Abstract
An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R ∥/R ⊥) whereR ∥ andR ⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10−3 in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.
Similar content being viewed by others
References
Ferruti P., Gill D., Harpold M.A., Klein M.P.: J. Chem. Phys.50, 4545–4550 (1969)
Morsy M.A., Oweimreen G.A., Hwang J.S.: J. Phys. Chem.100, 8331–8337 (1996)
Pudzianowski A.T., Stillman A.E., Schwartz R.N., Bales B.L., Lesin E.S.: Mol. Cryst. Liq. Cryst.34, 33–41 (1976)
Hwang J.S., Pollet P., Saleem M.: J. Chem. Phys.84, 577–583 (1986)
Goldman S.A., Bruno G.V., Freed J.H.: J. Phys. Chem.76, 1858–1860 (1972)
Smith P.M.: Eur. Polym. J.15, 147–152 (1974)
Hamada K., Iijima T., McGregor R.: Macromolecules19, 1443–1448 (1986)
Freed J.H., Bruno G.V., Polnaszek C.F.: J. Phys. Chem.75, 3385–3399 (1971)
Freed J.H. in: Spin Labeling: Theory and Applications (Berliner L.J., ed.), chapt. 3. New York: Academic Press 1976.
Oweimreen G.A., Martire D.E.: J. Chem. Phys.72, 2500–2510 (1980)
Oweimreen G.A., Hwang J.S.: Chem. Phys. Lett.334, 83–88 (2001)
Hwang J.S., Morsy M.A., Oweimreen G.A.: J. Phys. Chem.98, 9056–9062 (1994)
Li A.S.W., Hwang J.S.: J. Phys. Chem.89, 2556–2260 (1985)
Hwang J.S., Mason R., Hwang L.P., Freed J.H.: J. Phys. Chem.79, 489–511 (1979)
Luckhurst G.R. in: Liquid Crystals and Plastic Crystals (Gray G.W., Winsor P.A., eds.), vol. 2, chapt. 7. New York: Wiley 1974.
Kelker H., Hatz R.: Handbook of Liquid Crystals, chapt. 2, p. 48, Weinheim: Verlag Chemie 1980.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Hwang, J.S., Oweimreen, G.A. Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals. Appl. Magn. Reson. 26, 387–393 (2004). https://doi.org/10.1007/BF03166810
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF03166810