TIME-REVERSE ODESSA - A 1D EXCHANGE EXPERIMENT FOR ROTATING SOLIDS WITH SEVERAL GROUPS OF EQUIVALENT NUCLEI

A one-dimensional exchange experiment is proposed for magic-angle-spin ning samples with several groups of equivalent nuclei undergoing inter nal exchange, such as pure reorientation, as opposed to mutual exchang e. The method, which we term time-reverse ODESSA, is an extension of t he recently proposed 1D ODESSA experiment for a single group of exchan ging nuclei. When several different groups of spins are present, as is usually the case for carbon-13 in polymers and molecular crystals, th e normal ODESSA spectrum yields phase-twisted spectra which are diffic ult to analyze quantitatively. This problem is solved in the time-reve rse ODESSA experiment which yields pure absorption spectra for all fam ilies of side bands, as long as only internal exchange need be conside red. The experiment consists of the usual three pulse sequence of 2D e xchange, P1-t(1)-P2-tau(m)-P3-t(2) (acquisition), except that the evol ution time is fixed at half a rotation period, t(1) = T-R/2, the mixin g time is set to an odd number of half rotation periods, tau(m) = (2G - 1)T-R/2, and the acquisition starts at t(2) = T-R/2 after the detect ion pulse, P3. The method is demonstrated using the carbon-13 spectra of dimethyl sulfone and an enriched sample of tropolone, and is applie d to the study of the pi flip of the inner benzene ring of 1,4-dipheno xybenzene. The scope and limitations of the method are discussed. (C) 1997 Academic Press.