Showing papers by "Jack H. Freed published in 1987"

Calculation of ESR spectra and related Fokker–Planck forms by the use of the Lanczos algorithm. II. Criteria for truncation of basis sets and recursive steps utilizing conjugate gradients

Abstract: The complex symmetric Lanczos algorithm (LA) has proven to be a very efficient means of calculating magnetic resonance line shapes and spectral densities associated with Fokker–Planck forms. However, the relative importance of the various components of the basis set in an accurate representation of the spectrum and the proper number of recursive steps are not easily assessed in practice using the Lanczos algorithm. A systematic and objective procedure for the determination of optimal basis sets and number of recursive steps is developed using a generalization of the conjugate gradient method (CGM) appropriate for the type of complex symmetric matrices occuring in these problems. The relative importance of the individual basis vectors is determined by using the CGM to obtain the ‘‘solution vector’’ from the set of algebraic equations defining the spectrum. This is done at several values of the sweep variable (e.g., the frequency or the magnetic field). The maximum (over these values of sweep variable) for ...

Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in two-dimensional fluids

Abstract: The dynamic effects of pair correlation functions (pcf) on spin relaxation by translational diffusion in infinite two‐dimensional fluids are considered explicitly through a Smoluchowski equation, for the usual conditional probability, with appropriate boundary conditions, especially at the contact separation of the interacting pair of molecules. The solution of this equation by finite difference techniques permits the calculation of time correlation functions, spectral densities, and spin‐relaxation rates associated with a dipolar relaxation mechanism between the spin‐bearing molecules. Comparison of the two‐dimensional spin‐relaxation results obtained with different pcf is presented. The spectral densities and spin‐relaxation rates are indeed found to be significantly altered by the pcf. For example, for a nonuniform pcf, the two‐dimensional spectral densities, at (and above) the Larmor frequency ω0, are greater for translational correlation times τt that are an order of magnitude faster than τt=ω−10 whi...

Comments on the interpretation of dynamic deuterium NMR spectra from solid inclusion compounds

Abstract: Guest molecules trapped within the regular voids of crystalline adducts, called inclusion compounds, are likely to experience mobility. Recent solid-state /sup 2/H NMR studies indicated these motions to be primarily discrete jumps related to the molecular symmetry, to that of the environment, or to some other geometric feature of the system investigated. In this work we offer a more consistent picture based primarily on the symmetry of the voids, but also including the molecular symmetry. The authors show that the /sup 2/H NMR spectra from a whole class of guest molecules can be interpreted in terms of planar threefold jumps between unequally populated sites, such that the populations follow a simple Boltzmann law. The enthalpy difference between these sites is of the order of 1-2 kcal reflecting a rather weak but nonuniform guest-host interaction.

Deuterium NMR study of the structure and dynamic of the side chains of several solid polyglutamates

Abstract: A /sup 2/H NMR study of polycrystalline powders and oriented films of several selectively deuterium-labeled poly(..gamma..-benzyl L-glutamate) (PBLG) and poly(..gamma..-ethyl L-glutamate) (PELG) homopolymers is reported. The deuterium labels on the ester side chain of PBLG indicate the presence of two polymer species below 330 K. A phase transition is detected at 330 K. The phase transition observed at 330 K, associated with the side-chain melting phenomenon, is likely to be accompanied by rotational or translational diffusion of the helices while they maintain their axial orientation. Furthermore, a comparison with the authors conformational analysis suggests a high degree of cooperativity between the adjacent helices. With powders and films of PELG, the dominant motion at low temperatures (besides rapid methyl rotation) can be interpreted either as a discrete jump rotation with 3-fold symmetry but about a diffusion axis that does not coincide with the O/sup eta/-C/sup zeta/ bond, or in terms of a three-site jump motion about this bond among unevenly populated sites. Motional averaging becomes very effective and isotropic in nature at about room temperature. This abrupt change from anisotropic to isotropic motion is indicative of a phase transition. Results obtained with oriented films indicate that the PBLG benzene rings orientmore » preferentially parallel to the axes of the polymer helices, consistent with theoretical predictions, whereas the PELG ethyl groups are distributed randomly in space.« less

Studies of Nonlinear Spin Oscillations in a Rarefied Quantum Gas: Spin Polarized Atomic Hydrogen

Abstract: We describe pulsed nuclear magnetic resonance experiments probing the evolution of the spin wave frequency spectrum from the linear into nonlinear regimes. Results compare favorably with numerical simulations based on the theoretical equations of motion and indicate the presence of distinct modes not present in the linear regime.

Measurement of the Longitudinal Spin Diffusion Coefficient in Spin Polarized Atomic Hydrogen

Abstract: We report on the measurement of the longitudinal spin diffusion coefficient in doubly spin polarized atomic hydrogen. Measurements were made using pulsed nuclear magnetic resonance to probe the transport of magnetization between a sealed container and a large reservoir. Results are presented for densities between 1.4×1015 and 4.8×1016 cm-3 at temperatures between 246 and 470 mK in a magnetic field of 7.7 Tesla and are found to be in reasonable agreement with theoretical calculations by Lhuillier.