NMR characterisation of dynamics in solvates and desolvates of formoterol fumarate

Abstract

Solid-state NMR is used to characterise dynamics in the ethanol solvate of the pharmaceutical material formoterol fumarate and its associated desolvate. Jump rates and activation barriers for dynamic processes such as phenyl ring rotation and methyl group rotational diffusion are derived from 1D-EXSY and ¹³C spin–lattice relaxation times respectively. ²H and ¹³C spin–lattice relaxation times measured under magic-angle spinning conditions are used to show that the fumarate ion in the desolvate is undergoing small-amplitude motion on a frequency scale of 100s of MHz at ambient temperature with an activation parameter of about 32 kJ mol⁻¹. Exact calculations of relaxation times under MAS provide a simple and robust means to test motional models in cases where relaxation rate maxima are observed, including for systems where the crystal structure of the material is unknown.