Dynamic Nuclear Polarization: Collision Mechanics in Fluorocarbon Solutions

Abstract

Dynamic polarization measurements at three widely different magnetic field strengths are presented for an array of chemically different free radicals and fluorocarbons. Observed fluorine NMR signal enhancements ranged from − 225 to + 430. The scalar hyperfine correlation time, which is shown to be the major factor governing the enhancement, is systematically longer for systems showing higher positive enhancement at low field. A generalized pulse-transform collision model is applicable to all cases, and it is found that the slope of the scalar spectral cutoff becomes progressively less steep as low-field scalar coupling increases. In extreme cases, the spectrum may be interpreted as a superposition of at least two component spectra, each corresponding to a pulse from a stereospecific collision attitude. The observed behavior is consistent with molecular orbital calculations. The range of computed hyperfine coupling energies is also in good qualitative agreement with observed differences between fluorocarbons of diverse structure and chemical nature. Over all, the results demonstrate the usefulness of measurements of this sort for understanding the microscopic character of molecular collisions.