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Proton magnetic resonance measurements in gases Lipsicas-Lipschitz, Max Menachem Mordechai

Abstract

The work reported here is a proton magnetic resonance investigation of some polyatomic gases. Pulsed nuclear magnetic resonance techniques were used throughout. Little systematic work on gases using this technique has been reported hitherto, and the present study clearly indicates the potentialities of nuclear magnetic resonance experiments for the investigation of certain physical processes occurring in gases. The gases studied were hydrogen (H₂), methane (CH₄), ethane (C₂H₆) and ethylene (C₂H₄). A spectrometer and ancilliary equipment have been constructed enabling measurements to be made at 30 Mc/s, over a temperature range from 35K° to 300K° using gas pressures up to 200 atmospheres. In hydrogen, the spin-lattice relaxation time, T₁, was studied as a function of the density, temperature and ortho-para concentration of the gas. In the "dilute" gas region, T₁ was found to increase linearly with density at fixed temperature, indicating that the relaxation process is of intra-molecular origin and that studies of T₁ are able to yield valuable information on the asymmetric part of the intermolecular interaction. The temperature dependence of T₁ (at fixed density) was studied in the "dilute" gas between 35K° and 300K°, and quantum effects were discovered at low temperatures. In the dense gas, at low temperatures, T₁ was found to be a very rapidly increasing function of density at fixed temperature. In the dilute gas, T₁, at fixed density and temperature, was found to increase with increasing ortho-hydrogen concentration, indicating that the cross-section for a reorientation collision of two orthohydrogen molecules is greater than that for a collision between an ortho-molecule and a paramolecule. Diffusion measurements in hydrogen were carried out, at 78K°, as a function of density in the "dilute" gas, and it has been shown that the coefficient of self-diffusion is inversely proportional to density. The spin-spin relaxation time, T₂, for hydrogen at 78K° has been found to vary linearly with density, in the "dilute" gas, and T₂ = 0.8 T₁. In all the hydrocarbon gases studied, the relaxation process was governed by a single exponential function of time, thus enabling a unique relaxation time T₁ to be defined. In the "dilute" gas, T₁ was found to vary linearly with density. An estimate has been made of the mean square angle of rotation of a methane molecule and of an ethylene molecule per collision, with a view to examining the applicability of a rotational diffusion model for molecular reorientation in these gases. In methane, T₁ has been studied as a function of density just above the critical point for this gas and at room temperature. The effect of traces of oxygen on the relaxation process ("impurity relaxation") in the "dilute" and "dense" gas regions has been studied also. T₁ has been studied in ethane above and below the critical point, and in ethylene above the critical point, in the "dilute" and "dense" gas regions. The oxygen impurity effects have been found to be very marked indeed in the case of ethylene. In the course of this work, a n.m.r. method has also been evolved for the measurement of the compressibility of a gas. A programme of future n.m.r. work on gases, suggested by the results obtained and experience gained in this work, is outlined.

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