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

Abstract

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 T_1ρ) 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 T_1ρ in phase III gave the activation parameters E_a= 20.1 ± 1.1 kJ mol^–1 and τ₀=(2.68^+2.31 _–1.25)× 10^–14 s for methyl reorientation [E_a= 18.7 ± 0.9 kJ mol^–1, τ₀=(4.63^+3.11 _–1.86)× 10^–14 s in phase II] and E_a= 18.3 ± 0.3 kJ mol^–1 and τ₀=(6.35^+2.15 _–1.60)× 10^–14 s for uniaxial molecular reorientation [E_a= 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 V_m, where V_m is the molar volume calculated from neutron-diffraction data. On the other hand T_1ρ 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 V_m, 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.