全文获取类型
收费全文 | 247608篇 |
免费 | 2579篇 |
国内免费 | 877篇 |
专业分类
化学 | 140203篇 |
晶体学 | 3530篇 |
力学 | 9002篇 |
综合类 | 7篇 |
数学 | 29096篇 |
物理学 | 69226篇 |
出版年
2021年 | 2136篇 |
2020年 | 2523篇 |
2019年 | 2822篇 |
2018年 | 3583篇 |
2017年 | 3466篇 |
2016年 | 5142篇 |
2015年 | 3152篇 |
2014年 | 4761篇 |
2013年 | 10847篇 |
2012年 | 8901篇 |
2011年 | 10679篇 |
2010年 | 7314篇 |
2009年 | 7023篇 |
2008年 | 10190篇 |
2007年 | 10105篇 |
2006年 | 9559篇 |
2005年 | 8715篇 |
2004年 | 7688篇 |
2003年 | 6880篇 |
2002年 | 6712篇 |
2001年 | 6404篇 |
2000年 | 5133篇 |
1999年 | 3854篇 |
1998年 | 3380篇 |
1997年 | 3409篇 |
1996年 | 3237篇 |
1995年 | 2613篇 |
1994年 | 2757篇 |
1993年 | 2766篇 |
1992年 | 2883篇 |
1991年 | 2918篇 |
1990年 | 2794篇 |
1989年 | 2730篇 |
1988年 | 2662篇 |
1987年 | 2648篇 |
1986年 | 2526篇 |
1985年 | 3381篇 |
1984年 | 3494篇 |
1983年 | 2822篇 |
1982年 | 3138篇 |
1981年 | 2954篇 |
1980年 | 2771篇 |
1979年 | 2936篇 |
1978年 | 3035篇 |
1977年 | 3053篇 |
1976年 | 3021篇 |
1975年 | 2857篇 |
1974年 | 2813篇 |
1973年 | 2927篇 |
1972年 | 1951篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
851.
Coulometric titrations of mineral acids, sulphonic acids, carboxylic acids, enols, imides and phenols have been carried out in t-butanol or in acetone with electrically generated tetrabutylammonium hydroxide. Either a potentiometric titration or a visual indicator end-point may be used. The amount of acid titrated ranges from 10 to 60 muequiv, and the precision and accuracy of the method are excellent. 相似文献
852.
In o,o'-dihydroxyazo-dye metal-ion indicators, one hydroxyl group is 10(4) times more acidic than the other. The location of the more acidic group has been a mystery. By methylation of the acidic group, reduction of the monomethylated compound, isolation and characterization of the split methoxyamine and hydroxyamine, the location of the acidic group has been established. Indicators examined include Calmagite, Eriochrome Black T, Eriochrome Blue Black R and 4-(2-pyridylazo)resorcinol (PAR). 相似文献
853.
An unambiguous synthesis of Cypridina etioluciferamine was accomplished in order to prove the structure of this important bioluminescent natural product. Several 2-aminopyrazine 1-oxides were synthesized in order to establish a spectroscopic method for determining the placement of substituents on the pyrazine nucleus of Cypridina etioluciferamine. Titanium tetrachloride was used to improve the yields of these compounds; for example, the yield of 2-amino-3-methyl-5-phenylpyrazine 1-oxide (19) from reaction of phenylglyoxal 1-oxime and α-aminopropionitrile was raised from 3% to 51% by the use of titanium tetrachloride. The pyrazine ring proton is found at τ 1·37 (DMSO-d6). The isomeric 2-amino-3-methyl-6-phenylpyrazine 1-oxide (22) was similarly prepared and its pyrazine ring proton is found at τ 2·18. This large difference (0·81 ppm) in chemical shift was used to determine whether a 2-aminopyrazine 1-oxide was 5- or 6- substituted. Prepared in an analogous fashion were 2-amino-5-(indol-3-yl)-3-methylpyrazine 1-oxide (23) and 2-amino-5-(indol-3-yl)-3-(3-phthalimidopropyl)pyrazine 1-oxide (16). The structures of these compounds were verified by NMR spectroscopy. By treatment with Raney nickel and hydrogen gas, then 100% hydrazine hydrate, 16 was converted to 2-amino-3-(3-aminopropyl)-5-indol-3-ylpyrazine (5), isolated as the dihydrochloride. This compound, with the indole moiety definitely placed at C-5, is identical with Cypridina etioluciferamine dihydrochloride (IR, UV, TLC). These results show that the structures of Cypridina etioluciferamine and luciferin are correct as published. 相似文献
854.
E. D. Sverdlov A. P. Krapivko E. I. Budovskii 《Chemistry of Heterocyclic Compounds》1971,7(9):1188-1190
The 1-β-D-ribofuranosides of 2-keto-4-(N-methoxyamino)pyrimidine, 2-keto-4-(N-methyl-N-methoxyamino)pyrimidine, and 2-keto-3-methyl-4-(N-methoxyamino)pyrimidine were synthesized, and their pKa values were determined by spectrophotometry. The pKa values of the compounds are evidence that the tautomeric equilibrium between the oxime and hydroxyamine forms of 1-β-D-ribofuranosyl-2-keto-4-(N-methoxyamino)pyrimidine in aqueous solutions is shifted to favor the oxime form (KT?25). 相似文献
855.
Nature and proportions of the products formed in the nitrations mentioned in the title are affected by the process of mixing. Pentamethylnitrobenzene, formed initially in the nitration of pentamethylbenzene, is attacked by a nitronium ion at a position meta to the nitro-group. In the σ-complex formed, loss of a proton from a methyl group leads to production of by-products and release of nitrite ions. The nitrous acid formed is protonated, and water is displaced from the nitrous acidium ion by the anion, PF6?, giving the nitrosonium ion which forms a 1:1 symmetric molecular complex with pentamethylbenzene. A similar complex is formed in the reaction between durene and nitronium salts. The results are consistent with the view that no σ-bond is formed between the aromatic compound and the nitrosonium ion. 相似文献
856.
An electrochemical method for the simultaneous determinations of HgII concentration and total AsIII and AsV concentration has been developed. The method does not require the additional preliminary step of the chemical reduction of AsV to AsIII, or oxidation of AsIII to AsV before stripping analysis takes place. Also, the method for the simultaneous determination of HgII concentration and AsIII concentration is described. Measurements were performed in 0.1 M HCl using a gold-plated graphite electrode as sensor. Detection limits for both methods are below 0.4 ppb. Relative standard deviation did not exceed 15%. The possible interference by other trace metals was investigated. Analyses of natural water and industrial solutions were made using proposed methods and AAS. The t-test demonstrates that there was no significant difference between the results obtained with these methods. Proposed methods decrease the time of analysis because concentrations of the HgII and arsenic ions were measured simultaneously. Also, the removal of the additional step of chemical reduction of AsV to AsIII or oxidation of AsIII to AsV decreases analysis time, and also reduces the chance of contamination due to the use of additional reagents. 相似文献
857.
T. Jagodzinski E. Jagodzinska T. Dziembowska B. Szczodrowska 《Chemistry of Heterocyclic Compounds》1988,24(3):337-344
The positions of the tautomeric equilibria for a number of thioacyl derivatives of 2-aminothiazole and 2-aminobenzothiazole were determined by UV and IR spectroscopy with the use of model compounds. Quantum-chemical calculations by the CNDO/2 method were made for some of the acyl and thioacyl derivatives of 2-aminothiazole and 2-iminothiazole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 24, No. 3, pp. 410–417, March, 1988. 相似文献
858.
Due to the fact that in a system undergoing spinodal decomposition the long wavelength fluctuations are enhanced, we expect that multiple events can be observed when the passage of light through the sample is measured. We study here the intensity of double scattering for both polarized and depolarized measurements, as a function of time and angle of detection. The time dependence is nonexponential and the dependence on the scattering wave vector is less pronounced than in the case of single scattering. For systems with spherical molecules it is possible to separate double and single scattering contributions if both polarized and depolarized measurements are made. This increases the amount and quality of information about the dynamics of the spinodal decomposition that can be obtained from light scattering experiments. 相似文献
859.
W. Sawka-Dobrowolska G. Bator L. Sobczyk E. Grech J. Nowicka-Scheibe A. Pawlukoj 《Structural chemistry》2005,16(3):281-286
In searching for new molecular materials the crystals of 1:1 complexes of tetramethylpyrazine (TMP) with chloranilic acid (CLA) were grown. In the crystalline lattice of TMP.CLA infinite chains of hydrogen bonded components are formed. Very strong (d(O·sN) = 2.692(2) {?}) hydrogen bonds without proton transfer are present. The components of the complex form stacks of parallel arrangement with the distance typical of van der Waals interaction. One can conclude that in packing, interactions via hydrogen bonds play a decisive role. In the IR spectrum one observes a broad, continuous absorption with well resolved trio typical of strong hydrogen bonds. A detailed analysis of IR and Raman spectra in the low frequency region related to deformation vibrations of CH3 groups shows that strong interaction via hydrogen bonding only slightly affects the dynamics of these groups. 相似文献
860.
Klyueva M. E. Stuzhin P. A. Berezin B. D. 《Russian Journal of Coordination Chemistry》2003,29(3):189-195
Complexes of manganese(III) and manganese(V) with octaphenyltetraazaporphine (H2OPTAP) were synthesized. Acid–base interactions of these complexes in the CH2Cl2–CF3COOH system and kinetics of their dissociation in concentrated sulfuric acid, as well as kinetics of octaphenyltetraazaporphine destruction in sulfuric acid solutions were studied by spectrophotometric methods. Acid–base interactions in CH2Cl2–CF3COOH were shown to involve two macrocyclic meso-nitrogen atoms in succession. Concentration stability constants of the acid forms obtained pK
1 = 0.29 ± 0.01 and pK
2 = –0.62 ± 0.08 for (chlorine)manganese(III)octaphenyltetraazaporphine ((Cl)MnOPTAP); pK
1 = 0.99 ± 0.02 and pK
2 = – 0.70 ± 0.03 for (nitrido)manganese(V)octaphenyltetraazaporphine ((N)MnOPTAP). The rate of dissociation of the complexes in 94–98% H2SO4 does not depend on the acid concentration. The manganese(V) complex is three times as stable as the manganese(III) complex. 相似文献