A role for low concentration reaction intermediates in the signal amplification by reversible exchange process revealed by theory and experiment

A route to monitor the involvement of less abundant species during the catalytic transfer of hyperpolarisation from parahydrogen into a substrate is detailed. It involves probing how the degree of hyperpolarisation transfer catalysis is affected by the magnetic field experienced by the catalyst during this process as a function of temperature. The resulting data allow the ready differentiation of the roles played by hard to detect and highly reactive complexes, such as [Ir(H)2(NHC)(substrate)2(methanol)]Cl, from dominant species such as [Ir(H)2(NHC)(substrate)3]Cl. The difference in behaviour results from changes in the interligand spin–spin coupling network within the active SABRE catalysts.