A one-dimensional exchange experiment is proposed for rotor period, and the resulting cross peaks provide quantitamagic-angle-spinning samples with several groups of equivalent tive information on dynamic processes even if only equivanuclei undergoing internal exchange, such as pure reorientation, lent nuclei are involved (6-8).
as opposed to mutual exchange. The method, which we term Such two-dimensional experiments are often very time time-reverse ODESSA, is an extension of the recently proposed consuming, and it is therefore desirable to use, whenever 1D ODESSA experiment for a single group of exchanging nuclei. possible, equivalent 1D methods. In these 1D experiments, When several different groups of spins are present, as is usually the spin system is prepared in a nonequilibrium state that is the case for carbon-13 in polymers and molecular crystals, the modified during the following mixing time by some dynamic normal ODESSA spectrum yields phase-twisted spectra which process. The first such 1D exchange experiment for equivaare difficult to analyze quantitatively. This problem is solved in lent nuclei in rotating solids was designed by Yang et al.
the time-reverse ODESSA experiment which yields pure absorption spectra for all families of side bands, as long as only internal (9). It consists of a preparation period during which the exchange need be considered. The experiment consists of the spinning side bands are suppressed by a TOSS sequence, a usual three pulse sequence of 2D exchange, P1-t 1 -P2-t mvariable mixing period t m , during which spin exchange will P3 -t 2 ( acquisition ) , except that the evolution time is fixed at mismatch the TOSS conditions, followed by a detection pehalf a rotation period, t 1 Å T R / 2, the mixing time is set to an riod in which the restoration of the spinning side bands is odd number of half rotation periods, t m Å ( 2G 0 1) T R /2, and monitored. The experiment is, however, very sensitive to the acquisition starts at t 2 Å T R / 2 after the detection pulse, the exact setting of the TOSS conditions. P3. The method is demonstrated using the carbon-13 spectra of An alternative MAS 1D exchange experiment, suitable dimethyl sulfone and an enriched sample of tropolone, and is for monitoring spin exchange between equivalent nuclei, was applied to the study of the p flip of the inner benzene ring of recently proposed by Ge ´rardy-Montouillout et al. (10). The 1,4-diphenoxybenzene. The scope and limitations of the method experiment was termed ODESSA for one-dimensional exare discussed. ᭧ 1997 Academic Press change spectroscopy by sideband-alternation. It consists of the usual three-pulse sequence of 2D exchange spectroscopy, P1-t 1 -P2-t m -P3-t 2 (acquisition), but with the 245