Pure absorption-mode chemical exchange nuclear magnetic resonance spectroscopy with suppression of spinning sidebands in a slowly rotating solid

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Abstract
A new technique is presented for studying chemical exchange in solids under conditions of magic angle spinning (MAS); it is based on the original two‐dimensional exchange experiment but uses the combination of a TOSS (total suppression of sidebands) sequence with its time‐reversed counterpart in the first dimension to effect evolution under the isotropic chemical shift alone. Pure absorption‐mode spectra can then be obtained in the usual way. One version of this experiment also uses TOSS at the start of acquisition, producing an exchange spectrum that has isotropic chemical shifts in both frequency dimensions. This is useful for isotropic powder samples with anisotropies that are too large to be averaged completely by sample spinning but are still within the regime where TOSS can be applied. A second experiment allows free MAS evolution during acquisition, giving a spectrum with isotropic shifts in the first dimension and both shifts and sidebands in the second dimension. This version has two important features: (i) it does not suffer from the intensity losses normally inherent in TOSS when the anisotropy is large compared to the spinning speed; and (ii) it is applicable even to samples that have an anisotropic distribution of crystallites. The tautomeric hydrogen shift in solid tropolone is used to illustrate how chemical exchange can be readily monitored, irrespective of the number of spinning sidebands present in the one‐dimensional MAS spectrum. This method of obtaining ‘‘isotropic evolution’’ can be used in any two‐ or three‐dimensional MAS experiment and provides a practical alternative to high‐speed MAS.