全文获取类型
收费全文 | 4366篇 |
免费 | 93篇 |
国内免费 | 7篇 |
专业分类
化学 | 2993篇 |
晶体学 | 45篇 |
力学 | 104篇 |
数学 | 287篇 |
物理学 | 1037篇 |
出版年
2021年 | 31篇 |
2020年 | 27篇 |
2019年 | 48篇 |
2016年 | 68篇 |
2015年 | 55篇 |
2014年 | 85篇 |
2013年 | 161篇 |
2012年 | 107篇 |
2011年 | 162篇 |
2010年 | 109篇 |
2009年 | 91篇 |
2008年 | 149篇 |
2007年 | 131篇 |
2006年 | 138篇 |
2005年 | 132篇 |
2004年 | 118篇 |
2003年 | 102篇 |
2002年 | 95篇 |
2001年 | 79篇 |
2000年 | 77篇 |
1999年 | 72篇 |
1998年 | 53篇 |
1997年 | 68篇 |
1996年 | 78篇 |
1995年 | 84篇 |
1994年 | 76篇 |
1993年 | 99篇 |
1992年 | 63篇 |
1991年 | 56篇 |
1990年 | 53篇 |
1989年 | 59篇 |
1988年 | 40篇 |
1987年 | 60篇 |
1986年 | 57篇 |
1985年 | 58篇 |
1984年 | 58篇 |
1983年 | 46篇 |
1982年 | 49篇 |
1981年 | 60篇 |
1980年 | 65篇 |
1979年 | 44篇 |
1978年 | 60篇 |
1977年 | 37篇 |
1976年 | 68篇 |
1975年 | 62篇 |
1974年 | 51篇 |
1973年 | 61篇 |
1972年 | 34篇 |
1969年 | 28篇 |
1967年 | 44篇 |
排序方式: 共有4466条查询结果,搜索用时 15 毫秒
991.
Zusammenfassung Die Darstellung eines neuartigen, zum gas-chromatographischen Nachweis geeigneten Phosphorderivats von Oestron wird beschrieben. Der Oestron-dimethylphosphinsäureester läßt sich sehr empfindlich mit dem Alkaliflammenionisationsdetektor (AFID) nachweisen. Die Umsetzung von Dimethylaminodimethylphosphin mit Oestron ist unter den angegebenen Bedingungen quantitativ und sehr gut reproduzierbar. Bezugskurven mit Oestronkonzentrationen von 2–50 g Oestron im Ansatz hatten bei Nachweis mit dem FID einen Korrelationskoeffizienten von 0,999, bei Nachweis mit dem AFID von 0,998. 200 pg des Derivats konnten noch gut gemessen werden. Die Reaktionsbedingungen werden diskutiert. Auf die erfolgreiche Derivatisierung anderer Steroidhormone wird abschließend hingewiesen.
Estrone-dimethylphosphinic ester, a new type of derivatives for the sensitive, gas chromatographic determination of steroids
The synthesis of estrone-dimethyl phosphinic ester, with good gas chromatographic properties is described. This ester is very sensitively detectable with the alkali detector. The reaction of dimethyl-amino-dimethylphosphine with estrone is quantitative and reproducible. Calibration curves with concentrations of estrone varying from 2 to 50 g per sample detected by a FID yielded a correlation coefficient of 0.999, detected by an AFID one of 0.998. The smallest amount which could be measured was 200 pg. The reaction conditions are discussed. The possibility of the derivatisation of other steroid hormones is shown.相似文献
992.
S. Mlinko E. Fischer und J. F. Diehl 《Fresenius' Journal of Analytical Chemistry》1972,261(3):203-208
Zusammenfassung Zur Ermittlung des natürlichen Tritiumgehaltes von Lebensmitteln wurde eine Verbrennungsanlage entwickelt, in der aus Lebensmitteln beliebiger Zusammensetzung in einem Arbeitsvorgang etwa 100 ml Wasser gewonnen werden. Zur Messung der Tritiumaktivität dieses Verbrennungswassers im Flüssigkeitsszintillationsspektrometer wird das Wasser von störenden Verbrennungsprodukten durch Destillation gereinigt. Aufbau und Arbeitsweise der Verbrennungsanlage und des Wasserreinigungssystems werden beschrieben.Die Teilnahme von S. Mlinko an diesen Untersuchungen wurden durch ein Stipendium der Alexander von Humboldt-Stiftung ermöglicht, wofür wir hier unseren herzlichen Dank sagen wollen.Frl. I. Kuschel wird für die wertvolle Mitarbeit bei den Untersuchungen gedankt. 相似文献
993.
Patrick Chaumette Hubert Mimoun Lucien Saussine Jean Fischer Andre Mitschler 《Journal of organometallic chemistry》1983,250(1):291-310
Novel oxoperoxomolybdenum(VI) complexes with the general formula MoO(O2)L2X2 (III, L = DMF, HMPT) and MoO(O2)Cl(ON)L(IV, ON) = pyridin-2-carboxylate (Pic), 8-hydroxyquinolinate (Quin) were prepared from the reaction of Ph3COOH or H2O2 with the corresponding cis-dioxo complexes. In the reaction with Ph3COOH both oxygen atoms of the peroxo moiety were found, by 18O labeling experiments, to come from the hydroperoxide. The X-ray crystal structure of MoO(O2)Cl(Pic)(HMPT) revealed a bipyramidal pentagonal surounding with a rather short OO distance (1.41 Å). Complexes III were found to be more reactive than MoO(O2)2,HMPT for the epoxidation of olefins (oxidative cleavage products are consecutively formed) but react by the same cyclic peroxymetalation mechanism. The absence of reaction in the case of complexes IV illustrates the necessity for the metal to possess an equatorial releasable coordination site adjacent to the peroxo group for the oxygen transfer to occur. Catalytic oxidation of olefins using Ph3COOH gave a selectivity in oxygenated products very different from that using t-BuOOH, and 18O labeling studies showed that alkyl-peroxidic rather than peroxo species are intermediates in this latter reaction. The mechanism of epoxidation of olefins by alkyl hydroperoxides catalyzed by d0 metal complexes is discussed. 相似文献
994.
Li QS Schwaneberg U Fischer P Schmid RD 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(9):1531-1536
The self-sufficient cytochrome P450 BM-3 enzyme from Bacillus megaterium catalyzes subterminal hydroxylation of saturated long-chain fatty acids and structurally related compounds. Since the primary structure of P450 BM-3 is homologous to that of mammalian P450 type II, it represents an excellent model for this family of enzymes. During studies on the directed evolution of P450 BM-3 into a medium-chain fatty-acid hydroxylase, several mutants, in particular the triple mutant Phe87Val, Leu188Gln, Ala74Gly, were observed to hydroxylate indole, producing indigo and indirubin at a catalytic efficiency of 1365 M(-1)s(-1) (kcat=2.73 s(-1) and Km=2.0 mM). Both products were unequivocally characterized by NMR and MS analysis. Wild-type P450 BM-3 is incapable to hydroxylate indole. These results demonstrate that an enzyme can be engineered to catalyze the transformation of substrates with structures widely divergent from those of its native substrate. 相似文献
995.
Karel Cizek Jiri Barek Jan Fischer Karolina Peckova Jiri Zima 《Electroanalysis》2007,19(12):1295-1299
The voltammetric behavior of 3‐nitrofluoranthene and 3‐aminofluoranthene was investigated in mixed methanol‐water solutions by differential pulse voltammetry (DPV) at boron doped diamond thin‐film electrode (BDDE). Optimum conditions have been found for determination of 3‐nitrofluoranthene in the concentration range of 2×10?8–1×10?6 mol L?1, and for determination 3‐aminofluorathnene in the concentration range of 2×10?7–1×10?5 mol L?1, respectively. Limits of determination were 3×10?8 mol L?1 (3‐nitrofluoranthene) and 2×10?7 mol L?1 (3‐aminofluoranthene). 相似文献
996.
Molar conductances of dilute aqueous benzoic acid solutions are presented for temperatures from 5 to 80°C. The data have been analyzed to give acid dissociation constants as well as ΔH o, ΔS o, and ΔC p o for the ionization process and the limiting conductance of the benzoate ion. The conductance-viscosity product changes less than 4% over the temperature range, indicating that the interaction of the benzoate ion with the solvent changes little if at all with increasing temperature. The pK a(m) vs.T data show that ΔH o decreases quadratically while ΔC p o increases linearly withT although, over the 75°C range, ΔC p o increases only about 6 cal-mole?1 deg?1 around an average of ?37 cal-mole?1deg?1. The acid dissociation constants as derived from the conductance-molal concentration analysis show an average uncertainty of about 0.1% and are fitted to within about 0.01% by the equation $$p{\text{K}}_{\text{a}} (m) = - 75.5422 + 3136.34/T + 28.7965 log T - 6.8139 {\text{x}} 10^{ - 3{\text{T}}} $$ whereT is the absolute temperature. 相似文献
997.
The salt elimination reaction of the transition carbonyl metal-lates [L(CO)nM](Na/K) (M = Cr, Mo, W, Mn, Re, Fe, Co, Ni; L= CO, n5-C5R5, PR3; n= 1-4; R= alkyl, aryl) with the base-stabilized galliumhalides ClaGaR3 -a(Do) (R = H, alkyl, halide; Do = THF, N(CH3)3, NC7H13) or ClaGa[(CH2)3N-R2](R)2 - a yielded almost quantitatively the transition metal-substituted, gallanes [L(CO)nM]aGaR3 - a(Do) and [L(CO)n-M]aGa[(CH2)3NR2](R)2 - a, respectively. Residual halide functionalities in these complexes were selectively replaced by various other groups. The new compounds were characterized by means of elemental analysis, 1H-, 13C-, 31P-NMR, MS, and lR v(CO) data. The single-crystal X-ray structure analysis of trans-(Ph3P)(CO)3Co-Ga[(CH2) 3N(C2H5)2](R)( 6s : R = Cl, 6t : R= CH3) showed s̀(Co-Ga) lengths of 237.78(4) and 249.5(1) pm, respectively. A short s̀(Fe-Ga) contact of 236.18(3) pm was found for (n5-C5H5)(CO)2Fe-Ga-Cl2[N(CH 3)3] ( 5a ). Low-pressure MOCVD experiments were performed to give thin films of analytically pure CoGa alloy. 相似文献
998.
Hans Schuh Edwin J. Hamilton Henning Paul Hanns Fischer 《Helvetica chimica acta》1974,57(7):2011-2024
The rate constants of the unimolecular decomposition of the pivaloyl radical (kD) and of the bimolecular self terminations of pivaloyl (k1) and t-butyl radicals (k2) in liquid methylcyclopentane are determined by ESR.-spectroscopy: The viscosity dependence of (k2) is analysed with respect to diffusion control of the reaction. Comparison of (kD) values of different acyl radicals reveals a strong dependence of the activation energies on radical structure. 相似文献
999.
A sensitive and selective high-performance liquid chromatography method has been developed for the determination of the new monocyclic beta-lactam antibiotic carumonam in plasma and urine. The method for plasma involves protein precipitation with acetonitrile and removal of lipids with dichloromethane; urine is diluted with buffer. Separation and quantification are achieved using a mobile phase based on either ion-suppression or ion-pair chromatography on a reversed-phase column with UV detection. The limit of determination is 0.5 micrograms/ml plasma, using a 0.5-ml specimen, and 25 micrograms/ml urine, using a 50-microliter specimen. The inter-assay reproducibility is generally better than 4% when an internal standard is used. Since beta-lactam antibiotics may degrade on storage, close attention must be paid to the stability of these drugs in biological fluids; novel measures to prevent degradation on storage are described. The assay has been successfully applied to the analysis of several thousand samples from pharmacokinetic studies, including a study involving patients with impaired renal function. 相似文献
1000.