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981.
The redox-active ligand N-(2-hydroxy-5-methylbenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hptp1) was prepared and complexed to manganese(II). The isolated [Mn(Hptp1)(MeCN)](2+) serves as a magnetic resonance imaging contrast agent, with an r(1) value comparable to those of other mononuclear gadolinium(III) and manganese(II) complexes. The metal and ligand are stable in aerated aqueous solutions, but the addition of H(2)O(2) causes the complex to oxidatively couple to itself through a bimolecular reaction involving the phenol groups of two Hptp1 ligands. The binuclear product is less paramagnetic per manganese(II) than its mononuclear precursor, lowering the measured r(1) per manganese(II). The manganese(II) complex with Hptp1 can thereby serve as a sensor for oxidative stress. 相似文献
982.
Statistical theory based on a mean isotropic potential deduced from a full potential energy surface is applied to the complex-forming, reactive H(+) + H(2) system in the interval of collision energies E(c) = 10(-3) to 0.5 eV. We present expressions for the reaction probabilities that incorporate the full permutation symmetry of the protons and compare our results with other statistical models and full quantum mechanical approaches that take account this symmetry correctly, approximately, or erroneously for the exchange rearrangement mechanism of the reaction. 相似文献
983.
Kaiser RI Parker DS Zhang F Landera A Kislov VV Mebel AM 《The journal of physical chemistry. A》2012,116(17):4248-4258
The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry. 相似文献
984.
985.
Beliefs constitute a central part of a person’s professional competencies and are crucial to the perception of situations
as they influence our choice of actions. This paper focuses on epistemological beliefs about the nature of mathematics of
future primary teachers from an international perspective. The data reported are part of a larger sample originating from
the TEDS-M study which compares primary mathematics teacher education in 15 countries. In this paper we examine the pattern
of beliefs of future teachers aiming to teach mathematics at primary level. We explore whether and to what extent beliefs
concerning the nature of mathematics are influenced by cultural factors, in our case the extent to which a country’s culture
can be characterized by an individualistic versus collectivistic orientation according to Hofstede’s terminology. In the first
part of the paper, the literature on epistemological beliefs is reviewed and the role of culture and individualism/collectivism
on the formation of beliefs concerning the nature of mathematics will be discussed. In the empirical part, means and distributions
of belief ratings will be reported. Finally, multilevel analyses explore how much of the variation of belief preferences between
countries can be explained by the individualistic orientation of a country. 相似文献
986.
RA Kastelein R Gransier L Hoek CA de Jong 《The Journal of the Acoustical Society of America》2012,132(2):607-610
The distance at which harbor porpoises can hear underwater detonation sounds is unknown, but depends, among other factors, on the hearing threshold of the species for impulsive sounds. Therefore, the underwater hearing threshold of a young harbor porpoise for an impulsive sound, designed to mimic a detonation pulse, was quantified by using a psychophysical technique. The synthetic exponential pulse with a 5?ms time constant was produced and transmitted by an underwater projector in a pool. The resulting underwater sound, though modified by the response of the projection system and by the pool, exhibited the characteristic features of detonation sounds: A zero to peak sound pressure level of at least 30?dB (re 1?s(-1)) higher than the sound exposure level, and a short duration (34?ms). The animal's 50% detection threshold for this impulsive sound occurred at a received unweighted broadband sound exposure level of 60?dB re 1?μPa(2)s. It is shown that the porpoise's audiogram for short-duration tonal signals [Kastelein et al., J. Acoust. Soc. Am. 128, 3211-3222 (2010)] can be used to estimate its hearing threshold for impulsive sounds. 相似文献
987.
We develop a bead–spring Brownian dynamics model for simulating the topological interactions between polymers and thin obstacles
and apply this method to electrophoretically translating DNA strands interacting with an immovable post. The use of a bead–spring
method allows for the simulation of entanglement interactions of polymer chains too long to be simulated using bead–rod or
pearl necklace models. Using stiff “FENE-Fraenkel” springs, we are able to model short chains as well. Our new method determines
the shortest distance between a spring and the post, calculates a repulsive force inversely related to this distance using
an exponential potential, and corrects for the rare situation when a spring passes beyond the post despite the repulsive interaction.
As an example problem, we consider single-chain collisions with a single post in weak electric fields. We explore a wide range
of chain lengths (25–1,515 Kuhn steps), and we find that the average delay produced by the collision is a function of both
the chain length and the Peclet number. Chains of all lengths reach the same upper limit at high Peclet number, but they follow
separate curves with similar slopes at lower Peclet number. Our results are consistent with published results for a 25-Kuhn-step
chain at Peclet number Pe = 10. Our new method is a general one that allows us to compute the effects of entanglements in
systems with rare entanglements and long chains that cannot be simulated by other more microscopic methods. 相似文献
988.
A comparative study of the length scales and morphology of dissipation fields in turbulent jet flames and non-reacting jets
provides a quantitative analysis of the effects of heat release on the fine-scale structure of turbulent mixing. Planar laser
Rayleigh scattering is used for highly resolved measurements of the thermal and scalar dissipation in the near fields of CH4/H2/N2 jet flames (Re
d
= 15,200 and 22,800) and non-reacting propane jets (Re
d
= 7,200–21,700), respectively. Heat release increases the dissipation cutoff length scales in the reaction zone of the flames
such that they are significantly larger than the cutoff scales of non-reacting jets with comparable jet exit Reynolds numbers.
Fine-scale anisotropy is enhanced in the reaction zone. At x/d = 10, the peaks of the dissipation angle PDFs in the Re
d
= 15,200 and 22,800 jet flames exceed those of non-reacting jets with corresponding jet exit Reynolds numbers by factors
of 2.3 and 1.8, respectively. Heat release significantly reduces the dissipation layer curvature in the reaction zone and
in the low-temperature periphery of the jet flames. These results suggest that the reaction zone shields the outer regions
of the jet flame from the highly turbulent flow closer to the jet axis. 相似文献
989.
Abstract. Shock tubes often experience temperature and pressure nonuniformities behind the reflected shock wave that cannot be neglected
in chemical kinetics experiments. Because of increased viscous effects, smaller tube diameters, and nonideal shock formation,
the reflected-shock nonidealities tend to be greater in higher-pressure shock tubes. Since the increase in test temperature
() is the most significant parameter for chemical kinetics, experiments were performed to characterize in the Stanford High Pressure Shock Tube using infrared emission from a known amount of CO in argon. From the measured change
in vibrationally equilibrated CO emission with time, the corresponding ddt (or for a known time interval) of the mixture was inferred assuming an isentropic relationship between post-shock temperature
and pressure changes. For a range of representative conditions in argon (24–530 atm, 1275–1900 K), the test temperature 2
cm from the endwall increased 3–8 K after 100 s and 15–40 K after 500 s, depending on the initial conditions. Separate pressure measurements using a shielded piezoelectric transducer confirmed
the isentropic assumption. An analytical model of the reflected-shock gas dynamics was also developed, and the calculated
's agree well with those obtained from experiment. The analytical model was used to estimate the effects of temperature and
pressure nonuniformities on typical chemical kinetics measurements. When the kinetics are fast (s), the temperature increase is typically negligible, although some correction is suggested for kinetics experiments lasting
longer than 500 s. The temperature increase, however, has a negligible impact on the measured laser absorption profiles of OH (306 nm) and
CH (216 nm), validating the use of a constant absorption coefficient. Infrared emission experiments are more sensitive to temperature
and density changes, so nonuniformities should be taken into account when interpreting ir-emission data.
Received 25 April 2000 / Accepted 8 September 2000 相似文献
990.
Wolfgang Desch Ronald Grimmer 《Transactions of the American Mathematical Society》2001,353(12):5095-5120
We consider a material with memory whose constitutive law is formulated in terms of internal state variables using convex potentials for the free energy and the dissipation. Given the stress at a material point depending on time, existence of a strain and a set of inner variables satisfying the constitutive law is proved. We require strong coercivity assumptions on the potentials, but none of the potentials need be quadratic.
As a technical tool we generalize the notion of an Orlicz space to a cone ``normed' by a convex functional which is not necessarily balanced. Duality and reflexivity in such cones are investigated.