This work was intended to understand spin interactions, rotation, and diffusion of molecules in novel hydrogen clathrate by in-situ proton NMR. In hydrogen clathrate, one and four H2 molecules occupy small and large cages, respectively, yielding a¼3.8 weight% of H2. To prepare the samples, ground heavy ice was pressurized with H2 gas at 1000--1500 bar and 250 K for three hours in a high-pressure NMR probe. After the formation of the clathrate, the excess H2 gas was evacuated before performing NMR at 8.32 T. Lineshapes and the relaxation times, T1, T2, and T1rho, were recorded from 2--160 K. The NMR spectrum consists of a single broad line despite the occupation of H2 in two different cages, indicating the two environments are indistinguishable by NMR at most temperatures. The spectrum at 1.9 K appears to have some features of Pake-like lineshape with 164 kHz cusps-to-cusp splitting, resembling solid H2 at relevant temperatures. Before the decomposition of clathrate was reached at a¼160 K, two components of the resonance lineshape appeared at a¼130 K; the narrow (broad) part is from mobile (immobile) H2 in large (small) cages. This reveals that the linewidth arises due to incomplete time-average of intramolecular dipole interactions of H2 modulated with reorientation. The linewidth is proportional to 1/T from 12--120 K, indicating nearly temperature independent crystal fields produced by frozen-in orientations of cage-wall D2O. The dramatic fall of linewidth at 120 K leads to estimate activation energy of 3.8 kcal/mol for diffusion of H2 between large cages. T1 undergoes its minimum value of 22 ms at 10 K when the rate of reorientation becomes comparable to the resonance frequency, denoting fast reorientation of H2. T2 is in reasonable accord with second-moment calculations considering H2-H2 dipole interactions for most temperatures. Hysteresis behavior of T2 at low temperatures reflects the ortho-para conversion of H2. The T1rho follows T1 at low temperatures from 45--85 K, as expected from BPP theory, and starts falling at 85 K due to the cage-to-cage diffusion of H2 in large cages. These interpretations may provide insight into the dynamic properties of hydrogen clathrate.I warmly thank my Sri Lankan friends Bandula, Janaki, Ashoka, Thanura, Isuru, and Nilushi for their sincere friendship, great help, and being a part of my happy family during this work. I owe my loving thanks to my uncles Gamini, Premasiri, anbsp;...
|Title||:||Proton NMR Studies of Hydrogen Clathrate Hydrates|
|Publisher||:||ProQuest - 2008|