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