Session: Undergraduate Research Posters: Physical Chemistry:

NMR and MRI are cherished for their ability to characterize chemical and morphological structures and they are invaluable tools for fundamental science and medicine alike. However, both NMR and MRI suffer from very poor concentration sensitivity because of low thermal nuclear spin polarization, even in strong magnetic fields of modern NMR and MRI magnets. To address this problem, hyperpolarization methods are being developed that polarize much larger fractions of the nuclear spins. NMR and MRI signal enhancements of up to seven orders of magnitude have been demonstrated. Signal Amplification by Reversible Exchange (SABRE) stands out as a particularly simple and robust hyperpolarization modality, which relies on parahydrogen as the source of spin order. Polarization is transferred in transition metal complexes that undergo reversible exchange with hydrogen and substrate molecules. In this work, an in-depth theoretical analysis governing the chemical equilibria in SABRE is provided. Ab initio DFT calculations implemented via the FHI-aims code package reveal a detailed reaction network. For all reactions in the identified network, the potential energy surface is probed using the string method and the climbing image technique to determine the minimum energy paths and the transition states. Harmonic Transition State Theory (HTST) is applied to calculate exchange rates that approximate experimental values. Furthermore, the reaction network energy surface can be modulated by chemical potentials that account for concentration, temperature and pressure dependences. Accordingly, this theoretical analysis identified the rate-determining step in the SABRE process, and provides guidance for optimizing experimental approaches, and the development of future SABRE catalysts.

The reaction network for the Ir-IMes and pyridine SABRE catalyst system. Gas-phase relative Born-Oppenheimer energies for each complex is labeled on the y-axis. The energy barriers for the exchange reactions are labeled above or below the corresponding reaction arrows.

The reaction network for the Ir-IMes and pyridine SABRE catalyst system. Gas-phase relative Born-Oppenheimer energies for each complex is labeled on the y-axis. The energy barriers for the exchange reactions are labeled above or below the corresponding reaction arrows.

Volker Blum

Appearances