全文获取类型
收费全文 | 343篇 |
免费 | 16篇 |
专业分类
化学 | 280篇 |
力学 | 9篇 |
数学 | 32篇 |
物理学 | 38篇 |
出版年
2023年 | 6篇 |
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 8篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 10篇 |
2015年 | 13篇 |
2014年 | 14篇 |
2013年 | 10篇 |
2012年 | 25篇 |
2011年 | 20篇 |
2010年 | 14篇 |
2009年 | 18篇 |
2008年 | 11篇 |
2007年 | 14篇 |
2006年 | 12篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 4篇 |
2002年 | 7篇 |
2001年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 13篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 8篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 8篇 |
1970年 | 2篇 |
1966年 | 2篇 |
1965年 | 1篇 |
1906年 | 2篇 |
1905年 | 2篇 |
1904年 | 2篇 |
1880年 | 2篇 |
1869年 | 2篇 |
排序方式: 共有359条查询结果,搜索用时 15 毫秒
1.
Patricia D. Morrell Camille Wainwright Lawrence Flick 《School science and mathematics》2004,104(5):199-213
The purposes of this study were to observe the teaching practices occurring in student teachers' science and mathematics K‐12 classrooms, compare the student teachers' perceptions of their teaching with what was actually occurring in their classrooms, and determine which college faculty members and courses these student teachers felt contributed to the teaching methods they used. Data on each student teacher were gathered via field notes of three classes, an observation protocol completed after each lesson, and an interview. Composites were written for each of the students. The total data set of all composites was examined to see if any patterns generalizable to the whole were evident. Differences between and among grade levels and content areas surfaced and are discussed. 相似文献
2.
3.
Douglas R. Hurst Kristen L. French April J. Angel Angela R. Williams Mary E. Rampey Tina S. Guion Kam W. Chan Camille M. Kassis Shannon L. Studer Martinez Charles F. Beam 《Journal of heterocyclic chemistry》1998,35(6):1357-1359
Several acetoacetanilides were trilithiated in excess lithium diisopropylamide, and the resulting polylithiated intermediates were regioselectively condensed with lithiated methyl salicylates followed by acid cyclization to substituted 4-oxo-N-aryl-4H-1-benzopyran-2-acetamides (benzopyranone-2-acetamides). 相似文献
4.
Two synthetic approaches to the novel C10H16 hydrocarbon tricyclo[4.4.0.03,9]decane ( 1 ; 2-homotwistbrendane), one of the 19 members of the adamantaneland, and its Lewis-acid-catalyzed rearrangement are described. One route starts from tricyclo[4.3.0.03,8]nonan-2-one ( 2 ; 2-twistbrendanone). The missing tenth C-atom is introduced by ring enlargement (Tiffeneau-Demjanov method). Starting from methyl 8,9,10-trinorborn-5-ene-2-endo-carboxylate ( 8 ), ring enlargement by one C-atom, regio- and stereoselective introduction of a C1 unit to a 2-endo,6-endo-disubstituted bicyclo[3.2.1]octane, and ring closure by acyloin condensation are the key steps in the second approach. 相似文献
5.
Stevens SM Chung AY Chow MC McClung SH Strachan CN Harmon AC Denslow ND Prokai L 《Rapid communications in mass spectrometry : RCM》2005,19(15):2157-2162
A fluorescent affinity tag (FAT) was synthesized and was utilized to selectively modify phosphorylated serine and threonine residues via beta-elimination and Michael addition chemistries in a 'one-step' reaction. This labeling technique was used for covalent modification of both phosphoproteins and phosphopeptides, allowing identification of these molecular species by fluorescence imaging after solution- or gel-based separation methods. In addition to the strong fluorescence of the rhodamine tag, a commercially available antibody can be used to enrich low-abundance post-labeled phosphopeptides present in complex mixtures. Application of this methodology to phosphorylation-site mapping has been evaluated for a phosphoprotein standard, bovine beta-casein. Initial results demonstrated low femtomole detection limits after fluorescence image analysis of FAT-labeled proteins or peptides. 相似文献
6.
C. Sandorfy 《International journal of quantum chemistry》1984,26(5):907-915
The primary photochemical step in the mechanism of vision is likely to involve cis → trans isomerization as well as proton transfer. Our understanding of this is connected with the problem as to whether the nitrogen of the chromophore Schiff base is protonated or not. More precisely we need to know the shape of the potential in the C?N···H+···X group, where X is a proton donor. It is proposed that this potential is double well. The consequences of this relating to the resonance Raman spectra are discussed. 相似文献
7.
Abstract— Quantum chemical calculations have been performed on the retinylidene Schiff base assumed to be protonated or hydrogen bonded at the nitrogen and perturbed by a carboxylate ion at its β-ionone end. The results indicate that weak charge transfer type bands might exist due to electron transfer from the carboxylate ion to the Schiff base and that the intense (π, π*) band in the visible spectrum might acquire a significant degree of charge transfer character. 相似文献
8.
Fluorocarbons containing higher halogens have the potency of breaking certain hydrogen bonds in solutions. The relative strength of this potency varies in the series F < Cl < Br < I and the presence of hydrogen atoms in the fluorocarbon increases it. There is a striking parallelism between the hydrogen bond breaking and the anaesthetic potency of these molecules. It is suggested that the breaking of hydrogen bonds is an important step in the mechanism of anaesthesia. 相似文献
9.
Ab initio Gaussian type calculations are reported for the ground and excited states of the hypothetical molecule formaldimine (H2CNH). The C=N group is compared with the C=O group.
Zusammenfassung Die Ergebnisse einer ab initio Rechnung mit Gau\-Orbitalen für das hypothetische Formaldimin-Molekül (H2CNH) werden mitgeteilt. Es werden sowohl Grundzustand als auch einige angeregte ZustÄnde untersucht und die beiden Gruppen C=O und N=O verglichen.
Résumé On présente les résultats de calculs ab initio utilisant une base Gaussienne pour la molécule hypothétique de formaldimine (H2CNH). L'étude porte sur l'état fondamental et quelques états excités. Une comparaison est faite entre le groupement C=N et le groupement C=O.相似文献
10.
The far-ultraviolet absorption spectrum of the triethylamine-perfluoro-tert-butanol complex has been measured in the gas phase. The photoelectron band of lowest energy has also been determined. It is about 13000 cm?1 higher than for free triethylamine. Up to at least 60000 cm?1, the UV spectrum is readily interpreted as a triethylamine spectrum shifted to higher frequencies by about 13000 cm?1. The bands of lowest frequency are the 3s and 3p Rydberg bands which follow the ionization potential. Up to 60000 cm?1, no charge transfer type band has been found. 相似文献