Tooru Hasebe

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

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 Torreys 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.

Molecular motions in (CH3)3CCl by 1H spin–lattice relaxation

Molecular motions in (CH3)3CCl have been further investigated by 1H n.m.r. relaxation times (T1 and T1ρ) measured in all solid and liquid phases. In the lowest temperature phase (phase IV), two minima are observed at T1ρ and are attributed to uniaxial molecular and methyl reorientation. Two minima at T1 were also indicated but only one was clearly visible. Analysis of the T1ρ and T1 data in phase IV gives the activation parameters Ea= 20.3 ±1.71.4kJ mol–1 and τ0=(2.8±4.11.9)× 10–14 s for methyl reorientation and Ea= 15.8 ± 0.1 kJ mol–1 and τ0=(6.7 ±0.21.1)× 10–15 s for uniaxial molecular reorientation. Uniaxial molecular reorientation is found to be faster than methyl reorientation as in the case of (CH3)3CCN. In phase III one minimum at T1 is observed due to methyl reorientation with activation parameters of Ea= 17.6±1.60.7kJ mol–1 and τ0=(1.7±1.01.1)× 10–13 s. In phase II we could not obtain enough variation in T1 and T1ρ to be able to establish the details of the molecular motions since the existence of this phase only extends over ca. 2 K, between 217.7 and 219.5 K. In phase I (plastic phase), T1 and T1ρ are governed by translational self-diffusion with an activation energy of 36.2 ±1.00.7 kJ mol–1 and τ0=(1.8±0.80.7)× 10–14 s. The mean jump time of the molecules at the melting point is 7.4 × 10–7 s. The activation energy for the molecular motion in the liquid phase is 10.9 ± 1.4 kJ mol–1.

Molecular motions in 2-iodo-2-methylpropane, (CH3)3CI. Proton nuclear magnetic resonance measurements at variable temperatures

Polymorphism of (CH3)3CI has been confirmed by differential thermal analysis. There are two solid phases; one is a plastic-crystalline phase, phase I (235.7–240.4 K) and the other is a brittle crystal phase, phase II (below 235.7 K). Molecular motion in each phase has been studied by 1H n.m.r. relaxation times (T1, T2 and T1ρ). In phase II, the uniaxial molecular reorientation (C′3 reorientation) is faster than the methyl reorientation (C3 reorientation). The activation parameters of these motions are found to be Ea= 23.2 ± 0.4 kJ mol–1 and τ0=(2.9 ± 0.7)× 10–15 s for the C′3 reorientation, and Ea= 31.4+3.7–2.1 kJ mol–1 and τ0=(1.1+2.4–1.0)× 10–15 s for the C3 reorientation, respectively. In the plastic (phase I) overall molecular reorientation and molecular self-diffusion are believed to be responsible for T1 and T1ρ, and their activation parameters are found to be Ea= 7.9 ± 0.9 kJ mol–1 for the formation motion and Ea= 35.3+1.8–1.4 kJ mol–1 and τ0=(2.1+2.3–1.3)× 10–14 s for the latter motion, respectively. An activation energy of 11.4 ± 0.3 kJ mol–1 is found for the molecular motion in the liquid phase.

Molecular motion in t-butyl cyanide. Nuclear magnetic resonance measurements at variable temperature and pressure

The three solid phases of (CH3)3CCN have been further investigated by differential thermal analysis. Phase I is the highest-temperature phase. Phase II is fairly stable to the lowest temperature unless it is rewarmed. 1H n.m.r. relaxation times (T1 and T1ρ) were measured in all the solid and the liquid phases. In phases II and III two minima in T1 were observed owing to methyl and uniaxial molecular reorientation. The analysis of the T1 and T1ρ in phase III gave the activation parameters Ea= 20.1 ± 1.1 kJ mol–1 and τ0=(2.68+2.31–1.25)× 10–14 s for methyl reorientation [Ea= 18.7 ± 0.9 kJ mol–1, τ0=(4.63+3.11–1.86)× 10–14 s in phase II] and Ea= 18.3 ± 0.3 kJ mol–1 and τ0=(6.35+2.15–1.60)× 10–14 s for uniaxial molecular reorientation [Ea= 16.9 ± 0.3 kJ mol–1, τ0=(1.14+0.31–0.24)× 10–14 s in phase II]. Uniaxial molecular reorientation is found to be faster than methyl-group reorientation. In phase I the T1 is governed by methyl reorientation with an activation energy of 16.3 ± 0.7 kJ mol–1 and activation volume of 0.04 Vm, where Vm is the molar volume calculated from neutron-diffraction data. On the other hand T1ρ is governed by translational self-diffusion with an activation energy of 78.8 ± 3.7 kJ mol–1 and τ0(3.5+14.4–2.8)× 10–18 s and an activation volume of 0.9 Vm, which suggests a monovacancy diffusion mechanism. The mean jump time of the molecules at the melting point is three orders of magnitude slower than in plastic crystals. The activation energy for the molecular motion in the liquid phase is 11.9 ± 1.4 kJ mol–1.