全文获取类型
收费全文 | 3350篇 |
免费 | 261篇 |
国内免费 | 220篇 |
专业分类
化学 | 1939篇 |
晶体学 | 34篇 |
力学 | 67篇 |
综合类 | 38篇 |
数学 | 673篇 |
物理学 | 1080篇 |
出版年
2023年 | 13篇 |
2022年 | 27篇 |
2021年 | 17篇 |
2020年 | 49篇 |
2019年 | 52篇 |
2018年 | 57篇 |
2017年 | 58篇 |
2016年 | 89篇 |
2015年 | 69篇 |
2014年 | 92篇 |
2013年 | 277篇 |
2012年 | 245篇 |
2011年 | 142篇 |
2010年 | 116篇 |
2009年 | 184篇 |
2008年 | 247篇 |
2007年 | 216篇 |
2006年 | 187篇 |
2005年 | 176篇 |
2004年 | 164篇 |
2003年 | 131篇 |
2002年 | 115篇 |
2001年 | 117篇 |
2000年 | 105篇 |
1999年 | 110篇 |
1998年 | 94篇 |
1997年 | 63篇 |
1996年 | 63篇 |
1995年 | 55篇 |
1994年 | 63篇 |
1993年 | 40篇 |
1992年 | 58篇 |
1991年 | 41篇 |
1990年 | 33篇 |
1989年 | 23篇 |
1988年 | 33篇 |
1987年 | 28篇 |
1986年 | 23篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 20篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1973年 | 7篇 |
1972年 | 4篇 |
排序方式: 共有3831条查询结果,搜索用时 15 毫秒
31.
The stability of hexacyanoferrate(II)-amine(methylamine, ethylenediamine, diethylenetriamine and tetraethylenepentamine) was determined potentiometrically. Species Fe(CN)6(A)H
j
(j–4)
(A=amine) are formed in all the systems investigated, with j=1...n+2 (n=number of aminogroups). Some other complexes Fe(CN)6(A)iHj (with i>1) were also found. The stability of these complexes is fairly high: the full protonated amine species, show for the reaction Fe(CN)6
4- + HnAn+ = Fe(CN)6(A)Hn
(n-4) an equilibrium constant given by logK=0.686+2.10n. Factors affecting the stability are discussed in comparison with similar systems, together with the importance of interferences. 相似文献
32.
Apparent molar heat capacities and volumes of amylamine (PentNH2) 0.02m, capronitrile (PentCN) 0.02m and nitropentane (PentNO2) 0.009m in decyltrimethylammonium bromide (DeTAB) micellar solutions, in water and in octane were measured at 25°C. By assuming that their concentration approaches the standard infinite dilution state, heat capacities and volumes were rationalized by means of previously reported equations following which the distribution constant between the aqueous and the micellar phase and heat capacity and volume of the additives in both phases are simultaneously derived. The present results are compared to those we have previously obtained for pentanol (PentOH). The thermodynamic properties of PentNH2 in water and in micellar phase are substantially identical to those of PentOH but different from those of PentCN and PentNO2 whereas the opposite behavior was observed in their pure liquid state and in octane. The nature of the solvent medium seems to affect the thermodynamic behavior of PentNH2. Also, the study of the apparent molar heat capacities of the amyl compounds investigated here in micellar solutions as a function of surfactant concentration shows evidence of a maximum at about 0.4m DeTAB, which can be attributed to a micellar structural transition. Accordingly, the solubilities of PentCN and PentNO2 as a function of the DeTAB concentration drop in the neighborhood of the concentration where heat capacities display the maximum. 相似文献
33.
The excess Gibb's free energy of mixing, GE, for ethyl iodide+toluene at 25°C have been obtained from the measured vapor pressuure data. The HE and GE values for ethyl iodide+toluene are positive throughout the ethyl iodide concentration range and GE>HE. The results have been analyzed in terms of Flory and ideal associated model theory of nonelectrolyte solutions. It has been observed that the ideal associated model approach which assumes the presence of AN and A2B molecular species describes well (within±10 J-mol–1 in the worst case) the general dependence of HE on XA (mole fraction of ethyl iodide) over the whole composition range for ethyl iodide+toluene mixtures. The equilibrium constants for A+A AB and 2A+BA2B reactions along with the enthalpies of formation of AB and A2B molecular species have been calculated. 相似文献
34.
I. Brandariz F. Arce X. L. Armesto F. Penedo M. Sastre de Vicente 《Monatshefte für Chemie / Chemical Monthly》1993,124(3):249-257
Summary A systematic study of the dependence of acid-base stoichiometric constants on the ionic strength has been carried out for the aminoacidL-valine in ClK and BrK solutions. The observed dependence has been interpreted by using Guggenheim, Scatchard and Pitzer models for the activity coefficients of the species involved in the equilibria.
Die Abhängigkeit von Gleichgewichtskonstanten desL-Valin von der Ionenstärke entsprechend den Modellen von Guggenheim, Scatchard und Pitzer
Zusammenfassung Es wurde anL-Valin in KCl- und KBr-Lösungen eine systematische Studie der Abhängigkeit der Säure-Base-Stöchiometrie von der Ionenstärke durchgeführt. Die beobachteten Abhängigkeiten wurden aufgrund der nach den Modellen von Guggenheim, Scatchard und Pitzer erhaltenen Aktivitätskoeffizienten der an den Gleichgewichten beteiligten Spezies interpretiert.相似文献
35.
E. N. Ushakov L. G. Kuz’mina A. I. Vedernikov M. S. Kapichnikova J. A. K. Howard M. Wörner A. M. Braun M. V. Alfimov S. P. Gromov 《Russian Chemical Bulletin》2006,55(1):94-100
The structures of new butadienyl dyes of the benzothiazole series containing the dithia-15-crown-5 (2a) or dithia-18-crown-6 (2b) fragments were established by X-ray diffraction. Complexation of dyes 2a,b with Hg2+, Pb2+, Cd2+, Ag+, Zn2+, and alkaline-earth cations in aqueous-acetonitrile solutions was studied by spectrophotometry. At a high percentage of water
in solutions (P
w ≈ 50%), these dyes have a very low ability to bind Pb2+ cations (logK < 2) and virtually do not bind Cd2+, Zn2+, and alkaline-earth cations. At the same time, these dyes form stable 1: 1 complexes with Hg2+ and Ag+ cations at all P
w. The stability constants of complexes with the Ag+ cation increase with increasing P
w because the free energy of hydration of this cation is much lower than the free energy of solvation in acetonitrile. In the
P
w range from 0 to 75%, the stability constants of the complexes of dyes 2a,b with the Hg2+ cation are larger than those of the corresponding complexes with the Ag+ cation by more than four orders of magnitude.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 90–96, January, 2006. 相似文献
36.
Jasmina M. Dimitrić Marković Zoran S. Marković Jelisaveta M. Baranac Marina L. Dašić 《Monatshefte für Chemie / Chemical Monthly》2007,138(12):1225-1232
Summary. The study of delphinidin complexation with trivalent aluminum in acidic aqueous buffered (pH 3.0 and 3.8) and methanolic solutions was performed utilizing electronic absorption spectroscopy and quantum chemical calculations.
In its structure delphinidin possesses several chelating sites in competition towards aluminum(III). Molar ratio plots denoted
the formation of only one aluminum(III):delphinidin complex of stoichiometry of 1:1 in both investigated media. Semiempirical
calculations, performed at the restricted HF AM1 level, enabled the determination of the structural features of free delphinidin and structural modifications caused by
chelation of aluminum(III). Considering the pigment molecular structure and the results of the theoretical calculations it
is possible to equally implicate C3′–C4′ and C4′–C5′ hydroxyl groups as those with the predominant chelating power. 相似文献
37.
38.
Gabelica V Galic N Rosu F Houssier C De Pauw E 《Journal of mass spectrometry : JMS》2003,38(5):491-501
A method for determining the equilibrium association constant of a complexation reaction A + B left harpoon over right harpoon AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A-B solutions at different concentrations C(0). The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant K(a) and a factor R, defined by I(AB)/I(A) = R x [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with alpha,omega-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes. 相似文献
39.
100公里以上的大气分子离子主要为NO~+。它的辐射特点及它与电子、原子或分子的相互作用,对于理解大气的化学过程具有特别重要的意义。为了研究这些过程,确定NO~+分子离子基态及其各个激发态的分子势能函数是非常重要的。精确的X~1Σ~+,A~1Ⅱ和a~3∑~+势能曲线已发表;基于光电子谱的研究发现了NO~+的其它激发态,但对于这些激发态的研究尤其是势能函数的研究不多。本文研究并导出NO~+的基态和10个激发态的势能函数。 相似文献
40.
Abdul -Latif K. Al-Jiburi Kisma H. Al-Niami Muthana Shanshal 《Theoretical chemistry accounts》1979,53(4):327-335
The harmonic force constants, vibrational frequencies and integrated intensity ratios of CH2, H2O, CH2O, C2H2, CO2, HCN, CH3, CH4, and C2H4 have been calculated using the MINDO—FORCES program and the Pulay method for the calculation of the molecular force constants. The results obtained are in general quite satisfactory when compared with available literature values. The results are, however, not as satisfactory in case of molecules containing heteroatoms, due to the neglect of some dipolar repulsion integrals for the heteroatoms by the MINDO/3 method. Calculated integrated intensities for CH3 and C2H4 agree well with experimental results. The calculated integrated intensities for other molecules are obtained for the first time and no comparison with published data is therefore possible.Part of the M.Sc. Thesis of K. H. A. 1978. 相似文献