Intermolecular interactions of benzene and carbon tetrachloride with selected free radicals in solution as studied by13C and1H dynamic nuclear polarization

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Abstract
Intermolecular hyperfine coupling between 13C nuclei and free-radical unpaired electron spins has been examined for six combinations of three free radicals and two solvent molecules. From magnetic field and temperature-dependent dynamic nuclear polarization measurements, scalar and dipolar coupling contributions have been separated. Interpretation of these results in terms of a modified diffusion model revealed scalar spectral density functions with two or three frequency components, each characterized by a distinct contact time. Collisions with the shortest time constant were found to be nearly independent of the system and accounted for approximately 90 per cent of all radical-receptor collisions and 30 per cent of the scalar relaxation rate. These have been related to random elastic collisions. In contrast, collisions with long contact times were found to be highly system dependent. Although few in number, they accounted for a relatively large fraction of the scalar relaxation rate because of their long duration. These have been interpreted as stereospecific collisions which reflect the tendency of the colliding partners to form weak transient complexes. Lastly, relative scalar coupling energies were obtained and related to the availability of the unpaired electron and to the structure of both colliding molecules.