Spin Exchange Rate Constants Research Articles

Electron Spin-Spin Interaction and Translational Diffusion of Nitroxide Radicals in Sodium Dodecyl Sulfate Micelles

Abstract The line width measurements of ESR spectra of 2,2,6,6-tetramethyl-4-benzoyloxypiperidinyl-1-oxyl (BzONO) in sodium dodecyl sulfate (SDS) micelles have been carried out at various concentrations of BzONO. The plot of the line width against the concentration of BzONO shows a line with a break at mole fraction ca. 0.05, as opposed to the theoretical prediction that the plot would give a straight line. In order to explain this anomalous behavior, three models for the solubilization of BzONO are considered. On the basis of two of these models, spin exchange rates of BzONO in the micelles are estimated. Translational diffusion coefficients of BzONO in the SDS micelles at 20, 31, and 38.5 °C are calculated by using the spin exchange rate constants and an appropriate diffusion model. The obtained values are of the order of 10−7 cm2/s.

Electron spin exchange in isoviscous solvent mixtures

Electron spin exchange rate constants and hyperfine coupling constants have been measured for two nitroxide spin probes in a number of isoviscous mixed solvents. Collision rate constants, normalized to 1 cP, are lower for solvents in which the major component is water. Further anomalies in the coupling constant for the systemtert-butanol-water are explained in terms of the marked concentration fluctuations known to occur in this solvent mixture. Substitution of a hydrophobic spin probe by one containing an −OH group leads to lowering of the spin exchange rate, possibly due to solvation of the probe.

Study of the dynamic interactions of macromolecules in concentrated polymer solutions using spin labelling

The intermolecular spin exchange rate constants and the rotational diffusion correlation times have been measured for spin labels (side groups of poly(glycidyl methacrylate)) in concentrated polymer solutions ( c p ⩽ 0.50 g/ml). The spin exchange was measured with average spacings of ∼30Å between the labels. It is shown that the rate of translational and rotational mobility of the labels at c p ⩽ 0.25 g/ml decreases in thermodynamically “good” solvents by a factor of ∼1.6 as the polymer concentration rises. On this basis it is proposed that in concentrated polymer solutions the “freely permeating coils” model (average degree of polymerization of the polymer P=690 ) should hold up to c p=0.25 g/ml. An analysis of the results obtained and of data in the literature leads to the tentative proposal of a threshold polymer concentration in thermodynamically “good” solvents at which solutions may become “insensitive” to their quality (at the iso point).

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

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.

The Dependence of the Electron-transfer Rates of Aromatic Hydrocarbon Anion Radicals upon the Molecular Radii of the Reactants

Abstract The applicability of the theory of R. A. Marcus is tested, the behavior of the rate constant being examined as a function of the molecular radii of the reactants. The rate constants of the electron-transfer reactions from the anion radicals of pyrene, perylene, and tetracene to their respective parent molecules are measured under experimental conditions in which the anion radicals are in a free ion or loose ion-pair state. The rate constants thus obtained and subsequently corrected for diffusion are 2.8×109, 3.0×109, and 2.6×109 M−1sec−1 respectively, in 1,2-dimethoxyethane at 25°C. These rate constants, as well as others obtained elsewhere for naphthalene and anthracene, are compared with those predicted theoretically, assuming a spherical and an oblate spheroidal molecular shape. The consistence between the theoretical and experimental results is not satisfactory; the variation in the observed rate constants with the molecular radii of the reactants is much smaller than that expected theoretically. The spin-exchange rate constants for anion radicals of pyrene, perylene, and tetracene are also determined to be 7.0×109, 5.1×109, and 4.8×109 M−1sec−1 respectively, in the same solvent at 25°C.