C. Hackel, C. Franz, A. Achilles, K. Saalwächter, D. Reichert.
loss in 1D magic-angle spinning exchange NMR (CODEX): radio-frequency
limitations and intermediate motions.
Phys. Chem. Chem. Phys.
11, 7022-7030 (2009). DOI 10.1039/b906527j.
The popular 1D MAS exchange experiment CODEX suffers limitations due to signal
loss during the finite recoupling periods, during which the magnetization
evolves in the transverse plane. Here, we address the origins and possible
improvements of this problem, aimed at (i) an optimization of the
signal-to-noise ratio in the experiments, as well as harnessing
intermediatemotion induced signal loss for obtaining approximate information on
(ii) correlation times and (iii) potential distributions, where the latter are
often found in polymeric systems. First, we show that the intensity of the
signal is sensitive to the radiofrequency (rf) parameters of the carbon
recoupling and proton decoupling, and care must be taken to gain optimal signal
intensity. Optimum conditions are found for recoupling pulses being as short as
possible for large chemical shift anisotropy (CSA) values, and approaching a
ratio of 3 between the nutation frequencies for protonated carbons, calling for
an individual adjustment in each case. Second, we demonstrate that the effect of
intermediate motions can be studied semi-quantitatively by combining CODEX data
with its constant-time modification CONTRA, which allows for a tuning of the
signal loss due to intermediate motions. Third, for the case of samples
featuring a distribution of correlation times, we propose a procedure to obtain
an estimate of the proportion of molecular segments in the sample for which the
CODEX data are representative, i.e., which share of segments moves truly in the
slow-motion regime. The procedure involves the combination of CODEX data with a
cross-polarisation (CP) reference experiment for an estimate of the full sample
magnetization; it is demonstrated on the example of semi-crystalline