全文获取类型
收费全文 | 1455篇 |
免费 | 14篇 |
国内免费 | 13篇 |
专业分类
化学 | 921篇 |
晶体学 | 5篇 |
力学 | 19篇 |
综合类 | 1篇 |
数学 | 297篇 |
物理学 | 239篇 |
出版年
2021年 | 11篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 21篇 |
2015年 | 10篇 |
2014年 | 19篇 |
2013年 | 71篇 |
2012年 | 61篇 |
2011年 | 70篇 |
2010年 | 41篇 |
2009年 | 29篇 |
2008年 | 67篇 |
2007年 | 65篇 |
2006年 | 68篇 |
2005年 | 65篇 |
2004年 | 61篇 |
2003年 | 55篇 |
2002年 | 62篇 |
2001年 | 24篇 |
2000年 | 22篇 |
1999年 | 22篇 |
1998年 | 8篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 20篇 |
1986年 | 13篇 |
1985年 | 23篇 |
1984年 | 39篇 |
1983年 | 18篇 |
1982年 | 28篇 |
1981年 | 22篇 |
1980年 | 9篇 |
1979年 | 26篇 |
1978年 | 29篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 26篇 |
1974年 | 23篇 |
1973年 | 20篇 |
1972年 | 14篇 |
1971年 | 8篇 |
1968年 | 9篇 |
排序方式: 共有1482条查询结果,搜索用时 15 毫秒
91.
Johann-Peter Melder Klaus Weber Andreas Weiler Emmerich Sackers Hans Fritz Dieter Hunkler Horst Prinzbach 《Research on Chemical Intermediates》1996,22(7):667-702
Methodological alternatives for the preparation of highly strained, highly pyramidalized dodecahedrene 2 (Estr=87.3 kcal mol?1; ?=43.5°, MM2) and 1,16-dodecahedradiene 3 (Estr=105.3 kcal mol?1; ?=42.9°, MM2) have been explored, protection/deprotection strategies have been tested—with the eye on their utilization for the generation of higher unsaturated dodecahedranes (e.g. 1,4, 16-triene 4, 1,4,10 (14),16-tetraene 5). For the acquisition of preparative quantities of monoene 2 the “P2F” catalyzed cis-β-elimination in bromododecahedrane, of diene 3 the FVP fragmentation of a “twofold protected” precursor (bis-furan adduct) have become the protocols of choice, which both profit from the recent synthetic advances along the pagodane → dodecahedrane scheme. Because of unusually effective steric protection the highly tilted C=C double bonds of 2 (λmax (CH3CN) = 254 nm, ν C=C = 1658 cm?1, δC=C = 164.4) and 3 (δC=C = 170.5) enter into thermal stabilization pathways (dimerization, oligomerization) only at higher temperatures (for 2 ca. 50% consumption after 5 h at 100°C in a 3·10?3 molar toluene solution); extreme sensitivity to oxygen is primarily attributed to kinetically and thermodynamically promoted allylic hydrogen abstraction. 相似文献
92.
Xiao D Rajian JR Hines LG Li S Bartsch RA Quitevis EL 《The journal of physical chemistry. B》2008,112(42):13316-13325
This article reports a study of the effect of anions on the optical Kerr effect (OKE) spectra of binary ionic liquid mixtures with one mixture comprising the 3-methyl-1-pentylimidazolium ([C 5mim] (+)) cation and the anions PF 6 (-) and CF 3CO 2 (-) (TFA (-)), and another mixture comprising the [C 5mim] (+) cation and the anions Br (-) and bis(trifluomethanesulfonyl)imide (NTf 2 (-)). The spectra were obtained by the use of optical heterodyne-detected Raman-induced Kerr Effect Spectroscopy at 295 K. The OKE spectra of the mixtures are compared with the calculated mole-fraction weighted sum of the normalized OKE spectra of the neat liquids. The OKE spectra are nearly additive for [C 5mim]Br/[C 5mim][NTf 2] mixtures, but nonadditive for [C 5mim][PF 6]/[C 5mim][TFA] mixtures. In the case of the equimolar [C 5mim][PF 6]/[C 5mim][TFA] mixture, the nonadditivity is such that the experimental OKE spectrum is narrower than the calculated OKE spectrum. The additivity or nonadditivity of OKE spectra for IL mixtures can be explained by assuming ionic liquids are nanostructurally organized into nonpolar regions and ionic networks. The ionic networks in mixtures will be characterized by "random co-networks" for anions that are nearly the same in size (PF 6 (-) and TFA (-)) and by "block co-networks" for anions that differ greatly in size (Br (-) and NTf 2 (-)). 相似文献
93.
Silva AM Filipe P Seixas RS Pinto DC Patterson LK Hug GL Cavaleiro JA Mazière JC Santus R Morlière P 《The journal of physical chemistry. B》2008,112(37):11456-11461
In micellar solutions, one-electron reduction of (*)O 2 (-) radical-anions by 3-alkylpolyhydroxyflavones (FnH) with alkyl chains of n = 1, 4, 6, 10 carbons produces phenoxyl radicals ( (*)Fn) identical to those obtained by one-electron oxidation by (*)Br 2 (-) radical-anions or by repair of tryptophan radicals. In cetyltrimethylammonium bromide (CTAB), F1H localizes in the Stern layer, and alkyl chains of other FnH solubilize in the hydrophobic interior, interacting with cetyl tails. This interaction produces more compact micelles with lower intramicellar fluidity, as suggested by the increase in the pseudo-first-order rate constant of (*)Fn formation ( k 1) from approximately 390 s (-1) for n = 1 to 610 s (-1) for n = 10, leading to an intramicellar bimolecular rate constant of 1 x 10 (5) M (-1) s (-1). Additionally, (*)F1 and (*)F4 decay by intermicellar bimolecular reaction (2 k = 20 and 2 x 10 (5) M (-1) s (-1), respectively) whereas other (*)Fn radicals are stable over seconds due to increased localization with regards to the Stern layer. In contrast, the thick uncharged hydrophilic palisade layer and the compact hydrophobic core of Triton X100 micelles are responsible for a much higher microviscosity resulting in a decrease in k 1 from approximately 15.6 s (-1) for n = 1 to 9.6 s (-1) for n = 10. 相似文献
94.
Janzen DE Vanderveer DG Mehne LF da Silva Filho DA Brédas JL Grant GJ 《Dalton transactions (Cambridge, England : 2003)》2008,(14):1872-1882
We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6). 相似文献
95.
Wilkes JG Rafii F Sutherland JB Rushing LG Buzatu DA 《Rapid communications in mass spectrometry : RCM》2006,20(16):2383-2386
Pyrolysis mass spectrometry (PyMS) was investigated as a rapid tool to distinguish potential bioterror hoax materials from samples containing pathogenic bacteria. A pyrolysis time-of-flight (TOF) mass spectrometer equipped with an alternative ionization technique, metastable atom bombardment (MAB), was used to produce sample spectra. These spectra were analyzed by principal component and discriminant analysis for pattern recognition. Materials investigated were two strains of Vibrio parahaemolyticus, one of which produced the tdh toxin, two Salmonella enterica serotypes, a biological mosquito control product containing spores of Bacillus thuringiensis, and several white to off-white powders (which could be used as hoax materials), such as flour, corn starch, methyl cellulose, and xanthan gum. PyMS distinguished bacterial samples from hoax materials. Furthermore, pattern analysis differentiated Vibrios from Salmonellae, Salmonella enterica Anatum from S. enterica Heidelberg, and the two V. parahaemolyticus strains from each other. The B. thuringiensis mixture was distinguished from other bacteria and powders, suggesting that PyMS with pattern recognition may differentiate samples containing pathogens, including Bacillus spp., from nonbiological agents and that it can be a rapid method for detection of bacteria. MS data acquisition took only 7 min for each sample. 相似文献
96.
Coated-wire ion-selective electrodes based on dinonylnaphthalene sulfonic acid (DNNS) are prepared for methadone, methylamphetamine, cocaine and protriptyline in protonated form. In each set, nearly-Nernstian responses are obtained while detection limits range from 10-5.5M for cocaine and methylamphetamine electrodes, to 10-6.0M for methadone, and 10-6.5 M for protriptyline electrodes. Selectivity is found to decrease in the order methadone, protriptyline, cocaine and methylamphetamine; these results are consistent with systematic selectivity studies reported earlier for electrodes in this family. 相似文献
97.
Kusari U Li Y Bradley MG Sneddon LG 《Journal of the American Chemical Society》2004,126(28):8662-8663
In contrast to reactions that have been observed in traditional organic solvents, decaborane olefin-hydroboration and alkyne-insertion reactions have been found to proceed in ionic liquid solvents without the need of a catalyst. These reactions now provide important new, high-yield synthetic pathways to functionalized decaborane and o-carborane clusters. 相似文献
98.
Frank J. Manganiello Larry W. Christensen W.M. Jones 《Journal of organometallic chemistry》1982,235(3):327-334
Contrary to earlier reports [1,2] cyclopropyliron σ complexes can be conveniently prepared in fair to excellent yields by photodecarbonylation of the corresponding acyl complexes. 相似文献
99.
100.
Andrews KW Schweitzer A Zhao C Holden JM Roseland JM Brandt M Dwyer JT Picciano MF Saldanha LG Fisher KD Yetley E Betz JM Douglass L 《Analytical and bioanalytical chemistry》2007,389(1):231-239
As part of a study initiating the development of an analytically validated Dietary Supplement Ingredient Database (DSID) in
the United States (US), a selection of dietary supplement products were analyzed for their caffeine content. Products sold
as tablets, caplets, or capsules and listing at least one caffeine-containing ingredient (including botanicals such as guarana,
yerba mate, kola nut, and green tea extract) on the label were selected for analysis based on market share information. Two
or three lots of each product were purchased and analyzed using high-pressure liquid chromatography (HPLC). Each analytical
run included one or two National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) and two
products in duplicate. Caffeine intake per serving and per day was calculated using the maximum recommendations on each product
label. Laboratory analysis for 53 products showed product means ranging from 1 to 829 mg caffeine/day. For products with a
label amount for comparison (n = 28), 89% (n = 25) of the products had analytically based caffeine levels/day of between −16% and +16% of the claimed levels. Lot-to-lot
variability (n = 2 or 3) for caffeine in most products (72%) was less than 10%. 相似文献