A Unified Description for Polarization-Transfer Mechanisms in Magnetic Resonance in Static Solids: Cross Polarization and DNP

TL;DR

This paper presents a unified theoretical framework for polarization transfer mechanisms in solid-state NMR, specifically cross polarization and DNP, offering new insights and potential applications in hyperpolarization and distance measurements.

Contribution

The authors develop a unified theory based on average Hamiltonian theory that describes both CP and DNP mechanisms, providing new insights and design principles.

Findings

Unified description of CP and DNP mechanisms.

Enhanced understanding of the cross effect DNP process.

Potential for new electron-nucleus distance measurement techniques.

Abstract

Polarization transfers are crucial building blocks in magnetic resonance experiments, i.e., they can be used to polarize insensitive nuclei and correlate nuclear spins in multidimensional NMR spectroscopy. The polarization can be transferred either across different nuclear spin species or from electron spins to the relatively low-polarized nuclear spins. The former route occurring in solid-state NMR (ssNMR) can be performed via cross polarization (CP), while the latter route is known as dynamic nuclear polarization (DNP). Despite having different operating conditions, we opinionate that both mechanisms are theoretically similar processes in ideal conditions, i.e., the electron is merely another spin-1/2 particle with a much higher gyromagnetic ratio. Here, we show that the CP and DNP processes can be described using a unified theory based on average Hamiltonian theory (AHT) combined with fictitious operators. The intuitive and unified approach has allowed new insights on the cross effect (CE) DNP mechanism, leading to better design of DNP polarizing agents and extending the applications beyond just hyperpolarization. We explore the possibility of exploiting theoretically predicted DNP transients for electron-nucleus distance measurements--like routine dipolar-recoupling experiments in solid-state NMR.