全文获取类型
收费全文 | 1906篇 |
免费 | 38篇 |
国内免费 | 9篇 |
专业分类
化学 | 1377篇 |
晶体学 | 31篇 |
力学 | 41篇 |
数学 | 244篇 |
物理学 | 260篇 |
出版年
2022年 | 9篇 |
2021年 | 16篇 |
2020年 | 23篇 |
2019年 | 20篇 |
2018年 | 15篇 |
2017年 | 18篇 |
2016年 | 38篇 |
2015年 | 24篇 |
2014年 | 45篇 |
2013年 | 101篇 |
2012年 | 108篇 |
2011年 | 133篇 |
2010年 | 54篇 |
2009年 | 62篇 |
2008年 | 130篇 |
2007年 | 127篇 |
2006年 | 145篇 |
2005年 | 114篇 |
2004年 | 96篇 |
2003年 | 83篇 |
2002年 | 89篇 |
2001年 | 39篇 |
2000年 | 19篇 |
1999年 | 25篇 |
1998年 | 22篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 33篇 |
1994年 | 36篇 |
1993年 | 16篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1973年 | 5篇 |
1972年 | 4篇 |
排序方式: 共有1953条查询结果,搜索用时 15 毫秒
11.
12.
Limonene is a common component found in consumer goods ranging from beverages to cleaning compounds. Limonene oxidation products, however, have a less desirable flavor and fragrance. Early detection of limonene oxide formation would aid quality control. A method is developed to determine the concentration of limonene oxide in essential oils and beverages using solid-phase microextraction (SPME). A headspace sampling technique is used to reduce or eliminate the presence of less volatile components. Several different SPME fibers are tested, varying in polymer thickness, polymer cross-linking and bonding, and polarity of the polymer. For each fiber tested, the sampling time is optimized for reproducible results. The 7-microm-thick bonded poly(dimethylsiloxane) fiber provides the best results. External standards are used for quantitation. 相似文献
13.
Ballou S Goodpaster J MacCrehan W Reeder D 《Analytical and bioanalytical chemistry》2003,376(8):1149-1150
14.
Angelo C. Pinto Susan K. Do Prado Raimundo Braz Filho William E. Hull Andras Neszmelyi Gabor Lukacs 《Tetrahedron letters》1982,23(50):5267-5270
With the help of Natural Abundance 13C - 13C Coupling Constants, observed via double quantum coherence, the structural determination of VELLOZIOLONE (1), a new diterpene, has been carried out. 相似文献
15.
16.
Plumb RS Stumpf CL Gorenstein MV Castro-Perez JM Dear GJ Anthony M Sweatman BC Connor SC Haselden JN 《Rapid communications in mass spectrometry : RCM》2002,16(20):1991-1996
The application of liquid chromatography/mass spectrometry (LC/MS) followed by principal components analysis (PCA) has been successfully applied to the screening of rat urine following the administration of three candidate pharmaceuticals. With this methodology it was possible to differentiate the control samples from the dosed samples and to identify the components of the mass spectrum responsible for the separation. These data clearly show that LC/MS is a viable alternative, or complementary, technique to proton NMR for metabonomics applications in drug discovery and development. 相似文献
17.
Buffers containing triethanolamine hydrochloride and EDTA decrease the sensitivity of the protein determination by the biuret reaction. This effect is not caused by changes in the pH but is due to an increase in the optical density (O.D. at 540 nm) of the biuret reagent by formation of a triethynolamine-copper complex. By increasing the concentration of the biuret reagent the sensitivity of the biuret method can be increased by about 33%. In the range between 0.05 and 0.23 the ionic strength of the buffers does not influence the O.D. of the biuret reagent. EDTA causes a remarkable decrease in the O.D. of the biuret reagent only at concentrations higher than 5 mM (in the buffer). This effect can be eliminated by increasing the amount of added biuret reagent. 相似文献
18.
Jaehun Park Doug S. English Yvonne Wannemuehler Susan Carpenter Jacob W. Petrich 《Photochemistry and photobiology》1998,68(4):593-597
The light-induced antiviral activity of hypericin and hypocrellin in the presence and absence of oxygen was examined under experimental conditions where the effect of oxygen depletion could be quantified. There was a significant reduction of light-induced antiviral activity of hypericin and hypocrellin under hypoxic conditions. Interestingly, antiviral activity of hypocrellin was not observed at low oxygen levels at which hypericin retained measurable virucidal activity. This suggests that additional pathways, such as the generation of protons from excited states of hypericin, may enhance the biological activity of activated oxygen species. 相似文献
19.
Conway B Hevia E Kennedy AR Mulvey RE Weatherstone S 《Dalton transactions (Cambridge, England : 2003)》2005,(8):1532-1544
Synthesised either by an unusual tert-butyl metathesis between tert-butyllithium and a n,s-butylmagnesium amide or by reaction of an alkyl Grignard reagent and a sodium amide, five tert-butylmagnesium amides, Bu(t)MgDBA (5)(DBA=dibenzylamide), Bu(t)MgDA (6)(DA=diisopropylamide), Bu(t)MgHMDS (7)(HMDS=1,1,1,3,3,3-hexamethyldisilazide), Bu(t)MgTMP (8)(TMP=2,2,6,6-tetramethylpiperidide) and Bu(t)MgNCy2 (9)(cy=cyclohexyl) have been isolated as crystalline solids. All five amides have been characterised by X-ray crystallography and solution NMR spectroscopic studies. The former studies reveal a common dimeric molecular structure with amido bridges in a planar (MgN)2 ring and terminal Bu(t) ligands on the Mg atoms. Also described is the dodecameric primary amide [Bu(n)MgN(H)Dipp]12 (10a) and its monomeric solvate Bu(n)MgN(H)Dipp.TMEDA (10b)(Dipp=2,6-diisopropylphenyl; TMEDA=N,N,N',N'-tetramethylethylenediamine). The crystal structures of the oxo-insertion products Bu(t)MgOBu(t).THF (11), Bu(t)Mg(mu-OBu(t))(mu-TMP)MgTMP (12) and Mg(OBu(n))HMDS.solv [solv=THF (13a) or Et2O (13b)], made fortuitously during the course of this work, are also presented. 相似文献
20.
By using low temperatures and largely deuterated solvents, the rate of OH proton exchange for aqueous solutions of alcohols is reduced sufficiently to give separate NMR signals from water and alcohol OH protons. The limiting shifts for dilute alcohols in water are down-field of both the water resonance and those of the pure alcohols. This contrasts with the limiting shift for water in the alcohols, which is to high field of the bulk water resonance. The resonance shifts initially to low fields as [ROH] increases, the rate of shift being greatest for t-butyl alcohol. For dilute aqueous solutions, all the alcohols reduce the total concentration of free OH groups, as judged by the overtone infrared spectra. Some of these results are interpreted in terms of a scavenging of free OH groups by the excess lone-pairs of the alcohol molecules. An extra, temperature dependent, down-field shift in the water proton resonance induced by t-butyl alcohol is assigned to a clathrate cage effect. 相似文献