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71.
The absolute configurations of 2-cyclohexenone cis-diol metabolites resulting from the biotransformation of the corresponding phenols have been determined by comparison of their experimental and calculated circular dichroism spectra (TDDFT at the PCM/B2LYP/Aug-cc-pVTZ level), optical rotations (calculated at the PCM/B3LYP/Aug-cc-pVTZ level) and by stereochemical correlation. It is found that circular dichroism spectra and optical rotations of 2-cyclohexenone derivatives are strongly dependent on the ring conformation (M or P sofa S(5) or half-chair), enone non-planarity and the nature and positions of the hydroxy and alkyl substituents. The effect of non-planarity of the enone chromophore, including the distortion of the C=C bond, is determined for the model structures by TDDFT calculations at the PCM/B2LYP/6-311++G(2d,2p) level. Non-planarity of the C=C bond in the enone chromophore is commonly encountered in 2-cyclohexenone derivatives and it is a source of significant rotatory strength contribution to the electronic circular dichroism spectra. It is shown that the two lowest-energy transitions in acrolein and 2-cyclohexenone and its derivatives are n(C=O)-π(C=O)* and π(C=C)-π(C=O)*, as expected, while the shorter-wavelength (below 200 nm) transitions are of more complex nature. In 2-cyclohexenone and its alkyl derivatives it is predominantly a mixture of π(C=C)-π(C=C)* and π(C=C)-σ* transitions, whereas the presence of hydroxy substituent results in a dominant contribution due to the n(OH)-π(C=O)* transition. A generalized model for correlation of the CD spectra of 2-cyclohexenones with their structures is presented. 相似文献
72.
Kudavalli JS Boyd DR Coyne D Keeffe JR Lawlor DA MacCormac AC More O'Ferrall RA Rao SN Sharma ND 《Organic letters》2010,12(23):5550-5553
Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ? HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation. 相似文献
73.
Accurate size measurement during nanoparticle production is essential for the continuing innovation, quality and safety of
nano-enabled products. Size measurement by analysing a number of separate particles individually has particular advantages
over ensemble methods. In the latter case nanoparticles have to be well dispersed in a fluid and changes that may occur during
analysis, such as agglomeration and degradation, will not be detected which could lead to misleading results. Atomic force
microscopy (AFM) allows imaging of particles both in air and liquid, however, the strong interactions between the probe and
the particle will cause the broadening of the lateral dimension in the final image. In this paper a new procedure to measure
the size of spherical nanoparticles from AFM images via vertical height measurement is described. This procedure will quickly
analyse hundred of particles simultaneously and reproduce the measurements obtained from electron microscopy (EM). Nanoparticles
samples that were difficult, if not impossible, to analyse with EM were successfully measured using this method. The combination
of this procedure with the use of a metrological AFM moves closer to true traceable measurements of nanoparticle dispersions. 相似文献
74.
N.-C. Yeh M.-L. Teague S. Yeom B.L. Standley R.T.-P. Wu D.A. Boyd M.W. Bockrath 《Surface science》2011,605(17-18):1649-1656
Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice structures and electronic density of states (DOS). In particular, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. Additionally, the large and non-uniform strain induces pseudo-magnetic field up to ~ 50 T, as manifested by the DOS peaks at quantized energies that correspond to pseudo-magnetic field-induced integer and fractional Landau levels. In contrast, for graphene transferred from copper to SiO2 substrates after the CVD growth, the average strain on the whole diminishes, so do the corresponding charging effects and pseudo-magnetic fields except for sample areas near topological defects. These findings suggest feasible nano-scale “strain engineering” of the electronic states of graphene by proper design of the substrates and growth conditions. 相似文献
75.
J.P. Boyd 《Applied Mathematics Letters》1998,11(6):27-31
W(z) is defined implicitly as the root of W exp(W) = z. It is shown that a simple analytic approximation has a relative error of less than 5% over the whole domain z ε [−exp(−1), ∞] of the principle branch—sufficiently accurate so that four Newton iterations refine this approximation to a relative error smaller than 1.E-12. As a second form of global approximation, the W-function is expanded as a series of rational Chebyshev functions TBj in a shifted, logarithmic coordinate with an error that decreases exponentially fast with the series truncation. 相似文献
76.
We introduce the classes of locally convex spaces with the local Dunford-Pettis property and locally dual Schur spaces. We
examine their properties and their relationship to other classes of locally convex spaces. In the class of locally convex
spaces with the local Dunford-Pettis property all polynomials are weakly sequentially continuous whereas in the class of locally
dual Schur spaces all polynomials are weakly continuous on bounded sets. 相似文献
77.
We introduce the classes of locally convex spaces with the local Dunford-Pettis property and locally dual Schur spaces. We
examine their properties and their relationship to other classes of locally convex spaces. In the class of locally convex
spaces with the local Dunford-Pettis property all polynomials are weakly sequentially continuous whereas in the class of locally
dual Schur spaces all polynomials are weakly continuous on bounded sets.
Research supported by Science Foundation Ireland, Basic Research Grant 2004. 相似文献
78.
79.
We report results from particle-in-cell simulations of the interaction of intense laser light with overdense plasma designed to examine the effects of plasma waves generated by pulses of fast electrons on high-order harmonic emission from the plasma. We show that the emission spectrum is modulated at the plasma frequency and identify combinations of parameters and circumstances favorable for modulation. In particular, the observed modulation is shown to depend not only on the chosen plasma electron density and intensity of the incident light but on the density profile and pulse shape. 相似文献
80.
Zhai J Waddington L Wooster TJ Aguilar MI Boyd BJ 《Langmuir : the ACS journal of surfaces and colloids》2011,27(24):14757-14766
Lipid liquid crystalline nanoparticles such as cubosomes and hexosomes have unique internal nanostructures that have shown great potential in drug and nutrient delivery applications. The triblock copolymer, Pluronic F127, is usually employed as a steric stabilizer in dispersions of lipid nanostructured particles. In this study, we investigated the formation, colloidal stability and internal nanostructure and morphology of glyceryl monooleate (GMO) and phytantriol (PHYT) cubosome dispersions on substituting β-casein with F127 in increasing proportion as the stabilizer. Internal structure and particle morphology were evaluated using small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM), while protein secondary structure was studied using synchrotron radiation circular dichroism (SRCD). The GMO cubosome dispersion stabilized by β-casein alone displayed a V(2) (Pn3m) phase structure and a V(2) to H(2) phase transition at 60 °C. In comparison, F127-stabilized GMO dispersion had a V(2) (Im3m) phase structure and the H(2) phase only appeared at higher temperature, that is, 70 °C. In the case of PHYT dispersions, only the V(2) (Pn3m) phase structure was observed irrespective of the type and concentration of stabilizers. However, β-casein-stabilized PHYT dispersion displayed a V(2) to H(2) to L(2) transition behavior upon heating, whereas F127-stabilized PHYT dispersion displayed only a direct V(2) to L(2) transition. The protein secondary structure was not disturbed by interaction with GMO or PHYT cubosomes. The results demonstrate that β-casein provides steric stabilization to dispersions of lipid nanostructured particles and avoids the transition to Im3m structure in GMO cubosomes, but also favors the formation of the H(2) phase, which has implications in drug formulation and delivery applications. 相似文献