全文获取类型
收费全文 | 1847篇 |
免费 | 77篇 |
国内免费 | 37篇 |
专业分类
化学 | 1096篇 |
晶体学 | 8篇 |
力学 | 22篇 |
综合类 | 12篇 |
数学 | 196篇 |
物理学 | 627篇 |
出版年
2020年 | 37篇 |
2019年 | 21篇 |
2017年 | 19篇 |
2016年 | 32篇 |
2015年 | 45篇 |
2014年 | 25篇 |
2013年 | 43篇 |
2012年 | 81篇 |
2011年 | 89篇 |
2010年 | 45篇 |
2009年 | 50篇 |
2008年 | 73篇 |
2007年 | 78篇 |
2006年 | 68篇 |
2005年 | 48篇 |
2004年 | 52篇 |
2003年 | 36篇 |
2002年 | 37篇 |
2001年 | 56篇 |
2000年 | 53篇 |
1999年 | 49篇 |
1998年 | 24篇 |
1997年 | 33篇 |
1996年 | 18篇 |
1995年 | 25篇 |
1994年 | 42篇 |
1993年 | 36篇 |
1992年 | 35篇 |
1991年 | 27篇 |
1990年 | 30篇 |
1989年 | 23篇 |
1988年 | 28篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 34篇 |
1984年 | 34篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 28篇 |
1978年 | 25篇 |
1977年 | 19篇 |
1976年 | 37篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1973年 | 23篇 |
1972年 | 15篇 |
1968年 | 13篇 |
1967年 | 16篇 |
排序方式: 共有1961条查询结果,搜索用时 171 毫秒
71.
Preparation, Crystal and Molecular Structure of Triphenylphosphineoxide Hydrogen - fluoride (C6H5)3PO · HF (C6H5)3PO · HF was prepared from hydrofluoric acid (40%) and (C6H5)3PO in benzene. It crystallizes in the monoclinic space group P21/c with a = 1 032.8(3), b = 1 051.0(7), c = 1695.5(2) pm, β = 121.95(2)° and Z = 4; d (calc./obs.) 1.27/1.26 g ° cm?3. The structure was determined by direct methods from 2 709 independent reflections and has been refined by full matrix least squares methods to R = 0.049. In the compound HF and (C6H5)3PO are linked by a short H-bond. Some distances: O? F 238.4(5), O? H 142.3, H? F 99.8, P? O 149.5(4) pm. Angle O? H? F 159.8°. 相似文献
72.
R. H. Dekker B. N. Srinivasan J. R. Huber ‡ K. Weiss 《Photochemistry and photobiology》1973,18(6):457-466
Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λmax 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λmax 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH2 *) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines. 相似文献
73.
Walter Huber 《Mikrochimica acta》1969,57(5):897-904
Zusammenfassung Durch Hydrolyse in siedender, hochkonzentrierter Phosphorsäure läßt sich anscheinend aus allen Kohlensäurederivaten quantitativ Kohlendioxid abspalten. Dieses wird nach Reinigung und Trocknung gravimetrisch bestimmt. Folgende Verbindungen wurden analysiert: lineare und cyklische Harnstoffe, Thioharnstoffe, Urethane, Carbamate, Kohlensäureester, Phenylisocyanat, Melamin, Guanidin, Cyanursäure und Dicyandiamid. Die Anwendung verschiedener Phosphorsäurekonzentrationen erlaubt auch selektive Bestimmungen.
Herrn Prof. Dr.B. Timm zum 60. Geburtstag gewidmet. 相似文献
The determination of derivatives of carbonic acid via an acidic hydrolysis
Summary Hydrolysis in boiling highly concentrated phosphoric acid seemingly results in the quantitative splitting off of carbon dioxide from all derivatives of carbonic acid. The latter is determined gravimetrically after purification and drying. The following compounds were analyzed: linear and cyclic ureas, thioureas, urethanes, carbamates, esters of carbonic acid, phenylisocyanate, melamine, guanidine, cyanuric acid, dicyandiamide. The use of various concentrations of phosphoric acid permits selective determinations.
Herrn Prof. Dr.B. Timm zum 60. Geburtstag gewidmet. 相似文献
74.
75.
Zusammenfassung Es wurde untersucht, warum Indol und viele seiner Derivate bei der Aminostickstoffbestimmung ein Gasvolumen liefern, obwohl sie keine primäre Aminogruppe besitzen. An Hand von präparativen Befunden wird gezeigt, daß die anomale Reaktion bei Indol in folgender Weise abläuft: Bei Einwirkung von salpetriger Säure bildet sich 3-Nitrosoindol, das sieh mit freiem Indol zu Indolrot polymerisiert. Gleichzeitig lagert sich die Nitrosogruppe zur Isonitrosogruppe um. Diese reagiert nun mit überschüssiger salpetriger Säure unter Entwicklung von Stickstoff und Distickstoffoxyd. Ist die Stellung 2 und 3 besetzt, so bleibt die anomale Gasentwicklung aus, da sich keine Isonitrosoverbindung bilden kann. Es wurde auch das Verhalten von 3-Methylindol und 3-Indolylessigsäure untersucht und die anomale Reaktion erörtert.
I: Anomalie der Isonitrosoverbindungen1; II: Anomalie der Verbindungen mit NH-CO-Gruppierung2; III: Anomalie der Verbindungen mit aktiven Methylengruppen3; IV: Anomalie von Glycin4; V: Anomalie der Phenole5. 相似文献
Summary A study was made to learn why indole and many of its derivatives deliver a gas volume during the determination of the amino nitrogen, even though they contain no primary amino group. On the basis of preparative findings, it was shown that in the case of indole the anomalous reaction proceeds in the following fashion. When the nitrous acid reacts, 3-nitrosoindole is formed, which then polymerizes with free indole to indole red. At the same time, the nitroso group rearranges into the isonitroso group. The latter reacts with excess nitrous acid with evolution of nitric oxide and nitrogen dioxide. If positions 2 and 3 are occupied, there is no anomalous generation of gas, because no isonitroso compound can form. The behavior of 3-methylindole and 3-indylacetic acid was also studied and the anomalous reaction noted.
Résumé Les auteurs ont recherché pourquoi l'indol et nombre de ses dérivés donnent lieu à un dégagement gazeux lors de la détermination de l'azote aminé bien qu'ils ne contiennent aucun groupement aminé primaire. Des constatations effectuées au cours de travaux de chimie préparative ont montré que la réaction anormale de l'indol a le cours suivant: lors de l'action de l'acide nitreux il se forme du nitroso-3 indol qui en présence d'indol libre se polymerise en rouge d'indol. Simultanément le groupement nitrosé se transpose en groupement isonitrosé. Ce dernier réagit alors avec l'acide nitreux en excès en donnant lieu à un dégagement d'azote et de protoxyde d'azote. Si les positions 2 et 3 sont occupées il ne se produit pas de dégagement gazeux anormal car la combinaison isonitrosée ne peut se former. Les auteurs ont également examiné le comportement du méthyl-3 indol et de l'acide indoyl-3 acétique et discuté leurs réactions anormales.
I: Anomalie der Isonitrosoverbindungen1; II: Anomalie der Verbindungen mit NH-CO-Gruppierung2; III: Anomalie der Verbindungen mit aktiven Methylengruppen3; IV: Anomalie von Glycin4; V: Anomalie der Phenole5. 相似文献
76.
Comparative study of capillary zone electrophoresis and high-performance liquid chromatography in the analysis of oligonucleotides and DNA. 总被引:4,自引:0,他引:4
Capillary zone electrophoresis and high-performance liquid chromatography were compared with regard to the separation of oligonucleotides and double-stranded DNA. Both anion-exchange and reversed-phase high-performance liquid chromatography on non-porous particles are considered to be superior to capillary electrophoresis in terms of speed and selectivity in the analysis of oligonucleotides up to 30 bases in length. Moreover, reversed-phase chromatography allows the simultaneous purification of detritylated oligonucleotides with recoveries > 90%. Compared with anion-exchange chromatography, there is no need for a subsequent desaltation step because the volatile buffer system can be readily evaporated. With regard to dsDNA, however, the resolving power of capillary electrophoresis cannot be matched by anion-exchange chromatography at present. Moreover, the combined use of hydroxyethylcellulose and ethidium bromide not only yielded a separation efficiency equal to that achieved by means of gel-filled capillaries but also avoids some of their limitations such as the destruction of the gel matrix at high current densities and the bias involved in electrokinetic injection. 相似文献
77.
78.
Host-guest antenna materials 总被引:2,自引:0,他引:2
Calzaferri G Huber S Maas H Minkowski C 《Angewandte Chemie (International ed. in English)》2003,42(32):3732-3758
The focus of this review is on host-guest composites with photonic antenna properties. The material generally consists of cylindrical zeolite L crystals the channels of which are filled with dye molecules. The synthesis is based on the fact that molecules can diffuse into individual channels. This means that, under the appropriate conditions, they can also leave the zeolite by the same way. In some cases, however, it is desirable to block their way out by adding a closure molecule. Functionalization of the closure molecules allows tuning of, for example, wettability, refractive index, and chemical reactivity. The supramolecular organization of the dyes inside the channels is a first stage of organization. It allows light harvesting within a certain volume of a dye-loaded nanocrystalline zeolite and radiationless transport to both ends of the cylinder or from the ends to the center. The second stage of organization is the coupling to an external acceptor or donor stopcock fluorophore at the ends of the channels, which can trap or inject electronic excitation energy. The third stage of organization is the coupling to an external device through a stopcock molecule. The wide-ranging tunability of these highly organized materials offers fascinating new possibilities for exploring excitation-energy-transfer phenomena, and challenges for developing new photonic devices. 相似文献
79.
The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S2 of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φf (295 K) = 5.1 × 10?3 and φf(77 K) = 1.0 × 10?2 for XS, and φf (295 K) = 1.5 × 10?3 and φf (77 K) = 2.5 × 10?3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S2 is derived. It is concluded that internal conversion S2 ? S1 is the dominating deactivation channel of S2 with k77 Knr(S2 ? S1) = 1.0 × 1010 s?1 for XS and k77 Knr (S2→S1) = 2.2 × 1010 s?1 for TXS. Between 295 and 77 K, φf increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φf is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant knr(S2?S1) with an activation energy of 190 ± 20 cm?1 and a frequency factor k′nr = 3 × 1010 s?1 for the XS molecule in 3-methylpentane. 相似文献
80.
Zusammenfassung Es wurde die anomale Reaktion von Glycin mit salpetriger Säure erneut bearbeitet, da das vonAustin angegebene Reaktionsschema nicht zur Erklärung der anomalen Gasmengen ausreicht. Zunächst wurde wahrscheinlich gemacht, daß sich aus dem Carbeniumkation, das bei der Desaminierung von Glycin entsteht, unmittelbar Methylnitrolsäure bildet. Nitroessigsäure oder Nitromethan kommen als Zwischenprodukte nur in untergeordneter Menge in Betracht. Die Methylnitrolsäure ist Ausgangspunkt für die anomale Reaktion, die in zwei Konkurrenzreaktionen abläuft. Bei der einen Konkurrenzreaktion reagiert die Isonitrosogruppe von Methylnitrolsäure mit salpetriger Säure, wobei 1 Mol (N2 + N2O) sowie Nitroformaldehyd entstehen. Nitroformaldehyd reduziert salpetrige Säure zu N2O und N2 und wird zu Nitroameisensäure oxydiert. Letztere zerfällt unter CO2-Entwicklung. Die zweite Konkurrenzreaktion wurde im Prinzip bereits vonAustin angegeben und von uns entsprechend den Ergebnissen über den Abbau von Isonitrosoverbindungen modifiziert. Demnach wird Methylnitrolsäure thermisch unter Bildung von Knallsäure und salpetriger Säure zersetzt. Knallsäure lagert Essigsäure an. Dieses Addukt wird von salpetriger Säure unter Entwicklung von N2 und N2O zu Ameisensäure zersetzt. Ameisensäure wird in geringer Menge noch zu CO2 weiter-oxydiert.
Summary The anomalous reaction of glycine with nitrous acid was reinvestigated because the reaction scheme given byAustin is not adequate to explain the anomalous amounts of gas. At the start it was made probable that methylnitrolic acid is formed directly from the carbenium cation which results from the deamination of glycine. Nitroacetic acid or nitromethane are involved to only a minor extent as intermediate products. Methylnitrolic acid is the starting point for the anomalous reaction, which proceeds in two competing reactions. In one of the competing reactions the isonitroso group of methylnitrolic acid reacts with nitrous acid, whereby one mol (N2 + N2O) and also nitroformaldehyde are produced. Nitroformaldehyde reduces nitrous acid to N2O and N2, and is oxidized to nitroformic acid. The latter decomposes with evolution of CO2.The second competing reaction has previously been given, in principle, byAustin and modified by the present writers to correspond with the results of the degradation of the isonitroso compounds. Accordingly, methylnitrolic acid is decomposed thermally to produce fulminic acid and nitrous acid. Fulminic acid adds acetic acid. This adduct is decomposed by nitrous acid with generation of N2 and N2O and formic acid results. A slight proportion of the formic acid is also oxidized further to CO2.
Résumé Les auteurs ont à nouveau étudié la réaction anormale de la glycine sur l'acide nitreux car le schéma réactionnel donné parAustin ne permet pas de rendre compte des dégagements gazeux anormaux qui se produisent. II a tout d'abord été établi selon toute vraisemblance qu'à partir du cation carbénium qui se forme lors de la désamination de la glycine il se produit immédiatement de l'acide méthylnitrolique. L'acide nitroacétique ou le nitrométhane n'apparaissent comme produits intermédiaires qu'en quantités minimes. L'acide méthylnitrolique est le point de départ de la réaction anormale qui se partage en fait en deux réactions concurrentielles. Lors de l'une de ces réactions, le groupement isonitrosé de l'acide méthylnitrolique réagit avec l'acide nitrolique avec formation d'une molécule du mélange (N2 + N2O) ainsi que de nitroformaldéhyde. Le nitroformaldéhyde réduit l'acide nitreux en N2O et N2 tandis qu'il s'oxyde en acide nitroformique. Ce dernier se décompose avec dégagement de CO2. La seconde de ces réactions a été déjà indiquée en principe parAustin et modifiée par les auteurs compte tenu des données relatives à la dégradation des composés isonitrosés. D'après cette réaction l'acide nitrolique se décompose thermiquement avec formation d'acide fulminique et d'acide nitreux. L'acide fulminique se fixe par addition sur l'acide acétique. Le produit obtenu est décomposé par l'acide nitreux avec dégagement de N2 et N2O et formation d'acide formique. L'acide formique s'oxyde ensuite en faibles proportions en CO2.相似文献