2D exchange NMR spectra under slow MAS: A simplified scheme to obtain pure-phase spectra without unwanted cross peaks

A simplified method for acquiring pure-phase two-dimensional exchange spectra under slow magic-angle spinning (MAS) is introduced, It combines rotor-synchronized 2D exchange spectroscopy with whole-echo acquisition leading to a simplification in data acquisition and data processing compared to the States-type data sampling using "time reversal" (A. Hagemeyer et al., Adv. Magn, Reson, 13, 85 (1989)). As an added benefit, it allows for well-defined mixing times of an arbitrary integer multiple of the MAS rotor period. The proposed method is, however, only applicable to samples where an echo of the free-induction decay can be obtained, i.e., where the inhomogenous linewidth is larger than the homogeneous linewidth. This is, for example, the case in rare-spin spectroscopy of samples with natural isotopic abundance. The usefulness of the new method is demonstrated, using C-13 spectroscopy, on two model compounds. (C) 1999 Academic Press.