Tuesday, February 22, 2011

Dynamic Nuclear Polarization Assited Spin Diffusion for the Solid Effect Case

Hovav et al., Dynamic Nuclear Polarization Assited Spin Diffusion for the Solid Effect Case, J. Chem. Phys, 2011, 134, 074509

The dynamic nuclear polarization (DNP) process in solids depends on the magnitudes of hyperfine interactions between unpaired electrons and their neighboring (core) nuclei, and on the dipole–dipole interactions between all nuclei in the sample. The polarization enhancement of the bulk nuclei has been typically described in terms of a hyperfine-assisted polarization of a core nucleus by microwave irradiation followed by a dipolar-assisted spin diffusion process in the core–bulk nuclear system. This work presents a theoretical approach for the study of this combined process using a density matrix formalism. In particular, solid effect DNP on a single electron coupled to a nuclear spin system is considered, taking into account the interactions between the spins as well as the main relaxation mechanisms introduced via the electron, nuclear, and crossrelaxation rates. 

The basic principles of the DNP-assisted spin diffusion mechanism, polarizing the bulk nuclei, are presented, and it is shown that the polarization of the core nuclei and the spin diffusion process should not be treated separately. To emphasize this observation the coherent mechanism driving the pure spin diffusion process is also discussed. In order to demonstrate the effects of the interactions and relaxation mechanisms on the enhancement of the nuclear polarization, model systems of up to ten spins are considered and polarization buildup curves are simulated. A linear chain of spins consisting of a single electron coupled to a core nucleus, which in turn is dipolar coupled to a chain of bulk nuclei, is considered. The interaction and relaxation parameters of this model system were chosen in a way to enable a critical analysis of the polarization enhancement of all nuclei, and are not far from the values of 13C nuclei in frozen (glassy) organic solutions containing radicals, typically used in DNP at high fields. Results from the simulations are shown, demonstrating the complex dependences of the DNP-assisted spin diffusion process on variations of the relevant parameters. In particular, the effect of the spin lattice relaxation times on the polarization buildup times and the resulting end polarization are discussed, and the quenching of the polarizations by the hyperfine interaction is demonstrated.

Wednesday, February 16, 2011

What is a Gyrotron?

Have you been wondering what a Gyrotron is and how it works? Check out this short description by Bridge12 Technologies.  For more information on Gyrotrons for DNP-enhanced NMR spectroscopy (DNP-NMR) visit dnp.bridge12.com.

Monday, February 7, 2011

The DNP-NMR Worldmap

If you are curious about who in the world performs DNP-enhanced NMR Spectroscopy (DNP-NMR) have a look at the DNP Worldmap. This map was create using Google Maps and features laboratories that perform DNP-NMR spectroscopy and vendors of DNP-NMR instrumentation.

The DNP-NMR Worldmap (at Google Maps)

This is an interactive map. Follow one of the two links above and start investigating who is performing DNP-NMR. Each location is tagged by a color-coded pin (green - Bridge12, red - home built, blue - Bruker BioSpin, yellow - Oxford Instruments HyperSense). Each pin has a short description and a hyperlink pointing to the website of the lab.