Time-Reverse ODESSA. A 1D Exchange Experiment for Rotating Solids with Several Groups of Equivalent Nuclei

摘要

A one-dimensional exchange experiment is proposed for magic-angle-spinning samples with several groups of equivalent nuclei undergoing internal exchange, such as pure reorientation, as opposed to mutual exchange. The method, which we term time-reverse ODESSA, is an extension of the recently proposed 1D ODESSA experiment for a single group of exchanging nuclei. When several different groups of spins are present, as is usually the case for carbon-13 in polymers and molecular crystals, the normal ODESSA spectrum yields phase-twisted spectra which are difficult to analyze quantitatively. This problem is solved in the time-reverse ODESSA experiment which yields pure absorption spectra for all families of side bands, as long as only internal exchange need be considered. The experiment consists of the usual three pulse sequence of 2D exchange,P1—t1—P2—τm—P3—t2(acquisition), except that the evolution time is fixed at half a rotation period,t1= TR/2, the mixing time is set to an odd number of half rotation periods, τm= (2G? 1)TR/2, and the acquisition starts att2= TR/2 after the detection pulse,P3. The method is demonstrated using the carbon-13 spectra of dimethyl sulfone and an enriched sample of tropolone, and is applied to the study of the π flip of the inner benzene ring of 1,4-diphenoxybenzene. The scope and limitations of the method are discussed.