No abstract

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors e ( 1 H) in cross-effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP-optimized glycerol/water matrix ("DNP juice") have been studied. We observe that e ( 1 H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e-e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e-e distance or too long a T 1e can dramatically limit e ( 1 H). Our study also shows that the molecular structure of AMUPol is not optimal and its e ( 1 H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better e ( 1 H) than AMUPol itself (by a factor of ca. 1.2).

TinyPol binitroxides provide significantly higher DNP enhancement factors for solid-state NMR spectroscopy at high magnetic fields than today's reference radical AMPUPol.