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Browsing by Author "Song, Li"

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    Liquid Phase Carbon-Thirteen Dynamic Nuclear Polarization Study of Monosubstituted Aromatic Compounds
    Song, Li (Virginia Tech, 1997-09-18)
    In this work, liquid phase Carbon-thirteen DNP experimental data were collected in a flow transfer system for different mono-substituted aromatic compounds, such as, anisole, nitrobenzene and halobenzenes. These substitute groups covered a wide range of electronic effects from strong electron donating groups to strong electron withdrawing groups. Hammett linear free energy relationship was used to quantitatively study the electronic effect of substituent on the carbon-thirteen DNP enhancement. It was found that the carbon-thirteen DNP enhancements at meta and para positions exhibits a reasonable correlation. A better correlation of carbon-thirteen DNP enhancements with the Hammett inductive factor was observed. A large scalar dominated enhancement at ipso (C-1) position was observed for iodobenzene. This suggests that facile transfer of spin polarization to the C-1 carbon via the highly polarizable iodine atom is possible and the dominate electron transfer mechanism for this system. A model of polarization transfer via spin diffusion of abundant proton spins was examined. The results indicate that it is not important.
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    Optimization and prediction of the electron-nuclear dipolar and scalar interaction in 1H and 13C liquid state dynamic nuclear polarization
    Wang, Xiao; Isley, William C., III; Salido, Sandra I.; Sun, Z.; Song, Li; Tsai, K. H.; Cramer, Christopher J.; Dorn, Harry C. (The Royal Society of Chemistry, 2015-07-29)
    During the last 10–15 years, dynamic nuclear polarization (DNP) has evolved as a powerful tool for hyperpolarization of NMR and MRI nuclides. However, it is not as well appreciated that solution-state dynamic nuclear polarization is a powerful approach to study intermolecular interactions in solution. For solutions and fluids, the 1H nuclide is usually dominated by an Overhauser dipolar enhancement and can be significantly increased by decreasing the correlation time (τc) of the substrate/nitroxide interaction by utilizing supercritical fluids (SF CO2). For molecules containing the ubiquitous 13C nuclide, the Overhauser enhancement is usually a profile of both scalar and dipolar interactions. For carbon atoms without an attached hydrogen, a dipolar enhancement usually dominates as we illustrate for sp2 hybridized carbons in the fullerenes, C60 and C70. However, the scalar interaction is dependent on a Fermi contact interaction which does not have the magnetic field dependence inherent in the dipolar interaction. For a comprehensive range of molecular systems we show that molecules that exhibit weakly acidic complexation interaction(s) with nitroxides provide corresponding large scalar enhancements. For the first time, we report that sp hybridized (H–C) alkyne systems, for example, the phenylacetylene–nitroxide system exhibit very large scalar dominated enhancements. Finally, we demonstrate for a wide range of molecular systems that the Fermi contact interaction can be computationally predicted via electron–nuclear hyperfine coupling and correlated with experimental 13C DNP enhancements.