Stephen B. W. Roeder

Molecular Motions in Several Solids Studied by Nuclear Magnetic Relaxation in the Rotating Frame

Rotating frame nuclear magnetic relaxation times have been measured as a function of temperature from 117 to 290°K for perfluorocyclohexane, cyclohexane, 2,2‐dichloropropane, neopentane, and tetramethylammonium iodide. The data are interpreted in terms of molecular rotation and diffusion in the solid state.

Spurious ringing in pulse NMR

Abstract The theory of electromagnetic generation of ultrasonic standing waves in metals is reviewed and applied to the problem of spurious ringing detected in pulse NMR. The theory predicts that the spurious signal is proportional to the transmitter pulse amplitude and the square of the static field intensity. It also depends on some material properties and geometrical factors. A table of pertinent properties for various metals is presented showing that aluminum is one of the worst materials. Experiments confirm that replacement of aluminum with other metals in the probe attenuates the spurious signal and that the ringing of the coil is attenuated by a judicious choice of wire size.

NMR coils with segments in parallel to achieve higher frequencies or larger sample volumes

Abstract The question of using multisegment coils, connected in parallel, for NMR is investigated. It is shown theoretically and experimentally that such coils offer significant improvements over equivalent single coils in several ways: (1) For a given size, they are usable at much higher frequencies; (2) for a given Larmor frequency, they can be made larger to contain more sample; and (3) the axial component of the rf electric field can be reduced for certain coil geometries.

Opposed coil magnet calculations for large sample and unilateral nuclear‐magnetic resonance

A magnet configuration has been devised, consisting of two nested coaxial coils with the current in the inner coil circulating in an opposite sense from that in the outer coil. The magnetic field generated by such an opposed pair exhibits a small region of homogeneity outside of the magnet itself, creating the possibility of constructing a unilateral nuclear‐magnetic‐resonance device capable of examining an object from only one side. Calculations on one special case, the inside‐out Helmholtz configuration, indicate a volume of 10 cm3 with homogeneity of ±1% for a coil of outer radius 10 cm, and 0.1 cm3 with 100‐ppm homogeneity. A second special case, consisting of two sets of opposed solenoids arranged in a quasi‐Helmholtz manner, offers homogeneity comparable to a conventional Helmholtz pair of similar diameter but with an increase in separation of as much as a factor of 2.

A Single‐Coil Probe Damper for Pulsed Nuclear Magnetic Resonance

A circuit design is presented for a probe damper suitable for single coil NMR probes which improves the time for ringdown following an rf pulse by a factor of 2.5. The damping is provided for a precisely controlled time and removed positively to enable the use of high‐Q single‐coil NMR probes for samples with very short T2s. The damper can be adapted for use with negative or positive transmitter gating pulses.

A spatially selective opposed-loop surface coil

Abstract We describe a variation of the conventional surface coil for use in large-sample NMR. It consists of two concentric and coplanar circular loops of wire with different diameters and possibly different numbers of turns, and with currents flowing in opposite directions. The ratio of currents may be chosen so that the field near the center of the coil is zero, resulting in a region at some distance from the coil in which the radiofrequency field intensity is uniform. A parallel winding scheme is described which allows operation at high frequencies. A 5 cm overall diameter opposed-loop coil was built to operate at 80 MHz and exhibited an axial maximum in sensitivity between 1.5 and 2 cm from the plane of the coil.

Spin–lattice relaxation in cross relaxed systems: Anisole

Spin−lattice relaxation has been monitored as a function of temperature in liquid anisole and its partially deuterated analogues C6D5OCH3 and C6H5OCD3, as pure liquids and in solution in C6D5OCD3. Conventional pulse and Fourier transform techniques have been used. The high resolution spectrum of C6H5OCH3 was found to contain three groups of lines, one associated with the methyl protons, one associated with the meta phenyl protons, and one associated with the ortho and para phenyl protons. The two groups of aromatic lines were found to have, within experimental error, identical spin−lattice relaxation times and activation energies. The contribution of intermolecular interactions to the relaxation mechanism was found to decrease with increasing temperature and was ? 36% for T ? 312°K. A method is discussed for determining the cross relaxation rate between the differently relaxing methyl and phenyl spins. This method utilizes average T1 values for C6H5OCH3, C6D5OCH3, and C6H5OCD3 and yields for anisole a pos...

Boron‐11 NMR study of the large angle anisotropic motion of B3H−8 in (CH3)4NB3H8

Steady state nuclear magnetic resonance was performed in powdered (CH3)4NB3H8. The quadrupole coupling e2qQ/2h of boron‐11 decreases montonically at a decreasing rate from 70 kHz at 278 K to 16 kHz at 370 K. The birefringence also decreases with increasing temperature in the same temperature range and becomes undetectable at 473 K. We conclude that the B3H−8 anions are undergoing large angle wobbling motion in this phase between 270 and 473 K. The wobbling amplitude gradually increases with temperature until the motion becomes isotropic at an order–disorder phase transition near 473 K.

A Simple Computer Interface for a Pulsed NMR Spectrometer

Described here is a simple and relatively inexpensive (less than

Nuclear magnetic resonance apparatus having semitoroidal rf coil for use in topical NMR and NMR imaging

500) computer interface between a pulsed NMR spectrometer and the Varian 620/i computer. The basic design should be suitable for interfacing most minicomputers and pulsed spectrometers (NMR and others). It features an oscilloscope display, a fast ADC, and circuitry for proper pulse and signal conditioning.