Charles S. Johnson

Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications

Nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

A PFG NMR experiment for accurate diffusion and flow studies in the presence of eddy currents

Abstract A pulsed-field-gradient NMR experiment that permits accurate measurements of diffusion and flow rates in the presence of eddy currents is presented. The experiment employs short transverse evolution times and is thus especially useful for the study of systems exhibiting fast transverse relaxation or homonuclear scalar coupling. The insensitivity of the experiment to the presence of eddy currents is demonstrated with experiments on 5% polyacrylamide gels.

High‐Resolution ESR Spectra of Photochemically Generated Free Radicals: The Viologens

High‐resolution electron spin resonance spectra have been obtained for free radicals produced from methyl, ethyl, and benzyl viologen and 4–4′‐bipyridyl in ethanol—water solutions by irradiation with sunlight. From the analysis of these spectra we find that the hyperfine splittings for the protons ortho and meta to the nitrogen atoms are almost equal. The spin density distributions are discussed in terms of McLachlans approximate SCF theory and we conclude that |QNHH|≈|QN CH3H|≈25.

Factors involved in the determination of rotational correlation times for spin labels

Abstract The determination of rotational correlation times and activation energies by means of ESR line widths has been investigated in detail for di- t -butyl nitroxide and for four spin labels in solution. Consistent correlation times can be obtained only if the parameters b and Δγ in the spin Hamiltonian are determined separately for each radical solvent system. Line width corrections to account for unresolved hyperfine structure were found to affect significantly the apparent activation energies for rotation.

Nuclear Relaxation in Gases: Mixtures of Methane and Oxygen

Three previously proposed mechanisms of nuclear relaxation in fluids are qualitatively discussed: (1) modulation of intramolecular fields by collisions, (2) transient, and (3) diffusively modulated dipolar interactions with paramagnetic impurities. It is shown that the dependence of T1 for the protons in methaneoxygen mixtures on composition, density, and temperature is adequately described by a superposition of these contributions to 1/T1, and that the magnitudes involved are consistent with a priori estimates. It is suggested that the spin‐rotation interaction provides an important relaxation mechanism in both gaseous and liquid methane.

Nuclear Relaxation in Adulterated Hydrogen

The proton spin‐lattice relaxation time T1 has been measured in normal hydrogen and in mixtures with 12 other gases as a function of density and composition at room temperature. Cross sections for mJ transitions of the H2 molecule deduced from the Bloembergen—Schwinger formula are interpreted qualitatively on the basis of electric multipole moments as measures of the anisotropy of the intermolecular forces, using the transient approximation. This approximation is justified in the case of H2–CO2 mixtures on the basis of the temperature dependence of T1. The assumptions of the Bloemberger—Schwinger formula are discussed, and an experiment is proposed to determine the connection between two correlation times which appear in the theory.

The effect of torsional vibrations on the minimum value of T1 for methyl protons in solids

Abstract It is demonstrated that currently accepted theories of spin-lattice relaxation for protons in methyl groups, undergoing hindered rotation, predict minimum values of T1 which are significantly smaller than those found experimentally. It is shown that the decrease in the efficiency of spin relaxation can result from the partial averaging of the magnetic dipolar Hamiltonian by torsional oscillations. A calculation of this effect, based on the harmonic approximation, is presented, which improves the agreement with experiment.

Imaging of transient magnetization gratings in nmr. analogies with laser-induced gratings and applications to diffusion and flow

Abstract Pulsed field gradient NMR (PFGNMR) experiments for the measurement of diffusion and flow are described in terms of magnetization gratings. These gratings are analogous to the transient refractive index gratings in optical holographic relaxation experiments, and the analogy between spin echoes and diffraction spots is discussed. One-dimensional NMR imaging of a sample containing a magnetization grating permits the absolute calibration of both magnetic field gradients that were used to create and to read the grating. This method, which is well known in imaging applications, is also useful in PFGNMR diffusion studies. Calibration and diffusion measurements are presented as illustrations. The application of the imaging sequences to the measurement of flow rates is discussed, and a two-dimensional version of the experiment is suggested.

Structural and dynamic origins of intensity in holographic relaxation spectroscopy

Recent holographic relaxation spectroscopy (HRS) experiments can be explained in terms of multiple-laser-induced gratings. It is shown that diffraction may result either from intrinsic differences in the optical properties of ground and excited states or from differences in their transport properties. The effects of photoinduced changes in diffusion coefficients and electrophoretic mobilities are treated. It is suggested that photoinduced molecular dissociation can be studied by means of HRS without the necessity of photochromic labeling.

The determination of tracer diffusion coefficients for proteins by means of pulsed field gradient NMR with applications to hemoglobin

Abstract A pulsed magnetic field gradient NMR spectrometer system was assembled for the measurement of diffusion coefficients for proteins. This systems was applied to study of hemoglobin in the concentration range from 5 to 33 g/dl. The results compare favorably with classical diffusion cell measurements of tracer diffusion coefficients for hemoglobin, and complement quasi-elastic light-scattering measurements of mutual diffusion coefficients.