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 T_1ρ) measured in all solid and liquid phases. In the lowest temperature phase (phase IV), two minima are observed at T_1ρ 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 T_1ρ and T₁ data in phase IV gives the activation parameters E_a= 20.3 ±^1.7_1.4kJ mol^–1 and τ₀=(2.8±^4.1_1.9)× 10^–14 s for methyl reorientation and E_a= 15.8 ± 0.1 kJ mol^–1 and τ₀=(6.7 ±^0.2_1.1)× 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 E_a= 17.6±^1.6_0.7kJ mol^–1 and τ₀=(1.7±^1.0_1.1)× 10^–13 s. In phase II we could not obtain enough variation in T₁ and T_1ρ 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 T_1ρ are governed by translational self-diffusion with an activation energy of 36.2 ±^1.0_0.7 kJ mol^–1 and τ₀=(1.8±^0.8_0.7)× 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.