全文获取类型
收费全文 | 234篇 |
免费 | 5篇 |
专业分类
化学 | 158篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 27篇 |
物理学 | 50篇 |
出版年
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 5篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 9篇 |
2012年 | 9篇 |
2011年 | 9篇 |
2010年 | 2篇 |
2009年 | 8篇 |
2008年 | 13篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 8篇 |
2004年 | 11篇 |
2003年 | 6篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 9篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1984年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1966年 | 1篇 |
1965年 | 2篇 |
1964年 | 3篇 |
1963年 | 3篇 |
1932年 | 2篇 |
1928年 | 1篇 |
1914年 | 1篇 |
排序方式: 共有239条查询结果,搜索用时 84 毫秒
101.
The Penney-Dirac calculation of bond orders in conjugated hydrocarbons is reconsidered, and a method of computing them, which does not involve perturbation theory, is developed. This method is general, in that it permits the exchange and Coulomb integrals for the various bonds to have values appropriate to their lengths, and not necessarily all equal. Detailed application to 1,3-butadiene and fulvene leads to results in close correspondence with molecular-orbital calculations. 相似文献
102.
The 57Fe Mössbauer effect in two samples (A and B) of [Fe(papt)2] and in its solvates with CHCl3 and C6H6 has been studied between 4.2 and 343 K and clearly indicates a temperature induced high-spin (5T2) ? low-spin (1A1) transition in these compounds [paptH = 2-(2-pyridylamino)-4-(2-pyridyl) thiazole]. At 343 K, sample B shows a doublet with ΔEQ = 2.03 mm s?1 and δIS = +0.87 mm s?1, characteristic of a 5T2 ground state. At 257 K, a second doublet, typical for a 1A1 ground state, is observed and its intensity increases as the transition progresses but levels off below ~ 100 K. At 4.2 K, 83% of the intensity is due to the 1A1 state, and ΔEQ(1A1) = 1.56 mm s?1 and δIS(1A1 = +0.32 mm s?1. In an applied magnetic field, Vzz(1A1) < 0 and η ≈ 0.7 have been determined, whereas for the sT2 ground state, Vzz(sT2) > 0, η ≈ 0.75, and an internal hyperfine field Hn ≈ ?13 kG have been observed. Similar results have been obtained with the other samples.Debye-Waller factors f5T2 and f1A1 were determined from the saturation corrected areas in the Mössbauer spectra, assuming Curie-Weiss dependence of the magnetic susceptibility for the 5T2 and constant υcff for the 1A1 ground state. The temperature dependence of ?In f1A1 closely follows the Debye model with Θ1A1 = 165 K, whereas the same applies to ?ln f5T2 only above ~ 210 K and Θ5T2 = 134 K. The nature of the observed transition is discussed and the data presented are shown to be incompatible with a model based on a Boltzmann distribution between the two states. 相似文献
103.
T. H. Goodwin 《Theoretical chemistry accounts》1964,2(4):315-323
A valency bond study of naphthalene has been made in which, assuming the molecular symmetry to be D
2h, successively more canonical structures up to the total of 42 have been included. All four chemically differentiable bonds have been independently varied in length to find the minimum molar energy and thus the equilibrium ground state molecular structure. The result is remarkably close to the observed structure and also agrees well with those deduced by other types of calculation.
The author gratefully acknowledges helpful discussions with Professor C. A. Coulson (Oxford), Professor D. H. W. Den Boer and Mevrouw P. C. Den Boer (Utrecht) and Dr. E. Clar (Glasgow). 相似文献
Zusammenfassung Im Rahmen der Methode der Valenzstrukturen wurde eine Untersuchung des Naphthalins durchgeführt, wobei als Symmetriegruppe des Moleküls D 2hangenommen wurde. Die Anzahl der einbezogenen Strukturen wurde schrittweise auf die Gesamtzahl von 42 erhöht. Bei der Minimisierung der molekularen Energie zur Bestimmung der Gleichgewichtsstruktur des Grundzustandes wurden auch die Längen aller vier chemisch unterscheidbaren Bindungen variiert. Diese Gleichgewichtsabstände stimmen sowohl mit den Meßwerten als auch mit den Ergebnissen anderer Rechenverfahren gut überein.
Résumé Une étude du naphtalène a été faite par la méthode de la mésomérie. On a supposé que la symétrie soit D 2het on a compris successivement de plus en plus formules canoniques jusqu'au total de 42. On a fait varier indépendamment les longueurs de toutes les quatre liaisons qu'on peut distinguer chimiquement afin qu'on peut estimer l'énergie moléculaire minimale et ainsi la structure stable de la molécule en état fondamental. Le résultat s'approche remarquablement de la structure observée et s'accorde bien à celles qu'on a déterminées par des autres types de calculs.
The author gratefully acknowledges helpful discussions with Professor C. A. Coulson (Oxford), Professor D. H. W. Den Boer and Mevrouw P. C. Den Boer (Utrecht) and Dr. E. Clar (Glasgow). 相似文献
104.
105.
Interaction of a low-pressure NO2 with sublimed layers of (meso-tetraphenylporphyrinato)cobalt(II) (Co(TPP)) leads to formation of 5-coordinate nitro complex Co(III)(TPP)(NO2). Upon exposure of these layers to pyridine vapors, the fast reaction with formation of 6-coordinate nitro-pyridine porphyrins (Py)Co(III)(TPP)(NO2) occurs. By means of IR spectroscopy and use of nitrogen oxide isotopomers, it is shown that an oxo-transfer reaction occurs from 5-coordinate species to free nitric oxide (NO) while the 6-coordinate complex is rather inert. It is also demonstrated that the stepwise addition of low-pressure NO2 to nitrosyl complex Co(TPP)(NO) leads to formation of the nitro complex most likely by an exchange reaction. 相似文献
106.
A number of studies have shown that providing point forecasts to decision makers can lead to improved production planning decisions. However, point forecasts do not convey information about the level of uncertainty that is associated with forecasts. In theory, the provision of prediction intervals, in addition to point forecasts, should therefore lead to further enhancements in decision quality. To test whether this is the case in practice, participants in an experiment were asked to decide on the production levels that were needed to meet the following week’s demand for a series of products. Either underproduction cost twice as much per unit as overproduction or vice versa. The participants were supplied with either a point forecast, a 50% prediction interval, or a 95% prediction interval for the following week’s demand. The prediction intervals did not improve the quality of the decisions and also reduced the propensity of the decision makers to respond appropriately to the asymmetry in the loss function. A simple heuristic is suggested to allow people to make more effective use of prediction intervals. It is found that applying this heuristic to 85% prediction intervals would lead to nearly optimal decisions. 相似文献
107.
Sara Goodwin Alexandra M. Gade Michelle Byrom Baine Herrera Camille Spears Dr. Eric V. Anslyn Dr. Andrew D. Ellington 《Angewandte Chemie (International ed. in English)》2015,54(21):6339-6342
Differential sensing (DS) methods traditionally use spatially arrayed receptors and optical signals to create score plots from multivariate data which classify individual analytes or complex mixtures. Herein, a new approach is described, in which nucleic acid sequences and sequence counts are used as the multivariate data without the necessity of a spatial array. To demonstrate this approach to DS, previously selected aptamers, identified from the literature, were used as semi‐specific receptors, Next‐Gen DNA sequencing was used to generate data, and cell line differentiation was the test‐bed application. The study of a principal component analysis loading plot revealed cross‐reactivity between the aptamers. The technique generates high‐dimensionality score plots, and should be applicable to any mixture of complex and subtly different analytes for which nucleic acid‐based receptors exist. 相似文献
108.
109.
Goodwin KD Lewis MA Tanious FA Tidwell RR Wilson WD Georgiadis MM Long EC 《Journal of the American Chemical Society》2006,128(24):7846-7854
A general strategy for the rapid structural analysis of DNA binding ligands is described as it was applied to the study of RT29, a benzimidazole-diamidine compound containing a highly twisted diphenyl ether linkage. By combining the existing high-throughput fluorescent intercalator displacement (HT-FID) assay developed by Boger et al. and a high-resolution (HR) host-guest crystallographic technique, a system was produced that was capable of determining detailed structural information pertaining to RT29-DNA interactions within approximately 3 days. Our application of the HT/HR strategy immediately revealed that RT29 has a preference for 4-base pair (bp), A.T-rich sites (AATT) and a similar tolerance and affinity for three A-T-bp sites (such as ATTC) containing a G.C bp. On the basis of these selectivities, oligonucleotides were designed and the host-guest crystallographic method was used to generate diffraction quality crystals. Analysis of the resulting crystal structures revealed that the diphenyl ether moiety of RT29 undergoes conformational changes that allow it to adopt a crescent shape that now complements the minor groove structure. The presence of a G.C bp in the RT29 binding site of ATTC did not overly perturb its interaction with DNA-the compound adjusted to the nucleobases that were available through water-mediated interactions. Our analyses suggest that the HT/HR strategy may be used to expedite the screening of novel minor groove binding compounds leading to a direct, HR structural determination. 相似文献
110.
The quantum yields of triplet formation by some α,ω-dinaphthylalkanes and related compounds which exhibit intramolecular excimer fluorescence and/or intramolecular fluorescence quenching have been determined. Although most compounds have quite high quantum yields (~ 0.4) others are much lower and no single reason could be found to explain this variance. 相似文献