John W. Logan

Rotary resonance recoupling for half-integer quadrupolar nuclei in solid-state nuclear magnetic resonance spectroscopy

Investigations were made of rotary resonance recouplings (R3) of chemical shift anisotropy (CSA), heteronuclear dipolar (HTD), and homonuclear dipolar (HMD) couplings involving half-integer quadrupolar nuclei under magic-angle sample spinning condition. Under rotary resonance conditions provided by a low amplitude rf field and a high spinning speed, the spectrum of the central transition coherence of half-integer quadrupolar nuclei shows recouplings of CSA, HTD, and HMD interactions that depend on the ratio of the rf field to the spinning speed. These new properties can be used to extract electronic and structural information about the sample that are otherwise difficult to extract in the presence of a dominant quadrupolar interaction. An average Hamiltonian theory is used to explain the recoupling properties of various interactions. Experimental implementations of the R3 are demonstrated on model compounds with spin-3/2 systems.

Permanent Replacements and the End of Labor's “Only True Weapon”

This article analyzes the origins and impact of one of the most powerful antiunion weapons used by American employers during the past four decades: the right to use and threaten to use permanent replacement workers during economic strikes. It examines the policy debate over replacements in the 1930s and 1940s, the increasing use of permanent replacements in the 1970s and 1980s, the growth of a powerful and sophisticated “strike management industry,” and the unsuccessful efforts of organized labor and its political allies to amend the National Labor Relations Act to outlaw permanent replacements. The article concludes with a brief discussion of the relationship between the “striker replacement doctrine” and declining strike levels in the postwar decades.

Theoretical investigations of I = 5/2 quadrupolar spin dynamics in the sudden-passage regime

The theoretical approach utilizing bimodal Floquet theory in the quadrupolar/central-transition interaction frame, presented in an earlier article [J. D. Walls, K. H. Lim, and A. Pines, J. Chem. Phys. 116, 79 (2002)], is extended to describe the more complicated spin dynamics of I=5/2 spin systems. Rotary resonance effects occur when the strength of the radio-frequency irradiation, ω1, matches the sample spinning speed, ωr, at the conditions ω1=23nωr (n integral). At these conditions, conversions of both triple-quantum and five-quantum coherences to central-quantum coherence are observed. Between rotary resonance conditions [2n3ωr<ω1<[2(n+1)]/3ωr], five-quantum as well as triple-quantum coherences can be created from equilibrium z-magnetization via a nutation mechanism. In addition, effective transfer between five-quantum and triple-quantum coherences also is observed in between rotary resonance conditions. These effects have been investigated theoretically and verified by both numerical calculations and experimental results.The theoretical approach utilizing bimodal Floquet theory in the quadrupolar/central-transition interaction frame, presented in an earlier article [J. D. Walls, K. H. Lim, and A. Pines, J. Chem. Phys. 116, 79 (2002)], is extended to describe the more complicated spin dynamics of I=5/2 spin systems. Rotary resonance effects occur when the strength of the radio-frequency irradiation, ω1, matches the sample spinning speed, ωr, at the conditions ω1=23nωr (n integral). At these conditions, conversions of both triple-quantum and five-quantum coherences to central-quantum coherence are observed. Between rotary resonance conditions [2n3ωr<ω1<[2(n+1)]/3ωr], five-quantum as well as triple-quantum coherences can be created from equilibrium z-magnetization via a nutation mechanism. In addition, effective transfer between five-quantum and triple-quantum coherences also is observed in between rotary resonance conditions. These effects have been investigated theoretically and verified by both numerical calculations and ...

“Lighting Up” NMR and MRI in Colloidal and Interfacial Systems

By means of optical pumping with laser light, the nuclear spin polarization of gaseous xenon can be enhanced by many orders of magnitude. The enhanced polarization has allowed an extension of the pioneering experiments of Fraissard and coworkers to novel applications of NMR and MRI in chemistry, materials science and biomedicine. Examples are presented of developments and applications of laser-polarized xenon NMR and MRI on distance scales from nanometers to meters. The size of the xenon atom is similar to that of small organic molecules, such as methane, yet the nuclear magnetic resonance (NMR) signal from xenon proves a more sensitive probe for the local environment. Laser-polarized xenon NMR has been used, in collaboration with Sozzani and coworkers, to investigate the interactions present in an effectively one- dimensional gas phase inside nanochannels. Small changes in channel size and/or structure lead to very different modes of diffusion. Optically pumped Xe NMR can distinguish between these different diffusion modes out to unparalleled time scales (several tens of seconds). These studies are particularly useful for gaining a fundamental understanding of the laws that govern heterogenous mass transport such as gas transport into porous catalysts or molecular sieves, or liquid transport through pore-forming transmembrane proteins in biological systems. The understanding of mass transport inside microporous materials is crucial for many industrial and commercial processes. Recent experiments will also be described in which xenon has been used to investigate the cavities of biological nanosystems and in which polarization has been transferred to molecules on surfaces and in solution. As an example, in collaboration with Wemmer and coworkers, xenon has been used as a molecular probe to investigate the hydrophobic surfaces and interiors of macrocyclic molecules and proteins; recent results show evidence for binding of xenon to the outside of a protein, a proposed cause of the anesthetic mechanism of xenon. Indeed, localized injection of polarized xenon solutions into human blood has provided observations of the real-time process of xenon penetrating red blood cells. The injection technique also makes it possible to provide enhanced magnetic resonance images of localized areas in living organisms. Furthermore, the use of laser-polarized xenon also opens an exciting new frontier in the possibility of “functionalized xenon” as a biosensor of analytes and metabolites in chemistry, materials science and biomedicine. The novel biosensor offers advantages of multiplexing capabilities and the possibility of detection in-vivo.