; or |
(b) | not of general type and non-special (i.e. h1(O C(1))=0. |
116.
Dov Talmor Joshua Hermolin Emilia Kirowa-Eisner 《Journal of Electroanalytical Chemistry》1982,139(1):81-95
The applications of the reverse pulse method are extended in order to distinguish between different paths of electrodimerization. The method for determination of rate constants of the chemical reaction coupled with charge transfer (ec2) is further developed. Measurements of K′d vs. tp provide valuable information for the determination of rate constants.The reverse pulse method is applied in the study of the electrodimerization of 1-alkyl-4-t-butylpyridinium ions (1-alkyl=?CH3; ?CH2CH3; ?CH(CH 3)2) in acetonitrile. It is found that the electrodimerization mechanism is consistent with the sequence: Py+ + e = Py ? Py2. The formal potential Eo′ is measured directly for 1-CH(CH3)2, and indirectly for the other pyridium ions. All values are identical within experimental error (Eo′ = ?1.835 V vs. Ag/Ag+).The values of kd and km are measured. With an increase in the size of the l-alkyl group, kd decreases and km increases. This is consistent with the expected “stabilization” of the radicals due to the increase of the l-alkyl group size which hinders the dimerization site (2- and 6-positions). 相似文献
117.
Martinez M del Carmen Michelini M Rivalta I Russo N Sicilia E 《Inorganic chemistry》2005,44(26):9807-9816
The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined. 相似文献
118.
Quagliotto P Viscardi G Barolo C Barni E Bellinvia S Fisicaro E Compari C 《The Journal of organic chemistry》2003,68(20):7651-7660
A new series of pyridinium cationic gemini surfactants was prepared by quaternization of the 2,2'-(alpha,omega-alkanediyl)bispyridines with N-alkylating agents, whose reactivity is briefly discussed. Particularly useful was the use of long-chain alkyl triflates (trifluoromethanesulfonates) for both overcoming the sterical hindrance in the pyridines and obtaining higher synthetic yields. Well-known 4,4'-(alpha,omega-alkanediyl)bis(1-alkylpyridinium) structures showed narrow temperature ranges for practical applications, due to their high Krafft points, while the new 2,2'-(alpha,omega-alkanediyl)bis(1-alkylpyridinium) series, accounted for good surface active properties. Due to the Krafft points below 0 degrees C, they could be exploited as solutions in water at any temperature. The characterization of the behavior of the series was performed by conductivity measurements. Some of the proposed structures exhibited unusual surface active behavior, which was interpreted in terms of particular conformational arrangements. 相似文献
119.
Emilia Bertolo Rufina Bastida David E. Fenton Carlos Lodeiro Alejandro Macias Adolfo Rodroguez 《Journal of inclusion phenomena and macrocyclic chemistry》2003,45(1-2):155-160
The ability of lanthanide(III) ions to form stable complexeswith three different macrocyclic ligands, L1 , L2 and L3 , has been investigated.The Schiff base macrocycle L1 and its corresponding reduced ligand L2 arederived from 2,6-bis(2-formylphenoxymethyl)pyridine and diethylentriamine;the reduced ligand L3 is derived from 2,6-diformylpyridine and N,N-bis(3-aminopropyl)methylamine. Lanthanide nitrate complexes of L1 and L2 have beenprepared by direct reaction between each ligand and the appropriate hydrated lanthanidenitrate; attempts to obtain the corresponding perchlorate complexes have been unsuccessful.All nitrate complexes of L1 give the expected [1:1, Ln:L1 ] stoichiometry; however, complexes obtained with L2 show a [2:1, Ln:L2 ] stoichiometry. Finally, complexation reactions with L3 have been carried out in order to investigatethe coordination capability of this small and flexible ligand towards the Ln(III) ions. 相似文献
120.
Price JM Nizzi KE Campbell JL Kenttämaa HI Seierstad M Cramer CJ 《Journal of the American Chemical Society》2003,125(1):131-140
A negatively charged analogue of meta-benzyne, 3,5-didehydrobenzoate, was synthesized in a Fourier transform ion cyclotron resonance mass spectrometer, and its reactivity was compared to that of the same ion generated previously in a flowing afterglow apparatus and to its positively charged cousin, N-(3,5-didehydrophenyl)-3-fluoropyridinium. 3,5-Didehydrobenzoate was found to react as a nucleophile with electrophilic reagents. In contrast, N-(3,5-didehydrophenyl)-3-fluoropyridinium does not react with the same electrophilic reagents but reacts instead with nucleophilic reagents. Neither ion is able to abstract hydrogen atoms from typical hydrogen atom donors. The absence of any radical reactivity for these meta-benzynes is consistent with predictions that radical reactions of singlet biradicals should be hindered as compared to their monoradical counterparts. High-level calculations predict that the carboxylate moiety does not significantly perturb the singlet-triplet splitting of 3,5-didehydrobenzoate relative to the parent meta-benzyne. 相似文献