James P. Yesinowski

Multiple-quantum NMR dynamics in the quasi-one-dimensional distribution of protons in hydroxyapatite

Abstract Multiple-quantum NMR dynamics are reported for the quasi-one-dimensional distribution of uniformly spaced proton spins in hydroxyapatite, Ca 5 (OH)(PO 4 ) 3 , using a phase-incremented even-order selective time-reversal pulse sequence. The initial increase of the effective size N with the preparation time for creation of multiple-quantum coherences is linear, in qualitative (but not quantitative) agreement with calculations based upon an “incremental shell” model recently developed by Levy and Gleason. An upward curvature observed at longer preparation times is qualitatively ascribed to the incomplete isolation of the linear chains. Slight deficiencies of hydroxyl groups in the linear chain lead to a measurable decrease in the slope of the linear portion of the curve.

Chemical-shift-selective multiple-quantum NMR as a probe of the correlation length of chemical shielding tensors

Abstract A method for obtaining multiple-quantum NMR spectra of a selected homogeneously broadened region of an inhomogeneously broadened powder pattern (CHESHIRE) is demonstrated with 19 F NMR results for polycrystalline calcium fluorapatite, Ca 5 F(PO 4 ) 3 , and a 9 1 calcium/strontium fluorapatite solid solution. The slower growth rate of 19 F multiple-quantum coherences at the same selected chemical shift in the latter compound is interpreted in terms of a decrease in the average correlation length of the shielding tensor in the one-dimensional chain of fluoride ions in the solid-solution, due to the 19 F chemical- shift perturbation caused by the strontium ion substitutions.

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about129Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report129Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by129Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the129Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of129Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The129Xe spin-lattice relaxation timesT1 range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT1 values (35 ms) are discussed. Preliminary131Xe and1H NMR results are presented, as well as a method for greatly increasing the sensitivity of129Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.