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Heteronuclear Cross-Relaxation Effects in the NMR Spectroscopy of Hyperpolarized Targets



Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as 15N and 13C. This report explores certain features arising in solution-state 1H NMR upon polarizing low-γ nuclear species. Following solid-state hyperpolarization of both 13C and 1H, solution-phase 1H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring 13C nuclei appeared larger than the typically dominant 12C-bonded 1H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from 13C→1H polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the 13C-bound 1H NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity 1H NMR traces was explored.