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.