the NMR FID of the 1H nuclei, for example, in the presence of continuous microwave irradiation as a function of the microwave frequency offset. Carbonaceous materials such as cellulose char contain large numbers of stable free electrons delocalized in large polycyclic aromatic hydrocarbons. Such electrons can give large polarization enhancements to nearby protons via proton-proton spin-diffusion if they are not so close together that the electron-nuclear dipolar interaction does not broaden the proton resonance beyond detection. For small isolated clusters, the free electrons are fixed and give rise to solid-state enhancements (SS). The maximal proton solid-state enhancement is observed at microwave offsets of ω ≈ ωe ± ωH, where ωe and ωH are the electron and nuclear Larmor frequencies, respectively. For larger and more densely concentrated aromatic clusters, the free electrons can undergo rapid electron exchange interactions. These electrons give rise to an Overhauser enhancement centered at a microwave offset of ωe - ωH = 0. The cellulose char also exhibits electrons undergoing thermal mixing effects (TM). While the enhancement curve reveals the types electron-nuclear spin interactions in a material, it is not quantitative and the relative abundance of the different types of nuclei cannot be determined directly from the curve. References Further reading Review articles Books Carson Jeffries, "Dynamic Nuclear Orientation", New York, Interscience Publishers, 1963 Anatole Abragam and Maurice Goldman, "Nuclear Magnetism: Order and Disorder", New York : Oxford University Press, 1982 Tom Wenckebach, "Essentials of Dynamic Nuclear Polarization", Spindrift Publications, The Netherlands, 2016 Special issues Dynamic Nuclear Polarization: New Experimental and Methodology Approaches and Applications in Physics, Chemistry, Biology and Medicine, Appl. Magn. Reson., 2008. 34(3-4) High field dynamic nuclear polarization - the renaissance, Phys. Chem. Chem. Phys., 2010. 12(22) Blogs The DNP-NMR blog (Link) Chemical physics Nuclear magnetic resonance