Damping by bulk and shear viscosity of confined acoustic phonons for nanostructures in aqueous solution

A nanoparticle in aqueous solution is modeled as a homogeneous elastic isotropic continuum sphere in contact with an infinite viscous compressible Newtonian fluid. The frequencies and damping of the confined vibrational modes of the sphere are calculated for the material parameters of a CdSe nanoparticle in water and a poly(methyl methacrylate) nanosphere in water. Although the effects of viscosity are found to be negligible for macroscopic objects, for nanoscale objects, both the frequency and damping of the vibrational modes are significantly affected by the viscosity of the liquid. Furthermore, both shear viscosity and bulk viscosity play an important role. A model of the spherical satellite tobacco mosaic virus consisting of outer solid layers with a water core is also investigated, and the viscosity of the water core is found to significantly damp the free vibrational modes. The same approach can be applied for nonspherical geometries and also to viscoelastic nanoparticles.     

Fragmentation study of iridoid glucosides through positive and negative electrospray ionization, collision-induced dissociation and tandem mass spectrometry

Mass spectrometric methodology based on the combined use of positive and negative electrospray ionization, collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS) has been applied to the mass spectral study of a series of six naturally occurring iridoids through in-source fragmentation of the protonated [M+H]+, deprotonated [M--H]- and sodiated [M+Na]+ ions. This led to the unambiguous determination of the molecular masses of the studied compounds and allowed CID spectra of the molecular ions to be obtained. Valuable structural information regarding the nature of both the glycoside and the aglycone moiety was thus obtained. Glycosidic cleavage and ring cleavages of both aglycone and sugar moieties were the major fragmentation pathways observed during CID, where the losses of small molecules, the cinnamoyl and the cinnamate parts were also observed. The formation of the ionized aglycones, sugars and their product ions was thus obtained giving information on their basic skeleton. The protonated, i.e. [M+H]+ and deprotonated [M--H]-, ions were found to fragment mainly by glycosidic cleavages. MS/MS spectra of the [M+Na]+ ions gave complementary information for the structural characterization of the studied compounds. Unlike the dissociation of protonated molecular ions, that of sodiated molecules also provided sodiated sugar fragments where the C0+ fragment corresponding to the glucose ion was obtained as base peak for all the studied compounds.     

Efficient thermal field computation in phase-field models

We solve the phase-field equations in two dimensions to simulate crystal growth in the low undercooling regime. The novelty is the use of a fast solver for the free space heat equation to compute the thermal field. This solver is based on the efficient direct evaluation of the integral representation of the solution to the constant coefficient, free space heat equation with a smooth source term. The computational cost and memory requirements of the new solver are reasonable and no artificial boundary conditions are needed. This allows one to solve for the thermal field in a computational domain whose size depends only on the size of the growing crystal and not on the extent of the thermal field, which can result in significant computational savings in the low undercooling regime.     

Simultaneous measurement of proline and related compounds in oak leaves by high-performance ligand-exchange chromatography and electrospray ionization mass spectrometry for environmental stress studies

A mass spectrometer was coupled to high-performance ligand-exchange liquid chromatography (HPLEC) for simultaneous analysis of stress associated solutes such as proline, hydroxyproline, methylproline, glycine betaine and trigonelline extracted from leaves of drought stressed oaks and an internal standard namely N-acetylproline. Methanol/chloroform/water extracts were analyzed using an Aminex HPX-87C column and specifically quantified by the positive ion mode of an electrospray ionisation-mass spectrometry (ESI-MS) in single ion monitoring (SIM) mode. The recovery of N-acetyl proline added to oak leaf extracts ranged from 85.2 to 122.1% for an intra-day study. Standard calibration curves showed good linearity in the measured range from 0.3125 to 10 μmol L⁻¹ with the lowest correlation coefficient of 0.99961 for trigonelline. The advantages of this alternative procedure, compared to previously published methods using fluorescence or amperometric detections, are the simultaneous and direct detection of osmoprotectants in a single chromatographic run, a minimal sample preparation, a good specificity and reduced limits of quantification, ranging from 0.1 to 0.6 μmol L⁻¹. Fifty-six days of water deficit exposure resulted in increased foliar free proline levels (2.4-fold, P < 0.001, 155 μmol g⁻¹ FW) and glycine betaine contents (2.5-fold, P < 0.05, 175 μmol g⁻¹ FW) of drought stressed oak compared to control.     

Estimating the intensity of a random measure by histogram type estimators

The purpose of this paper is to estimate the intensity of some random measure N on a set ${\mathcal{X}}$ by a piecewise constant function on a finite partition of ${\mathcal{X}}$ . Given a (possibly large) family ${\mathcal{M}}$ of candidate partitions, we build a piecewise constant estimator (histogram) on each of them and then use the data to select one estimator in the family. Choosing the square of a Hellinger-type distance as our loss function, we show that each estimator built on a given partition satisfies an analogue of the classical squared bias plus variance risk bound. Moreover, the selection procedure leads to a final estimator satisfying some oracle-type inequality, with, as usual, a possible loss corresponding to the complexity of the family ${\mathcal{M}}$ . When this complexity is not too high, the selected estimator has a risk bounded, up to a universal constant, by the smallest risk bound obtained for the estimators in the family. For suitable choices of the family of partitions, we deduce uniform risk bounds over various classes of intensities. Our approach applies to the estimation of the intensity of an inhomogenous Poisson process, among other counting processes, or the estimation of the mean of a random vector with nonnegative components.     

Involatile nanodroplets: an asymptotic analysis

The structure of nanometer-scale droplets of weakly volatile liquids arises through the interplay of strong intermolecular attraction, and core intermolecular repulsion, interfacial forces, and the large, negative chemical potential of the low density vapor with which it is in equilibrium. Using a van der Waals equation of state and a mesoscopic multiphase model, the structure of such nanodroplets is determined via an asymptotic analysis in terms of the ambient to critical temperature ratio. The structure of a spherical droplet is obtained as the solution of a simple "shooting" problem. The intradroplet pressure profile and a minimal droplet size are determined. The high pressure in the core of the droplet gives evidence for the preferred melting there for systems like water with a negative volume of melting. Our methodology can be generalized to multiphase droplets, as well as to composite structures wherein viruses or other nanoparticles are embedded.     

Iridoid glucosides from the aerial parts of Globularia alypum L. (Globulariaceae)

From the hydromethanolic extract of the aerial parts of Globularia alypum grown in Morocco, a new chlorinated iridoid glucoside, globularioside has been isolated beside 5 known iridoid glycosides, globularin, globularicisin, globularidin, globularinin and globularimin. This is the first report of a chlorinated iridoid in G. alypum and in the Globulareaceae. Unlike all other known 7-chlorinated iridoid glucosides where the chlorine atom exhibits an alpha configuration, globularioside incorporate the chlorine atom as a 7beta substituent. The structures of the isolated compounds were established on the basis of ESI-MS, MS-MS, 1D and 2D NMR spectral analysis.     

Risk bounds for model selection via penalization

Performance bounds for criteria for model selection are developed using recent theory for sieves. The model selection criteria are based on an empirical loss or contrast function with an added penalty term motivated by empirical process theory and roughly proportional to the number of parameters needed to describe the model divided by the number of observations. Most of our examples involve density or regression estimation settings and we focus on the problem of estimating the unknown density or regression function. We show that the quadratic risk of the minimum penalized empirical contrast estimator is bounded by an index of the accuracy of the sieve. This accuracy index quantifies the trade-off among the candidate models between the approximation error and parameter dimension relative to sample size. If we choose a list of models which exhibit good approximation properties with respect to different classes of smoothness, the estimator can be simultaneously minimax rate optimal in each of those classes. This is what is usually called adaptation. The type of classes of smoothness in which one gets adaptation depends heavily on the list of models. If too many models are involved in order to get accurate approximation of many wide classes of functions simultaneously, it may happen that the estimator is only approximately adaptive (typically up to a slowly varying function of the sample size). We shall provide various illustrations of our method such as penalized maximum likelihood, projection or least squares estimation. The models will involve commonly used finite dimensional expansions such as piecewise polynomials with fixed or variable knots, trigonometric polynomials, wavelets, neural nets and related nonlinear expansions defined by superposition of ridge functions. Received: 7 July 1995 / Revised version: 1 November 1997     

Determination of the isomerization rate constant HOCH2CH2CH2CH(OO·)CH3 →HOC·HCH2CH2CH(OOH)CH3. Importance of intramolecular hydroperoxy isomerization in tropospheric chemistry

The rate constant of the title reaction is determined during thermal decomposition of di-n-pentyl peroxide C₅H₁₁O( )OC₅H₁₁ in oxygen over the temperature range 463–523 K. The pyrolysis of di-n-pentyl peroxide in O₂/N₂ mixtures is studied at atmospheric pressure in passivated quartz vessels. The reaction products are sampled through a micro-probe, collected on a liquid-nitrogen trap and solubilized in liquid acetonitrile. Analysis of the main compound, peroxide C₅H₁₀O₃, was carried out by GC/MS, GC/MS/MS [electron impact EI and NH₃ chemical ionization CI conditions]. After micro-preparative GC separation of this peroxide, the structure of two cyclic isomers (3S*,6S*)3α-hydroxy-6-methyl-1,2-dioxane and (3R*,6S*)3α-hydroxy-6-methyl-1,2-dioxane was determined from ¹H NMR spectra. The hydroperoxy-pentanal OHC( )(CH₂)₂( )CH(OOH)( )CH₃ is formed in the gas phase and is in equilibrium with these two cyclic epimers, which are predominant in the liquid phase at room temperature. This peroxide is produced by successive reactions of the n-pentoxy radical: a first one generates the CH₃C·H(CH₂)₃OH radical which reacts with O₂ to form CH₃CH(OO·)(CH₂)₃OH; this hydroxyperoxy radical isomerizes and forms the hydroperoxy HOC·H(CH₂)₂CH(OOH)CH₃ radical. This last species leads to the pentanal-hydroperoxide (also called oxo-hydroperoxide, or carbonyl-hydroperoxide, or hydroperoxypentanal), by the reaction HOC·H(CH₂)₂CH(OOH)CH₃+O₂→O()CH(CH₂)₂CH(OOH)CH₃+HO₂. The isomerization rate constant HOCH₂CH₂CH₂CH(OO·)CH₃→HOC·HCH₂CH₂CH(OOH)CH₃ (k₃) has been determined by comparison to the competing well-known reaction RO₂+NO→RO+NO₂ (k₇). By adding small amounts of NO (0–1.6×10¹⁵ molecules cm⁻³) to the di-n-pentyl peroxide/O₂/N₂ mixtures, the pentanal-hydroperoxide concentration was decreased, due to the consumption of RO₂ radicals by reaction (7). The pentanal-hydroperoxide concentration was measured vs. NO concentration at ten temperatures (463–523 K). The isomerization rate constant involving the H atoms of the CH₂( )OH group was deduced: or per H atom: The comparison of this rate constant to thermokinetics estimations leads to the conclusion that the strain energy barrier of a seven-member ring transition state is low and near that of a six-member ring. Intramolecular hydroperoxy isomerization reactions produce carbonyl-hydroperoxides which (through atmospheric decomposition) increase concentration of radicals and consequently increase atmospheric pollution, especially tropospheric ozone, during summer anticyclonic periods. Therefore, hydrocarbons used in summer should contain only short chains (<C₄) hydrocarbons or totally branched hydrocarbons, for which isomerization reactions are unlikely. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 875–887, 1998