Influence of ¹³C Isotopic Labeling Location on Dynamic Nuclear Polarization of Acetate

Dynamic nuclear polarization (DNP) via the dissolution method has alleviated the insensitivity problem in liquid-state nuclear magnetic resonance (NMR) spectroscopy by amplifying the signals by several thousand-fold. This NMR signal amplification process emanates from the microwave-mediated transfer of high electron spin alignment to the nuclear spins at high magnetic field and cryogenic temperature. Since the interplay between the electrons and nuclei is crucial, the chemical composition of a DNP sample such as the type of free radical used, glassing solvents, or the nature of the target nuclei can significantly affect the NMR signal enhancement levels that can be attained with DNP. Herein, we have investigated the influence of ¹³C isotopic labeling location on the DNP of a model ¹³C compound, sodium acetate, at 3.35 T and 1.4 K using the narrow electron spin resonance (ESR) line width free radical trityl OX063. Our results show that the carboxyl ¹³C spins yielded about twice the polarization produced in methyl ¹³C spins. Deuteration of the methyl ¹³C group, while proven beneficial in the liquid-state, did not produce an improvement in the ¹³C polarization level at cryogenic conditions. In fact, a slight reduction of the solid-state ¹³C polarization was observed when ²H spins are present in the methyl group. Furthermore, our data reveal that there is a close correlation between the solid-state ¹³C T₁ relaxation times of these samples and the relative ¹³C polarization levels. The overall results suggest the achievable solid-state polarization of ¹³C acetate is directly affected by the location of the ¹³C isotopic labeling via the possible interplay of nuclear relaxation leakage factor and cross-talks between nuclear Zeeman reservoirs in DNP.