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Nuclear spin relaxation and Overhauser effects in polyatomic gases Dong, Yi-Yam Ronald

Abstract

Using modern signal averaging techniques, the proton and fluorine spin-lattice relaxation time T₁ has been measured in CH₄, CF₄, CHF₃, CH₃F, and CF₃Cl gases at low densities and 297°K. By measuring the dependence of T₁ on density near the characteristic T₁ minimum, we have been able to obtain new information on the spin-rotation interaction coupling constants in CF₄, CHF₃, and CH₃F. The CH₄ system was used to test the validity of this method since the spin-rotation coupling constants are accurately known for CH₄. The correlation function for the spin-rotation interaction was found to be exponential within experimental error for all of the molecules studied. The temperature dependence of the fluorine T₁ in CHF₃, CH₃F, and CF₄ was investigated at higher densities. The proton T₁ in CHF₃ and CH₃F have also been studied in the same density region and at several temperatures. A very striking density dependence of the proton T₁ in these two symmetric-top molecules was discovered. A plot of T₁/ρ versus ρ shows "steps". Steady-state Overhauser effects have been studied in experiments performed at 297°K in both CHF₃ and CH₃F gases to demonstrate the importance of the intra-molecular magnetic dipolar interaction at moderate densities. This interaction in CHF₃ and CH₃F is found to be responsible for the peculiar density dependence of the proton T₁. A phenomenological interpretation of the above proton results was given using a high temperature approximation relaxation theory in which the correlations between the spin-dependent interactions of the different nuclei and the existence of three distinct molecular symmetry species in CX₃Y molecules were properly accounted for. A detailed molecular theory for polyatomic molecules is still needed to extract information on the anisotropic part of the inter-molecular potential.

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