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 T_1ρ) 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 T_1ρ were observed owing to methyl-group and uniaxial molecular reorientations. The analyses of T₁ and T_1ρ in phase III gave the activation parameters E_a= 13.9 ± 0.2 kJ mol^–1 and τ₀=(2.39 ± 0.35)× 10^–14 s for methyl reorientation and E_a= 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 T_1ρ 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 T_1ρ results in phase I were analysed by Torrey's diffusion model, and the activation parameters were determined to be E_a= 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.