全文获取类型
收费全文 | 5466篇 |
免费 | 176篇 |
专业分类
化学 | 3270篇 |
晶体学 | 51篇 |
力学 | 176篇 |
数学 | 840篇 |
物理学 | 1305篇 |
出版年
2023年 | 36篇 |
2021年 | 39篇 |
2020年 | 77篇 |
2019年 | 71篇 |
2018年 | 82篇 |
2017年 | 71篇 |
2016年 | 150篇 |
2015年 | 139篇 |
2014年 | 148篇 |
2013年 | 272篇 |
2012年 | 298篇 |
2011年 | 333篇 |
2010年 | 193篇 |
2009年 | 170篇 |
2008年 | 246篇 |
2007年 | 242篇 |
2006年 | 225篇 |
2005年 | 211篇 |
2004年 | 210篇 |
2003年 | 165篇 |
2002年 | 163篇 |
2001年 | 88篇 |
2000年 | 75篇 |
1999年 | 67篇 |
1998年 | 59篇 |
1997年 | 63篇 |
1996年 | 46篇 |
1995年 | 76篇 |
1994年 | 37篇 |
1993年 | 41篇 |
1992年 | 53篇 |
1991年 | 38篇 |
1990年 | 45篇 |
1989年 | 39篇 |
1988年 | 58篇 |
1987年 | 53篇 |
1985年 | 38篇 |
1984年 | 36篇 |
1983年 | 31篇 |
1982年 | 39篇 |
1981年 | 51篇 |
1980年 | 38篇 |
1979年 | 30篇 |
1978年 | 43篇 |
1977年 | 34篇 |
1976年 | 36篇 |
1975年 | 34篇 |
1974年 | 34篇 |
1973年 | 28篇 |
1908年 | 27篇 |
排序方式: 共有5642条查询结果,搜索用时 15 毫秒
61.
7-Benz[c] acridinemethanols as tetracyclic analogs of the 2-phenyl-4-quinolinemethanol antimalarials
Andre Rosowsky Michael Chaykovsky Sandra A. Yeager Richard A. St. Amand May Lin Edward J. Modest 《Journal of heterocyclic chemistry》1971,8(5):809-820
A series of new 7-benz[c]acridinemethanols and 5,6-dihydro-7-benz[c]acridinemethanols was prepared as rigid, tetracyclic analogs of the antimalarial 2-phenyl-4-quinolinemethanols. Condensation of 5,7-dichloroisatin with 6-chloro-, 7-chloro-, and 6,7-dichloro-1-tetralone furnished halogenated 5,6-dihydro-7-benz[c]acridinecarboxylic acids, which were transformed into the corresponding acid chlorides, acyl malonates, α-bromomethyl ketones, and epoxides. Fully aromatic members of the series obtained via dehydrogenation of the 5,6-dihydro acids were likewise converted into epoxides via the acylmalonate route. Although all the epoxides studied proved to be exceptionally resistant to ring-opening by di-n-butylamine, probably on account of steric effects, they could be cleaved readily with piperidine or morpholine. Nmr spectra of the resulting amino alcohols suggest that these compounds exist in a single preferred conformation stabilized by internal O-H····N hydrogen bonding, and that free rotation about the side chain C-C bond does not occur at room temperature. 相似文献
62.
Griffith GA Percy JM Pintat S Smith CA Spencer N Uneyama E 《Organic & biomolecular chemistry》2005,3(15):2701-2712
Highly-functionalised difluorinated cyclooctenones were synthesised from trifluoroethanol using either metallated difluoroenol acetal or carbamate chemistry, followed by a [2,3]-Wittig rearrangement or aldol reaction. Efficient RCM reactions afforded the title compounds which showed rather restricted fluxional behaviour by VT (19)F NMR. Topological characterisation by molecular modelling and NOESY/ROESY experiments offered a number of challenges, but allowed the identification of two favoured boat-chair conformers which interconverted by pseudorotation with relatively large activation barriers. 相似文献
63.
Stølen S Mohn CE Ravindran P Allan NL 《The journal of physical chemistry. B》2005,109(25):12362-12365
We discuss the importance of the topography of the potential energy hypersurface for the ionic conductivity of perovskite-related A(2)B(2)O(5) oxides. A correlation between the energetic preference of the cations for different coordination geometries and the ionic conductivity is proposed based on a first principles periodic density functional theory study of selected possible structures for Ba(2)In(2)O(5), Sr(2)Fe(2)O(5), Sr(2)Mn(2)O(5), and La(2)Ni(2)O(5). There are a large number of low-energy local minima on the potential energy hypersurfaces of the two first compounds due to an energetic preference for BO(4) tetrahedra. Tetrahedral environments are energetically unfavorable for Mn(III) in Sr(2)Mn(2)O(5) and for Ni(II) in La(2)Ni(2)O(5), and the number of low-energy configurations is relatively low in these two cases. Consistent with our findings, in contrast to Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5), Sr(2)Mn(2)O(5) and La(2)Ni(2)O(5) do not exhibit transitions to disordered phases on heating, and there appear to be no reports of enhanced ionic conductivity for these compounds. Thus we suggest that the possibility of many different oxygen orderings associated with a variety of low-energy connectivity schemes within tetrahedral layers such as in the brownmillerite-based structures of Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5) is a prerequisite for high ionic conductivity in perovskite-related A(2)B(2)O(5) oxides. 相似文献
64.
Novel large polyazadioxime molecules 4,5,8,9-tetraaza-3,6,7,10-tetramethyl -3,5,7,9-dodecatetraene-2, 11-dione-2, 11-dioxime (H2doxN4) and 4,5,8,9,12, 13-hexaaza-3,6,7,10,11,14-hexamethyl-3, 5,7,9,11,13-hexadecahexane-2,15-dione-2, 15-dioxime (H2doxN6) were synthesized. The molecular geometries of these molecules as well as smaller dioxime molecules, H2dox and H2doxN2 were optimized by using modified intermediate neglect of differential overlap (MNDO) calculations. The optimized conformations for all the molecules under study are close to the all-E, all-s trans conformation of C2h symmetry group. However, the energy barriers of internal rotation around the N-N single bonds were found to be low. Therefore some distortions of the polyazabackbone through internal rotation the N-N bonds have been evidenced. By infrared and Raman spectroscopies in the solid state as well as in solution. From the MNDO calculations and vibrational spectroscopy, the polyaza chain molecules under study appear as a poorly conjugated system and can be represented as a sequence of single and double bond alternation. 相似文献
65.
Capillary electrophoresis coupled to mass spectrometry (CE-MS) is reported for the first time as an alternative and powerful analytical method for the characterization and monitoring of N-substituted 1-amino-1-deoxyketoses (Amadori compounds). It allows rapid separation and identification of Amadori compounds, while benefiting from the well-known advantages of MS, such as specificity and sensitivity. Amadori compounds of several amino acids, such as glycine, valine, isoleucine, methionine, proline, and phenylalanine, as well as a cysteine-derived compound, were separated and/or discriminated using CE-MS/MS under standard conditions. The technique may also be useful to study the stability and degradation kinetics of other labile charged Maillard intermediates that play an important role in food and medical science. 相似文献
66.
Complex systems of paramagnetic centres existing in demineralised flame coal (71.4 wt% C), medium-rank coal (85.6 wt% C) and
anthracite (94.9 wt% C) were analysed by electron paramagnetic resonance spectroscopy (EPR). Different groups of paramagnetic
centres were searched in macerals (exinite, vitrinite, inertinite) of coals with carbon contents of 73.8 wt% C and 85.6 wt%
C. Experimental EPR spectra were fitted by different superposition of theoretical lines. Total concentration of paramagnetic
centres both in coal and macerals increases with carbon content in the sample. Total concentration of paramagnetic centres
in macerals increases from exinite to inertinite. Four groups of paramagnetic centres were found in medium-rank coal, and
only two groups exist in flame coal and anthracite. The EPR spectrum of medium-rank coal is a superposition of two broad and
two narrow lines. Both broad and narrow, and two narrow lines were measured in EPR spectra of flame coal and anthracite, respectively.
Two types of paramagnetic centres with broad lines and one group with narrow lines exist in exinite and vitrinite from medium-rank
coal. Two narrow components were observed in EPR spectra of its inertinite. Two different broad electron paramagnetic resonance
signals were detected for macerals from low-coalificated coal. Paramagnetic centres systems of medium-rank coal samples reveal
the most complex character. Broad EPR lines were not observed for the higher coalificated samples, i.e., anthracite and inertinite
from medium-rank coal. Narrow lines do not occur in EPR spectra of low-coalificated macerals. Strong dipolar interactions
and fast spin-lattice relaxation processes are characteristic for paramagnetic centres with broad lines. Strong exchange interactions
and short spin-lattice relaxation time were measured for paramagnetic centres with the narrowest EPR signals. 相似文献
67.
Guilard R Gryko DT Canard G Barbe JM Koszarna B Brandès S Tasior M 《Organic letters》2002,4(25):4491-4494
[reaction: see text] We have developed a new method that affords regioisomerically pure corroles possessing up to three different substituents at the meso positions. The corrole formation reaction involves the acid-catalyzed condensation of a dipyrromethane-dicarbinol with pyrrole followed by oxidation with DDQ. ABC-Type corroles were synthesized for the first time according to this procedure. 相似文献
68.
Thermal degradation at 180° in air of PVC stabilized with an equimolar mixture of Ba and Cd stearates has been studied. The “unreacted” stearates were separated by filtering tetrahydrofuran solutions of samples. The filtration and thermal degradation were examined by means of infrared spectroscopy in the range 1400–1800 cm?1. It has been demonstrated that, during the thermal stress, CdCl2 is accumulated in the sample and that the presence of Ba stearate supports only the consumption of Cd stearate. 相似文献
69.
Ovcharenko VV Pihlaja K Stájer G 《Journal of the American Society for Mass Spectrometry》2003,14(9):1049-1056
The mass spectrometric behavior of stereo- and regioisomeric, partially saturated isoindoloquinazolines was studied by positive-ion electron ionization (EI) and fast-atom bombardment (FAB/LSIMS) mass spectrometry combined with collision-induced dissociation (CID). A highly stereospecific retro-Diels-Alder process was observed in the cyclohexene-fused isomers under the EI conditions, and a corresponding (although less specific) fragmentation was observed in their FAB spectra. In the absence of RDA fragmentations, regio- and stereoisomers of the cyclohexane-fused heterocycles could be distinguished based on their FAB/CID spectra. 相似文献
70.
B.?Stark C.?Lach B.?Farago H.?Frey C.?Schlenk B.?StühnEmail author 《Colloid and polymer science》2003,281(7):593-600
A series of carbosilane dendrimers with perfluorinated end groups has been prepared. The structure of these molecules in dilute solution is studied using small angle neutron scattering. For generations g<3 we find a non-spherical shape of the particles and a tendency for aggregation. This result is supported by the analysis of the diffusion coefficient obtained from photon correlation spectroscopy. The overall shape of the molecules is that of a core-shell particle. The generation 4 molecule is obtained as a compact sphere. Neutron spin echo spectroscopy reveals a relaxation time which is attributed to the form fluctuations of this particle. 相似文献