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Nitroxide spin exchange due to re-encounter collisions in a series of n-alkanes.

ABSTRACT: Bimolecular collisions between perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl molecules in three alkanes have been studied by measuring the electron paramagnetic resonance (EPR) spectral changes induced by spin exchange. We define an "encounter" to be a first-time collision followed by a series of re-encounters prior to the diffusing pair's escaping each other's presence. The present work stems from a recent proposal [B. L. Bales et al., J. Phys. Chem. A 107, 9086 (2003)] that an unexpected linear dependence of the spin-exchange-induced EPR line shifts on spin-exchange frequency can be explained by re-encounters of the same probe pair during one encounter. By employing nonlinear least-squares fitting, full use of the information available from the spectral changes allows us to study encounters and re-encounters separately. The encounter rate constants appear to be dominated by hydrodynamic forces, forming a common curve for hexane, decane, and hexadecane when plotted against T/eta, where eta is the shear viscosity. Unexpectedly, encounters are not dependent on the ratio mu = a/a(s), where a and a(s) are the van der Waals radii of the nitroxide probe and the solvent, respectively. It is argued that the near coincidence of the resulting encounter rate constant with the hydrodynamic prediction is likely due to a near cancellation of terms in the general diffusion coefficient. Thus, the semblance of hydrodynamic behavior is coincidental rather than intrinsic. In contrast, the mean times between re-encounters do depend on the relative sizes of probe and solvent. For hexane at lower temperatures, the Stokes-Einstein equation apparently describes re-encounters well; however, at higher temperatures and for decane and hexadecane, departures from the hydrodynamic prediction become larger as mu becomes smaller. This is in qualitative agreement with the theory of microscopic diffusion of Hynes et al. [J. Chem. Phys. 70, 1456 (1979)]. These departures are well correlated with the free volume available in the solvent; thus, the mean times between re-encounters form a common curve when plotted versus the free volume. Because free volume is manifested macroscopically by the isothermal compressibility, it is expected and observed that the re-encounter rate also forms a common curve across all three solvents when plotted with respect to compressibility. The existence of a common curve for alkanes raises the prospect of using EPR to determine the compressibility of substances such as fossil fuels and biological membranes.

SUBMITTER: Kurban MR

PROVIDER: S-EPMC2674375 | biostudies-other | 2008 Aug

REPOSITORIES: biostudies-other

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Project description:EPR spectra of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (PDT) are studied as functions of molar concentration, c, and temperature, T, in water and 70 wt % glycerol in water. The increase of the intrinsic line width averaged over the three hyperfine lines, B(tot), varies linearly with c with zero intercept in both solvents at all temperatures; therefore dB(tot)/dc is independent of c. The spin exchange induced dispersion, from which the spin exchange frequency, omega(e), may be computed, increases linearly with B(tot), passing through the origin in water and in 70% glycerol at high temperatures; however, at low temperatures, where dipolar interactions broaden the spectra, linearity does not prevail until B(tot) > 1 G due to a contribution of dipolar interactions to the dispersion. The broadening constant due to spin exchange, dB(e)/dc, is found from the slope of the linear region, permitting a computation of the dipolar constant, dB(dip)/dc = dB(tot)/dc - dB(e)/dc. Thus, the separation of concentration broadening into spin exchange and dipolar contributions is effected without having to appeal to some supposed temperature dependence of the two interactions. The fractional broadening by spin exchange, Omega(T), is near unity at high temperatures in both solvents, decreasing to zero in 70% glycerol at 273 K. Omega(T) is a continuous function of the inverse rotational correlation time of PDT but is discontinuous as a function of T/eta where eta is the shear viscosity. Omega(T) = 0.5, where spin exchange and dipolar interactions contribute equally to the line width occurs at T/eta = 20 +/- 1 K/cP in 70% glycerol. Hydrodynamic predictions of dB(e)/dc via the Stokes-Einstein (SE) equation are remarkably accurate in 70% glycerol comparable with the results in a series of alkanes. In water, dB(e)/dc is linear with T/eta with zero intercept as required by the SE; however, with slope a factor of 0.73 smaller. dB(dip)/dc is reasonably predicted by the SE only at very small values of eta/T very quickly following an approximately logarithmic dependence rather that the linear prediction. Values of dB(dip)/dc approach a plateau above eta/T = 0.20 cP/K that is about one-half the solid state limit. Line shifts due to spin exchange are not yet useful to deduce values of Omega(T) due to a lack of knowledge of the time between re-encounters; however, they may be used to verify the values determined from line broadening and spin exchange induced dispersion. Some effects at low temperatures in 70% glycerol suggest that the effects of dipolar interaction are inadequately described by the widely accepted theory.

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Nitroxide spin exchange due to re-encounter collisions in a series of n-alkanes.

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