Proton spin relaxation, internal motion and structure in solid 1,2,4,5-tetraisopropylbenzene

Abstract

The temperature dependence of the solid-state proton Zeeman relaxation rate in polycrystalline 1,2,4,5-tetraisopropylbenzene has been measured at Larmor frequencies of 8.5 and 53 MHz. The isopropyl groups are immobile on the NMR timescale. The data are interpreted with a very small distribution of barriers for all eight methyl groups and this is consistent with a molecular geometry which has the isopropyl lone protons in neighbouring pairs, facing each other in the plane of the aromatic ring. It was also possible to interpret the data with a two-site model consistent with the much less sterically hindered arrangement whereby the isopropyl group lone protons are rotated by ca. 45° in opposite directions out of the plane of the ring. These matters are discussed and the models are compared with results from other spectroscopic measurements. This and other isopropyl systems are compared with related ethyl and tert-butyl systems. Also, the observation of non-exponential nuclear spin relaxation is discussed.