Proton NMR relaxation and molecular motion of long-chain cyclic paraffins in the solid state

Temperature-dependent measurements have been made on the proton NMR relaxations and proton linewidths of the cyclic paraffins (c-C24H48, c-C28H56, and c-C128H256) in the solid state. It is found that in c-C24H48 and c-C128H256 there exist two transitions, one due to the melting process and the other due to a rapid transition between trans and gauche conformers in a compact structure that consists of two parallel trans zigzag straight chains bridged at both ends in the crystalline state. In c-C28H56 there exists only the transition due to melting. Proton NMR relaxations have also been measured for c-C40H80 and c-C160H320 at room temperature, in addition to those for c-C24H48, c-C28H56, and c-C128H256. It is found that the extent of molecular motion decreases as the chain length increases.