Nuclear Magnetic Resonance in Liquids under High Pressure

The work reported here is an application of free precession or ``spin echo'' techniques in nuclear magnetic resonance to a study of the properties of liquids under high pressure. The proton relaxation time T1 was measured as a function of pressure, over the range 1 to 10 000 atmospheres, for six liquids: water, n-pentane, n-hexane, toluene, ethyl iodide, and methyl iodide. In addition to the relaxation time, the normalized values D(P)/D(1) of the self-diffusion constant were measured as a function of pressure for water and methyl iodide. In each liquid T1 decreases with increasing pressure. However, comparison of the observed variation of T1 with pressure with Bridgman's data on the variation of viscosity η with pressure shows in all cases (except that of methyl iodide for which no viscosity data are available) that T1 is not simply proportional to 1/η, but instead T1η increases with pressure. Similarly, for water and methyl iodide, the diffusion constant D varies in such a way that T1/D increases with pressure. For water the Stokes-Einstein relation Dη = cT is obeyed within the errors of observation. The behavior of T1 can be understood if one assumes that the dominant relaxation mechanism involves intramolecular fields, whose correlation time is determined by the rotational freedom of the molecule, and that compression of the liquid inhibits migrational freedom more drastically than rotational freedom. The necessity of making the present measurements has led to the discovery that BeCu, a heat treatable alloy, makes a very satisfactory high pressure, non-magnetic bomb which might well be used for other magnetic investigations under high pressure.