Quasi-Resonance Fluorine-19 Signal Amplification By Reversible Exchange.

We report on an extension of the quasiresonance (QUASR) pulse sequence used for signal amplification by reversible exchange (SABRE), showing that we may target distantly J-coupled F-19-spins. Polarization transfer from the parahydrogen-derived hydrides to the F-19 nucleus is accomplished via weak five-bond J-couplings using a shaped QUASR radio frequency pulse at a 0.05 T magnetic field. The net result is the direct generation of hyperpolarized F-19 z-magnetization, derived from the parahydrogen singlet order. An accumulation of F-19 polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized F-19 signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for N-15 QUASR-SABRE. Moreover, the hyperpolarized F-19 signals of 3-F-19-N-14-pyridine and 3-F-19-(15)-N-pyridine isotopologues are similar, suggesting that (i) polarization transfer via QUASR-SABRE is irrespective of the nitrogen isotopologue and (ii) the presence or absence of the spin-1/2 N-15 nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to F-19 (P-19F approximate to 0.1%) was not the goal of this study, we show that high-field SABRE can be efficient and broadly applicable for direct hyperpolarization of F-19 spins.