Michael S. Greenfield

Bio: Michael S. Greenfield is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Quadrupole & Pulse sequence. The author has an hindex of 3, co-authored 4 publications receiving 313 citations.

Deuterium relaxation and vibrationally averaged quadrupole coupling in an alkane/urea clathrate

Abstract: Measurements of the temperature dependence of deuterium relaxation rates for the methylene groups of perdeuterated n‐nonadecane in a urea clathrate single crystal are reported Individual spectral densities J1(ω0) and J2(2ω0) were determined using Jeener–Broekaert experiments, and data obtained at two orientations of the main crystal axis were fit to a motional model which combines unrestricted rotational diffusion about the alkane long axis with restricted wobbling of the methylene group planes Both motions are fast on the NMR time scale and both affect the observed, time averaged quadrupole coupling constant (e2qQ/h)eff=163 kHz However, quadrupole tensor components appearing in the relaxation Hamiltonian should not be averaged over that part of the motion which is in fact responsible for the relaxation, and by fitting the relaxation data together with the observed quadrupolar splitting to the motional model described above we obtain a value ≥ 185 kHz for the methylene quadrupole coupling constant Thi

Deuterium Relaxation and Molecular Motion in Channel Clathrates

Abstract: The use of magnetic resonance techniques in studies of structure and molecular dynamics of inclusion compounds has increased dramatically in recent years. Considering the importance of many types of clathrates in catalysis this is to be expected. Considering the inherent charming physical chemical properties of many clathrates, it is surprising that physical chemists among the magnetic resonance community have waited so long to take an in depth look at the properties of these materials. It turns out that deuterium NMR is very well suited for investigating the dynamics of clathrates.

Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors

Abstract: A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr−Purcell−Meiboom−Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87Rb (RbClO4 and RbVO3) and 59Co spectra (Co(NH3)5 Cl3), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.