Quasi-Resonance Fluorine-19 Signal Amplification by Reversible Exchange

We report on an extension of the quasi-resonance (QUASR) pulse sequence used for signal amplification by reversible exchange (SABRE), showing that we may target distantly J -coupled 19 F-spins. Polarization transfer from the parahydrogen-derived hydrides to the 19 F 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 19 F z -magnetization, derived from the parahydrogen singlet order. An accumulation of 19 F polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized 19 F signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for 15 N QUASR-SABRE. Moreover, the hyperpolarized 19 F signals of 3- 19 F- 14 N-pyridine and 3- 19 F- 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 15 N nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to 19 F ( P 19 F ≈ 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 19 F spins.