全文获取类型
收费全文 | 1528篇 |
免费 | 109篇 |
国内免费 | 2篇 |
专业分类
化学 | 1216篇 |
晶体学 | 3篇 |
力学 | 32篇 |
综合类 | 1篇 |
数学 | 194篇 |
物理学 | 193篇 |
出版年
2023年 | 28篇 |
2022年 | 19篇 |
2021年 | 31篇 |
2020年 | 51篇 |
2019年 | 34篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 80篇 |
2015年 | 75篇 |
2014年 | 69篇 |
2013年 | 75篇 |
2012年 | 101篇 |
2011年 | 102篇 |
2010年 | 73篇 |
2009年 | 54篇 |
2008年 | 99篇 |
2007年 | 103篇 |
2006年 | 97篇 |
2005年 | 89篇 |
2004年 | 78篇 |
2003年 | 57篇 |
2002年 | 34篇 |
2001年 | 27篇 |
2000年 | 16篇 |
1999年 | 21篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1916年 | 3篇 |
1911年 | 2篇 |
排序方式: 共有1639条查询结果,搜索用时 31 毫秒
131.
Jens Langer Sven Krieck Reinald Fischer Helmar Görls Matthias Westerhausen Prof. Dr. 《无机化学与普通化学杂志》2010,636(7):1190-1198
Iodide is a very soft and large anion and as such its extreme ability to be polarized leads to a flat energy surface with respect to the variation of the Ca–I distances in [(L)nCaI2] and [(L)nCa(R)I]. The influence of the donor strength and the bulkiness of the neutral coligands L on the Ca–I distances is studied. The base adducts of calcium diiodide can be isolated after the addition of L to CaI2 or from the Schlenk equilibrium after the direct synthesis of calcium powder with aryl iodides. As L the ethers diethyl ether (Et2O), tetrahydrofuran (thf), tetrahydropyran (thp), 1,2‐dimethoxyethane (dme), 18‐crown‐6 (18C6), bis(methoxyethyl)ether (diglyme), and amines tetramethylethylenediamine (tmeda), and hexamethyltriethylenetetramine (hmteta) are studied yielding the adducts [(thp)4Ca(Ph)I] ( 1a ), [(thf)4Ca(Ph)I] ( 1b ), [(dme)2(thf)Ca(Ph)I] ( 1c ), [(18C6)Ca(Ph)I] ( 1d ), and [(tmeda)2Ca(Ph)I] ( 1e ), as well as [(thp)4CaI2] ( 2a ), [(thf)4CaI2] ( 2b ), [(Et2O)4CaI2] ( 2c ), [(diglyme)(thf)2CaI2] ( 2d ), [(diglyme)(dme)CaI2] ( 2e ), [(dme)2(thf)CaI2] ( 2f ), [(18C6)CaI2] ( 2g ), [(tmeda)2CaI2] ( 2h ), and [(hmteta)CaI2] ( 2i ). For comparison reasons, [(thf)4Ca(Ph)Br] ( 3a ), [(thp)4CaBr2] ( 4a ), [(thf)4CaBr2] ( 4b ), and [(dme)2(AcOH)CaBr2] ( 4c ) with AcOH being acetic acid are included as well. The comparison shows that the coordination number of calcium itself only plays an insignificant role whereas bulkiness and donor strength of L represent the key influences. 相似文献
132.
133.
134.
Nga H. Nguyen Brad M. Rosen Xuan Jiang Sven Fleischmann Virgil Percec 《Journal of polymer science. Part A, Polymer chemistry》2009,47(21):5577-5590
SET‐LRP is mediated by a combination of solvent and ligand that promotes disproportionation of Cu(I)X into Cu(0) and Cu(II)X2. Therefore, the diversity of solvents suitable for SET‐LRP is limited. SET‐LRP of MA in a library of solvents with different equilibrium constants for disproportionation of Cu(I)X such as DMSO, DMF, DMAC, EC, PC, EtOH, MeOH, methoxyethanol, NMP, acetone and in their binary mixtures with H2O was examined. H2O exhibits the highest equilibrium constant for disproportionation of Cu(I)X. The apparent rate constant of the polymerization exhibits a linear increase with the addition of H2O. This is consistent with higher equilibrium constants for disproportionation generated by addition of H2O to organic solvents. Furthermore, with the exception of alcohols and carbonates, the rate constant of polymerization in binary mixtures could be correlated with the Dimroth‐Reichardt solvent polarity parameter. This is consistent with the single‐electron transfer mechanism proposed for SET‐LRP that involves a polar transition state. These experiments demonstrate that the use of binary mixtures of solvents with H2O provides a new, simple and efficient method for the elaboration of a large diversity of reaction media that are suitable for SET‐LRP even when one of the two solvents does not mediate disproportionation of Cu(I)X. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5577–5590, 2009 相似文献
135.
A sonic spray ionization (SSI) interface for liquid chromatography-mass spectrometry (LC-MS) analysis was optimized for analysis of 2-[(1R)-3-[bis(1-methylethyl)amino]-1-phenylpropyl]-4-methyl-phenol (tolterodine), used as a model drug substance, and the influence of different parameter settings was evaluated using factorial design. A comparison between SSI and electrospray ionization (ESI) was made for tolterodine, tolterodine metabolites, and a set of steroids.SSI was found to give slightly poorer repeatability and broader peaks for tolterodine compared to ESI. However, there was no significant difference in chromatographic peak shape, and the repeatability using SSI was similar to that obtained using ESI if a ratio (area of tolterodine/area of metabolite) was used. In this study, the sensitivity was higher using SSI. For the analysis of pregnanolone, less water loss was obtained using SSI, probably due to less energy being transferred to the analyte upon ionization. 相似文献
136.
137.
The electronic coupling between symmetrically placed π electrons through a cyclohexane spacer is calculated using ab initio methods. In particular we have performed a multiconfigurational configuration interaction study involving the broken symmetry Hartree—Fock solutions. The value of Δ, in a simplified model the splitting between the symmetric and antisymmetric π orbitals, is moderately changed compared to the Koopmans theorem value. The influence of the basis set on the calculated splittings is also analyzed. 相似文献
138.
Functionalized 2,3-benzoxepins were prepared by cyclopropanation of benzopyrylium triflates with diazoesters and subsequent TFA-mediated ring enlargement. 相似文献
139.
Spitzer SM Liss JM Mattys SL 《The Journal of the Acoustical Society of America》2007,122(6):3678-3687
It has been posited that the role of prosody in lexical segmentation is elevated when the speech signal is degraded or unreliable. Using predictions from Cutler and Norris' [J. Exp. Psychol. Hum. Percept. Perform. 14, 113-121 (1988)] metrical segmentation strategy hypothesis as a framework, this investigation examined how individual suprasegmental and segmental cues to syllabic stress contribute differentially to the recognition of strong and weak syllables for the purpose of lexical segmentation. Syllabic contrastivity was reduced in resynthesized phrases by systematically (i) flattening the fundamental frequency (F0) contours, (ii) equalizing vowel durations, (iii) weakening strong vowels, (iv) combining the two suprasegmental cues, i.e., F0 and duration, and (v) combining the manipulation of all cues. Results indicated that, despite similar decrements in overall intelligibility, F0 flattening and the weakening of strong vowels had a greater impact on lexical segmentation than did equalizing vowel duration. Both combined-cue conditions resulted in greater decrements in intelligibility, but with no additional negative impact on lexical segmentation. The results support the notion of F0 variation and vowel quality as primary conduits for stress-based segmentation and suggest that the effectiveness of stress-based segmentation with degraded speech must be investigated relative to the suprasegmental and segmental impoverishments occasioned by each particular degradation. 相似文献
140.