全文获取类型
收费全文 | 50篇 |
免费 | 1篇 |
专业分类
化学 | 37篇 |
晶体学 | 4篇 |
力学 | 2篇 |
数学 | 1篇 |
物理学 | 7篇 |
出版年
2022年 | 2篇 |
2020年 | 3篇 |
2016年 | 2篇 |
2014年 | 3篇 |
2013年 | 1篇 |
2012年 | 1篇 |
2011年 | 1篇 |
2009年 | 1篇 |
2008年 | 3篇 |
2006年 | 3篇 |
2005年 | 1篇 |
2004年 | 2篇 |
2003年 | 1篇 |
2000年 | 1篇 |
1999年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1966年 | 1篇 |
1935年 | 2篇 |
1931年 | 2篇 |
排序方式: 共有51条查询结果,搜索用时 0 毫秒
1.
Steven P. Hopper Michael J. Tremelling Emma W. Goldman 《Journal of organometallic chemistry》1978,156(2):331-340
The heterogeneous gas-solid reaction of trimethylchlorosilane and an equal molar mixture of potassium t-butoxide and sodium methoxide gave exclusively trimethyl-t-butoxysilane. A similar reaction utilizing dimethyldichlorosilane as the vapor phase substrate gave predominant incorporation of butoxide. The heterogeneous reaction of chloromethyldimethylchlorosilane with these two bases was extensively studied. The parameters investigated for the latter system include the dependence of the product distribution on temperature, nitrogen flow rate and the relative hydroxide content of the solid alkoxide base. These studies allow the assessment of the relative reactivities of these bases under the conditions of heterogeneous gas/solid reaction. Under these conditions and with partially hydrolyzed bases a substantial amount of displacement of the chloromethyl group from chloromethyldimethylchlorosilane is observed. The role of hydroxide ion in this reaction is discussed. Convenient syntheses of t-butyl methyl ether and dimethylchloromethoxysilane are reported. 相似文献
2.
The rates of mechanochemical reactions are generally found to increase exponentially with applied stress. However, a buckling theory analysis of the effect of a normal stress on an adsorbate that is oriented perpendicularly to the surface that reacts by tilting suggests that a critical value of the stress should be required to initiate a mechanochemical reaction. This concept is verified by using density functional theory calculations to simulate the effect of compressing a homologous series of alkyl thiolate species on copper by a hydrogen-terminated copper counter-face. This predicts that a critical stress is indeed needed to initiate methyl thiolate decomposition, which has a perpendicular C–CH3 bond. In contrast, no critical stress is found for ethyl thiolate with an almost horizontal C–CH3 bond, while a critical stress is required to isomerize propyl thiolate from a trans to a cis configuration. These predictions are tested by measuring the mechanochemical reaction rates of these alkyl thiolates on a Cu(100) substrate by sliding an atomic force microscope tip over the surface and finding a critical stress of ∼0.43 GPa for methyl thiolate, ∼0.33 GPa for propyl thiolate, but no evidence of a critical stress for ethyl thiolate, in accord with the predictions. These results provide insights not only into mechanochemical reaction mechanisms on surfaces, but also on the origin of critical phenomena in stress-induced processes in general. It also suggests novel approaches to designing robust surface films that can resist wear and damage.The rates of mechanochemical reactions are generally found to increase exponentially with applied stress. 相似文献
3.
4.
5.
6.
7.
A rapid, sensitive, and specific method was developed for the determination of perchlorate anion in foods. The foods included high moisture fruits and vegetables, low moisture foods (e.g. wheat flour and corn meal), and infant foods. Improvements to existing procedures were made in sample preparation that reduced sample test portion size from 100 to 5 or 10 g, extraction solvent volume from 150 to 20-40 ml, and replaced blending extraction-vacuum filtration and their associated large glassware with a simple shakeout-centrifugation in a small conical tube. Procedures common to all matrices involved: extraction, centrifugation, graphitized carbon solid phase extraction (SPE) cleanup, and ion chromatography-tandem mass spectrometry (IC-MS/MS) analysis. A Waters IC-Pak Anion HR column (4.6 mm × 75 mm) was eluted with 100 mM ammonium acetate in 50:50 (v/v) acetonitrile/water mobile phase at a rate of 0.35 ml/min. A triple stage quadrupole mass spectrometer, equipped with electrospray ionization (ESI) in the negative ion mode, was used to detect perchlorate anion. An 18O4-labeled perchlorate anion internal standard was used to correct for any matrix effects. The method limit of quantitation (LOQ) was: 1.0 μg/kg in fruits, vegetables, and infant foods; 3.0 μg/kg in dry products. Fortified test portions gave 80-120% recoveries. Determination of incurred perchlorate anion residues agreed well with results for comparable commodities or products analyzed by published methods. 相似文献
8.
Abstract The eigenvalue problems resulting from stiffness matrix formulations of structural vibration and buckling problems are nonlinear if substructures are analyzed exactly, or if classical frequency (vibration problems) or load factor (buckling problems) dependent member equations are used. This makes rapid calculation of accurate free vibration or buckling modes difficult. This paper presents several techniques which might overcome this difficulty, examines them theoretically and experimentally, and gives some of the ways in which the more successful techniques can be incorporated in mode finding methods. Coincident eigenvalues (i.e., natural frequencies or critical load factors) are included. 相似文献
9.
Idlir Liko Jonathan T. S. Hopper Timothy M. Allison Justin L. P. Benesch Carol V. Robinson 《Journal of the American Society for Mass Spectrometry》2016,27(6):1099-1104
Membrane protein complexes are commonly introduced to the mass spectrometer solubilized in detergent micelles. The collisional activation used to remove the detergent, however, often causes protein unfolding and dissociation. As in the case for soluble proteins, electrospray in the positive ion mode is most commonly used for the study of membrane proteins. Here we show several distinct advantages of employing the negative ion mode. Negative polarity can yield lower average charge states for membrane proteins solubilized in saccharide detergents, with enhanced peak resolution and reduced adduct formation. Most importantly, we demonstrate that negative ion mode electrospray ionization (ESI) minimizes subunit dissociation in the gas phase, allowing access to biologically relevant oligomeric states. Together, these properties mean that intact membrane protein ions can be generated in a greater range of solubilizing detergents. The formation of negative ions, therefore, greatly expands the possibilities of using mass spectrometry on this intractable class of protein. 相似文献
10.
D.G. Hopper 《Chemical physics letters》1975,31(3):446-450
The 14A″(4Π) state of N2O+ is found to be the most stable at bent conformations of the nuclei. Ab initio SCF, MC SCF, and MC SCF CI calculations with a double-zeta basis and typical bond-length values for RNN and RNO all yield minimum energy angles near 126°. The energy lowering is such that the total energy of N2O+ (14A″, 126°) is very near that of the lowest O, N2 asymptote, 4S, X 1Σg+. These results are shown to imply that the ionospheric reaction O+ + N2 → NO+ + N, the rate-determining step in the electron removal reaction network, is adiabatic on the potential surface of the 14A″ state. 相似文献