Bimolecular Encounters and Re-Encounters (Cage Effect) of a Spin-Labeled Analogue of Cholestane in a Series of n-Alkanes: Effect of Anisotropic Exchange Integral

Electron paramagnetic resonance (EPR) spectra of the nitroxide spin probe 3 beta-doxyl-5 alpha-cholestane (CSL) are studied as functions of the molar concentration, c, and the temperature, T, in a series of n-alkanes. The results are compared with a similar study of a much smaller spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDT). The Heisenberg spin exchange (HSE) rate constants, K-ex, of CSL are similar in hexane, octane, and decane and are about one-half of those for pDT in the same solvents. They are also about one-half of the Stokes-Einstein-Perrin prediction. This reduction in HSE efficiency is attributed to an effective steric factor, f(eff), which was evaluated by comparing the results with the Stokes-Einstein-Perrin prediction or with pDT, and it is equal to 0.49 +/- 0.03, independent of temperature. The unpaired spin density in CSL is localized near one end of the long molecule, so the exchange integral, J, leading to HSE, is expected to be large in some collisions and small in others; thus, J is modeled by an ideal distribution of values of J = J(0) with probability f and J = 0 with probability (1 - f). Because of rotational and translation diffusion during contact and between re-encounters of the probe, the effective steric factor is predicted to be f(eff) = f(1/2). Estimating the fraction of the surface of CSL with rich spin density yields a theoretical estimate of f(eff) = 0.59 +/- 0.08, in satisfactory agreement with experiment. HSE is well described by simple hydrodynamic theory, with only a small dependence on solvent-probe relative sizes at the same value of T/eta, where eta is the viscosity of the solvent. This result is probably due to a fortuitous interplay between long- and short-range effects that describe diffusion processes over relatively large distances. In contrast, dipole-dipole interactions (DD) as measured by the line broadening, B-dip, and the mean time between re-encounters within the cage, tau(RE), vary significantly with the solvent-probe size ratio at the same value of T/eta. For these phenomena, dominated by short-range diffusion, the reciprocal fractional free volume V-0/V-f provides a better description of the diffusion. Thus, B-dip and tau(RE) form common curves when plotted vs V-0/V-f. As a result, the fractional broadening by DD occurs at an order of magnitude higher values of T/eta for CSL compared with pDT.