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991.
Jiali Gao 《Journal of Chemical Sciences》1994,106(2):507-519
The origin of the solvent effects on the free energy of activation for the isomerization of N,N-dimethylformamide in water,
CHCl3 and CCl4 has been investigated through statistical mechanical simulations using the combined quantum mechanical and molecular mechanical
AM1/OPLS potential. The differential solvations between the ground state and transition state in various solvents can be attributed
to the differences in molecular dipole moments in solution, and to the solvent polarization effects. In polar solvents, DMF
is polarized more favorably in the ground state than in the rotated conformers, leading to greater solvent contributions.
The modest solvent effects in CCl4 are a reflection of its much smaller dielectric constant. 相似文献
992.
Recent measurements show substantial differences in levels of Reynolds stress in otherwise nominally similar separations. No explanation for these differences is know, but it appears they are not associated with end effects. End effects appear to be significant only for unexpectedly small flow width. 相似文献
993.
Lydéric Bocquet Jean-Pierre Hansen Jaroslaw Piasecki 《Journal of statistical physics》1997,89(1-2):321-346
In this work, we show thatin any finite system, the binary friction tensor for two Brownian particlescannot be directly estimated from an evaluation of the microscopic Green-Kubo formula, involving the time integral of force-force
autocorrelation functions. This pitfall is associated with a subtle inversion of the thermodynamic and long-time limits and
leads to spurious results for the estimates of the friction matrix based on molecular dynamics simulations. Starting from
a careful analysis of the coupled Langevin equations for two interacting Brownian particles, we derive a method to circumvent
these effects and extract the binary friction tensor from the correlation function matrix of the instantaneous forces exerted
by the bath particles on the fixed Brownian particles, and from the relaxation of the total momentum of the bath in afinite system. The general methodology is applied to the case of two hard or soft Brownian spheres in a bath of light particles.
Numerical estimates of the relevant correlation functions and of the resulting self and mutual components of the matrix of
friction tensors are obtained by molecular dynamics simulations for various spacings between the Brownian particles.
This paper is dedicated to B. Jancovici on the occassion of his 65th birthday. 相似文献
994.
We measured the wavelength-dependent intensity decays of 4,9-dihydro-4,6-dimethyl-9-oxo-1H-imidazo-1,2a-purine (Yt-base) in propanol to determine the time-resolved emission spectra and rates of spectral relaxation. We found that resolution of the spectral relaxation times was dramatically improved by global analysis of the frequency-domain data with increasing amounts of the collisional quencher CCl4. Collisional quenching preferentially decreases the longer-lived relaxed component of the emission, thereby increasing the fractional contribution of the incompletely relaxed portion of the emission. The data could not be explained by a single spectral relaxation time, and at least two relaxation times are needed to describe the time-dependent emission center of gravity of Yt-base.Dedicated to Professor Stefan Paszyc on the occasion of his 70th birthday. 相似文献
995.
Summary Kubelka-Munk theory of radiation transfer in turbid media is applied to determine the influence of skin optical losses on
the efficiency of phototherapy of neonatal hyperbilirubinemia. By using a multilayer model of the skin and a rate equation
analysis of bilirubin photo-isomerization, the photon absorption rates of birilubin and of its configurational photoisomers
and, in turn, the photoequilibrium concentrations and rise time are calculated for spectrally Gaussian light sources and fluorescent
lamps used in phototherapy. It turns out that light absorption and scattering processes in the skin layers produce a 30 nm
red-shift of the optimum value of the peak excitation wave-length of bilirubin absorption and photoequilibrium rise time.
The comparative data on the phototherapeutical efficiency of different spectral regionsin vivo (Gunn rats and humans) are discussed.
Riassunto L'influenza delle perdite ottiche cutanee sul rendimento della fototerapia dell'iperblirubinemia neonatale è investigata mediante l'impiego della teoria di Kubelka-Munk sulla propagazione della luce in mezzi torbidi. L'evoluzione temporale delle concentrazioni di bilirubina e dei suoi fotoisomeri configurazionali è studiata per mezzo delle equazioni di bilancio nel caso del modello a multistrati della cute, assumendo una sorgente con distribuzione spettrale gaussiana e i profili spettrali delle principali lampade fluorescenti usate clinicamente. I risultati indicano che l'assorbimento e la diffusione della luce negli strati cutanei sposta di circa 30 nm verso lunghezze d'onda maggiori il valore ottimale della lunghezza d'onda d'assorbimento della bilirubina e del tempo di fotoequilibrio. Si discutono comparativamente i dati sul rendimento foroterapeutico di diverse regioni spettraliin vivo (su ratti Gunn e uomo).
Резюме Теория Кубелки-Мунка рационального переноса в мутной среде применяется для определения влияния оптических потерь в поверхностном слое на эффективность фототерапии. Используя многослойную модель кожи и анализ фотоизомеризации билирубина, вычисляются интенсивности поглощения фототонов билирубином и его конфигурационным фотоизомерами и, в свою очередь, фоторавновесные концентрации и времена возникновения для источников света с гауссовым спектром и для флуоресцентных ламп, используемых в фототерапии. Оказывается, что процессы поглощения и рассеяния света в поверхностных слоях кожи приводят к красному смещению 30 нм оптимальной величины длины волны для поглощения билирубином. Обсуждаются сравнительные данные об эффективности фототерапии в различных спектральных областях ?в естественных условиях?.相似文献
996.
超急速传热时球体内非稳态热传导的非傅里叶效应 总被引:13,自引:0,他引:13
本文分析了球形物体表面加有一突然温度变化这类超急速传热情形下的热波传播特性.基于超急速传热条件下,热流矢的传播和温度梯度的形成之间存在时间延迟的概念,本文采用双曲型热传导方程来描述该高度非稳态热传导问题,得到了球体内部温度分布的解析表达式.同时将所得解与抛物型热传导方程的解进行了比较,分析了其热传导的非傅里叶效应与产生条件. 相似文献
997.
Y. J. Wang H. A. Nickel B. D. McCombe F. M. Peeters J. M. Shi G. Q. Hai X. -G. Wu T. J. Eustis W. Schaff 《Physica E: Low-dimensional Systems and Nanostructures》1998,2(1-4)
A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al0.3Ga0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons. 相似文献
998.
Summary The post-buckling behavior of imperfect columns made of nonlinear viscoelastic materials is investigated, taking into account
the effect of shear deformation. The material is modeled according to the Leaderman representation of nonlinear viscoelasticity.
Solutions are developed, within the elastica and the shear deformation theories, in order to calculate the growth in time
of the total deflection. The numerical results establish the importance of the shear and the nonlinear viscoelasticity effects,
and of the h/ℓ ratio in the column post-buckling behavior.
Accepted for publication 11 November 1996 相似文献
999.
D. J. Harris T. M. Teo R. J. Batt S. C. Luk 《International Journal of Infrared and Millimeter Waves》1980,1(3):339-349
Interference effects between modes in open-resonators may seriously modify the resonance characteristics. Constructive and destructive interference may take place. For multimode conditions many path lengths exhibit resonance modification.Work supported by the U. K. Science Research Council. 相似文献
1000.
Summary A new method of dental fluorine prophylaxis based on the chemical reaction induced by UV irradiation in dental enamel has
been presented. Fluorine ions from a gel topic can be retained to the dental apatite in the lamp-irradiated samples at about
70% and in the laser-irradiated samples at about 80% of the maximum deposited value. The19F(p,α)16O nuclear reaction was used to measure the fluorine concentrations in the first 3 μm of the enamel of healthy teeth before
and after the gel topic applications with and without UV irradiation. This method of dental prefenction resolves the problem
of the traditional fluorine prophylaxis which brings fluorine in the enamel without binding the apatite.
To speed up publication, the authors of this paper have agreed to not receive the proofs for correction. 相似文献