Intermolecular Nuclear Spin Hyperpolarization Transfer via Cross-Relaxation Triggers RASER of Solute Molecules.

Radiofrequency amplification by stimulated emission of radiation (RASER) is a phenomenon that is observed in systems of nuclear spins with strong inverse polarization beyond the thermal equilibrium. RASER is of fundamental interest and also possesses several features of possible practical importance, such as very narrow NMR lines and background-free detection. However, so far, the effect was limited to molecules directly polarized by corresponding hyperpolarization techniques. In this work, we found that strongly hyperpolarized allylic compounds transferred polarization via nuclear Overhauser effect (NOE) to other solutes, triggering their RASER. In stark contrast to previous observations, the parahydrogen addition and intermolecular NOE transfer engendered RASER (PAINTER) does not require direct hyperpolarization of the target molecule. This way polarized and detected solutes had 10-20 times narrower lines compared to their classical NMR spectra, providing a useful analysis tool for various molecules beyond standard NMR limitations.