Parahydrogen‐Induced Polarization of ¹⁴N Nuclei

Abstract

Hyperpolarization techniques provide a dramatic increase in sensitivity of nuclear magnetic resonance spectroscopy and imaging. In spite of the outstanding progress in solution‐state hyperpolarization of spin‐1/2 nuclei, hyperpolarization of quadrupolar nuclei remains challenging. Here, hyperpolarization of quadrupolar 14N nuclei with natural isotopic abundance of >99% is demonstrated. This is achieved via pairwise addition of parahydrogen to tetraalkylammonium salts with vinyl or allyl unsaturated moieties followed by a subsequent polarization transfer from 1H to 14N nuclei at high magnetic field using PH‐INEPT or PH‐INEPT+ radiofrequency pulse sequence. Catalyst screening identified water‐soluble rhodium complex [Rh(P(m‐C6H4SO3Na)3)3Cl] as the most efficient catalyst for hyperpolarization of the substrates under study, providing up to 1.3% and up to 6.6% 1H polarization in the cases of vinyl and allyl precursors, respectively. The performance of PH‐INEPT and PH‐INEPT+ pulse sequences was optimized with respect to inter‐pulse delays, and the resultant experimental dependences were in good agreement with simulations. As a result, 14N NMR signal enhancement of up to 760‐fold at 7.05 T (corresponding to 0.23% 14N polarization) was obtained.