Chapter 6: CODEX-based Methods for Studying Slow Dynamics Check Access

The 1D MAS exchange experiment CODEX and its variants are dynamic solid-state MAS NMR experiments that detect and quantify molecular reorientations with correlation times τ_c between a few milliseconds to some hundreds of milliseconds (“slow motions”). The method is based on the dependence of the NMR resonance frequency on the orientation of interaction tensors, and compares phases acquired by the precessing magnetizations before and after an adjustable time during which the dynamic process happens. Though the dynamic range of the method covers only about two orders of magnitude, it can provide very precise and detailed information on kinetic parameters: for jumps between a small number of discrete sites, τ_c can be read-off from the data directly, while the apparent τ_c for diffusive motions depends additionally on experimental parameters. Furthermore, the amplitude of motions (jump angles etc.) can be extracted by comparison with numerical calculation of the signal. The presence of faster processes (so-called “intermediate motions”) is detectable from the CODEX data as well. We suggest here a convenient experimental approach to schedule the experiment and to treat the data in the most efficient way. We discuss a number of competing effects requiring attention for the proper interpretation of the data and demonstrate the experimental approach on two examples.