H-1 and F-19 multiple-quantum NMR dynamics in quasi-one-dimensional spin clusters in apatites

The H-1 multiple-quantum NMR dynamics of quasi-one-dimensional distributions of clusters of uniformly spaced proton spins in hydroxyapatite, Ca-5(OH)(PO4)(3), and related fluorine-containing apatites have been studied. Two different growth exponents characterizing the power-law dependence of the effective size upon preparation time are observed at short and long times for the hydroxyapatite samples. The corresponding growth exponents are 0.98 and 1.78 for the most stoichiometric sample, which suggest a possible transition from 1-D to 2-D growth at longer preparation times. A slight deficiency of hydroxyl groups in another sample of hydroxyapatite leads to a decreased rate of MQ coherence growth and a measurable decrease in the short-time growth exponent to 0.83. The H-1 MQ dynamics of two fluorohydroxyapatite solid solutions, Ca5Fx(OH)(1-x)(PO4)(3), exhibit both decreasing rates of MQ coherence growth as x increases and decreased values of the short-time growth exponent. A I-D cluster model is developed to account for the effects upon the MQ dynamics of randomly distributed defects (either vacancies or substitutions) in the linear chain of proton spins. The F-19 MQ NMR dynamics of a single crystal of mineral fluorapatite, Ca-5(PO4)(3), at different orientations with respect to the external magnetic field exhibit oscillatory behavior. These oscillations are reproduced in density matrix calculations for the finite 1-D clusters of fluoride ions that are present in a defect-containing sample. Implications of these single-crystal results for future studies of the effects of dimensionality upon MQ NMR dynamics are discussed.