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- Creator:
- Meyer, Michelle D., Peric, Miroslav, Smith, Steven A., and Bales, Barney L.
- Description:
- EPR line shifts due to spin exchange of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (14N−PDT) in aqueous solutions and the same probe isotopically substituted with 15N (15N−PDT) were measured from 293 to 338 and 287 to 353 K, respectively. Nonlinear least-squares fits of the EPR spectra yielded the resonance fields of the nitrogen hyperfine lines to high precision from which the shifts were deduced. The shifts are described by two terms: one linear and the other quadratic in the electron spin-exchange frequency, ωe. The quadratic term is due to spin exchange that occurs when two spin probes diffuse together and collide. A linear term is predicted for spin exchanges that occur upon re-encounter of the same two probes while they occupy the same “cage” before diffusing apart. The quadratic term has no adjustable parameters, while the linear term has one: the mean time between re-encounters, τRE. The theory is cast in terms of the spin-exchange-induced line broadening that can be measured from each spectrum independently of the line shifts, thereby removing the explicit dependence of ωe on the temperature and the spin-probe concentration. In this form, theoretically, the value of the linear term is about a factor of 2 larger for 15N−PDT than for 14N−PDT for all temperatures; however, τRE must be the same. Experimentally, we find that both of these expectations are fulfilled, providing strong support that the linear term is indeed due to re-encounter collisions. Values of τRE derived from 14N−PDT and 15N−PDT are of the same order of magnitude and show the same trend with temperature as a hydrodynamic estimate based on the Stokes−Einstein equation.
- Resource Type:
- Article
- Identifier:
- 1089-5639
- Campus Tesim:
- Northridge
- Creator:
- Peric, Miroslav, Smith, Steven A., Schwartz, Robert N., and Bales, Barney L.
- Description:
- The sensitivity of the 14N-hyperfine splitting of nitroxide spin to the local electric field, Eloc, has often been used to probe the polarity of microenvironments; however, two uncertainties have been faced at once as follows: (i) the value of Eloc in a complex microenvironment and (ii) the effect of Eloc on the hyperfine splitting. In this paper, we study nitroxide free radicals that may be charged (positively or negatively) by varying the pH. The additional charge produces an additional Eloc, whose value we may calculate with confidence, allowing us to focus attention on the effect that Eloc has on the hyperfine splitting. A simple theory is developed that provides a link between the shift in the 14N-hyperfine splitting, ΔAN, and the local electric field, Eloc. To test this approach, we have calculated the ΔAN for several charged nitroxide spin probes and found good agreement (both magnitude and sign) with the measured values using ΔAN = −3.8 × 10-8Eloc (ΔAN in Gauss and Eloc in V/cm), where Eloc is the component of Eloc directed along the N−O bond from the nitrogen to the oxygen.
- Resource Type:
- Article
- Identifier:
- 1089-5647
- Campus Tesim:
- Northridge
- Creator:
- Meyer, Michelle D., Peric, Miroslav, Smith, Steven A., and Bales, Barney L.
- 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, ⟨Btot⟩, varies linearly with c with zero intercept in both solvents at all temperatures; therefore d⟨Btot⟩/dc is independent of c. The spin exchange induced dispersion, from which the spin exchange frequency, ωe, may be computed, increases linearly with ⟨Btot⟩, 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 ⟨Btot⟩ > 1 G due to a contribution of dipolar interactions to the dispersion. The broadening constant due to spin exchange, d⟨Be⟩/dc, is found from the slope of the linear region, permitting a computation of the dipolar constant, d⟨Bdip⟩/dc = d⟨Btot⟩/dc − d⟨Be⟩/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, Ω(T), is near unity at high temperatures in both solvents, decreasing to zero in 70% glycerol at 273 K. Ω(T) is a continuous function of the inverse rotational correlation time of PDT but is discontinuous as a function of T/η where η is the shear viscosity. Ω(T) = 0.5, where spin exchange and dipolar interactions contribute equally to the line width occurs at T/η = 20 ± 1 K/cP in 70% glycerol. Hydrodynamic predictions of d⟨Be⟩/dc via the Stokes−Einstein (SE) equation are remarkably accurate in 70% glycerol comparable with the results in a series of alkanes. In water, d⟨Be⟩/dc is linear with T/η with zero intercept as required by the SE; however, with slope a factor of 0.73 smaller. d⟨Bdip⟩/dc is reasonably predicted by the SE only at very small values of η/T very quickly following an approximately logarithmic dependence rather that the linear prediction. Values of d⟨Bdip⟩/dc approach a plateau above η/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 Ω(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.
- Resource Type:
- Article
- Identifier:
- 1089-5639
- Campus Tesim:
- Northridge
- Creator:
- Peric, Miroslav, Smith, Steven A., Schwartz, Robert N., and Bales, Barney L.
- Description:
- The sensitivity of the 14N-hyperfine splitting of nitroxide spin to the local electric field, Eloc, has often been used to probe the polarity of microenvironments; however, two uncertainties have been faced at once as follows: (i) the value of Eloc in a complex microenvironment and (ii) the effect of Eloc on the hyperfine splitting. In this paper, we study nitroxide free radicals that may be charged (positively or negatively) by varying the pH. The additional charge produces an additional Eloc, whose value we may calculate with confidence, allowing us to focus attention on the effect that Eloc has on the hyperfine splitting. A simple theory is developed that provides a link between the shift in the 14N-hyperfine splitting, ΔAN, and the local electric field, Eloc. To test this approach, we have calculated the ΔAN for several charged nitroxide spin probes and found good agreement (both magnitude and sign) with the measured values using ΔAN = −3.8 × 10-8Eloc (ΔAN in Gauss and Eloc in V/cm), where Eloc is the component of Eloc directed along the N−O bond from the nitrogen to the oxygen.
- Resource Type:
- Article
- Identifier:
- 1520-6106
- Campus Tesim:
- Northridge
- Creator:
- Meyer, Michelle D., Peric, Miroslav, Smith, Steven A., and Bales, Barney L.
- Description:
- EPR line shifts due to spin exchange of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (14N−PDT) in aqueous solutions and the same probe isotopically substituted with 15N (15N−PDT) were measured from 293 to 338 and 287 to 353 K, respectively. Nonlinear least-squares fits of the EPR spectra yielded the resonance fields of the nitrogen hyperfine lines to high precision from which the shifts were deduced. The shifts are described by two terms: one linear and the other quadratic in the electron spin-exchange frequency, ωe. The quadratic term is due to spin exchange that occurs when two spin probes diffuse together and collide. A linear term is predicted for spin exchanges that occur upon re-encounter of the same two probes while they occupy the same “cage” before diffusing apart. The quadratic term has no adjustable parameters, while the linear term has one: the mean time between re-encounters, τRE. The theory is cast in terms of the spin-exchange-induced line broadening that can be measured from each spectrum independently of the line shifts, thereby removing the explicit dependence of ωe on the temperature and the spin-probe concentration. In this form, theoretically, the value of the linear term is about a factor of 2 larger for 15N−PDT than for 14N−PDT for all temperatures; however, τRE must be the same. Experimentally, we find that both of these expectations are fulfilled, providing strong support that the linear term is indeed due to re-encounter collisions. Values of τRE derived from 14N−PDT and 15N−PDT are of the same order of magnitude and show the same trend with temperature as a hydrodynamic estimate based on the Stokes−Einstein equation.
- Resource Type:
- Article
- Identifier:
- 1089-5639
- Campus Tesim:
- Northridge