Principles of centerband-only detection of exchange in solid-state nuclearmagnetic resonance, and extension to four-time centerband-only detection of exchange

Theoretical principles and experimental details of the centerband-only dete ction of exchange (CODEX) nuclear magnetic resonance (NMR) experiment for c haracterizing slow segmental dynamics in solids are described. The experime nt, which is performed under magic-angle spinning, employs recoupling of th e chemical-shift anisotropy before and after a long mixing time during whic h molecular reorientations may occur. By an analysis in terms of the differ ence tensor of the chemical shifts before and after the mixing time, the de pendence on the reorientation angle is obtained analytically for uniaxial i nteractions, and a relation to two-dimensional exchange NMR patterns is est ablished; the same theory can also be applied for analyzing stimulated-echo and pure-exchange NMR data. A favorable linear dependence is derived gener ally for small rotations, which makes the experiment suitable for detecting small-amplitude motions. Quantification is excellent because the peaks are narrow and intense, unlike the broad powder or sideband spectra that are c haracteristic of all previous NMR experiments for probing slow segmental ro tations. We also introduce and demonstrate a four-time CODEX experiment tha t yields information previously obtained only in 3D (three-dimensional) and reduced 4D (four-dimensional) exchange NMR experiments, such as the number of orientational sites accessible to the mobile groups. Chemical-shift ani sotropies required in the CODEX analysis of motional amplitudes can be esti mated using a closely related chemical-shift recoupling experiment. The imp lementation of total suppression of sidebands before detection is also expl ained. The experiments are demonstrated on dimethylsulfone, isotactic polyp ropylene, and poly(methyl methacrylate), PMMA. In isotactic poly(1-butene), the signals of the amorphous and interfacial regions have been observed se lectively by using pure-exchange CODEX near the glass transition. The four- time CODEX experiment confirms that in the beta-relaxation process of glass y PMMA, fewer than half of the sidegroups perform jumps between two orienta tions. (C) 2000 American Institute of Physics. [S0021-9606(00)01319-2].