PROTON SPIN RELAXATION, INTERNAL MOTION AND STRUCTURE IN SOLID 1,2,4,5-TETRAISOPROPYLBENZENE

The temperature dependence of the solid-state proton Zeeman relaxation rate in polycrystalline 1,2,4,5-tetraisopropylbenzene has been measur ed at Larmor frequencies of 8.5 and 53 MHz. The isopropyl groups are i mmobile on the NMR timescale The data are interpreted with a very smal l distribution of barriers for all eight methyl groups and this is con sistent 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 mode l consistent with the much less sterically hindered arrangement whereb y the isopropyl group lone protons are rotated by ca. 45-degrees in op posite directions out of the plane of the ring. These matters are disc ussed and the models are compared with results from other spectroscopi c measurements. This and other isopropyl systems are compared with rel ated ethyl and tert-butyl systems. Also, the observation of non-expone ntial nuclear spin relaxation is discussed.