			Home About us Contact

Long-range Correlations (long-range + correlation)

Distribution by Scientific Domains

Chemistry	60%

Selected Abstracts

Long-Range Correlations between Aliphatic 13C Nuclei in Protein MAS NMR Spectroscopy,

ANGEWANDTE CHEMIE, Issue 31 2009
Marvin
Ein hocheffizienter Polarisationstransfer in MAS-NMR-Experimenten (MAS: Rotation im magischen Winkel) ist möglich durch eine Kombination von alternierenden 13C-Markierungen und bandselektiver Radiofrequenz(RF)-getriebener Wiedereinkopplung (BASE RFDR). Dabei wird die Bandselektivität schwacher RF-Strahlung genutzt, um die homonucleare Dipolwechselwirkung wieder einzuführen. [source]

Stochastic computational modelling of highly heterogeneous poroelastic media with long-range correlations

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 1 2004
Diego G. Frias
Abstract The compaction of highly heterogeneous poroelastic reservoirs with the geology characterized by long-range correlations displaying fractal character is investigated within the framework of the stochastic computational modelling. The influence of reservoir heterogeneity upon the magnitude of the stresses induced in the porous matrix during fluid withdrawal and rock consolidation is analysed by performing ensemble averages over realizations of a log-normally distributed stationary random hydraulic conductivity field. Considering the statistical distribution of this parameter characterized by a coefficient of variation governing the magnitude of heterogeneity and a correlation function which decays with a power-law scaling behaviour we show that the combination of these two effects result in an increase in the magnitude of effective stresses of the rock during reservoir depletion. Further, within the framework of a perturbation analysis we show that the randomness in the hydraulic conductivity gives rise to non-linear corrections in the upscaled poroelastic equations. These corrections are illustrated by a self-consistent recursive hierarchy of solutions of the stochastic poroelastic equations parametrized by a scale parameter representing the fluctuating log-conductivity standard deviation. A classical example of land subsidence caused by fluid extraction of a weak reservoir is numerically simulated by performing Monte Carlo simulations in conjunction with finite elements discretizations of the poroelastic equations associated with an ensemble of geologies. Numerical results illustrate the effects of the spatial variability and fractal character of the permeability distribution upon the evolution of the Mohr,Coulomb function of the rock. Copyright © 2004 John Wiley & Sons, Ltd. [source]

A robust method for determining 1H,15N long-range correlations: 15N optimized CIGAR-HMBC experiments,

MAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2003
Mike Kline
Abstract An examination of a variety of common nitrogen-containing systems was undertaken to optimize parameters for observation of 1H,15N long-range correlations. Because of the diversity of coupling constants encountered with 1H,15N correlations, a modified accordion-based sequence was used to provide the best results. Optimization of the values for the accordion delay revealed that a range between 3 and 10 Hz provided the best compromise between detection of weak correlations and loss of signal to T2 processes. Multiple bond correlations were readily detected for each class of compound with the exception of anilines. Correlations within heterocyclic systems revealed some general patterns. In general, stronger correlations were observed from protons to ,pyrrole-like' nitrogens than to the ,pyridine-type' nitrogens of imidazoles and pyrazoles. Very long-range (four- and five-bond) correlations were routinely observed between methyl groups and the nitrogens of aromatic heterocycles. Copyright © 2003 John Wiley & Sons, Ltd. [source]