Showing papers by "Jack H. Freed published in 1986"

Diffusion coefficients in anisotropic fluids by ESR imaging of concentration profiles

Abstract: We report on the use of ESR imaging for determining the translational diffusion constants of typical ESR spin probes in ordered and isotropic solvents. A discussion is given for a Fourier deconvolution method for determining the correct concentration profile if there is more than one hyperfine line in the spectrum of the radical as well as a spatial dependence of the spectrometer sensitivity. A simple but approximate subtraction deconvolution method is also presented. The diffusion constants for 4‐oxo‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPONE) in the nematic and isotropic phases of the liquid crystal p‐pentylbenzylidine‐p‐butylanaline (5,4) were determined. In the isotropic phase the diffusion coefficient is isotropic (D=2.5×10−6 cm2 s−1, at 50 °C), while in the nematic phase it was found to be mildly anisotropic. The diffusion coefficients for motion perpendicular and parallel to the director axis in the nematic phase are respectively, D⊥ =9.0×10−7 cm2 s−1, D∥ =6.4×10−7 cm2 s−1 at 27 °C. This is interpreted in terms of some smectic‐like character in the nematic phase of 5,4. Possible improvements in the technique are also discussed.

Two-dimensional Fourier transform ESR spectroscopy

Abstract: New Fourier transform ESR techniques, which permit rapid data acquisition of ESR spectra, now allow one to perform a variety of two‐dimensional ESR experiments. Two prototype experiments on a fast motional nitroxide spectrum are described. The first is based upon detection of the Han echo and may be used for discriminating the homogeneous broadening of an inhomogeneous spectrum (and/or the echo envelope modulation). The second is based upon detection of the FID after the three‐pulse Jeener–Ernst sequence, and it leads to cross peaks due to Heisenberg spin exhange among the hyperfine lines.

Linear prediction and resolution enhancement of complex line shapes in two‐dimensional electron‐spin‐echo spectroscopy

Abstract: A new type of two‐dimensional electron‐spin‐echo (2D‐ESE) spectroscopy was recently shown to be useful for studying slow molecular motions in liquids. A recently developed method of spectral enhancement based upon linear prediction with singular value decomposition (LPSVD) is applied in the present work to dramatically improve the signal‐to‐noise ratio and to correct for finite dead time in the data from this 2D‐ESE experiment. This permitted a more accurate comparison between theory and experiment. Good agreement is now obtained with a model of nearly isotropic Brownian motion for tempone in glycerol/water solvent.

The Lanczos Algorithm in Molecular Dynamics: Calculation of Spectral Densities

Abstract: In the past the Lanczos algorithm has proven to be a very useful tool in the study of molecular dynamics. It enables efficient calculation of the spectral densities which are essential in the interpretation of spectroscopic or scattering experiments. In the present work, the use of the Lanczos algorithm in the calculation of the spectral densities is analyzed in a general fashion. Its application to problems characterized by complex symmetric matrices, which are normally encountered in the analysis of magnetic resonance experiments, is then recovered as a particular case. After a discussion of the factors influencing the convergence in the calculation of spectral densities, the implementation of the Lanczos algorithm with non-orthogonal basis functions is considered in relation to molecular systems having hindered degrees of freedom.

Surface‐suppressed electron resonance spectroscopies

Abstract: Surface‐suppressed electron resonance spectroscopies (SSERS) refers to the phenomenon in which stable paramagnetic radicals adsorbed on clean (noble) metal surfaces have their ESR signal suppressed This phenomenon is studied in some detail by ultra‐high‐vacuum ESR (UHV‐ESR) and cyclotron resonance from microwave‐induced secondary electron emissions (CREMSEE) in combination with conventional thermal desorption The UHV‐ESR is performed in situ on the inner surface of the microwave cavity after leaking in stable nitroxides as previously described by Nilges and Freed In the SSERS phenomenon, the first layers of nitroxide deposited on the metal surface do not give rise to an observable ESR signal, even though a significant decrease in the CREMSEE microwave power threshold, Pt, is observed It is consistently found (for several nitroxides and noble metals) that an ESR signal is observed only when Pt has dropped to of the order of 20 (±10)% of its initial value The ESR signal then increases monotonically wit

Diffusion Coefficients in Anisotropic Fluids by ESR Imaging of Concentration Profiles.

Abstract: We report on the use of ESR imaging for determining the translational diffusion constants of typical ESR spin probes in ordered and isotropic solvents. A discussion is given for a Fourier deconvolution method for determining the correct concentration profile if there is more than one hyperfine line in the spectrum of the radical as well as a spatial dependence of the spectrometer sensitivity. A simple but approximate subtraction deconvolution method is also presented. The diffusion constants for 4‐oxo‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPONE) in the nematic and isotropic phases of the liquid crystal p‐pentylbenzylidine‐p‐butylanaline (5,4) were determined. In the isotropic phase the diffusion coefficient is isotropic (D=2.5×10−6 cm2 s−1, at 50 °C), while in the nematic phase it was found to be mildly anisotropic. The diffusion coefficients for motion perpendicular and parallel to the director axis in the nematic phase are respectively, D⊥ =9.0×10−7 cm2 s−1, D∥ =6.4×10−7 cm2 s−1 at 27 °C. This is interpreted in terms of some smectic‐like character in the nematic phase of 5,4. Possible improvements in the technique are also discussed.