Solid-State <sup>1</sup>H Spin Polarimetry by <sup>13</sup>CH<sub>3</sub> Nuclear Magnetic Resonance
Abstract. Dissolution-dynamic nuclear polarization is emerging as a promising means to prepare proton polarizations approaching unity. At present, 1H polarization quantification remains fastidious due to the requirement of measuring thermal equilibrium signals. Lineshape polarimetry of solid-state nuclear magnetic resonance spectra is used to determine a number of useful properties regarding the spin system under investigation. In the case of highly polarized nuclear spins, such as those prepared under the conditions of dissolution-dynamic nuclear polarization experiments, the absolute polarization of a particular isotopic species within the sample can be directly inferred from the characteristics of the corresponding resonance lineshape. In situations where direct measurements of polarization are complicated by deleterious phenomena, indirect estimates of polarization using coupled heteronuclear spins prove informative. We present a simple analysis of the 13C spectral lineshape asymmetry of [2-13C]sodium acetate based on relative peak intensities, which can be used to indirectly evaluate the proton polarization of the methyl group moiety, and very likely the entire sample in the case of rapid and homogeneous 1H-1H spin diffusion. 1H polarizations greater than ~10–25 % (depending on the sign of the microwave irradiation) were found to be linearly proportional to the 13C peak asymmetry, which responds differently to positive or negative microwave irradiation. These results suggest that, as a dopant, [2-13C]sodium acetate could be used to indirectly gauge 1H polarizations in standard sample formulations, which is potentially advantageous for: samples polarized in commercial dissolution-dynamic nuclear polarization devices that lack 1H radiofrequency hardware, measurements which are deleteriously influenced by radiation damping or complicated by the presence of large background signals, and situations where the acquisition of a thermal equilibrium spectrum is not feasible.
