The 1H and 13C dynamic nuclear polarization (DNP) enhancement for novel silica phase immobilized nitroxide (SPIN) samples

The solid/liquid intermolecular transfer (SLIT) flow dynamic nuclear polarization (DNP) experiment potentially provides new methodology for studying interfacial phenomena (e.g., weak hydrogen bonding). In addition, the high efficiency of the transfer also ensures dramatically enhanced NMR signals. These large DNP enhancements could alleviate sensitivity limitations in various flow NMR experiments. Previous studies have established that silica phase immobilized nitroxide (SPIN) radical system are advantageous in the SLIT experiment. In favorable cases (e.g. DCCI₃/SPIN system) a ¹³C DNP enhancement 60 times in excess of the high magnetic field (4.7 T) magnetization has been achieved.¹² However a number of factors still limit the SPIN system presently available. For example, low magnetogyric ratio nuclides, ¹³C, ¹⁵N, which are not dominated by scalar relaxation mechanism require high surface radical concentrations.

The focal point of the present study is the preparation and characterization of several new SPIN radical systems and can be divided into two parts:

1). Preparation, EPR, and DNP Characterization of Achiral SPIN Radicals: a number of SPIN samples were prepared in order to examine the dependence of the observed SLIT DNP enhancements as a function of the surface spin concentration and also isotope-substitution of the immobilized radicals. The SPIN samples were characterized by EPR and DNP. The results show that the increase in the spin concentration does not offer any advantage for ¹H DNP studies. In contrast, ¹³C SLIT DNP results in improved SPIN sample demonstrate the possibility of monitoring dipolar dominated ¹³C DNP enhancements as a result of better leakage factors and suppressed three-spin effects at higher radical concentration. The effect of substitution of deuterons for protons in the immobilized radical also suggest an appreciable contribution of a solid-state three-spin effect.

2). Preparation, EPR, and DNP Characterization of Chiral SPIN Samples: This part of the study provides a chiral SPIN radical suitable for monitoring enantioselective ¹³C DNP enhancements. The DNP results suggest that selective enantiomer/chiral SPIN interactions are feasible. Specifically, differences in the ¹³C DNP enhancements for a model system: (R)- and (S)- enantiomers of bromocamphor, and a (R) chiral SPIN sample were observed.