Eiichi Fukushima

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.

Anisotropy of the chemical shift tensor for solid carbon monoxide

Abstract The chemical shift anisotropy Δσ = σ|| − σ⊥ of the CO molecule has been measured by 13C NMR in the solid α phase of carbon monoxide. A polycrystalline sample, enriched to 93.7% in 13C, was used and Δσ was deduced from computer simulation of the experimental powder-pattern lineshape. A small amount of anomalous structure appears in the lineshape, which may be due either to preferred orientation of crystallites or to slow reorientational motion that is possibly associated with molecular ordering in the solid. The measured chemical shift anisotropy is 365 ± 20 ppm at 4.2 K, decreasing to 335 ± 20 ppm at 46 K. Rapid librational motion of the molecules produces motional averaging responsible for the temperature dependence. The averaging factor was estimated in the harmonic approximation using published Raman frequencies to be 0.90 ± 0.02 at 4.2 K and 0.83 ± 0.02 at 46 K, in excellent accord with the observed temperature dependence of Δσ. The chemical shift anisotropy for a static molecule is 406 ± 30 ppm. This value is in good agreement with theoretical estimations based on a molecular beam measurement of the spin-rotational interaction and ab initio calculations of the diamagnetic shielding.

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.

Carbon‐13 NMR of carbon monoxide. II. Molecular diffusion and spin–rotation interaction in liquid CO

Carbon‐13 NMR was used to measure the molecular diffusion coefficient D and the spin‐rotation correlation time τsr in liquid CO from its freezing point to about 110 K at several pressures up to 1.73 kbar. The behavior of D and τsr is similar for liquid CO and N2. Diffusion in CO at equilibrium vapor pressure takes place with an activation enthalpy of 2.9 kJ/mole and its pressure dependence leads to an activation volume which is a factor of 5 smaller than in the solid phase just below freezing. τsr is nearly equal for CO and N2 at T?80 K and its variation with pressure for CO is smaller than that of D by almost a factor of 2. We also present a method for determining the magnetic field gradient used in the measurement of D which is free from many of the usual problems.

Pressure dependence of the melting and ?-? phase transition temperatures of carbon monoxide

The pressure dependences of the melting temperatureTm and crystallographic phase transition temperatureTαβ of carbon monoxide have been measured as a function of hydrostatic pressure to 1.75 kbar by using associated changes in the nuclear spin-spin relaxation timeT2 of13C as an indicator of the transitions. From the Clausius-Clapeyron equation the volume increments at zero pressure are deduced to beΔVm=2.50±0.07 andΔVαβ=0.92±0.02 cm3/mole. The CO phase diagram is compared with that of N2.

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.

Structure and hindered motion of B3H8 anions in solids: An NMR study☆

Boron-11 quadrupole coupling parameters of (CH3)4NB3H8 and KB3H8 at 298 K are, respectively: νQ = 58 kHz, η = 0; νQ = 0.48 MHz, η = 0.76; where νQ = e2qzzQ/2h. The B3H8 anions undergo motions such that all like nuclei are equivalent at 298 K. First order phase transitions were discovered at approximately 270 and 316 K for (CH3)4NB3H8 and KB3H8, respectively.

NMR studies of indirect bismuth-fluorine coupling in polycrystalline KBiF6☆

Abstract A bismuth-fluorine coupling constant of magnitude 2.7 ± 0.3 kcps was obtained from the shape of the 19F NMR spectrum of polycrystalline KBiF6. The effect of the spin-spin coupling on the spectrum was pronounced only near 20° because of the presence of motional narrowing of 19F together with the absence of rapid quadrupolar relaxation of 209Bi at that temperature. The shape of the spectrum was independent of the Larmor frequency from about 30 to 65 Mcps at 20°.

Carbon-13 magnetic resonance shifts in triscyclopentadienyl uranium(IV) chloride

Abstract Carbon-13 NMR shift of (C 5 H 5 ) 3 UCl in THF for 160 ⩽ 320 K consists of a Curie-Weiss part with a Weiss temperature of 140 ± 10 K and a temperature-independent part of +59 ppm with respect to TMS. The contact shift is −309 ± 120 ppm at 290 K and is larger than the pseudocontact shift by about a factor of two.

NMR Study of Na3UF8 Structure and Hyperfine Effects

Nuclear magnetic resonance spectra of polycrystalline Na3UF8 were studied experimentally. The magnetic field dependence of the 19F second moment allowed a separation of its field‐dependent and field‐independent parts. The temperature dependence of the field‐independent part exhibits no discontinuities attributable to phase transitions and only a slow transition at about 250°K attributed to hindered molecular rotation of the isolated UF8 units. The temperature dependence of the field‐dependent part also exhibits no discontinuities. It does reflect the paramagnetic susceptibility which obeys, to within the accuracy of the experiment, the Curie law. The isotropic chemical shift is small, i.e., − 0.07% ± 0.01% with respect to F2 at 292°K and − 0.11% ± 0.01% at 175°K. The even smaller chemical‐shift anisotropy is approximately 25% of the isotropic shift. These shift data are consistent with a system containing one 5f electron as the unpaired spin. The 19F spectra and the temperature dependence of the field‐dep...