Coherent polarization transfer in chemically exchanging systems.

Signal Amplification by Reversible Exchange (SABRE) uses para-enriched dihydrogen, pH2, to boost the NMR signal by several orders of magnitude. Although the method was discovered more than a decade ago, the quest to optimize SABRE and to establish a complete description in silico is ever ongoing. The simulation of SABRE is exacerbated by a complex interplay of chemical exchange and coherent polarization transfer. Here, we compare two different simulation approaches, Markov chain Monte Carlo (MC) simulations and a modification of the Liouville von Neumann equation with superoperators of chemical exchange (SO). We apply these methods to simulate the fate of truncated and full, three and four spin-½ systems in two different experiments: continuous polarization at a constant or alternating B0 field. For all cases, MC and SO provided similar results, and previously reported experimental data were well reproduced. It appears that both methods are well suited to approach this matter, while SO was faster than MC by several orders of magnitude in the cases discussed.