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

A new technique is presented for studying chem. exchange in solids under conditions of magic angle spinning (MAS); it is based on the original 2-dimensional exchange expt. but uses the combination of a TOSS (total suppression of sidebands) sequence with its time-reversed counterpart in the 1st dimension to effect evolution under the isotropic chem. shift alone. Pure absorption-mode spectra can then be obtained in the usual way. One version of the expt. also uses TOSS at the start of acquisition, producing an exchange spectrum that has isotropic chem. 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 2nd expt. allows free MAS evolution during acquisition, giving a spectrum with isotropic shifts in the 1st dimension and both shifts and sidebands in the 2nd dimension. This version has 2 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 H shift in solid tropolone is used to illustrate how chem. exchange can be readily monitored, irresp. of the no. of spinning sidebands present in the 1-dimensional MAS spectrum. This method of obtaining isotropic evolution can be used in any 2- or 3-dimensional MAS expt. and provides a practical alternative to high-speed MAS. [on SciFinder (R)]

Published in:
Journal of Chemical Physics, 97, 5, 2928-37

 Record created 2006-02-22, last modified 2018-03-17

Rate this document:

Rate this document:
(Not yet reviewed)