Carnahan, Scott L., Yunhua Chen, James F. Wishart, Joseph W. Lubach, and Aaron J. Rossini. “Magic Angle Spinning Dynamic Nuclear Polarization Solid-State NMR Spectroscopy of γ-Irradiated Molecular Organic Solids.” Solid State Nuclear Magnetic Resonance 119 (June 2022): 101785.
In the past 15 years, magic angle spinning (MAS) dynamic nuclear polarization (DNP) has emerged as a method to increase the sensitivity of high-resolution solid-state NMR spectroscopy experiments. Recently, γ-irradiation has been used to generate signiﬁcant concentrations of homogeneously distributed free radicals in a variety of solids, including quartz, glucose, and cellulose. Both γ-irradiated quartz and glucose previously showed signiﬁcant MAS DNP enhancements. Here, γ-irradiation is applied to twelve small organic molecules to test the applicability of γ-irradiation as a general method of creating stable free radicals for MAS DNP experiments on organic solids and pharmaceuticals. Radical concentrations in the range of 0.25 mM–10 mM were observed in irradiated glucose, histidine, malic acid, and malonic acid, and signiﬁcant 1H DNP enhancements of 32, 130, 19, and 11 were obtained, respectively, as measured by 1H→13C CPMAS experiments. However, concentrations of free radicals below 0.05 mM were generally observed in organic molecules containing aromatic rings, preventing sizeable DNP enhancements. DNP sensitivity gains for several of the irradiated compounds exceed that which can be obtained with the relayed DNP approach that uses exogeneous polarizing agent solutions and impregnation procedures. In several cases, signiﬁcant 1H DNP enhancements were realized at room temperature. This study demonstrates that in many cases γ-irradiation is a viable alternative to addition of stable exogenous radicals for DNP experiments on organic solids.