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J . Phys. Chem. 1988, 92, 3630-3635
Anisotropic Rotational Reorientation of 2,2,6,6-Tetramethylpiperidine-l-oxyl in Anion-Exchange Resins: An EPR Line Shape Study J. S. Hwang,* W. A. Al-Rashid,+ and M. M. Saleem Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (Received: February 6 , 1987; In Final Form: November 4 , 1987)
Spin-probe investigations of Dowex 44324 and Dowex 44339 ion-exchange resins were carried out by electron paramagnetic resonance (EPR) spectroscopy as a function of temperature from 77 to 429 K. The spin-probe used in this study was 2,2,6,6-tetramethylpiperidine- 1-oxy1 (Tempo), and both resins were strong basic anion-exchange resins with quaternary ammonium type structure. The magnetic parameters of Tempo in the resins were determined and found to be typical of systems with hydrogen bonds. Variable-temperature EPR line shape analysis in the temperature range of 148-429 K revealed that Tempo was undergoing anisotropic rotational diffusion along the z ' = Yaxis with N = 15 f 1 at the higher temperature range (326-429 K) in both resins, while N = 2.7 at the lower temperature range (148-247 K). The decrease in N is a result of freezing out of the rotational degrees of freedom at lower temperatures. The rotational correlation times in the lower temperature s, indicating that the probe was highly immobilized. The activation energies range varied from 2.3 X lo-* to 1.0 X in Dowex 44324 and 44339 over the temperature range studied were 16.5 f 0.3 and 13.8 f 0.3 kJ/mol, respectively. The anisotropic interaction parameter K , which measures the coupling of rotation to translation, was found to be 0.02. The magnitude of the activation energies and the smallness of K are typical for spin-probes rotating in a clathrate-type environment. The EPR line shapes of Tempo in Dowex 44324 (mesh 20-50) and Dowex 44339 (mesh 200-400) were virtually identical, indicating that no surface effects were observed in this range of resin size.
Introduction The rotationid correlation times for a neutral spin-probe in the aqueous interior of a series of ion-exchange resins have been investigated at room temperature by Chestnut and Hower.' The experimental results from these studies showed that the correlation times increase with increasing cross-linking up to and including 8 and 10% cross-linking. In addition, it has been observed that the correlation times obtained from line width coefficient of M , the nuclear spin quantum number, and that of @, do not agree. This disagreement is probably due to anisotropic motion of the nitroxide spin-probe.2 Hence the study of ion-exchange resins by nitroxide as spin-probe should include anisotropic The nitroxide radical6 has paramagnetic properties due to the existence of an unpaired electron. A number of factors contribute to the popularity of these radicals: for example, many nitroxides can be easily synthesized7~* and are remarkably stable because the nitroxide group is protected by four bulky methyl groups which prevent disproportionation reactions, label and probe experiments have been conducted successfully at temperatures up to 440 K, they are soluble in a variety of solvents, nitroxides carrying a variety of functional groups to facilitate labeling are commercially available,' and they have well-defined g tensors and hyperfine tensors to the 14N nucleus. The last property is a fundamental requirement for obtaining quantitative dynamic information from a line width analysis of the EPR spectrum. There are two techniques related to the nitroxide radicals; the spin-labeling and spin-probe techniques.10-12 Spin-labeling is a technique which involves covalent bonding of different nitroxide radicals to diamagnetic polymers. Line width measurement of the EPR spectra of covalently bound nitroxide spin-label may give information on the mobility of macromolecules. The spin-probe technique is a method whereby different nitroxide radicals are mixed with the diamagnetic host (Le., unbound), or in our case ion-exchange resins (which do not contain unpaired electrons and do not give EPR spectra), and their tumbling behavior during relaxation and transition process in the diamagnetic host is studied. The spectral changes are the result of restrictions in motional freedom imposed upon the probe by its microenvironment. In the spin-probe technique the nitroxide radicals are present in the diamagnetic host at concentrations of 10-100 ppm. Nitroxide spin-probe studies have been performed in liquids and frozen *Author to whom correspondence should be addressed. 'Present address, Arabian American Oil Company, Box 2696, Dhahran 31311, Saudi Arabia.
0022-365418812092-3630$01.50/0
media,4~s~'3 and water and carbon t e t r a ~ h l o r i d e ,jojoba '~ oil,2 poiy(phenyla~etylene),~~ epoxy resin,16 poly(viny1 alcohol) gels," liquid crystals,'* micelle^,'^ phospholipids or synthetic vesicles,20 ion-exchange resins,' and zeolites.2' In this study two Dowex ion-exchange resins, Dowex 44324 with mesh 20-50 and Dowex 44339 with mesh 200-400 were inves-
(1) Chesnut, D. B.; Hower, J. F. J . Phys. Chem. 1971, 75, 907. (2) Hwang, J. S.;Pollet, P.; Saleem, M. M. J . Chem. Phys. 1986, 84, 577. (3) Li, A. S. W.; Hwang, J. S. J . Phys. Chem. 1985, 89, 2556. (4) Goldman, S. A.; Bruno, G. V.; Freed, J . H. J . Chem. Phys. 1972, 56.
716. (5) Hwang, J. S.; Mason, R. P.; Hwang, L. P.; Freed, J. H. J . Phys. Chem. 1975, 79, 489. (6) Rozantsev, E. G. Free Nitroxyl Radicals; Plenum: New York, 1970. (7) Keena, J. F. W. In Spin Labeling 11: Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1979; pp 115-172. (8) Gaffney, B. J. In Spin Labeling: Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1979; pp 183-238. (9) Molecular Probes, Inc. 24750 Lawrence Road, Junction City, OR 97448 U.S.A. (IO) Likhtenshtein, G. I. Spin Labeling Methods in Molecular Biology; Wiley: New York, 1976. (1 1) Berliner, L. J., Ed. Spin Labeling: Theory and Applications; Academic: New York, 1976. (12) Berliner, L. J., Ed. Spin Labeling II Theory and Applications; Academic: New York, 1979. ( 1 3 ) Goldman, S . A.; Bruno, G . V.; Freed, J. H. J . Chem. Phys. 1973,59, 307 1. (14) Windle, J. J. J . Magn. Reson. 1981, 45, 432. (15) (a) Hwang, J. S.; Tsonis, C. P. Macromolecules 1983, 16, 736. (b) Hwang, J. S.;Saleem, M. M.; Tsonis, C. P. Macromolecules 1985, 18, 2051. (16) Brown, I. M.; Sandreczki, T. C. Macromolecules 1985, 18, 2702. (17) Watanabe, T.; Yahagi, T.; Fujiwara, S. J . A m . Chem. SOC.1980, 102, 5187. (18) (a) Broido, M. S.; Belsky, I.; Meirovitch, E. J. Phys. Chem. 1982, 86, 4197. (b) Broido, M. S.; Meirovitch, E. J . Phys. Chem. 1983, 87, 1635. (19) (a) Spague, E. D.; Duecker, D. C.; Larrabee, C. E., Jr. J. A m . Chem. Soc. 1981, 103, 6797. (b) Pyter, R. A,; Ramachandran, C.; Mukerjee, P. J . Phys. Chem. 1982, 86, 3206. (c) Mukerjee, P. In Solution Chemistry of Surfactant; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1, p 153. (d) Ottaviani, M. F.; Baglioni, P.; Martini, G . J . Phys. Chem. 1983, 87, 3146. (e) Isshiki, S.; Uzu, Y. Bull. Chem. SOC.Jpn. 1981, 54, 3205. (f) Lasic, D. D.; Hauser, H . J . Phys. Chem. 1985, 89, 2649. (20) (a) Tanaka, H.; Freed, J. H. J . Phys. Chem. 1984, 88, 6633. (b) Kusumi, A.; Singh, M.; Tirell, Oehme, G.;Singh, A,; Samuel, N.K. P.; Hyde, J. S.; Regen, S. L. J . Am. Chem. SOC.1983, 105, 2975. (c) Lim, Y . Y . ; Fendler, J. H . J . A m . Chem. SOC.1979, 101, 4023. (d) Polnaszek, C. F.; Schreier, S.;Butler, K. W.; Smith, I. C. P. J . Am. Chem. SOC.1978, 100, 8223. (e) Schreier, S.;Polnaszek, C. F.; Smith, I. C. P. Biochim. Biophys. Acta 1978, 515, 375. (21) Eastman, M. P.; Gonzalez, J . A. J . Phys. Chem. 1985, 89. 488.
Q 1988 American Chemical Societv
EPR of Tempo in Anion-Exchange Resins tigated by using 2,2,6,6-tetramethylpiperidine- 1-oxy1 (Tempo) as the spin-probe. We chose Tempo as spin-probe because it is soluble in aqueous medium, because it has been employed in many earlier studies described above and in an earlier study of ion-exchange resin,] because it gives a three-line spectrum in the fast-motional region and its line width contributions can be calculated with the theory of Kubo and Tomita as applied by Kivelson22 or with Redfield relaxation matrix theory as applied by Freed and Fraenkel,23and because its spectrum in the slow-motional region can be analyzed and simulated by using the slow-tumbling theory of Freed.24 The slow-tumbling theory allows for the anisotropic rotational reorientation of Tempo in the resins. From a simulation of the EPR spectrum of the nitroxide spin-probe one can calculate the rotational correlation times of the spin-probe. Since the rotational correlation time is a direct measure of the motional freedom of the probe, we are able to discuss the environment inside the resin interior as seen by the probe.
Experimental Section The Dowex resins were purchased from Fluka AG, Buchs, Switzerland, in chloride form as 2C-50 mesh beads no. 44324 and 200-400 mesh beads no. 44339. Dowex 1-X8 is a strong basic anion exchanger. The active group in Dowex 1-X8 is trimethylbenzylammonium ion with standard cross-linkage of 8% by weight of divinylbenzene and special cross-linkage of 1% by weight of divinylbenzene. The average pore diameter is about 23.5 A for the ion exchange containing 8% divinylben~ene.~~ The nitroxide spin probe, 2,2,6,6-tetramethylpiperidine- 1-oxy1 (Tempo) was purchased from Molecular Probes, Junction City, OR. The resins were placed in columns and washed with methanol to remove impurities. After observing a colorless effluent, the resins were washed with deionized water, removed from the columns, and blotted dry. A sample of 0.25 g of the damp resin was allowed to equilibrate with 30 mL of M nitroxide aqueous solution. The equilibrium period was 3 days. After the equilibration period, the resins were removed from the solution and carefully washed with deionized water to remove any excess Tempo on the resin surface. The resins were then blotted dry and kept in samples of 2-mm i.d. by 3-mm-0.d. Pyrex tubing. The sample tube was adapted to 9-mm 0.d. at the open end to fit into the quick disconnect fitting (Seal structure, Sargent-Welch part no. S-76639-A, size 24/40) on the Pope vacuum manifold. The sample tube was immersed in liquid nitrogen and the valve opened to the vacuum system very slowly and carefully to prevent the resins from escaping. After several freeze-pumpthaw cycles, the sample tube was sealed at a point not more than 1 cm above the surface of the resins. As the Fluka catalog numbers for the two resins above are 44324 and 44339, respectively, and the Molecular Probes catalog number for Tempo is 500, we have given the notations 24/500 and 39/500 respectively to the two resins after doping them with Tempo. The EPR spectra were recorded over a range of temperatures from 77 to 429 K with a Varian E-109 spectrometer interfaced with an E-935 data acquisition system at the X-band frequency (9.1 13 GHz) and a modulation frequency of 100 kHz. The spectra were centered in a magnetic field of 3.2512 kG and had a scan range of 100 G. Magnetic field sweep was calibrated with a Varian E-500-2 self-tracking N M R gaussmeter. Microwave frequencies were measured with a Hewlett-Packard 5342A microwave frequency counter. The temperature was controlled by a Varian E-257 variable temperature unit to within f0.5 OC. Spectrometer scan speeds and time constants were chosen so as not to introduce any artifacts from scanning. Results and Discussion Figure 1 shows the two experimental spectra of Tempo in the two Dowex ion-exchange resins at 77 K. The rigid-limit (Le., (22) Kivelson, D. J . Chem. Phys. 1960, 33, 1094. (23) (a) Freed, J. H.; Fraenkel, G. K. J. Chem. Phys. 1963,39, 326. ( b ) Freed, J. H.; Fraenkel, G. K. J . Chem. Phys. 1964, 40, 1815. (24) Freed, J. H. In Spin Labeling: Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1976; Vol. 1, pp 53-132. (25) Seno, M.; Yamabe, T. Bull. Chem. SOC.Jpn. 1964, 37, 754.
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3631
Figure 1. Rigid-limit 77 K ESR spectra of Tempo in two resins denoted a s (a) Dowex 44324 and (b) Dowex 44339.
absence of fast rotation and a random spatial distribution of the axes of symmetry of the probe) spectrum is important because it can yield information on the magnetic parameters needed for an accurate analysis of the rotational anisotropy and the slowtumbling spectrum. The importance of accurate values of the magnetic parameters determined individually from the same system have been demonstrated in a careful study of PD-Tempone in toluene and glycerol-waters in which PD-Tempone is found to be rotating isotropically in both solvents. If we suppose the magnetic parameters of PD-Tempone had not been measured in glycerol-water solvent and we used the magnetic parameters of PD-Tempone obtained in toluene solvent to analyze the line widths of PD-Tempone in glycerol-water, then we would have obtained N, the anisotropy of molecular reorientation of 3.6, with the fastest rotation about the molecular Y axis, instead of the isotropic result obtained with the correct magnetic parameters. N is the ratio RII/R,, where Rllis the rotational diffusion constant along the molecular Z axis and R , is the rotational diffusion constant along the X and Y axes. The details of the analysis of the rigid-limit spectrum were given elsewhere.26 Briefly, the parameters A, and g, can be determined directly from the separation between the outer hyperfine extrema which is 2 4 , while g, corresponds to the midpoint of the two extrema. The remaining parameters, g,, gy, A,, and A,, and the line width are varied until the line shape of the central portion is simulated. The rigid-limit simulations were done according to the general methods of Lefebre and M a r ~ a n adapted i ~ ~ to nitroxide by Polnaszek.28 The simulations employ Simpson’s numerical integration over 0 in 85 intervals and over cp in 40 intervals. In all cases Lorentzian line shape gave the best overall fit. The reason that the center of the spectrum has a Lorentzian shape is perhaps a result of some residual motion at 77 K, and therefore the condition for the validity of a rigid limit IH,(f)TR/hl
