Using solid-state 1H nuclear magnetic resonance (NMR)
spin–lattice relaxation experiments, we have investigated the
effects of several solid–solid phase transitions on tert-butyl and methyl group rotation in solid 1,3,5-tri-tert-butylbenzene. The goal is to relate the dynamics of
the tert-butyl groups and their constituent methyl
groups to properties of the solid determined using single-crystal
X-ray diffraction and differential scanning calorimetry (DSC). On
cooling, the DSC experiments see a first-order, solid–solid
phase transition at either 268 or 155 K (but not both) depending on
thermal history. The 155 K transition (on cooling) is identified by
single-crystal X-ray diffraction to be one from a monoclinic phase
(above 155 K), where the tert-butyl groups are disordered
(that is, with a rotational 6-fold intermolecular potential dominating),
to a triclinic phase (below 155 K), where the tert-butyl groups are ordered (that is, with a rotational 3-fold intermolecular
potential dominating). This transition shows very different DSC scans
when both a 4.7 mg polycrystalline sample and a 19 mg powder sample
are used. The 1H spin–lattice relaxation experiments
with a much larger 0.7 g sample are very complicated and, depending
on thermal history, can show hysteresis effects over many hours and
over very large temperature ranges. In the high-temperature monoclinic
phase, the tert-butyl groups rotate with NMR activation
energies (closely related to rotational barriers) in the 17–23
kJ mol–1 range, and the constituent methyl groups
rotate with NMR activation energies in the 7–12 kJ mol–1 range. In the low-temperature triclinic phase, the
rotations of the tert-butyl groups and their methyl
groups in the aromatic plane are quenched (on the NMR time scale).
The two out-of-plane methyl groups in the tert-butyl
groups are rotating with activation energies in the 5–11 kJ
mol–1 range.