全文获取类型
收费全文 | 1427篇 |
免费 | 29篇 |
国内免费 | 6篇 |
专业分类
化学 | 896篇 |
晶体学 | 15篇 |
力学 | 31篇 |
数学 | 196篇 |
物理学 | 324篇 |
出版年
2022年 | 7篇 |
2021年 | 10篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 9篇 |
2016年 | 19篇 |
2015年 | 23篇 |
2014年 | 25篇 |
2013年 | 58篇 |
2012年 | 48篇 |
2011年 | 78篇 |
2010年 | 54篇 |
2009年 | 51篇 |
2008年 | 74篇 |
2007年 | 76篇 |
2006年 | 62篇 |
2005年 | 80篇 |
2004年 | 66篇 |
2003年 | 62篇 |
2002年 | 50篇 |
2001年 | 18篇 |
2000年 | 25篇 |
1999年 | 14篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 21篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 17篇 |
1990年 | 11篇 |
1989年 | 20篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 20篇 |
1984年 | 18篇 |
1983年 | 12篇 |
1982年 | 23篇 |
1981年 | 26篇 |
1980年 | 23篇 |
1979年 | 20篇 |
1978年 | 22篇 |
1977年 | 24篇 |
1976年 | 15篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1973年 | 15篇 |
1972年 | 6篇 |
排序方式: 共有1462条查询结果,搜索用时 15 毫秒
21.
Jean BourguignonUlf Bremberg Georges DupasKristina Hallman Lars HagbergLaurent Hortala Vincent LevacherSerghey Lutsenko Emmanuel MacedoChristina Moberg Guy QuéguinerFredrik Rahm 《Tetrahedron》2003,59(48):9583-9589
Chiral pyridinooxazoline, quinolinooxazoline, bis(oxazolino)pyridine (pybox), and bisoxazoline (box) derivatives containing crown ether residues were prepared. Some of the ligands were assessed in substrate binding studies and in palladium catalyzed allylic alkylations. 相似文献
22.
Overloaded band profiles of phenol were measured on a C18-Kromasil column in gradient elution conditions. The mobile phase used was a mixture of methanol and water. The volume fraction of methanol was allowed to vary between 0 and 0.5. A general adsorption model, which expresses the amount of phenol adsorbed q* as a function of both its concentration C and the composition phi of the organic modifier (methanol) in the mobile phase, was empirically derived from previous independent adsorption experiments based on frontal analysis (FA) and frontal analysis by the characteristic point (FACP). Accordingly, the general model was an extension of the simplest heterogeneous model, the Bilangmuir model, to non-isocratic conditions. The low-energy sites followed the classical linear solvent strength model (LSSM), but not the high-energy sites whose saturation capacity linearly decreased with phi. The general model was validated by comparing the experimental and simulated band profiles in gradient elution conditions, in linear and non-linear conditions, as well. The band profiles were calculated by means of the equilibrium-dispersive model of chromatography with a finite difference algorithm. A very good agreement was observed using steps gradient (delta phi) from 0 to 50% methanol and gradient times t(g) of 20, 25, 30, 40, 60, 80 and 100 min. The agreement was still excellent for steps gradient from 5 to 45% (t(g) = 25 min), 5 to 35% (t(g) = 50 min), 5 to 25% (t(g) = 50 min) and 5 to 15% (t(g) = 50 min). Significative differences appeared between experience and simulation when the slope of the gradient (delta phi/t(g)) became too strong beyond 3.3% methanol per minute. This threshold value probably mirrored the kinetic of arrangement of the G18-bonded chains when the methanol content increased in the mobile phase. It suggested that the chromatographic system was not in a full thermodynamic equilibrium state when very steep mobile phase gradients were applied. 相似文献
23.
The examination of 19F chemical shifts for ca. 650 F-alkylated compounds of general formula CF3(CF2)nCF2X led to the following conclusions: the CF2 groups α to X are very sensitive to the nature of X, and are spread over a range of 85 ppm. The effect of the length of the F-alkyl chain decreases rapidly, so that δCF2(α) can already be considered as characteristic of X for n = 1 or 2 for most practical purposes. Solvent effects (in 9 different solvents having ε = 1.8 to 52.1) were found to be rather small except for the F-alkyl iodides. A chart which indicates the domain in which the CF2(α) resonance signal is to be expected is given for 42 different series of F-alkylated compounds; it is expected to provide the synthetic chemist with a useful tool for the identification and characterization of such compounds. 相似文献
24.
The influence of the salt concentration (potassium chloride) on the retention and overloading behavior of the propranolol cation (R'-NH2+ -R) on an XTerra-C18 column, in a methanol:water solution, was investigated. The adsorption isotherm data were first determined by frontal analysis (FA) for a mobile phase without salt (25% methanol, v/v). It was shown that the adsorption energy distribution calculated from these raw adsorption data is bimodal and that the isotherm model that best accounts for these data is the bi-Moreau model. Assuming that the addition of a salt into the mobile phase changes the numerical values of the parameters of the isotherm model, not its mathematical form, we used the inverse method (IM) of chromatography to determine the isotherm with seven salt concentrations in the mobile phase (40% methanol, v/v; 0, 0.002, 0.005, 0.01, 0.05, 0.1 and 0.2 M). The saturation capacities of the model increase, q(s,1) by a factor two and q(s,2) by a factor four, with increasing salt concentration in the range studied while the adsorption constant b1 increases four times and b2 decreases four times. Adsorbate-adsorbate interactions vanish in the presence of salt, consistent with results obtained previously on a C18-Kromasil column. Finally, besides the ionic strength of the solution, the size, valence, and nature of the salt ions affect the thermodynamic as well as the mass transfer kinetics of the adsorption mechanism of propranolol on the XTerra column. 相似文献
25.
The CD. spectra of carotenoids in the 220-500 nm region may by inspection be classified as (a) essentially Conservative, (b) intermediate, or (c) essentially nonconservative. A conservative spectrum shows in that spectral region a sequence of 5-6 relatively sharp Cotton effects of alternating sign, the rotatory strengths of which roughly add to zero. In a non-conservative spectrum Δ? has the same sign over the whole region and its absolute value is in general somewhat smaller. In general, typical conservative spectra invert upon isomerization of the molecule from all-trans to mono-cis. Non-conservative spectra do not invert. The model of a chiral polyene of the length of the conjugated carotene chromophore reproduces well the main features of the conservative spectra. The theoretical predictions and the experimental data are shown to conform to the C2-rule [43] [44]. The particular nature of the longest-wavelength transition is interpreted. Based on a summary of the chiroptic data on about 50 naturally occurring compounds, the question is discussed of when conservative spectra arise and when not. 相似文献
26.
The equilibrium C-H bond length has been determined up to now for about 40 polyatomic molecules. These data are used to demonstrate the existence of quantitative correlations betweenr
e(C-H), isolated C-H bond stretching frequency and average distancer
g. It is also shown that ab initio calculations are often reliable to calculate the absolute value ofr
e(C-H), if an empirical correction is made. Some other correlations are also discussed. Finally, accurater
e(C-H) values are predicted for simple molecules. 相似文献
27.
28.
Georges Ripka 《Czechoslovak Journal of Physics》1996,46(7-8):721-750
These are introductory lectures to the Nambu Jona-Lasinio model with scalar interactions. It is applied to quark models of mesons and baryons. A critical survey is made of regularization methods. The collective rotation of solitons is briefly summarized.Lecture given at the 8th Summer School on Intermediate Energy Physics: Hadron Dynamics at Low and Intermediate Energies, Prague, July 10–14, 1995. 相似文献
29.
30.
Vasse JL Levacher V Bourguignon J Dupas G 《Chemical communications (Cambridge, England)》2002,(19):2256-2257
Asymmetric reduction of methyl benzoylformate by a new NADH mimic is reported; depending on the hydride source used to reduce the NAD+ precursor, NADH mimics so obtained lead to an inversion of enantioselectivity, affording either (R)-methyl mandelate in 88% ee or (S)-methyl mandelate in 78% ee. 相似文献