Nuclear magnetic resonance and dynamic polarization studies of liquid/liquid and liquid/solid interfaces
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In the present study, interactions at the liquid/liquid and liquid/solid interfaces have been investigated by the combination of both nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP) techniques. The 13C and 15N paramagnetic contact shifts, and 1H, 13C, and 15N relaxation times in CH3CN/2,2,6,6,-tetramethyl-1-piperidinyloxy (TEMPO) and CH3CONH2/TEMPO systems have been measured at high magnetic field (Bo = 1.9-9.4 T). The 13 DNP enhancements at low magnetic field (0.33 T) in the CH3CONH2/TEMPO system have been determined by the flow liquid-liquid intermolecular transfer (LLIT) DNP technique. The data can be understood in terms of transient hydrogen bond formation between closed shell diamagnetic molecules and the open shell free radical TEMPO.
A set of static and dynamic parameters, such as hyperfine coupling constants, correlation times, and free radicalnuclear internuclear distances in the hydrogen bonding complex, have also been determined. The scalar and dipolar contributions derived from the NMR study have been subsequently employed to predict the corresponding 1H, BC, and 15N low magnetic field (0. 33 T) DNP enhancements. Good agreement has been obtained between the NMR predicted and experimentally measured low magnetic field DNP results. The dynamic electron-nuclear intermolecular interactions between the newly discovered fullerene, C60 ' and the free radical TEMPO have been characterized by flow LLIT and solid liquid intermolecular transfer (SLIT) DNP techniques. A dipolar dominated ultimate DNP enhancement (-250) at 0.33 T magnetic field has been observed. The results are consistent with a model for C6~TEMPO interactions involving nonspecific complex formations.
In addition to DNP studies in the liquid state, the solid/liquid surface intermolecular interactions in solid samples of various activated carbon specimens have been monitored by using flow SLIT 'H and 13C DNP experiments. The activated carbon samples were prepared by pyrolysis of cellulose, and commercial samples were also employed. The surface-liquid interaction in these studies were monitored with the solvent benzene ( or d6-benzene). Both time dependent (Overhauser) and time-independent (solid-state) DNP enhancements were observed in these studies. Both chemisorption and physisorption processes of oxygen to the activated carbon were also monitored using the DNP approach.
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