Molecular Motion and Phase Transitions in 2-Methyl-2-nitropropane as Studied by Proton Magnetic Resonance and Relaxation

Abstract

Abstract Proton spin-lattice relaxation times (T1 and T1ρ) were measured in solid and liquid phases of 2-methyl-2-nitropropane. In the lowest temperature solid phase (Phase III) a broad, asymmetric minimum in T1 as well as a large reduction in the second moment was observed due to the coupled reorientations of methyl and t-butyl groups. The analyses of the T1 and T1ρ in Phase III gave the activation parameters Ea=14.6±0.2 kJ mol−1 and τ0=(6.45±0.10)×10−14 s for the methyl reorientation and Ea=15.1±0.3 kJ mol−1 and τ0=(1.35±0.10)×10−13 s for t butyl reorientation. In Phase II, the T1 is governed by the t-butyl reorientation with the activation parameters Ea=15.3±0.1 kJ mol−1 and τ0=(5.85±0.10)×10−14 s. In the low temperature region of the highest temperature plastic phase (Phase I) the molecular overall reorientation governs the T1 and the molecular translational self-diffusion becomes the dominant relaxation mechanism near melting point. The T1ρ results were analyzed with the isotropic translational diffusion model and the activation parameters were determined to be Ea=46.5±0.9 kJ mol−1 and τ0=(4.6±1.8)×10−15 s. The mean jump time of the molecule at the melting point, 6×10−7 s, is slightly longer than those in other plastic crystals. The activation energy for the translational diffusion is 9.7±1.5 kJ mol−1 in the liquid phase.