Proton NMR Relaxation Effects. Cross-Relaxation Processes in Pure Liquids

Spin–lattice relaxation in a liquid containing protons in two different molecular environments can proceed by three different routes. There is a route to relaxation associated with each of the environments resulting from interactions between protons within that same environment. The third route to relaxation results from interaction between protons in different environments and is called cross relaxation. In such a system, the total spin–lattice relaxation decay determined by pulsed NMR will be the sum of two exponentials. Three relaxation rate constants are derived from such data. When cross relaxation arises entirely from dipole–dipole interaction in the extreme narrowing limit, these rate constants may be associated with the three routes to relaxation. (The cross-relaxation rate can always be identified, regardless of the mechanism.) To illustrate such relaxation behavior and the methods for obtaining three independent relaxation rates, 1-phenylpropyne was studied as a function of temperature.