Determination of long-range scalar 1H-1H coupling constants responsible for polarization transfer in SABRE

SABRE (Signal Amplification By Reversible Exchange) nuclear spin hyperpolarization method can provide strongly enhanced NMR signals as a result of the reversible association of small molecules with para-hydrogen (p-H-2) at an iridium metal complex. The conversion of p-H-2 singlet order to enhanced substrate proton magnetization within such complex is driven by the scalar coupling interactions between the p-H-2 derived hydrides and substrate nuclear spins. In the present study these long-range homonuclear couplings are experimentally determined for several SABRE substrates using an NMR pulse sequence for coherent hyperpolarization transfer at high magnetic field. Pyridine and pyrazine derivatives appear to have a similar similar to 1.2 Hz (4)J coupling to p-H-2 derived hydrides for their ortho protons, and a much lower (5)J coupling for their meta protons. Interestingly, the (4)J hydride-substrate coupling for five-membered N-heterocyclic substrates is well below 1 Hz. (C) 2016 Elsevier Inc. All rights reserved.