Molecular motion in the plastic phase of pivalic acid studied by nuclear magnetic resonance

Abstract

Nuclear magnetic spin relaxation times T ₁, T ₂ and T _1ρ have been measured for methyl and acid protons in pivalic acid throughout the plastic crystalline phase, 6·9 to 36·5°C. T _1ρ and T ₂ of the methyl protons are interpreted in terms of translational self-diffusion of molecules. Diffusion coefficients are deduced which are in fair agreement with previous radiotracer measurements. T ₁ measurements at 10 MHz and 20·8 MHz indicate molecular reorientation and the temperature dependence of this process is investigated. T _1ρ and T ₂ measurements for the acid protons show that they move more rapidly through the lattice than the molecules. This is in agreement with previously reported tracer studies. A mechanism is proposed connecting the anomalously fast hydrogen atom diffusion and molecular reorientation, which is consistent with the N.M.R. and tracer measurements.