Nuclear Magnetic Relaxation in a Gas of Regular Molecules

Nuclear magnetic relaxation ( N M R ) in a gas of regular (tetrahedral and octahedral) molecules f rom the point of view of the kinetic theory of transport and relaxation processes based on the Waldmann-Snider equation is considered. A simple expression is obtained for the spin-rotation interaction based on considerations of the molecular symmetry. The form of the spin-rotation coupling which contains, in addition to the so-called "sca lar" coupling constant, also a scalar anisotropy part, Ac, of the spin-rotation coupling tensor determines, along with the collision operator, the form to be taken by an expansion of the nonequilibrium part of the distribution function-density matrix of the gas. Solutions of the set of coupled differential equations which fo l low upon application of the moment method to the linearized Waldmann-Snider equation lead to the Bloch equations which define the "spin-lattice" and transverse relaxation times in the gas. Those obtained here are comparable with those obtained using correlation function theory. However, in this work, due to explicit consideration of the molecular symmetry, it is shown that the "e f fect ive" spin-rotation coupling coefficient found in tetrahedral and octahedral molecules is not the same and depends in a simple way on the number of equivalent spins in the molecule.