全文获取类型
收费全文 | 380篇 |
免费 | 19篇 |
国内免费 | 2篇 |
专业分类
化学 | 325篇 |
力学 | 1篇 |
数学 | 26篇 |
物理学 | 49篇 |
出版年
2023年 | 1篇 |
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 6篇 |
2014年 | 11篇 |
2013年 | 16篇 |
2012年 | 20篇 |
2011年 | 34篇 |
2010年 | 22篇 |
2009年 | 5篇 |
2008年 | 31篇 |
2007年 | 20篇 |
2006年 | 30篇 |
2005年 | 24篇 |
2004年 | 20篇 |
2003年 | 22篇 |
2002年 | 15篇 |
2001年 | 11篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1934年 | 1篇 |
排序方式: 共有401条查询结果,搜索用时 765 毫秒
51.
The force fields for the in-plane vibrations of benzene and its fluoro derivatives are determined using the overlay technique. Between 36 and 39 parameters were allowed to vary in different fields and in all cases convergence on the experimental frequencies was smooth and rapid. Only one solution could be found. This solution agrees extremely well with the benzene field of Duinker and Mills. The variation of diagonal force constants in an internal valence coordinate basis set with fluorine substitution in the coordinate environment is determined. The variation in certain force constants is considerable. Thus for the CF stretching constant it is 12% for replacement of two hydrogens by two fluorines. Attempts are made to interpret these variations. Various aspects of the force constant problem for benzene are discussed. 相似文献
52.
Jenson F Padilla F Bousson V Bergot C Laredo JD Laugier P 《The Journal of the Acoustical Society of America》2006,119(1):654-663
Thirty-eight slices of pure trabecular bone 1-cm thickness were extracted from human proximal femurs. A pair of 1-MHz central frequency transducers was used to measure quantitative ultrasound (QUS) parameters in transmission [normalized broadband ultrasound attenuation (nBUA), speed of sound (SOS)] and in backscatter [broadband ultrasound backscatter (BUB)]. Bone mineral density (BMD) was measured using clinical x-ray quantitative computed tomography. Site-matched identical region of interest (ROIs) of 7 x 7 mm2 were positioned on QUS and QCT images. This procedure resulted in 605 ROIs for all the specimens data pooled together. The short-term precision of the technique expressed in terms of CV was found to be 2.3% for nBUA, 0.3% for SOS and 4.5% for BUB. Significant linear correlation between QUS and BMD were found for all the 605 ROIs pooled, with r2 values of 0.73, 0.77, and 0.58 for nBUA, SOS, and BUB, respectively (all p < 0.05). For the BUB, the best regression was obtained with a polynomial fit of second order (r2 = 0.63). An analysis of measurements errors was developed. It showed that the residual variability of SOS is almost completely predicted by measurements errors, which is not the case for BUA and BUB, suggesting a role for micro-architecture in the determination of BUA and BUB. 相似文献
53.
Hazan V Sennema A Faulkner A Ortega-Llebaria M Iba M Chunge H 《The Journal of the Acoustical Society of America》2006,119(3):1740-1751
This study assessed the extent to which second-language learners are sensitive to phonetic information contained in visual cues when identifying a non-native phonemic contrast. In experiment 1, Spanish and Japanese learners of English were tested on their perception of a labial/ labiodental consonant contrast in audio (A), visual (V), and audio-visual (AV) modalities. Spanish students showed better performance overall, and much greater sensitivity to visual cues than Japanese students. Both learner groups achieved higher scores in the AV than in the A test condition, thus showing evidence of audio-visual benefit. Experiment 2 examined the perception of the less visually-salient /1/-/r/ contrast in Japanese and Korean learners of English. Korean learners obtained much higher scores in auditory and audio-visual conditions than in the visual condition, while Japanese learners generally performed poorly in both modalities. Neither group showed evidence of audio-visual benefit. These results show the impact of the language background of the learner and visual salience of the contrast on the use of visual cues for a non-native contrast. Significant correlations between scores in the auditory and visual conditions suggest that increasing auditory proficiency in identifying a non-native contrast is linked with an increasing proficiency in using visual cues to the contrast. 相似文献
54.
Barker EJ Buttar D Cosgrove DA Gardiner EJ Kitts P Willett P Gillet VJ 《Journal of chemical information and modeling》2006,46(2):503-511
Similarity-based methods for virtual screening are widely used. However, conventional searching using 2D chemical fingerprints or 2D graphs may retrieve only compounds which are structurally very similar to the original target molecule. Of particular current interest then is scaffold hopping, that is, the ability to identify molecules that belong to different chemical series but which could form the same interactions with a receptor. Reduced graphs provide summary representations of chemical structures and, therefore, offer the potential to retrieve compounds that are similar in terms of their gross features rather than at the atom-bond level. Using only a fingerprint representation of such graphs, we have previously shown that actives retrieved were more diverse than those found using Daylight fingerprints. Maximum common substructures give an intuitively reasonable view of the similarity between two molecules. However, their calculation using graph-matching techniques is too time-consuming for use in practical similarity searching in larger data sets. In this work, we exploit the low cardinality of the reduced graph in graph-based similarity searching. We reinterpret the reduced graph as a fully connected graph using the bond-distance information of the original graph. We describe searches, using both the maximum common induced subgraph and maximum common edge subgraph formulations, on the fully connected reduced graphs and compare the results with those obtained using both conventional chemical and reduced graph fingerprints. We show that graph matching using fully connected reduced graphs is an effective retrieval method and that the actives retrieved are likely to be topologically different from those retrieved using conventional 2D methods. 相似文献
55.
William H. Perman Pratip Bhattacharya Jochen Leupold Alexander P. Lin Kent C. Harris Valerie A. Norton Jan-Bernd Hövener Brian D. Ross 《Magnetic resonance imaging》2010
Purpose
The goal of this work was to develop a fast 3D chemical shift imaging technique for the noninvasive measurement of hyperpolarized 13C-labeled substrates and metabolic products at low concentration.Materials and Methods
Multiple echo 3D balanced steady state magnetic resonance imaging (ME-3DbSSFP) was performed in vitro on a syringe containing hyperpolarized [1,3,3-2H3; 1-13C]2-hydroxyethylpropionate (HEP) adjacent to a 13C-enriched acetate phantom, and in vivo on a rat before and after intravenous injection of hyperpolarized HEP at 1.5 T. Chemical shift images of the hyperpolarized HEP were derived from the multiple echo data by Fourier transformation along the echoes on a voxel by voxel basis for each slice of the 3D data set.Results
ME-3DbSSFP imaging was able to provide chemical shift images of hyperpolarized HEP in vitro, and in a rat with isotropic 7-mm spatial resolution, 93 Hz spectral resolution and 16-s temporal resolution for a period greater than 45 s.Conclusion
Multiple echo 3D bSSFP imaging can provide chemical shift images of hyperpolarized 13C-labeled compounds in vivo with relatively high spatial resolution and moderate spectral resolution. The increased signal-to-noise ratio of this 3D technique will enable the detection of hyperpolarized 13C-labeled metabolites at lower concentrations as compared to a 2D technique. 相似文献56.
Fernando Ruiperez Cecile Danilo Florent Real Jean‐Pierre Flament Valerie Vallet Ulf Wahlgren 《ChemInform》2009,40(19):no-no
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option. 相似文献
57.
Eight bromoalkynes were prepared from substituted benzoic acids and treated with n-Bu3SnH to provide trans-fused perhydroindans or cis- and trans-fused perhydronaphthalenes. Atom-transfer reactions that accompany the free radical reactions resulted in several tandem radical cyclizations with formation of up to three carbon-carbon bonds in a single reaction. The relationship between these reactions and an approach to the quassinoid family of natural products is also described. 相似文献
58.
59.
60.
Jonas H. Muessig Melanie Thaler Rian D. Dewhurst Valerie Paprocki Jens Seufert James D. Mattock Alfredo Vargas Holger Braunschweig 《Angewandte Chemie (International ed. in English)》2019,58(13):4405-4409
The lability of B=B, B?P, and B–halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange. 相似文献