全文获取类型
收费全文 | 855篇 |
免费 | 35篇 |
国内免费 | 7篇 |
专业分类
化学 | 514篇 |
晶体学 | 4篇 |
力学 | 29篇 |
数学 | 117篇 |
物理学 | 233篇 |
出版年
2022年 | 9篇 |
2021年 | 8篇 |
2020年 | 17篇 |
2019年 | 12篇 |
2018年 | 13篇 |
2017年 | 9篇 |
2016年 | 25篇 |
2015年 | 23篇 |
2014年 | 20篇 |
2013年 | 23篇 |
2012年 | 37篇 |
2011年 | 37篇 |
2010年 | 15篇 |
2009年 | 19篇 |
2008年 | 35篇 |
2007年 | 22篇 |
2006年 | 27篇 |
2005年 | 26篇 |
2004年 | 22篇 |
2003年 | 29篇 |
2002年 | 32篇 |
2001年 | 19篇 |
2000年 | 14篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 11篇 |
1994年 | 24篇 |
1993年 | 20篇 |
1992年 | 20篇 |
1990年 | 10篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1977年 | 8篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 15篇 |
1973年 | 6篇 |
1972年 | 10篇 |
1971年 | 7篇 |
1968年 | 6篇 |
1967年 | 7篇 |
1966年 | 14篇 |
1943年 | 8篇 |
1927年 | 7篇 |
排序方式: 共有897条查询结果,搜索用时 15 毫秒
11.
12.
Anthony PL Arnold RG Band HR Borel H Bosted PE Breton V Cates GD Chupp TE Dietrich FS Dunne J Erbacher R Fellbaum J Fonvieille H Gearhart R Holmes R Hughes EW Johnson JR Kawall D Keppel C Kuhn SE Lombard-Nelsen RM Marroncle J Maruyama T Meyer W Meziani Z Middleton H Morgenstern J Newbury NR Petratos GG Pitthan R Prepost R Roblin Y Rock SE Rokni SH Shapiro G Smith T Souder PA Spengos M Staley F Stuart LM Szalata ZM Terrien Y Thompson AK White JL Woods M Xu J Young CC Zapalac G 《Physical review D: Particles and fields》1996,54(11):6620-6650
13.
Lung A Stuart LM Bosted PE Andivahis L Alster J Arnold RG Chang CC Dietrich FS Dodge WR Gearhart R Gomez J Griffioen KA Hicks RS Hyde-Wright CE Keppel C Kuhn SE Lichtenstadt J Miskimen RA Peterson GA Petratos GG Rock SE Rokni SH Sakumoto WK Spengos M Swartz K Szalata Z Tao LH 《Physical review letters》1993,70(6):718-721
14.
15.
16.
Zusammenfassung Fluoren reagiert in Gegenwart starker Basen mit Biphenylenvinylbromiden vom Typ (Biph)C=C(R)-Br; dabei bilden sich unter Halogenwasserstoffabspaltung direkt die Anionen acider Propene, deren Entladung mit K3Fe(CN)6 sehr stabile Kohlenstoffradikale liefert. Die Dehydrierung des 1,4-Bis-[bis-biphenylen-propenyl-(2)]-benzols ergibt das Biradikal V. Die 2-Arylbis-biphenylen-allyl-Radikale liegen in Lösung und im Fest-zustand bis 77° K monomer vor; ihre Elektronen- und ihre ESR-Spektren wurden aufgenommen. 相似文献
17.
The effect of bubbles on the measured electrochemical parameters during hydrogen evolution on nickel
The hydrogen evoluton reaction has been investigated at nickel rotating and vertical stationary electrodes in 1 M HC1. Steady state current—potential, impedance—potential and potential step—current—time transients results are reported. The impedance is analysed by equivalent circuit to produce double layr capacity—potential, ohrnic resistance—potential (and charge transfer resistance—potential) curves. Some deductions are made about the bubble layer and its effect on the electron transfer mechanism for the reaction. 相似文献
18.
Polymerase chain reaction techniques for food allergen detection 总被引:5,自引:0,他引:5
Food allergies represent an important health problem in industrialized countries. Undeclared allergenic foods as contaminants in food products pose a major risk for sensitized persons. Reliable detection and quantification methods for food allergens are necessary to ensure compliance with food labeling and improve consumer protection. The methods currently used for the detection of potential allergens in foods are to target either the allergen itself or a marker that indicates the presence of the offending food. As markers for the presence of potentially allergenic foods or ingredients, specific proteins or DNA fragments are targeted. In routine food analysis, the enzyme-linked immunosorbent assay (ELISA) and the polymerase chain reaction (PCR) in the form of a real-time PCR or in combination with an ELISA have been used. The availability, the characteristics, and some future aspects of DNA-based methods in the rapid and sensitive detection of potentially allergenic food constituents or contaminations are discussed in this review. 相似文献
19.
Vera A. Vil' Yana A. Barsegyan Leah Kuhn Maria V. Ekimova Egor A. Semenov Alexander A. Korlyukov Alexander O. Terent'ev Igor V. Alabugin 《Chemical science》2020,11(20):5313
How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer–Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF3·Et2O/H2O2 system. Although the primary effect (alignment of the migrating C–Rm bond with the breaking O–O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C–Rm bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol−1 Curtin–Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2–3 kcal mol−1 penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph3P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI – a ring-opening process.Protecting stereoelectronic effects prevent Baeyer–Villiger rearrangement and stabilize γ-OX-γ-peroxylactones (X = H, OH), the previously elusive non-strained Criegee intermediates. 相似文献
20.
Etiolated pea seedlings require transformation of Pr phytochrome to Pfr before they display optimal phototropic response to unilateral blue light. This study investigates the possible role of auxin transport in explaining these phenomena. Labeled [2-14C]IAA applied to the intact terminal buds of dark-grown and red light-treated pea seedlings was measured 210 min later on the shaded and illuminated sides of the epicotyl as a function of direction and duration of irradiation with blue light. Totally darkened epicotyls show an asymmetry in distribution of radioactivity in the upper growth zone of the epicotyl, in favor of the side under the concave part of the apical hook. Red light, which greatly potentiates curvature toward subsequent unilateral blue light, lowers this asymmetry. Blue light directed to the epicotyl of red-pretreated plants in a plane parallel to the hook and from the side bearing the convex portion of the hook induces positive phototropic curvature as well as a surplus of radioactivity on the illuminated side of the upper epicotyl and on the shaded side of the lower growth zone of the epicotyl. Light directed to the side bearing the concave part of the hook also causes an accumulation of counts in the upper part of the lighted side but produces neither curvature of the epicotyl nor accumulation of counts in the lower shaded side. Because of this built-in physiological asymmetry in the growth zone just below the apical hook, it is difficult to explain the effects of red and blue light on curvature in terms of patterns of auxin distribution alone. 相似文献