Polarizing agents for high-frequency Dynamic Nuclear Polarization : development and applications

Dynamic Nuclear Polarization (DNP) is utilized to enhance NMR signal intensities in systems including metals, ceramics, polymers and biological solids. The enhancement results from polarization transfers from unpaired electrons, usually carried by a polarizing agent such as TEMPO (2,2,6,6-tetramethylpiperydin-l-oxyl), to the target nuclei. In this thesis, polarizing agents were developed for efficient polarization mechanisms at 5 Tesla. DNP using biradicals yielded improvements of proton enhancement by about fourfold compared to an identical amount of monomeric TEMPO as used previously. The polarizing mechanism involved was the cross effect (CE), which relies on three-spin processes involving two electrons and one nucleus. Optimization of the CE requires the appropriate electron-electron interaction and the correct EPR frequency separation matching the nuclear Larmor frequency. Due to the relatively short inter-radical distance in interesting biradicals, multi-frequency EPR lineshape analyses are suitable to characterize, the distance and relative g-tensor orientations between electrons, revealing spectral parameters that explain the improvement of DNP efficiency. Alternatively, radical mixtures of TEMPO and Trityl, methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[ 1 ,2-d:4,5-d']bis( 1 ,3)dithiol-4-yl, improve the probability of the correct EPR frequency separation compared to TEMPO by itself.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Chemistry.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2006

Author dc:creator

• Hu, Kan-Nian

Advisor dc:contributor.advisor

• Robert G. Griffin.

• Chemistry.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/37887

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/37887