全文获取类型
收费全文 | 1365篇 |
免费 | 67篇 |
国内免费 | 1篇 |
专业分类
化学 | 682篇 |
晶体学 | 24篇 |
力学 | 57篇 |
数学 | 197篇 |
物理学 | 386篇 |
无线电 | 87篇 |
出版年
2023年 | 9篇 |
2022年 | 19篇 |
2021年 | 40篇 |
2020年 | 33篇 |
2019年 | 41篇 |
2018年 | 64篇 |
2017年 | 38篇 |
2016年 | 58篇 |
2015年 | 51篇 |
2014年 | 44篇 |
2013年 | 66篇 |
2012年 | 89篇 |
2011年 | 94篇 |
2010年 | 72篇 |
2009年 | 59篇 |
2008年 | 72篇 |
2007年 | 47篇 |
2006年 | 53篇 |
2005年 | 52篇 |
2004年 | 28篇 |
2003年 | 23篇 |
2002年 | 32篇 |
2001年 | 19篇 |
2000年 | 24篇 |
1999年 | 14篇 |
1998年 | 19篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 8篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 10篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1974年 | 7篇 |
1972年 | 9篇 |
排序方式: 共有1433条查询结果,搜索用时 171 毫秒
21.
I. L. Odinets O. I. Artyushin E. A. Antonov P. V. Petrovskii B. I. Freger T. A. Mastryukova M. I. Kabachnik 《Russian Chemical Bulletin》1994,43(1):121-124
It was shown by31P and13C NMR spectroscopy that methyl(3-carboxy-3-oxopropyl)phosphinic acid (4-methylhydroxyphosphinyl-2-oxobutyric acid) (1) and the amide (2) of the latter exist in keto forms in non-aqueous solutions. In aqueous solutions an equilibrium between the keto,gem-diol, and enol forms has been observed. The proportions of the diol and enol forms increase as the acidity of the media increases. Silylation of acid 1 with hexamethyldisilazane gives the tris(trimethylsilyl) derivative of enol form (3) (Z- andE-isomers).Translated fromIzyestiya Akadetnii Nauk. Seriya Khimicheskaya, No. 1, pp. 125–128, January, 1994. 相似文献
22.
23.
Voinov MA Polienko JF Schanding T Bobko AA Khramtsov VV Gatilov YV Rybalova TV Smirnov AI Grigor'ev IA 《The Journal of organic chemistry》2005,70(24):9702-9711
[reaction: see text] An approach to the synthesis of new imidazoline nitroxides bearing an N',N'-disubstituted amidine group is reported. The approach is based on the alkylation of diamagnetic 4-R-amino-1,2,2,5,5-pentamethyl-3-imidazolines with bromoacetic acid ethyl ester; the products of alkylation are further oxidized to the corresponding nitroxides. The approach allows a variety of functional groups to be introduced into the nitroxide molecule structure. Alkylation with bromoacetic acid ethyl ester was found to proceed with high regioselectivity and afford the products of exo-alkylation. The regiochemical assignment is made on the basis of 13C NMR spectra and confirmed by X-ray diffraction study. All of the nitroxides synthesized here were shown to have pH-dependent EPR spectra with pKa ranging from 3.5 to 6.2. For nitroxides 13 bearing the carboxylic group remote to the nitroxide moiety, the changes in isotropic magnetic parameters of EPR spectra due to reversible deprotonation of the carboxylic group were found to be small. For these nitroxides, we demonstrate an alternative approach for pKa determination that is based on measuring the peak-to-peak line width of the EPR spectrum in the presence of the paramagnetic broadening agent potassium ferricyanide. The partition coefficients of nitroxides in octanol/H2O and octanol/phosphate buffer solution mixtures were measured to reveal a range of their lipophilicities. 相似文献
24.
Peter Schwendt Michal Sivák Andrey E. Lapshin Yury I. Smolin Yury F. Shepelev Dalma Gyepesov 《Transition Metal Chemistry》1994,19(1):34-36
Summary The monoperoxo complexes, M2[VO(HEDTA)(O2)]· 4H2O, where M is K+ or NH
4
+
and H4EDTA is ethylene-diaminetetraacetic acid, were prepared and characterized by Raman and i.r. spectra in the solid state and in aqueous solution. The single crystal X-ray study revealed a pentagonal bipyramidal anion structure with a tetradentate HEDTA(3—) ligand. The decomposition of complexes in aqueous solution to blue vanadium(IV) complexes as end products proceeds via a nonperoxo complex of vanadium. 相似文献
25.
The temperature dependences of the equilibrium constants of two chain reversible reactions in quinonediimine (quinonemonoimine)—2,5-dichlorohydroquinone
systems in chlorobenzene were studied. The enthalpy of equilibrium of the reversible reaction of quinonediimine with 4-hydroxydiphenylamine
was estimated from these data (ΔH = − 14.4±1.6 kJ mol−1) and a more accurate value of the N-H bond dissociation energy in the 4-anilinodiphenylaminyl radical was determined (D
NH = 278.6±3.0 kJ mol−1). A chain mechanism was proposed for the reaction between quinonediimine and 2,5-dichlorohydroquinone, and the chain length
was estimated (ν = 300 units) at room temperature. Processing of published data on the rate constant of the reaction of styrylperoxy
radicals with 2,5-dichlorohydroquinone in the framework of the intersecting parabolas method gave the O-H bond dissociation
energy in 2,5-dichlorohydroquinone: D
OH = 362.4±0.9 kJ mol−1. Taking into account these data, the O-H bond dissociation energy in the 2,5-dichlorosemiquinone radical was found: D
OH = 253.6±1.9 kJ mol−1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1661–1666, October, 2006. 相似文献
26.
27.
The synthesis has been effected of the amide of the tetrapeptide forming the sequence 6–9 of oxytocin with the use of benzyl protection of the thiol function of cysteine by two main schemes 1+3 and 2+2. The advantageousness of performing the synthesis by the 2+2 scheme has been shown. The overall yield of tetrapeptide using the method of condensation with the formation of mixed anhydrides amounted to 51% by the scheme proposed.All-Union Scientific-Research Institute of the Technology of Blood Substitutes and Hormone Preparations, Moscow. Translated from Khimiya Prirodnykh Soedinenii, Nos. 3,4, pp. 393–400, May–August, 1992. 相似文献
28.
Tsirelson Vladimir Abramov Yury Zavodnik Valery Stash Adam Belokoneva Elena Stahn Jochen Pietsch Ullrich Feil Dirk 《Structural chemistry》1998,9(4):249-254
The critical points in the model electron density distributions of LiF, NaF, NaCl, and MgO crystals, constructed from accurate X-ray diffraction data, are determined. For LiF and MgO they are compared with those obtained from a Hartree–Fock electron density calculation. Both experiment and theory show the same type of critical points on the bond lines. The topological features in areas between structural units, where the electron density is low and near-uniform, turn out to be model dependent and cannot be established well with the data available. Topological analysis of procrystals (hypothetical systems consisting of spherical atoms or ions placed on the same sites as atoms in real crystal) show that (3, –1) critical points, usually connected with bonding interaction, are observed on interatomic lines in these nonbonded systems as well. 相似文献
29.
K. Laihia A. Valkonen E. Kolehmainen A. Antonov D. Zhukov I. Fedosov V. Nikiforov 《Journal of Molecular Structure》2006,800(1-3):100-105
Three spiro[pyrrolidine-2,3′-oxindoles], 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-phenyl-spiro[3H-indole-3,3′-[3H]-pyrrolizine]-2′-carboxylic acid methyl ester (1), 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-nitro-2′-phenyl-spiro[3H-indole-3, 3′-[3H]-pyrrolizine] (2) and 1,1′,2,2′,5′,6′,7′,7′a-octahydro-2-oxo-1′-nitro-2′-(4″-chlorophenyl)-spiro[3H-indole-3,3′-[3H]-pyrrolizine] (3) have been synthesized and their 1H, 13C and 15N spectra assigned. The chemical shift assignments are based on Pulsed Field Gradient (PFG) Double Quantum Filter (DQF) 1H, 1H correlation spectroscopy (COSY), PFG 1H, 13C Heteronuclear Multiple Quantum Coherence (HMQC) and PFG 1H,X (X = 13C and 15N) Heteronuclear Multiple Bond Correlation (HMBC) experiments. The single crystal X-ray structures of 1–3 have been determined. Compounds 1 and 2 crystallized in monoclinic space group C2/c and compound 3 in monoclinic space group P21/c, respectively. Also the ESI-TOF MS data of 1–3 are given. 相似文献
30.