Study of molecular motion in solid 2-bromo-2-methyl-propane by 1H nuclear magnetic resonance spectroscopy

Abstract

Proton spin–lattice relaxation times (T1 and T1ρ) have been measured over a wide temperature region in solid (CH3)3CBr. In the lowest-temperature solid phase (phase III) two minima in both T1 and T1ρ were observed owing to methyl-group and uniaxial molecular reorientations. The analyses of T1 and T1ρ in phase III gave the activation parameters Ea= 13.9 ± 0.2 kJ mol–1 and τ0=(2.39 ± 0.35)× 10–14 s for methyl reorientation and Ea= 22.0 ± 0.5 kJ mol–1 and τ0=(8.2 ± 2.5)× 10–15 s for uniaxial molecular reorientation. In phase II, T1 on the low-temperature side of the maximum is governed by uniaxial molecular reorientation with an activation energy of 9.5 ± 0.8 kJ mol–1. T1 at higher temperatures and T1ρ throughout the phase are governed by overall molecular tumbling with an activation energy of 18.4 ± 2.5 kJ mol–1. In the low-temperature region of the highest-temperature phase (phase I) overall molecular tumbling with an activation energy of 13.6 ± 4.9 kJ mol–1 governs T1. However, translational self-diffusion is the dominant T1 relaxation mechanism at 15 MHz throughout most of phase I. The T1ρ results in phase I were analysed by Torrey's diffusion model, and the activation parameters were determined to be Ea= 51.8 ± 3.6 kJ mol–1 and τ0=(1.2+5.9–1.0)× 10–17 s. The mean jump time of the molecules at the melting point is (4.4 ± 0.4)× 10–7 s, which is typical for plastic crystals with f.c.c. structure.