Determination of transverse relaxation rates in systems with scalar-coupled spins: The role of antiphase coherences

Homogeneous line-widths that arise from transverse relaxation tend to be masked by B-0 field inhomogeneity and by multiplets due to homonuclearf-couplings. Besides well-known spin-locking sequences that lead to signals that decay with a rate R-1p without any modulations, alternative experiments allow one to determine the transverse relaxation rates R-2 in systems with scalar-coupled spins. We evaluate three recent strategies by experiment and simulation: (i) moderate-amplitude SITCOM-CPMG sequences (Dittmer and Bodenhausen, 2006 [2]), (ii) multiple-quantum filtered (MQF) sequences (Barrere et al., 2011 [4]) and (iii) PROJECT sequences (Aguilar et al., 2012 [5]). Experiments where the J-evolution is suppressed by spin-locking measure the pure relaxation rate R2(15) of an in-phase component. Experiments based J-refocusing yield a mixture of in-phase rates R-2(I-x) and antiphase rates R-2(2I(y)S(z)), where the latter are usually faster than the former. Moderate-amplitude SITCOM-CPMG and PROJECT methods can be applied to systems with many coupled spins, but applications of MQF sequences are limited to two-spin systems since modulations in larger systems can only partly be suppressed. (C) 2013 Elsevier Inc. All rights reserved.