Pure-exchange solid-state NMR

Three exchange nuclear magnetic resonance (NMR) techniques are presented th at yield C-13 NMR spectra exclusively of slowly reorienting segments, suppr essing the often dominant signals of immobile components. The first techniq ue eliminates the diagonal ridge that usually dominates two-dimensional (2D ) exchange NMR spectra and that makes it hard to detect the broad and low o ff-diagonal exchange patterns. A modulation of the 2D exchange spectrum by the sine-square of a factor which is proportional to the difference between evolution and detection frequencies is generated by fixed additional evolu tion and detection periods of duration tau, yielding a 2D pure-exchange (PU REX) spectrum. Smooth off-diagonal intensity is obtained by systematically incrementing tau and summing up the resulting spectra. The related second t echnique yields a static one-dimensional (1D) spectrum selectively of the e xchanging site(s), which can thus be identified. Efficient detection of pre viously almost unobservable slow motions in a semicrystalline polymer is de monstrated. The third approach, a 1D pure-exchange experiment under magic-a ngle spinning, is an extension of the exchange-induced sideband (EIS) metho d. A TOSS (total suppression of sidebands) spectrum obtained after the same number of pulses and delays, with a simple swap of z periods, is subtracte d from the EIS spectrum, leaving only the exchange-induced sidebands and a strong, easily detected centerband of the mobile site(s). (C) 2000 Academic press.