Nuclear spin relaxation and internal motion in molecular solids containing isopropyl groups. Part 1.—An introductory Zeeman relaxation study

Abstract

We report proton Zeeman relaxation rates in solid 1,4-di-isopropylbenzene (DIB) and in 1,3,5-tri-isopropylbenzene (TIB) at Larmor frequencies of 8.5 and 53 MHz. The temperature ranged from 110 to 300 K for DIB and from 85 to 240 K for TIB. In DIB the free induction decay contains both a liquid-like and a solid-like component at higher temperatures, and the solid-like component relaxes non-exponentially. In TIB the free induction decay is typical of molecular solids with reorienting methyl groups and the relaxation is always exponential. The model used to fit the temperature and frequency dependence of the Zeeman relaxation rates R in both molecules suggests the occurrence of methyl reorientation only in the vicinity of and at temperatures below the observed maximum in R. Isopropyl reorientation does not occur except perhaps at the highest temperatures in TIB, where an additional motion begins to dominate the relaxation. These results are compared with the t-butyl analogues of these molecules in which the methyl and the t-butyl groups reorient at the same rate.