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Number Distributions (number + distribution)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Improving kp Data Originating from PLP Number Distributions

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 3 2006
Andreas Kornherr
Abstract Summary: Based on certain features, especially the width of the so-called extra peaks in the simulated number-chain-length distribution (CLD) of polymers prepared by pulsed laser polymerization (PLP), it is calculated by which factor the positions of the points of inflections and maxima deviate from the theoretical L0 data that are to be used for the evaluation of kp. These corrections are for slightly chain-length-dependent termination by disproportionation or combination and cover a wide range of chain-lengths and primary radical production. They can be applied either to the point of inflection on the low-molecular weight side of the extra peaks or to the peak maximum. On average, the mean error that is about ,2.5% for uncorrected data from first-order points of inflection is reduced to the order of less than 1% even if no assumption concerning the mode of termination is made. The situation is similar for the positions of the first-order peak maxima where the mean error of about +7% likewise can be cut down to less than 1% if the proper correction function is chosen. Second- and third-order peaks data, which are a priori less falsified, yield still better results after correction. Mass sensitivity of the detection process has comparatively little effect: it is only for unrealistically high extents of chain-length dependence in detection that considerable falsifications are to be expected. As an additional result it turned out that correction functions obtained for number distributions are also applicable to mass spectrometry raw distributions and even for mass distributions x(l),·,l provided Poissonian broadening is the only broadening process. Number distribution xC(l) calculated for termination by combination times attenuation function F1(l). [source]

Tl(I)-the strongest structure-breaking metal ion in water?

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2007
A quantum mechanical/molecular mechanical simulation study
Abstract Structural and dynamical properties of the Tl(I) ion in dilute aqueous solution have been investigated by ab initio quantum mechanics in combination with molecular mechanics. The first shell plus a part of the second shell were treated by quantum mechanics at Hartree-Fock level, the rest of the system was described by an ab initio constructed potential. The radial distribution functions indicate two different bond lengths (2.79 and 3.16 Å) in the first hydration shell, in good agreement with large-angle X-ray scattering and extended X-ray absorption fine structure spectroscopy results. The average first shell coordination number was found as 5.9, and several other structural parameters such as coordination number distributions, angular distribution functions, and tilt- and ,-angle distributions were evaluated. The ion,ligand vibration spectrum and reorientational times were obtained via velocity auto correlation functions. The TlO stretching force constant is very weak with 5.0 N m,1. During the simulation, numerous water exchange processes took place between first and second hydration shell and between second shell and bulk. The mean ligand residence times for the first and second shell were determined as 1.3 and 1.5 ps, respectively, indicating Tl(I) to be a typical "structure-breaker". The calculated hydration energy of ,84 ± 16 kcal mol,1 agrees well with the experimental value of ,81 kcal mol,1. All data obtained for structure and dynamics of hydrated Tl(I) characterize this ion as a very special case among all monovalent metal ions, being the most potent "structure-breaker", but at the same time forming a distinct second hydration shell and thus having a far-reaching influence on the solvent structure. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source]

Existence of a critical carbon number in the aging of a wax-oil gel

AICHE JOURNAL, Issue 9 2001
Probjot Singh
A fundamental study was carried out to understand the aging (or hardening) of the wax-oil gels formed in the subsea pipelines during the flow of crude oil from offshore wells to shore. The aging process is a counterdiffusion phenomenon where there exists a critical carbon number (CCN), and wax molecules with carbon numbers greater than the CCN diffuse into the gel matrices and vice versa. Using a careful analysis of carbon number distributions of gel deposits, collected from a cold finger after various deposition time intervals, the CCN for the wax-oil system was obtained. A mathematical model, based on a modified version of UNIQUAC model, was developed to predict the CCN for wax-oil systems. The size of the interaction units for n-alkanes in the solid-phase UNIQUAC model was found to be a strong function of the mean carbon number in the solid phase. [source]

Improving kp Data Originating from PLP Number Distributions

Temperature-induced phase transition in simulated amorphous Al2O3

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2006
Vo Van Hoang
Abstract Our previous simulation studies indicated the existence of pressure-induced phase transition in liquid and amorphous Al2O3. In the current paper, we present the structural transformation of simulated amorphous Al2O3 from an octahedral to a tetrahedral network structure by heating from the high-density amorphous (hda) model at constant pressure P = 0 GPa. The structure of the models was analyzed through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Furthermore, significant differences in structural characteristics of low-density amorphous (lda) models obtained by heating from the hda one and by cooling from the low-density melt have been found and are presented. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Observation of vanadyl porphyrins and sulfur-containing vanadyl porphyrins in a petroleum asphaltene by atmospheric pressure photonionization Fourier transform ion cyclotron resonance mass spectrometry

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 14 2008
Kuangnan Qian
Vanadyl (VO) porphyrins and sulfur-containing vanadyl (VOS) porphyrins of a wide carbon number range (C26 to C52) and Z-number range (,28 to ,54) were detected and identified in a petroleum asphaltene by atmospheric pressure photonionization (APPI) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). APPI provides soft ionization of asphaltene molecules (including VO and VOS porphyrins), generating primarily molecular ions (M+.). The ultra-high mass resolving power (m/,mFWHM ,500,K) of FTICR-MS enabled resolution and positive identification of elemental formulae for the entire family of VO and VOS porphyrins in a complicated asphaltene matrix. Deocophylerythro-etioporphyrin (DPEP) is found to be the most prevalent structure, followed by etioporphyrins (etio)- and rhodo (benzo)-DPEP. The characteristic Z-distribution of VO porphyrins suggests benzene and naphthene increment in the growth of porphyrin ring structures. Bimodal carbon number distributions of VO porphyrins suggest possible different origins of low and high molecular weight species. To our knowledge, the observation of VOS porphyrins in a petroleum product has not previously been reported. The work is also the first direct identification of the entire vanadyl porphyrin family by ultra-high resolution mass spectrometry without chromatographic separation or demetallation. Copyright © 2008 John Wiley & Sons, Ltd. [source]