Sine-modulated two-dimensional pure exchange solid-state NMR as a tool for characterizing dynamics in solids

We present a simple two-dimensional (2D) solid-state exchange NMR method to suppress significant undesirable diagonal signals. Two delays, τ, incorporated in standard 2D exchange experiments modulate the 2D spectra with sin{(Ω1−Ω2)τ}, where Ω1 and Ω2 are precession frequencies of a given site before and after the mixing time, respectively, yielding the spectra free of diagonal signals. The suppression of diagonal signals is demonstrated for dimethyl sulfone. This technique is combined with magic angle spinning and is applied to polyethylene. Not only the exchange signals between the crystalline and noncrystalline components but also those among the noncrystalline components in different states are clearly observed.