Heisenberg spin exchange and molecular diffusion in liquid crystals

Nayeem, Akbar; Rananavare, Shankar B.; Sastry, V. S. S.; Freed, Jack H.

doi:10.1063/1.457358

Cited by 52 publications

(84 citation statements)

References 41 publications

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Summarizing a great deal of work, a monograph 4 published in English in 1980 gathered a wealth of theoretical and experimental data and described the use of the spin-exchange technique to study problems in chemistry and biology. In a collision involving two radicals, the quantum mechanical probability for spin exchange is associated with an overlap of the orbitals of unpaired electrons on two probes.…”

Section: Introductionmentioning

confidence: 99%

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

Kurban

Perić

Bales

2008

The Journal of Chemical Physics

100

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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 ͑EPR͒ 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 ͑2003͔͒ 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 / , where is the shear viscosity. Unexpectedly, encounters are not dependent on the ratio = 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 becomes smaller. This is in qualitative agreement with the theory of microscopic diffusion of Hynes et al. ͓J. Chem. Phys. 70, 1456 ͑1979͔͒. 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.

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Section: Introductionmentioning

confidence: 99%

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

Kurban

Perić

Bales

2008

The Journal of Chemical Physics

100

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“…All of these effects are predicted by various theories, [2][3][4][5][6][7][8][9] and all of them have been observed experimentally. 3,7,[10][11][12][13][14][15][16][17][18][19] The original theory due to Kivelson 1 was based upon the general theory of motional narrowing due to Kubo and Tomita 21 and was able to predict the effects of spin exchange in the two limits of slow exchange and narrowing but could not describe the entire range. A rigorous theory covering the entire range was given by Currin 6 in 1962, and the modified Bloch equations were employed by Jones 3 the following year.…”

Section: Introductionmentioning

confidence: 99%

“…During these 37 years, much theoretical [2][3][4][5][6][7][8][9] and experimental 3,7,[10][11][12][13][14][15][16][17][18][19] work has been devoted to the EPR spectral changes induced by spin exchange between radicals in liquids. A monograph 4 published in English in 1980 gathered a wealth of theoretical and experimental data and described the use of the spin exchange technique to study problems in chemistry and biology.…”

Section: Introductionmentioning

confidence: 99%

EPR Line Shifts and Line Shape Changes Due to Spin Exchange of Nitroxide-Free Radicals in Liquids 4. Test of a Method to Measure Re-Encounter Rates in Liquids Employing ¹⁵N and ¹⁴N Nitroxide Spin Probes

et al. 2008

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An expression for the EPR line shape of a nitroxide free radical undergoing spin exchange in the slow exchange limit is derived and tested experimentally and against a rigorous theory. The line shape is the sum of the absorption and dispersion of Lorentzian lines, in which the dispersion component is of opposite signs for the outer lines and zero for the inner. The relative amplitude of the absorption and the dispersion is shown to be a linear function of the spin exchange frequency. Nonlinear least-squares fitting of the spectra allows the separation of the absorption and dispersion components determining their relative amplitudes to high precision yielding values of the spin exchange frequency that are of precision comparable to those derived from the more traditional line broadening. This new way to measure spin exchange frequencies is attractive because there are no complications from dipolar interactions and no measurements at low concentrations are required. The resonance fields of the absorption component of the outer lines shift toward the center of the spectrum as the square of the spin exchange frequency and the observed positions of the lines shift even further due to the absorption-dispersion overlap. The observed shift is also a quadratic function of the spin exchange frequency and involves the unbroadened line width. Both shifts may be used to measure the spin exchange frequency; however, only the observed shift yields values that are of precision comparable with broadening and dispersion-amplitude techniques. The predictions of the theory are tested experimentally using peroxylamine disulfonate in 50 mM K 2 CO 3 at 67°C. At this elevated temperature, the concentration of the radical decreases at a convenient rate making it possible to study a wide concentration range without disturbing the sample. The line broadening was linear with the concentration with a coefficient of correlation r ) 0.99996. The spin exchange frequency derived from the amplitude of the dispersion component was within less than 1% of that derived from line broadening and of comparable precision. Spin exchange frequencies derived from the shift of the lines were of lower precision yielding values of the spin exchange frequency about 2% lower than those found from line broadening.

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“…The theoretical description of kse has been thoroughly dealt with in literature [108,109]. relating it to the diffusional rate constant, kdiff, by…”

Section: Resultsmentioning

confidence: 99%

“…Spin exchange and its effect on the acquired ESR spectra is well understood, experimentally as well as theoretically [108][109][110][111][112][113][114][115]. A large amount work has been done using nitroxide radicals, e.g.…”

Section: Spin Exchange Reactionsmentioning

confidence: 99%