Proton Spin–lattice Relaxation in Polyatomic Gases

The proton spin–lattice relaxation time T 1 has been measured in gaseous samples of methane, ethylene, and ethane as a function of pressure at room temperature and also at 193° K for methane. In the pure gases T 1 is proportional to density, p, at low densities indicating that intramolecular interactions couple the spin systems to the lattice, as is the case in hydrogen gas. T 1 /p at low densities gives information on the mean square angle through which the molecules are rotated per collision. Relaxation due to paramagnetic O 2 is observed at higher densities when oxygen gas is added as an impurity. The relaxation probability per collision with an oxygen molecule is about 5 times larger for the ethylene–oxygen system than for the other two systems studied. This anomaly is discussed in terms of the theory of Oppenheim and Bloom. It is shown that a study of the temperature dependence of T 1 due to O 2 impurities provides a new way of obtaining detailed information on the Lennard–Jones parameters for the interaction between O 2 and the solvent molecules.