全文获取类型
收费全文 | 315篇 |
免费 | 14篇 |
国内免费 | 4篇 |
专业分类
化学 | 209篇 |
晶体学 | 3篇 |
力学 | 10篇 |
数学 | 54篇 |
物理学 | 57篇 |
出版年
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 2篇 |
2019年 | 6篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 6篇 |
2015年 | 10篇 |
2014年 | 6篇 |
2013年 | 16篇 |
2012年 | 18篇 |
2011年 | 22篇 |
2010年 | 26篇 |
2009年 | 20篇 |
2008年 | 20篇 |
2007年 | 17篇 |
2006年 | 19篇 |
2005年 | 18篇 |
2004年 | 11篇 |
2003年 | 14篇 |
2002年 | 7篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 5篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1965年 | 1篇 |
1962年 | 1篇 |
1961年 | 2篇 |
排序方式: 共有333条查询结果,搜索用时 31 毫秒
1.
J.T. Elizalde Galindo A.H. Adair C.E. Botez V. Corral Flores D. Bueno Baques L. Fuentes Cobas J.A. Matutes-Aquino 《Applied Physics A: Materials Science & Processing》2007,87(4):743-747
Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni1-xZnxFe2O4 nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature
T̄B were obtained from the characteristic temperature behavior of the imaginary susceptibility χimag. An Arrhenius-type law, which accurately describes the relationship between the observation time τobs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal Ueff. A Zn-content dependence of the energy barrier is observed, where Ueff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of Ueff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins
in the A and B sub-lattices of the ferrimagnetic inverse spinel.
PACS 75.50.Bb; 75.50.Gg; 75.30.Et 相似文献
2.
Catalina Soriano‐Correa Rodolfo O. Esquivel Robin P. Sagar 《International journal of quantum chemistry》2003,94(3):165-172
A theoretical study at the Hartree–Fock and density functional theory levels is performed on sulfonamide‐type bacteriostatic compounds with the aim to provide an insight into their structure–activity relationship. The basicity of the p‐amino group is analyzed by means of the proton affinities and the protonation energies, showing that molecules presenting bacteriostatic activity are less basic, i.e., they are characterized by larger protonation energies and smaller proton affinities. The acidity of the amide group is analyzed through the deprotonation energy. The results reveal that the more acidic molecules present a larger bacteriostatic activity. This result is also confirmed from a study of bond orders. A bond order analysis of the amide group suggests that the electron attracting group in these molecules is responsible for the increase in acidity. The charge of the SO2 group is also shown to be affected by the presence of the electron attracting group and consequently related to the acidity of the molecules. A geometric analysis shows that structures in which the amino group is more coplanar with respect to the benzenic ring possess larger bacteriostatic activity. A conformational analysis of these molecules illustrates that active molecules have relatively larger torsion energy barriers. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 165–172, 2003 相似文献
3.
A new one-step facile method for the synthesis of 1,12,3,4-tetrahydro5H-5-oxo-1,4-benzo-diuzepines is described. The method involves the addition of methyl anlhranilates to elhyl-enimine followed by intramolecular cyclizalion of the resulting intermediate compounds. Another attempted method starting from β-anilinoethylisocyanales is also described. 相似文献
4.
Santiago Conde Carlos Corral Jaime Lissavetzky 《Journal of heterocyclic chemistry》1980,17(5):937-940
The synthesis by two alternative routes of 1-(3-benzo[b]thienyloxy)-3-isopropylamino-2-propanol hydrochloride ( 1a HCl), a thiophenic isoster of Propranolol, and related compounds, is reported. The protecting and enolizing properties of the 2-methoxycarbonyl group on benzo[b]thiophene-3-one, along with its facile removal, are utilized in the first route. In the second one, conversion of 3-bromobenzo[b]thiophene in 1-(3-benzo[b]thienyloxy)-2,3-o-isopropylidenepropane is the key step. On the other hand, hydrolysis of 1-(3-benzo[b]furanyloxy)-2,3-o-isopropylidenepropane to the corresponding diol, in order to obtain a furanic isoster of Propranolol (17a ), was unsuccessful. 相似文献
5.
Verdejo B Aguilar J Doménech A Miranda C Navarro P Jiménez HR Soriano C García-España E 《Chemical communications (Cambridge, England)》2005,(24):3086-3088
L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes. 相似文献
6.
Carlos Corral Jaime Lissavetzky Ana M. Valdeolmillos 《Journal of heterocyclic chemistry》1985,22(5):1349-1352
A series of the title compounds has been synthesized starting from methyl 3-hydroxythiophene-2-carboxylate. Its condensation with 2,4-dinitrochlorobenzene provides through compound 4e a way to introduce 7-substituents by means of diazonium salts reactions. 相似文献
7.
Santiago Conde Carlos Corral Jaime Lissavetzky 《Journal of heterocyclic chemistry》1985,22(2):301-304
A study on the oximation of a number of 2-acetylthiophenes in order to ascertain the validity of contradictory results previously described is reported. The fact that the steric hindrance is smaller in 2-acetylthiophenes unsubstituted at position-3 than in acetylbenzenes allows in these cases the formation of Z oximes, which even can predominate on the E oximes in the case of a + M substitution at position-5. In the paper is also shown that the E/Z ratio of 2-acetylthiophene oximes can be deduced from the 1H-nmr spectrum of the crude oxime mixture. 相似文献
8.
The influence of the nature of the heteroatom on the Ni+ gas-phase binding energies of HCC–XH3 (X is C, Si, or Ge) compounds has been investigated through the use of high-level density functional theory methods. The structures of the corresponding Ni+ complexes were optimized at the B3LYP/6-311G(d,p) level of theory. Final energies were obtained in single-point B3LYP/6-311+G(2df,2p) calculations. Nonconventional complexes, in which the metal cation interacts simultaneously with the CC system and with one of the X–H bonds of the substituent XH3 group, play a significant role in the binding of Ni+ to HCC–XH3 (X is Si or Ge) derivatives. Conversely, such nonconventional complexes are not local minima of the propyne–Ni+ potential-energy surface. This establishes a clear distinction between unsaturated carbon derivatives and the Si- and Ge-containing analogues as far as bonding to transition-metal monocations is concerned. Actually, the attachment of Ni+ to HCC–XH3 (X is Si or Ge) compounds in the gas phase yields a mixture of conventional and nonconventional complexes. These agostic-type interactions can be viewed as a dative bond from the X–H bonding orbital toward the empty s orbital of the metal, and a back-donation from the valence electron pairs of the metal into the X–H antibonding orbital of the neutral species.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail 相似文献
9.
The twelve tri-N-sulphonylhexahydro-1,3,5-triazines (R = alkyl or aryl) exhibit a common fragmentation pattern; most of the fragments are directly or indirectly generated from the [M ? RSO2]+ ion. Tri-N-Benzylsulphonylhexahydro-1,3,5-triazine (R = PhCH2) departs from the others mainly because the key ion [M ? RSO2]+ suffers a skeletal rearrangement with loss of SO2. The rationalizations presented are supported by metastable evidence, accurate mass measurements and deuterium labelling. 相似文献
10.
David S. Soriano Kenneth F. Podraza Norman H. Cromwell 《Journal of heterocyclic chemistry》1980,17(3):623-624
L-Azetidine-2-carboxylic acid is a naturally occurring azetidine currently used in the synthesis of abnormally high molecular weight polypeptides. The synthesis of 2-carboxy-4-methylazetidine is now reported which is a novel isomeric analog of dl -proline. Reaction of α,β-dibromo carbonyl ester with three molar equivalents of benzylamine yields 1-benzyl-2-carbomethoxy-4-methyl-azetidine. Hydrolysis of the latter compound with barium hydroxide yields 1-benzyl-2-carboxyl-4-methylazetidine which, if subjected to catalytic hydrogenation., yields the title compound in practical yield. The cis configuration is tentatively assigned to the title compound on the basis of previously published arguments. 相似文献