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61.
Panda D Mishra PP Khatua S Koner AL Sunoj RB Datta A 《The journal of physical chemistry. A》2006,110(17):5585-5591
The photophysics of the fluorescent probe Lucifer yellow CH has been investigated using fluorescence spectroscopic and computational techniques. The nonradiative rate is found to pass through a minimum in solvents of intermediate empirical polarity. This apparently anomalous behavior is rationalized by considering the possibility of predominance of different kinds of nonradiative processes, viz. intersystem crossing (ISC) and excited-state proton transfer (ESPT), in solvents of low and high empirical polarity, respectively. The feasibility of the proton transfer is examined by the structure determined by the density functional theory (DFT) calculations. The predicted energy levels based on the time-dependent density functional theory (TD-DFT) method in the gas phase identifies the energy gap between the S(1) and nearest triplet state to be close enough to facilitate ISC. Photophysical investigation in solvent mixtures and in deuterated solvents clearly indicates the predominance of the solvent-mediated intramolecular proton transfer in the excited state of the fluorophore in protic solvents. 相似文献
62.
Ramesh-Babu Adusumalli Rejin Raghavan Rudy Ghisleni Tanja Zimmermann Johann Michler 《Applied Physics A: Materials Science & Processing》2010,100(2):447-452
The deformation and failure of the secondary cell wall of Spruce wood was studied by in-situ SEM compression of micropillars
machined by the focused ion beam technique. The cell wall exhibited yield strength values of approximately 160 MPa and large
scale plasticity. High resolution SEM imaging post compression revealed bulging of the pillars followed by shear failure.
With additional aid of cross-sectional analysis of the micropillars post compression, a model for deformation and failure
mechanism of the cell wall has been proposed. The cell wall consists of oriented cellulose microfibrils with high aspect ratio
embedded in a hemicellulose-lignin matrix. The deformation of the secondary wall occurs by asymmetric out of plane bulging
because of buckling of the microfibrils. Failure of the cell wall following the deformation occurs by the formation of a shear
or kink band. 相似文献
63.
Background
Tone languages such as Thai and Mandarin Chinese use differences in fundamental frequency (F0, pitch) to distinguish lexical meaning. Previous behavioral studies have shown that native speakers of a non-tone language have difficulty discriminating among tone contrasts and are sensitive to different F0 dimensions than speakers of a tone language. The aim of the present ERP study was to investigate the effect of language background and training on the non-attentive processing of lexical tones. EEG was recorded from 12 adult native speakers of Mandarin Chinese, 12 native speakers of American English, and 11 Thai speakers while they were watching a movie and were presented with multiple tokens of low-falling, mid-level and high-rising Thai lexical tones. High-rising or low-falling tokens were presented as deviants among mid-level standard tokens, and vice versa. EEG data and data from a behavioral discrimination task were collected before and after a two-day perceptual categorization training task.Results
Behavioral discrimination improved after training in both the Chinese and the English groups. Low-falling tone deviants versus standards elicited a mismatch negativity (MMN) in all language groups. Before, but not after training, the English speakers showed a larger MMN compared to the Chinese, even though English speakers performed worst in the behavioral tasks. The MMN was followed by a late negativity, which became smaller with improved discrimination. The High-rising deviants versus standards elicited a late negativity, which was left-lateralized only in the English and Chinese groups.Conclusion
Results showed that native speakers of English, Chinese and Thai recruited largely similar mechanisms when non-attentively processing Thai lexical tones. However, native Thai speakers differed from the Chinese and English speakers with respect to the processing of late F0 contour differences (high-rising versus mid-level tones). In addition, native speakers of a non-tone language (English) were initially more sensitive to F0 onset differences (low-falling versus mid-level contrast), which was suppressed as a result of training. This result converges with results from previous behavioral studies and supports the view that attentive as well as non-attentive processing of F0 contrasts is affected by language background, but is malleable even in adult learners. 相似文献64.
65.
Heat treatment of sodium silicate water glass of the nominal composition Na2O/SiO2 = 1:3 was carried out from 100 °C up to 800 °C and the advancement of the resulting phases was followed up by powder X-ray diffraction, scanning electron microscopy and thermogravimetry along with differential thermal analysis. The water glass, initially being an amorphous solid, starts to form crystals of β-Na2Si2O5 at about 400 °C and crystallizes the SiO2 modification cristobalite at about 600 °C that coexists along with β-Na2Si2O5 up to 700 °C. At 750 °C Na6Si8O19 appears as a separate phase and beyond 800 °C, the system turns into a liquid. 相似文献
66.
67.
The product of the reaction of (-C5 H5)Ru(PPh3)2 Cl and CuC2Ph has been characterized by single-crystal x-ray diffraction techniques and shown to have stoichiometry (-C5 H5)Ru(PPh3)2Cu(C2Ph)Cl-(CH3)2CO(I). The compound contains a phenylethynyl group -bonded to the ruthenium atom and simultaneously -bonded to a terminal Cu-Cl bond. Crystals of this compound from acetone are monoclinic,P21/c, witha = 12.914, = 22.111,c = 16.534 Å, = 110.77 °, andZ — 4. Full-matrix least-squares refinement constraining the phenyl rings of the triphenylphosphines, the cyclopentadienyl ring, and the solvent of crystallization (acetone) as rigid groups yieldedR and weightedR values of 0.084 and 0.075, respectively. 相似文献
68.
Strain-stiffening response in transient networks formed by reverse wormlike micelles 总被引:2,自引:0,他引:2
Strain-stiffening, that is, an increase in material stiffness at large deformations, is a property of many biological materials. Currently, model systems for the study of this phenomenon are elastic networks (gels) of semiflexible filamentous biopolymers such as actin, keratin, or fibrin. Here, we demonstrate strain-stiffening in a class of viscoelastic solutions, comprising reverse wormlike micelles. These structures are formed by the coassembly of the physiological surfactants, lecithin and bile salt, in an organic solvent, cyclohexane. In contrast to the biopolymer gels, the networks here are transient and are formed by the physical entanglement of relatively flexible worms. Our results suggest that neither a permanent network nor a high filament rigidity is required for strain-stiffening. We suggest a different origin, based on a temporary strain-induced increase in the volume fraction of entangled worms. Our system can also serve as a convenient synthetic model for future studies into this phenomenon. 相似文献
69.
Reverse vesicles are spherical containers in organic liquids (oils) consisting of an oily core surrounded by a reverse bilayer. They are the organic counterparts to vesicles in aqueous solution and could potentially find analogous uses in encapsulation and controlled release. However, few examples of robust reverse vesicles have been reported, and general guidelines for their formation do not exist. We present a new route for forming stable unilamellar reverse vesicles in nonpolar organic liquids, such as cyclohexane and n-hexane. The recipe involves mixing short- and long-chain lipids (lecithins) with a trace of a salt such as sodium chloride. The ratio of short- to long-chain lecithin controls the type and size of self-assembled structure. As this ratio is increased, a spontaneous transition from reverse micelles to reverse vesicles occurs. Small-angle neutron scattering (SANS) and transmission electron microscopy (TEM) confirm the presence of unilamellar vesicles in the corresponding solutions. Average vesicle diameters can be tuned from 60 to 250 nm depending on the sample composition. 相似文献
70.
Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches. 相似文献