Abstract
Although considerable progress has been made in simulating the dynamics of multiple coupled nuclear spins, predicting the evolution of nuclear magnetisation in the presence of radio-frequency decoupling remains challenging. We use exact numerical simulations of the spin dynamics under simultaneous magic-angle spinning and RF decoupling to determine the extent to which numerical simulations can be used to predict the experimental performance of heteronuclear decoupling for the CW, TPPM and XiX sequences, using the methylene group of glycine as a model system. The signal decay times are shown to be strongly dependent on the largest spin order simulated. Unexpectedly large differences are observed between the dynamics with and without spin echoes. Qualitative trends are well reproduced by modestly sized spin system simulations, and the effects of finite spin-system size can, in favourable cases, be mitigated by extrapolation. Quantitative prediction of the behaviour in complex parameter spaces is found, however, to be very challenging, suggesting that there are significant limits to the role of numerical simulations in RF decoupling problems, even when specialist techniques, such as state-space restriction, are used.
Highlights
Effective decoupling of the 1H nuclear spins is essential for achieving high resolution 13C NMR spectra from typical organic molecules
This number is larger than the practical limit for exact simulation, which is typically less than 12 spins
Frantsuzov et al / Solid State Nuclear Magnetic Resonance 70 (2015) 28–37 state, where very high spin orders can be observed amongst 1H nuclei [26,27,28], and so we investigate the role of higher spin orders in heteronuclear decoupling
Summary
Effective decoupling of the 1H nuclear spins is essential for achieving high resolution 13C NMR spectra from typical organic molecules. Different researchers have used slightly different methods for restricting the evolution of the spin system to coherences below a certain order, but it is argued that the success of such calculations relies on the populations of higher spin orders (i.e. the number of correlated spins involved in a coherence) remaining relatively small [22] This is clearly the case in solution-state NMR, where high spin order coherences relax relatively quickly, and some promising results have been obtained for simulations of 1H spin-diffusion in powder samples under MAS [23,24,25]. Frantsuzov et al / Solid State Nuclear Magnetic Resonance 70 (2015) 28–37 state, where very high spin orders can be observed amongst 1H nuclei [26,27,28], and so we investigate the role of higher spin orders in heteronuclear decoupling
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