全文获取类型
收费全文 | 3618篇 |
免费 | 448篇 |
国内免费 | 416篇 |
专业分类
化学 | 3040篇 |
晶体学 | 43篇 |
力学 | 9篇 |
综合类 | 14篇 |
数学 | 8篇 |
物理学 | 1368篇 |
出版年
2024年 | 5篇 |
2023年 | 27篇 |
2022年 | 100篇 |
2021年 | 79篇 |
2020年 | 129篇 |
2019年 | 116篇 |
2018年 | 75篇 |
2017年 | 106篇 |
2016年 | 135篇 |
2015年 | 133篇 |
2014年 | 151篇 |
2013年 | 319篇 |
2012年 | 205篇 |
2011年 | 212篇 |
2010年 | 162篇 |
2009年 | 206篇 |
2008年 | 239篇 |
2007年 | 253篇 |
2006年 | 230篇 |
2005年 | 190篇 |
2004年 | 166篇 |
2003年 | 162篇 |
2002年 | 116篇 |
2001年 | 148篇 |
2000年 | 96篇 |
1999年 | 116篇 |
1998年 | 94篇 |
1997年 | 73篇 |
1996年 | 63篇 |
1995年 | 62篇 |
1994年 | 46篇 |
1993年 | 36篇 |
1992年 | 30篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 24篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有4482条查询结果,搜索用时 31 毫秒
91.
M. Eddaoudi A. W. Coleman P. C. Junk 《Journal of inclusion phenomena and macrocyclic chemistry》1996,26(1-3):133-151
The solubilities of -, -, and -cyclodextrin have been measured in the presence of the first row transition metals: Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+; chlorides, nitrates and sulphates (in this case Fe2+), and, for companson, with CaCl2, the corresponding Group IIa salt. Where possible the measurements are reported as a function of the activity of the salts. In general, for the transition metals the sulphates all show a linear decrease in solubility with increasing salt activity: for the nitrates the solubility increases and then reaches a limiting value; and for the chlorides a small decrease in solubility is observed at low activity followed by an increase in solubility at higher salt activity. Circular dichroism measurements confirm that there is no direct complexation at non-basic pH.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995. 相似文献
92.
The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical. 相似文献
93.
Fluorescence signalling of the transition metal ions: Design strategy based on the choice of the fluorophore component 总被引:1,自引:0,他引:1
Transition metal ions are notorious for their fluorescence quenching abilities. In this paper, we discuss the design strategies
for the development of efficientoff-on fluorescence signalling systems for the transition metal ions. It is shown that even simplefluorophore-spacer-receptor systems can display excellentoff-on fluorescence signalling towards the quenching metal ions when the fluorophore component is chosen judiciously. 相似文献
94.
研究了Zn2+、Cd2+、和Hg2+离子在碱式纤维素基磁性聚偕胺肟树脂(BMAO)上的吸附动力学。结果表明,吸附速度受粒内扩散所控制。测得粒内扩散系数分别是1.22×10-4、2.42×10-4和3.90×10-4cm2·s-1;Hg2+离子在树脂上的吸附行为符合Freundlich和Langmuir等温吸附方程;浓集因数随Hg2+离子浓度降低而上升,有利于从稀溶液中除去Hg2+离子。将树脂加至Zn2+、Cd2+或Hg2+离子浓度为50ppm的溶液中,离子去除率分别是96.1、99.9和99.3%。 相似文献
95.
Dean C. Luehrs Richard E. Brown Kalpana A. Godbole 《Journal of solution chemistry》1989,18(5):463-469
A linear free energy relationship was found betweenv
max, the energy of the absorption maximum of chloride, bromide, or iodide ion in various solvents, and the solvatochromic parameter which is a measure of the hydrogen bond donor ability of the solvent. The relationships are: for chloride,v
max=6.99+50.16 kK; for bromide,v
max=5.52+45.20 kK; for iodide,v
max=5.02+40.33 kK. The correlation ofv
max for iodide ion with gave a larger correlation coefficient and smaller standard deviation than a correlation with the E
T
(30) parameter of Dimroth. From the values ofv
max for iodide ion and the solvated electron in liquid ammonia at 25°C, these correlations indicate a preliminary value of 0.00 for the parameter of liquid ammonia. This had not been reported before and is lower than expected from a general knowledge of the physical and chemical properties of liquid ammonia but is consistent with a more detailed consideration and recent gas phase measurements of hydrogen bond donor strength. 相似文献
96.
利用质量分析离子动能谱和碰撞诱导解离技采研究了邻、间、对二甲苯分子在电子轰击质谱中产生的双电荷离子[C8H10]2+、[C8H9]2+和单电荷离子[C8H10]+。根据测定的电荷分离反应的释放动能T和由此估算的双电荷离子电荷分离反应过渡态两电荷间距R,推测出过渡态的结构,利用单电荷离子[C8H10]+的MIKES/CID谱可区分邻二甲苯与间、对二甲苯异构体. 相似文献
97.
Maksymilian Ignaczak Andrzej Grzejdziak 《Monatshefte für Chemie / Chemical Monthly》1988,119(1):71-81
Predominance areas of various equilibria were identified and complex formation constants of Ag(I) with 1,10-phenanthroline were determined as well as the solubility product of the complex salt for Ag(phen)2ClO4 in propylene carbonate. The solubility product of AgNO3 in propylene carbonate was estimated. The value of the formal potential of the system Ag(phen)
2
2+
/Ag(phen)
2
+
inPC was determined by chronovoltammetry. Differences in the stability of analogous complexes in water andPC are discussed.
Die Gleichgewichtskonstante der Komplexierung und die Redoxpotentiale im System Ag(II)/Ag(I)-1,10-Phenantrolin in Propylencarbonat
Zusammenfassung Die Gebiete der Dominanz der einzelnen Gleichgewichte, die Bildungskonstanten 1 und 2 der Komplexe Ag(I) mit 1,10-Phenantrolin und der Wert des Löslichkeitsproduktes Ag(phen)2ClO4 in Propylencarbonat wurden bestimmt. Das Löslichkeitsprodukt von AgNO3 in Propylencarbonat wurde abgeschätzt. Die Werte des formalen Potentials des Systems Ag(phen) 2 2+ /Ag(phen) 2 + inPC wurden mit Hilfe cyclischer Voltametrie ermittelt. Es wurden weiterhin die Unterschiede in der Stabilität analoger Komplexe in Wasser und in Propylencarbonat diskutiert.相似文献
98.
硼离子对铕掺杂SiO2干凝胶发光性能的影响 总被引:3,自引:0,他引:3
采用溶胶-凝胶法制备了Al单掺和B,Al共掺的Eu掺杂SiO2干凝胶。利用荧光光谱、IR,XRD,DSC,TG/DTG等技术研究了硼离子、退火温度对样品发光性质的影响。经500℃以上退火处理用248nm激发的样品,产生Eu^3+离子^5D0→^7FJ的特征发射,^5D0→^7F1的跃迁分裂为两个峰。比较615nm处的发光强度,掺硼酸样品的发光强度是不加硼酸发光强度的3.3倍。这是因为B离子的加入,在材料中形成了Si—O—B键,破坏了网络的对称性,加强了Eu^3+的红光发射。当退火温度上升到850℃用350nm激发时,样品有很强的Eu^2+蓝光发射。Al单掺的发射中心在437nm处,发射半峰宽约为70nm,而B,Al共掺样品的发光中心蓝移到425nm处,单掺样品的蓝光强度几乎是共掺样品强度的2倍。这是由于硼酸的加入改变了基质的网络结构,从而导致单掺和共掺样品发射峰位和强度的改变。 相似文献
99.
V. Yu. Korotaev Yu. A. Skorik A. Yu. Barkov M. I. Kodess A. Ya. Zapevalov 《Russian Chemical Bulletin》2005,54(11):2545-2549
3,3,3-Trifluoro-N′-(3-trifluoromethylphenyl)-1,2-propanediamine (5) was synthesized by the reaction of 2-diazo-1,1,1-trifluoro-3-nitropropane or 3,3,3-trifluoro-1-nitropropene with 3-aminobenzotrifluoride
followed by the reduction of the nitro group. The Michael 1,4-addition of diamine 5 to acrylic acid occurs only at the N(1) atom and affords N-mono-or N,N-dicarboxyethyl derivatives 6 and 7, depending on the reactant ratio. Protolytic equilibria 5–7 in aqueous solutions were studied by pH-potentiometry and UV spectroscopy. Only the aliphatic amino group can be protonated
in an aqueous solution, while the aromatic amino group remains unprotonated even in 12 M HCl. The stability constants of transition metal (Cu2+, Ni2+, Zn2+) complexes with ligands 5–7 were determined by pH-potentiometric titration. The stability of the complexes and selectivity of the ligands toward Cu2+ ions increase with an increase in the number of N-carboxyethyl groups.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2465–2469, November, 2005. 相似文献
100.
Earlier work by Lindenbaum and Boyd has demonstrated the important role of hydrophobic interactions involving the water solvent in determining the osmotic coefficients and properties of aqueous solutions of the tetraalkylammonium halides. Osmotic coefficients of solutions of tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium bromides in the more highly structured solvent D2O have now been determined by the gravimetric isopiestic method, using reference solutions of NaCl in D2O. The data were fitted to the Rush-Johnson and Pitzer equations. Satisfactory agreement with the results for aqueous solutions at comparable concentrations indicates that the solution chemistry of these quaternary ammonium bromides is not highly dependent on the degree of structure of the pure solvents. Supplementary data for mixtures of Me4NBr with Et4NBr, Pr4NBr, or Bu4NBr in both H2O and D2O are consistent with this conclusion.On leave 1980–82 from Banaras Hindu University, India 相似文献