Scaling-down the CSA recoupling in S-CODEX 1D-MAS exchange experiments

We introduce a significant extension to the 1D-MAS exchange experiment CODEX that improves its ability to determine the amplitude and - in favorable cases - the reorientation angles of slow dynamic process in condensed matter. Large chemical-shift anisotropy (CSA) and/or large angle reorientation require small amounts of CSA recoupling in the CODEX experiment. This in turn prompts for sometimes inconveniently high MAS frequencies and leads to the necessity of using different MAS frequencies in the same dataset to determine the topology of the motion. We introduce a simple experimental approach that overcomes these difficulties by scaling-down the CSA recoupling by altering the timing of the NMR pulse sequence. We briefly present theory and calculations and demonstrate the usefulness of the approach for two examples: the slow twofold jump of the dimethylsulfone molecules as well as the slow threefold jump in the stacks of a pyramidic liquid crystal. (C) 2003 Elsevier B.V. All rights reserved.