Improving 13C Benchtop Hyperpolarization with TOTAPOL as a Polarizing Agent

Dissolution dynamic nuclear polarization (dDNP) offers a solution to the naturally low sensitivity of NMR spectroscopy thanks to signal enhancements by several orders of magnitude. However, this methodology usually requires high magnetic fields and cryogenic temperatures (typ. > 3 T and < 4.2 K), which implies complex and expensive experimental setups, usually hosted in dedicated facilities. In recent years, there has been a growing interest in more affordable and cost-effective systems, and our group has developed an approach based on a simple and affordable benchtop DNP polarizer. Mild experimental conditions (𝐵 0 = 1 T and 𝑇 = 77 K) are applied, together with continuous microwave irradiation, to transfer the high electron spin polarization from TEMPOL to nuclear spins, achieving typically 100-fold signal enhancement on 1 H nuclei in arbitrary samples. In the present study we aim at improving the NMR sensitivity of 13 C nuclei at benchtop conditions by means of cross-polarization DNP (CP-DNP) using a dinitroxide polarizing agent (PA), TOTAPOL, and demonstrate that we can increase the 13 C signal intensity by 50% compared to the use of TEMPOL. We rationalise the behaviour of the two PAs by analysing the interplay between 1 H and 13 C relaxation times during the CP contact time. Our study suggests that biradicals may offer a promising pathway to improve the 13 C signal enhancement under CP-DNP at benchtop conditions.