Characterization of electron spin exchange interactions in cellulose chars by means of ESR,1H NMR, and Dynamic Nuclear Polarization

Abstract

Cellulose chars heat treated under nitrogen atmosphere for six hours over a range of heating temperatures from 250 to 1000°C were studied by ESR, broadline1H NMR, and Dynamic Nuclear Polarization (DNP). Chars heated below 450°C exhibited DNP enhancements predominantly due to the solid state effect resulting from static electron-nuclear spinspin interactions, while chars heated at higher temperatures exhibited Overhauser enhancements, which result from time dependent interactions. It was found that, while the maximum number of unpaired electrons was obtained at a heating temperature of 700°C, the maximum Zeeman and rotating-frame1H relaxation rates were achieved at much lower temperatures. Moreover, small Overhauser enhancements were observed even at the lower heating temperatures, where the time dependent electron-nuclear interactions are expected to be minimal, and the solid state enhancements decrease more rapidly than expected for samples heated above 350°C. These effects are explained in terms of a distribution of rates of electron-electron spin-exchange interactions. The charred and carbonized cellulose samples provided a set of solids in which the number of unpaired electrons varied over a large range and exhibited a broad distribution of spin-exchange rates. It was shown that DNP-NMR is a powerful method for probing this distribution, and for detecting small fractions of rapidly exchanging and static electrons.