van Bentum, P.J.M., et al., Solid Effect DNP in a Rapid-melt setup. J. Magn. Reson., 2016. 263: p. 126-135.
Dynamic Nuclear Polarization (DNP) has become a key element in nuclear magnetic resonance (NMR). Recently, we developed a novel approach to DNP enhanced liquid-state NMR based on rapid melting of a solid hyperpolarized sample followed by ‘in situ’ liquid-state NMR detection. This method allows 1 H detection with fast cycling options for signal averaging. In nonpolar solvents, doped with BDPA radicals, proton enhancement factors were achieved of up to 400. A short recycling delay of about 5 s allows for a fast determination of the hyper-polarization dynamics as function of the microwave frequency and power. Here, we use the rapid melt dnp method to study the mechanisms for DNP in the solid phase in more detail. Solid Effect, Cross Effect, Solid Overhauser and Liquid-state (supercritical) Overhauser DNP enhancement can be observed in the same setup. In this paper, we concentrate on Solid Effect DNP observed with both homogeneous narrow line radicals such as BDPA and with wide line anisotropic nitroxide radicals such as TEMPOL. We find indications that BDPA protons play an important role in Solid Effect DNP with this radical. A simplified spin diffusion model for BDPA can give a semi-quantitative description of the enhancements as function of the microwave power and as function of the proton concentration in the solid solution. For aqueous frozen samples we observe a similar Solid Effect DNP enhancement, which is analyzed within the simplified spin diffusion model.