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排序方式: 共有125条查询结果,搜索用时 15 毫秒
81.
RE Tribble A Azhari HL Clark CA Gagliardi Y-W Lui AM Mukhamedzhanov A Sattaroy X Tang L Trache V Burjan J Cejpek V Kroha S Piskor J Vincour F Carstoiu 《Pramana》1999,53(3):585-594
S-factors for direct capture reactions can be found at astrophysical energies from asymptotic normalization coefficients which
provide the normalization of the tail of the overlap function. For example the overlap for 8B → 7Be+p defines the S-factor for 7Be (p, γ)8B. Peripheral transfer reactions offer a technique to determine these asymptotic normalization coefficients. As a test of
the technique, the 16O(3He, d)17F reaction has been used to determine asymptotic normalization coefficients for transitions to the ground and first excited
states of 17F. The S-factors for 16O(p, γ)17F calculated from these 17F → 16O+p asymptotic normalization coefficients are found to be in very good agreement with recent measurements. Following the same
technique, the 10B(7Be, 8B)9Be and 14N(7Be, 8B)13C reactions have been used to measure the asymptotic normalization coefficient for 7Be(p, γ)8B. This result provides an indirect determination of S
17(0). 相似文献
82.
F El Gabaly AH McDaniel M Grass WC Chueh H Bluhm Z Liu KF McCarty 《Chemical communications (Cambridge, England)》2012,48(67):8338-8340
We use spatially resolved photoelectron spectroscopy performed in operando to identify the reaction intermediates of the hydrogen electro-oxidation reaction on yttria-stabilized zirconia (YSZ) electrolytes with Pt electrodes. We find that hydroxyl on the zirconia electrolyte is a reaction intermediate in the hydrogen oxidation reaction and that it participates in the rate-determining step. In contrast to the general wisdom, the limiting step does not involve the transfer of charge. These results allow us to propose the detailed reaction pathway, which provides direct insight into how to accelerate the kinetics. 相似文献
83.
Kristina L. Stensaas Brent V. McCarty Natacha M. Touchette James B. Brock 《Tetrahedron》2006,62(46):10683-10687
The singlet oxygen (1Δg) photooxidations of 2-methyl-3-phenylthio-2-butene (1a), 1-[(4-nitrophenyl)thio]-2,3-dimethyl-2-butene (2c), 2-methyl-3-phenylsulfinyl-2-butene (3), 2-methyl-3-phenylsulfonyl-2-butene (6), and 1-[(4-nitrophenyl)sulfonyl]-2,3-dimethyl-2-butene (7c) were conducted in the following deuterated solvents: acetonitrile, benzene, chloroform, methanol, or methanol/water mixture. In each case the ene allylic hydroperoxide products and/or the [2+2] cycloaddition products were quantified and inspected for possible hydrogen bonding induced differences in product selectivity and regiochemistry. After comparison to literature values for related substrates, the results indicate that only photooxidations of vinyl sulfides are susceptible to hydrogen bonding solvent effects. 相似文献
84.
D'Souza F Chitta R Gadde S Zandler ME McCarty AL Sandanayaka AS Araki Y Ito O 《The journal of physical chemistry. A》2006,110(13):4338-4347
Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads. 相似文献
85.
86.
The objective of this study was to determine if topical anesthesia to the larynx and pharynx affects vocal fold motion during dynamic voice evaluation with transnasal flexible endoscopy. Transnasal dynamic laryngeal examinations of 10 patients with no voice complaints were evaluated by five blinded fellowship-trained laryngologists. Each patient was examined before and after application of topical anesthetic. Reviewers rated briskness of right and left vocal fold movement and longitudinal tension on a visual analogue scale. Statistical comparisons were made between individual subject scores before and after anesthetic application. Inter-rater reliability was also assessed. No statistical difference was observed between subject scores before and after anesthetic application. Average intraclass correlation coefficients were 0.643 and 0.591 for pre- and postanesthesia scores, respectively. Application of topical anesthesia to the larynx and pharynx does not affect vocal fold motion. 相似文献
87.
We use low-energy electron microscopy to image the reversible transformation of the TiO2(1 1 0) surface between a high-temperature 1 × 1 structure and a low-temperature 1 × 2 structure. The reconstruction dynamics are novel: 1 × 2 bands nucleated during cooling at the steps of the starting 1 × 1 surface and then grew laterally from the steps. The transformation kinetics are dominated by mass flow from the surface to the bulk, a process that facilitates converting the high-density 1 × 1 phase to the lower-density 1 × 2 phase. We have also imaged how the 1 × 1 surface reconstructs to 1 × 2 phase after sufficient oxygen is removed from the crystal’s bulk during vacuum annealing. 1 × 2 bands also nucleated and grew laterally from the initial 1 × 1-surface’s steps. However, because this isothermal 1 × 1-to-1 × 2 transition occurs largely by mass redistribution on the surface, the steps of the initial 1 × 1 surface and final 1 × 2 surface are offset. We propose models of mass redistribution during the 1 × 1/1 × 2 phase transition to explain this effect. We conclude that the phase transition is first-order because it always occurred by the nucleation and growth of discrete phases. Finally, we show that quenching can roughen TiO2’s surface by forming pits and that changing temperature causes step motion on 1 × 2 surfaces. 相似文献
88.
89.
MJ Lazaro AA Herod M Domin Y Zhuo CA Islas R Kandiyoti 《Rapid communications in mass spectrometry : RCM》1999,13(14):1401-1412
Three methods for determining a 'safe' estimate for high-mass limits of MALDI spectra of coal derived liquids were explored, using a sample of coal-tar pitch and its pyridine-insoluble fraction. Co-addition of increasing numbers of single-shot spectra (10, 30, 50 and 100 pulses) showed visually observable reductions in noise levels, consistent with robust and statistically meaningful signals. Three separate types of post-acquisition calculation were used to identify high-mass limits of the spectra. (i) A literature method indicated high-mass limits similar to those observed visually-as a shift from baseline at the highest masses, nearly 350 000 u for the coal tar pitch and about 390 000 u for its pyridine insoluble fraction. (ii) Comparing instrument signal with pre-selected multiples of the standard deviation, upper mass estimates of between 40-60 000 u for the coal-tar pitch and about 95 000 u for its pyridine-insoluble fraction were found. (iii) Calculation of the slope was used to identify 'lift-off' of the spectrum from baseline. The angle between the smoothed spectrum and the baseline was matched to a pre-selected value (e.g. 0.5 degrees and 1 degrees ). However, the arbitrary specification of the key parameter did not establish this last method on a firm basis. The choice of a criterion for estimating high-mass limits of MALDI spectra remains a semi-quantitative procedure; a reasonably conservative high-mass limit may be estimated by comparison of signal with five times the standard deviation. However, evaluation of size exclusion chromatograms of the present samples using polystyrene standards suggests that molecular mass distributions of pitch samples arrived at by MALDI mass spectrometry are, at least partly, determined by the limitations of available instruments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
90.
McCarty LS Winkleman A Whitesides GM 《Journal of the American Chemical Society》2007,129(13):4075-4088
This paper describes the fabrication and characterization of ionic electrets-materials that bear a long-lived electrostatic charge because of an imbalance between the number of cationic and anionic charges in the material. Crosslinked polystyrene microspheres that contain covalently bound ions and mobile counterions transfer some of their mobile ions in air, in the absence of bulk liquid, to another material upon contact. According to the ion-transfer model of contact electrification, this selective transfer of mobile ions yields microspheres that have a net electrostatic charge. A tool that operates on the principle of electrostatic induction measures the charge on individual microspheres (50-450 microm in diameter). Microspheres with a variety of covalently bound ionic functional groups (tetraalkylammonium, alkyltriphenylphosphonium, alkylsulfonate, and arylsulfonate) acquire charges consistent with this ion-transfer mechanism. The charge on a microsphere is proportional to its surface area (ca. 1 elementary charge per 2000 nm2) and close to the theoretical limit imposed by the dielectric breakdown of air. The charge density in an atmosphere of SF6 is more than twice that in an atmosphere of N2. These observations suggest that the charge density of these ionic electret microspheres is limited by the dielectric breakdown of the surrounding gas. Functionalizing the surfaces of glass or silicon with covalently bound ions and mobile counterions generates ionic electrets from these inorganic substrates. Soft lithography can pattern charge on a planar silicon surface (with oxide) and on the surface of 250-mum glass microspheres. 相似文献