全文获取类型
收费全文 | 8131篇 |
免费 | 284篇 |
国内免费 | 5篇 |
专业分类
化学 | 4800篇 |
晶体学 | 165篇 |
力学 | 175篇 |
数学 | 1634篇 |
物理学 | 1646篇 |
出版年
2023年 | 41篇 |
2022年 | 48篇 |
2021年 | 65篇 |
2020年 | 123篇 |
2019年 | 175篇 |
2018年 | 201篇 |
2017年 | 233篇 |
2016年 | 429篇 |
2015年 | 315篇 |
2014年 | 384篇 |
2013年 | 717篇 |
2012年 | 457篇 |
2011年 | 511篇 |
2010年 | 350篇 |
2009年 | 292篇 |
2008年 | 504篇 |
2007年 | 442篇 |
2006年 | 384篇 |
2005年 | 303篇 |
2004年 | 253篇 |
2003年 | 194篇 |
2002年 | 153篇 |
2001年 | 113篇 |
2000年 | 111篇 |
1999年 | 65篇 |
1998年 | 77篇 |
1997年 | 51篇 |
1996年 | 65篇 |
1995年 | 57篇 |
1994年 | 45篇 |
1993年 | 36篇 |
1992年 | 41篇 |
1991年 | 64篇 |
1990年 | 52篇 |
1989年 | 61篇 |
1988年 | 57篇 |
1987年 | 53篇 |
1986年 | 52篇 |
1985年 | 101篇 |
1984年 | 103篇 |
1983年 | 69篇 |
1982年 | 62篇 |
1981年 | 70篇 |
1980年 | 66篇 |
1979年 | 52篇 |
1978年 | 41篇 |
1977年 | 33篇 |
1976年 | 43篇 |
1975年 | 36篇 |
1974年 | 32篇 |
排序方式: 共有8420条查询结果,搜索用时 62 毫秒
91.
pH gradient high-performance liquid chromatography (HPLC) is a method of reversed-phase high-performance liquid chromatography suitable for ionogenic substances. It consists in programmed increase during the chromatographic process of the eluting strength of eluent with respect to the analytes separated. On the analogy of the conventional organic modifier gradient reversed-phase HPLC, in the pH gradient approach the eluting strength of the mobile phase increases due to its changing pH: increasing in case of acids or decreasing in case of bases. At the same time the content of organic modifier remains constant. A theory of the pH gradient HPLC has been elaborated. The resulting mathematical model is easily manageable. Its ability to predict changes in retention and separation of analytes following the changes in chromatographic conditions is demonstrated. The pH gradient method is uniquely suitable to determine pKa values of analytes. An equation is presented allowing to calculate pKa values basing on appropriate retention data. The effects on pKa are discussed of the concentration of methanol in the mobile phase. The RP HPLC-derived pKa data correlate to the reference pKa values (w(w)pKa) but are not identical. That may be explained by the effects on the chromatographically determined pKa of the specific interactions of analytes with stationary phases. The proposed pH gradient RP HPLC procedure offers a fast and convenient means to get comparable acidity parameters for larger series of compounds, like drug candidates, also when the analytes are available only in minute amounts and/or as complex mixtures. 相似文献
92.
The overall stability constantsK
1 andK
2 of NdNO
3
2+
and Nd(NO3)
2
+
complexes were determined (K
1=1.77;K
2=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio 1/2 of the stepwise stability constants is discussed. It was established that the Nd(NO3)
2
+
complex was an outer-sphere ion pair.
Die Assoziation von Nitrat-Ion mit Nd3+
Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK 1 undK 2 von NdNO 3 2+ - und Nd(NO3)+-Komplexen bestimmt (K 1=1.77;K 2=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis 1/2 der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO3)+-Komplex als ein Outer-Shere-Ionenpaar vorliegt.相似文献
93.
Tritium exchange at C-8 of xanthosine, theobromine, 1-, 3- and 7-methylxanthine in water has been studied. The rates of detritiation of these compounds have been determined over a pH range at constant temperature. Several mechanisms of exchange involving various ionic forms of substrate operating at different pH have been suggested. 相似文献
94.
The composition of the ion-association complex of Rhodamine B with silicomolybdate has been examined by Job's method and spectrophotometric titration. The ratio of Rhodamine B to silicon in the complex is 4 : 1. The same ratio is obtained by analysis of the crystalline complex. The composition of the complex is identical in aqueous medium, an organic solvent (ethanol) and in the crystalline state (C(28)H(30)N(2)O(3))(4)SiMo(12)O(40)). The complex is stable in the organic solvent and has its absorption maximum at 555 nm and a molar absorptivity of 5 x 10(5) 1.mole(-1)cm(-1). The complex can be advantageously used for the determination of silicon. 相似文献
95.
W. Gołkiewicz 《Chromatographia》1986,21(5):259-264
Summary Equations describing multi-step gradient elution with a mobile phase of constant composition in each step were derived. These
equations useful for calculating the retention volumes in both gradient HPLC and TLC were derived on the basis of the relationship
between the isocratic capacity factor and the volume fraction of the organic modifier. The validity of the equations was experimentally
verified in a LiChrosorbRP-18-water/methanol system for 11 methyl- and chlorobenzenes and phenols. A satisfactory agreement
between the theoretical and experimental k′ values was found. 相似文献
96.
Summary The [Rh(acac){P(OPh)3)}2] complex (Hacac = 2,4-pentadione) reacts in solution with gaseous HCN in the presence of P(OPh)3 to give [Rh{P(OPh)3}3CN]. Structural investigations of this complex including its31P n.m.r. spectra are reported. 相似文献
97.
Wójcik A Naumov S Marciniak B Hermann R Brede O 《The journal of physical chemistry. B》2005,109(31):15135-15144
Addition and elimination interaction of thiyl radicals with the C5-C6 double bond in pyrimidines was studied by the pulse radiolysis technique in aqueous solution with the use of different monitoring systems. For this purpose, p-thiocresol, cysteamine hydrochloride, and mercaptoethanol were used. The rate constants of addition and elimination of thiyl radicals were determined by applying the modified version of ACUCHEM (computer program for modeling complex reaction systems). Aliphatic thiyl radicals add to the pyrimidine C5-C6 double bond with k = 1.0-3.0 x 10(7) dm3 mol(-1) s(-1), whereas elimination takes place with k = 0.7-2.0 x 10(5) s(-1). Quantum chemical calculations at the B3LYP/6-31G(d)/PCM level show that the addition should occur at the C6 position of the pyrimidine ring and that the energy of interaction between thiyl radicals and the pyrimidine double bond C5-C6 is low. 相似文献
98.
We have studied the diffusion inside the silica network of sodium atoms initially located outside the surfaces of an amorphous silica film. We have focused our attention on structural and dynamical quantities, and we have found that the local environment of the sodium atoms is close to the local environment of the sodium atoms inside bulk sodo-silicate glasses obtained by quench. This is in agreement with recent experimental results. 相似文献
99.
Berezkin Victor G. Mardanov Robert G. Moiseew Aleksey A. Malinowska Irena Różyło Jan K. 《平面色谱法杂志一现代薄层色谱法》2002,15(5):377-379
JPC – Journal of Planar Chromatography – Modern TLC - 相似文献
100.
Kłonkowski Andrzej M. Widernik Teresa Grobelna Beata Jóźwiak Wojciech K. Proga Henryk Szubiakiewicz Ewa 《Journal of Sol-Gel Science and Technology》2001,20(2):161-180
The sol-gel process is a useful method for preparing two series of organically and co-ordinately modified xerogels of the types [CuN
n
]·N
5–n
·5xSiO4/2 (n < 4) and [Cu(N–N)n]·(N–N)2–n
·2x SiO4/2(n 2), where N = NH2(CH2)3 SiO3/2, N–N = NH2(CH2)2NH·(CH2)3SiO3/2 and x = [SiO4/2]/[N] or [SiO4/2]/[N–N]. The amino groups in the materials are coordinately active and participate partly in the coordination sphere of Cu(II) ions. The composition of the coordination sphere can be varied with the SiO4/2 content and also as a result of the thermal decomposition of the organic residues at higher temperatures.Because the xerogel materials are considered to be catalyst precursors, this study is focused on their coordination and thermal properties. The prepared xerogels, such as silica, aminated silicates with N and N–N, as well as those entities complexed with Cu(II), were characterised by FT-IR spectroscopy. During gelation and thermal decomposition the materials were analysed by electron paramagnetic resonance (EPR) spectroscopy. The xerogels were additionally studied by UV-Vis absorption spectroscopy. The gaseous products of the thermal decomposition of these materials in an Ar atmosphere were investigated by the use of FT-IR spectroscopy coupled with TG and DTG thermal analysis. These data were complemented by temperature-programmed decomposition (TPDec) in a 2% O2 + 98% Ar stream coupled with quadrupole mass spectroscopy. 相似文献