Derik K. Frantz

Topical Developments in High-Field Dynamic Nuclear Polarization.

We report our recent efforts directed at improving high-field DNP experiments. We investigated a series of thiourea nitroxide radicals and the associated DNP enhancements ranging from ε = 25 to 82 that demonstrate the impact of molecular structure on performance. We directly polarized low-gamma nuclei including 13C, 2H, and 17O using trityl via the cross effect. We discuss a variety of sample preparation techniques for DNP with emphasis on the benefit of methods that do not use a glass-forming cryoprotecting matrix. Lastly, we describe a corrugated waveguide for use in a 700 MHz / 460 GHz DNP system that improves microwave delivery and increases enhancements up to 50%.

Synthesis and properties of the 5,10,15-trimesityltruxen-5-yl radical.

The synthesis of a long-lived, truxene-based radical that is highly delocalized and exhibits a narrow EPR absorption is reported. The radical is stable for multiple hours in a solution exposed to air and remains for months in the solid state under inert gas. Characterization and properties are discussed.

A Tubular Structural Analogue of Helicene

Significance: Helical molecular systems, both natural and unnatural, continue to capture the interests of chemists. Using an enantiomerically pure bicyclic moiety to appropriately place kinks into the system, Wärnmark and co-workers report the synthesis of helical structure 1, an orthogonal topological analogue of helicene. Comment: The synthesis of 1 is accomplished by repeated employment of a two-step set of reactions consisting of (1) ring-opening hydrolysis in acid and (2) Friedländer condensation with a chiral bicyclic ketone. By this strategy, monomer 2 is converted into ring-opened trimer 3, which is converted into trimeric ketone 4. Condensation of 4 and 3 affords the helical target 1. N O

Expanded [n]Radialenes: Unusual Carbon-Rich Molecules

37 M . GH O L A M I , S . R A M SA Y W A C K , M . N . C H A U R , A . H . M U R R A Y , R . M C D O N A L D , M . J . F E RG U S ON , L . E C H E G O Y E N, R . R . TY K W I N SK I * ( UN I V E R S I T Y O F E R L A N G E N N U R E M B E RG , G E R M A N Y ; U NI V E R S I T Y O F A L B E R T A , E D M O NT ON , C A N AD A ; U N I V E R S I DA D D E L VA L L E , S A N T I A G O D E C A L I , C O L O M B I A ; U NI VE R S I T Y O F TE X AS A T E L P A S O , U SA ) Synthesis and Derivatization of Expanded [n]Radialenes (n = 3, 4) Chem. Eur. J. 2013, 19, 15120–15132.