全文获取类型
收费全文 | 1846篇 |
免费 | 399篇 |
国内免费 | 57篇 |
专业分类
化学 | 2085篇 |
晶体学 | 4篇 |
力学 | 1篇 |
综合类 | 7篇 |
数学 | 27篇 |
物理学 | 178篇 |
出版年
2024年 | 2篇 |
2023年 | 20篇 |
2022年 | 43篇 |
2021年 | 75篇 |
2020年 | 158篇 |
2019年 | 101篇 |
2018年 | 86篇 |
2017年 | 63篇 |
2016年 | 122篇 |
2015年 | 122篇 |
2014年 | 120篇 |
2013年 | 139篇 |
2012年 | 101篇 |
2011年 | 93篇 |
2010年 | 81篇 |
2009年 | 75篇 |
2008年 | 84篇 |
2007年 | 107篇 |
2006年 | 101篇 |
2005年 | 80篇 |
2004年 | 76篇 |
2003年 | 80篇 |
2002年 | 40篇 |
2001年 | 28篇 |
2000年 | 29篇 |
1999年 | 37篇 |
1998年 | 30篇 |
1997年 | 42篇 |
1996年 | 35篇 |
1995年 | 27篇 |
1994年 | 17篇 |
1993年 | 17篇 |
1992年 | 12篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1972年 | 2篇 |
1970年 | 3篇 |
1968年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有2302条查询结果,搜索用时 34 毫秒
71.
Volker Staemmler 《Theoretical chemistry accounts》1982,62(1):69-80
Ab initio calculations for the 2 ground states of SF and SCl have been performed on Hartree-Fock level and with inclusion of valence shell correlation effects by means of the CI and CEPA approaches. The calculated properties are: Equilibrium distances, vibration frequencies, and dipole moment curves in the vicinity of the respective equilibrium geometries. Our best estimates for the 0 1 infrared absorption frequencies o for SF and SCl are 786 cm–1 and 520 cm–1, respectively, both with an uncertainty of about 10 cm–1. This confirms a recent experimental value obtained by Willner for SF (791 cm–1), but indicates that for SCl both experimental values reported previously in the literature (617 cm–1 and 574 cm–1) are wrong. The S—F and S—Cl bonds in SF and SCl are very similar to the ones in SF2 and SCl2, being essentially single p-bonds in either case. In the analogous oxygen-halogen molecules the situation is different, the O—F and O—Cl bonds in the diatomic radicals OF and OCl have partial double bond character and are much stronger than those in OF2 and OCl2 or in HOF and HOCl. 相似文献
72.
G. V. Romanenko D. V. Ovcharenko S. F. Vasilevskii 《Journal of Structural Chemistry》2003,44(2):314-317
The crystal and molecular structures of the stable nitroxide radical 2,4,4,5,5pentamethyl2imidazoline1oxyl3oxide was determined. The N—O bond lengths are 1.279(2) and 1.280(2), respectively. The O-—N+=C—N— O fragment is nearly planar with carbon atoms of the ethyl fragment that deviated from the O—N+=C—N—O plane by –0.204(5) and +0.176(5). The minimum intermolecular distance between the oxygen atoms of NO groups is 4.094. 相似文献
73.
Wurche F Sicking W Sustmann R Klärner FG Rüchardt C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(11):2707-2721
The effect of pressure on the oxidation of hydroarenes 3-9 with 2,3-dichloro-5,6-dicyano-1,4-quinone (DDQ; 1 a) or o-chloranil (10), leading to the corresponding arenes, has been investigated. The activation volumes were determined from the pressure dependence of the rate constants of these reactions monitored by on-line UV/Vis spectroscopic measurements in an optical high-pressure cell (up to 3500 bar). The finding that they are highly negative and only moderately dependent on the solvent polarity (DeltaV( not equal ) = -13 to -25 in MTBE and -15 to -29 cm(3) mol(-1) in MeCN/AcOEt, 1:1) rules out the formation of ionic species in the rate-determining step and is good evidence for a hydrogen atom transfer mechanism leading to a pair of radicals in the rate-determining step, as was also suggested by kinetic measurements, studies of kinetic isotope effects, and spin-trapping experiments. The strong pressure dependence of the kinetic deuterium isotope effect for the reaction of 9,10-dihydroanthracene 5/5-9,9,10,10-D(4) with DDQ (1 a) can be attributed to a tunneling component in the hydrogen transfer. In the case of formal 1,3-dienes and enes possessing two vicinal C--H bonds, which have to be cleaved during the dehydrogenation, a pericyclic hydrogen transfer has to considered as one mechanistic alternative. The comparison of the kinetic deuterium isotope effects determined for the oxidation of tetralin 9/9-1,1,4,4-D(4)/9-2,2,3,3-D(4)/9-D(12) either with DDQ (1 a) or with thymoquinone 1 c indicates that the reaction with DDQ (1 a) proceeds in a stepwise manner through hydrogen atom transfer, analogously to the oxidations of 1,4-dihydroarenes, whereas the reaction with thymoquinone 1 c is concerted, following the course of a pericyclic hydrogen transfer. The difference in the mechanistic courses of these two reactions may be explained by the effect of the CN and Cl substituents in 1 a, which stabilize a radical intermediate better than the alkyl groups in 1 c. The mechanistic conclusions are substantiated by DFT calculations. 相似文献
74.
Houjou H Kanesato M Hiratani K Mandon D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4576-4583
Novel diiron complexes with an Fe2(mu-OMe)2 core were studied as models of the active site of nonheme iron-containing enzymes. X-ray crystal structures of the complexes showed the existence of two types of ligand folding-parallel and twisted-both of which have four virtually equivalent phenolato groups sticking out from the Fe2O2 rhombic plane. Cyclic voltammetry measurements revealed two or more distinct redox waves in a region of relatively high potential, in addition to known Fe(II)/Fe(III) redox waves in a region of lower potential. These new peaks were assigned to the high-valence state of iron atoms, that is, Fe(III)Fe(IV) and Fe(IV)Fe(IV), resonating with the phenoxyl radical(s). The split width of the redox waves ranged from 0.14 to 0.20 eV, which may be a measure of the electronic interaction of the phenolate groups through the Fe2(mu-OMe)2 core. 相似文献
75.
L. N. Novikova N. A. Ustynyuk B. L. Tumanskii P. V. Petrovskii A. A. Borisenko S. V. Kukharenko V. V. Strelets 《Russian Chemical Bulletin》1995,44(7):1306-1310
The oxidation of the carbon-centered [(6-C13H9)Cr(CO)3]– anion (1
–) results in formation of (-6:6-9,9-bifluorenyl)bis-chromiumtricarbonyl (3) due to coupling of the intermediate carbon-centered radical (1.). The oxidation of the metal-centered anion [(5-C13H9)Cr(CO)3]– (2
–), which is isomeric to the 1– anion, gives an equilibrium mixture of the chromium-centered radical {(5-C13H9)Cr(CO)3}. (2
.) and its dimer [(5-C13H9)Cr(CO)3]2 (6). Radical2
. readily reacts with MeI and the solvent (THF); the resulting derivatives, (5-C13H9)Cr(CO)3R (R=Me (10); R=H (7)), undergo fast ricochet inter-ring 56 rearrangements into (6-9R-C13H9)Cr(CO)3 (R=CH3 (9); R=H (4)).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1354–1358, July, 1995.The authors are grateful to D. V. Zagorevskii who recorded the mass spectra. This study was financially supported by the Russian Foundation for Basic Research (Grant No. 94-03-05209) and the International Science Foundation (Grant Nos. MQ 4000 and REV 000). 相似文献
76.
77.
Andriy B. Zaborovskiy Ruslan E. Prystansky Vitaliy I. Timokhin 《Journal of organometallic chemistry》2004,689(18):2912-2919
The interaction of (Me3Si)3SiH with O2 is known to afford (Me3SiO)2Si(H)SiMe3 in which the two oxygen atoms arise from the same oxygen molecule. In order to investigate the mechanism of this unusual reaction, the oxidation rates were measured in the temperature range 30-70 °C by following oxygen uptake in the presence and absence of hydroquinone as inhibitor. The rate constant for the spontaneous reaction of (Me3Si)3SiH with O2 was determined at 70 °C to be ∼3.5 × 10−5 M−1 s−1. A sequence of the propagation steps is proposed by combining the previous and present experimental findings with some theoretical results obtained at the semiempirical level. These calculations showed that the silylperoxyl radical (Me3Si)3SiOO undergoes three consecutive unimolecular steps to give (Me3SiO)2Si()SiMe3. Evidence has been obtained that the rate determining step is the rearrangement of silylperoxyl radical to a dioxirand-like pentacoordinated silyl radical. Our findings are of considerable importance for the understanding of the oxidation of hydrogen-terminated silicon surfaces. 相似文献
78.
The π-π stacking interactions between maleimide's and nitrone's aromatic rings during the 1,3-dipolar cycloaddition were assumed to control the exo-endo selectivity of the reaction. The exo-endo ratios change during the reactions until they reach a constant value, which depends on the substituent. Electron-withdrawing groups favour the exo adduct while electron-donating groups favour the endo adduct. The nitrone ZnBr2 complexes react much more slowly than the free nitrone and the cycloaddition is exo selective in all cases independent of the substituents on the maleimide's aromatic ring. Thermal retrocycloaddition of the cycloadducts produce the corresponding nitrones. The ring opening in the presence of secondary amines did not induce imine formation. endo Adducts were shown for the first time to be the stable paramagnetic compounds. 相似文献
79.
Koji Nakabayashi Masaki Kawano Prof. Dr. Tatsuhisa Kato Prof. Dr. Ko Furukawa Dr. Shin‐ichi Ohkoshi Prof. Dr. Toshiya Hozumi Dr. Makoto Fujita Prof. Dr. 《化学:亚洲杂志》2007,2(1):164-170
We show a new approach to manipulating the through‐space spin–spin interaction by utilizing the confined cavity of a self‐assembled M6L4 coordination cage. The coordination cage readily encapsulates stable organic radicals in solution, which brings the spin centers of the radicals closer to each other. In sharp contrast to the fact that the radical in solution in the absence of the cage is in a doublet state, in the presence of the cage through‐space spin–spin interaction is induced through cage‐encapsulation effects in solution as well as in the solid state, resulting in the triplet state of the complex. These results were confirmed by ESR spectroscopy and X‐ray crystallography. The quantity of triplet species generated by encapsulation in the cage increases with increasing affinity of the radicals to the cage. We estimated the affinity between several types of guests and the cage in solution by cyclic voltammetry. We also demonstrate that the through‐space interaction of organic radicals within the self‐assembled coordination cage can be controlled by external stimuli such as heat or pH. 相似文献
80.
Interaction of 1-hydroxy-3-imidazoline and 3-imidazoline 3-oxide derivatives with esters in the presence of LDA gives enaminones, derivatives of imidazolidine. Oxidation of these compounds with MnO2 leads to4H-imidazoleN-oxides, oxidative dirnerization products, or stable nitroxyl radicals, depending on the structure of the initial compound.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1789–1795, July, 1996. 相似文献