Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Abstract

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k c ′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k c ′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek c ′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.