J. Phys. Chem. 1990, 94, 949-953
949
Paramagnetic Relaxation of Radical Species in y-Irradiated Glassy Ethanol Solution of Tetracyanoethylene Anion Radical. Role of Spectral Diffusion in Cross Relaxation Tsuneki Ichikawa* and Hiroshi Yoshida Faculty of Engineering, Hokkaido University, Sapporo, 060 Japan (Received: May 2, 1989)
The mechanism of paramagnetic relaxation for hydroxyethyl radicals and chemically prepared tetracyanoethylene anion radicals in y-irradiated solid ethanol has been studied at 77 K by an electron spin echo method. Cross relaxation between anion radicals and hydroxyethyl radicals is much faster than that between anion radicals themselves. The cross relaxation between the anion radicals and hydroxyethyl radicals is induced by spectral diffusion of off-resonant hydroxyethyl radicals into the on-resonant spectral region of the anion radicals, which in turn is induced by the time fluctuation of the hyperfine fields due to the rotation of methyl groups on the hydroxyethyl radicals. Spectral diffusion by rotation of the methyl group is so fast that only hydroxyethyl radicals with methyl proton spin states of ‘I2, ‘I2) are observed on the electron spin echo detected ESR spectrum. The theoretical cross relaxation rate of the anion radicals agrees with the observed one.
Introduction Due to the recent development of pulsed microwave techniques, pulsed ESR techniques such as the electron spin echo (ESE) method have become popular for studying the nature of paramagnetic species. Since paramagnetic relaxation directly affects the shape and intensity of an ESE signal, it is important in ESE spectroscopy to know the detailed mechanism of paramagnetic relaxation. Paramagnetic relaxation is the recovery of electron spin states after the excitation by microwave to the thermal equilibrium states and is divided into transverse and longitudinal relaxations. Transverse relaxation is the dephasing of the precession motion of on-resonant spin packets that is induced both by the static distribution of the precession frequencies and by the time fluctuation of the local magnetic field of each spin packet. The static distribution of the precession frequencies determines the relative shape of an ESE signal, whereas the time fluctuation of the local field determines the intensity of the ESE signal. The relaxation induced by the latter process is called “transverse relaxation of one spin packet” or “phase relaxation”. In the present study, we use “transverse relaxation” to be synonymous with transverse relaxation of one spin packet because the rate of transverse relaxation thus defined is independent of the intensity and the width of the microwave pulses used for the ESE measurement. On the other hand, the rate of transverse relaxation induced by the static distribution of the precession frequencies depends on the number of the spin packets and is therefore strongly dependent on the experimental condition of the ESE measurement. Longitudinal relaxation is the recovery of electron spin energies at an on-resonant spectral position from the perturbed distribution by microwave to the thermal equilibrium distribution and is induced either by the change of the spin states of on-resonant spins or by the diffusion of on-resonant spins to the off-resonant spectral region and the concomitant diffusion of off-resonant spins into the on-resonant spectral region. Deposition of radiation energy in molecular substances causes the pairwise formation of reactive free radicals. We have recently shown’-3 that the intrapair distance in irradiated solids can be determined by analyzing the cross relaxation rate of generated free radicals that is measured by means of an ESE method. Cross relaxation is the transfer of magnetic energy between on-resonant and off-resonant radicals through magnetic dipolar interactions. Since the cross relaxation rate is highly dependent on the distance between radicals, by extracting the rate of cross relaxation by the paired radical from the entire longitudinal relaxation rate, it is possible to determine the intrapair distance. ~
During the course of these studies, we have found that the cross relaxations between tetracyanoethylene anion radicals and the solvent radicals of ethanol and between biphenyl anion radicals and the solvent radicals of 2-methyltetrahydrofuran are much faster than that between the anion radicals themselves. The cross relaxation rate generally increases with increasing spectral overlapping between on-resonant spins and off-resonant spins. Since the continuous wave (CW) ESR spectra of the anion radicals are much sharper than those of the solvent radicals, at first glance, the cross relaxation rate between the anion radicals seems faster. However, this is not the case. Very recently, we found that the rotation of the methyl group adjacent to the unpaired electron of an alkyl radical causes fast fluctuation of the hyperfine field and, accordingly, fast paramagnetic r e l a x a t i ~ n . ~This . ~ suggests that the cross relaxation between the anion radicals and the solvent radicals is induced by the rotation of methyl groups, because the solvent radicals of 2-methyltetrahydrofuran and ethanol have methyl groups adjacent to the unpaired electrons. The present study is aimed at elucidating the mechanism of fast cross relaxation between tetracyanoethylene anion radicals and hydroxyethyl radicals in solid ethanol.
Experimental Section Analytical-grade ethanol was dried over magnesium amalgam in vacuo, vacuum distilled, and then degassed by several freezepump-thaw cycles. Tetracyanoethylene was purified by sublimation in vacuo. Tetracyanoethylene anion radical (TCNE’-) was prepared by the reduction of tetracyanoethylene with lithium iodide.6 Ethanol solutions of TCNE’- radical were sealed under vacuum into high-purity quartz tubes and then plunged into liquid nitrogen to obtain glasslike samples. A neat ethanol sample without TCNE‘- radical was also prepared. These samples were irradiated at 77 K with 6oCoy-rays at a dose rate of 1 Mrad/h. The samples were then illuminated at 77 K with visible light for converting all the trapped electrons to hydroxyethyl radicals.’ The concentration of hydroxyethyl radicals was calculated from the absorbed dose with use of the G value (number of product molecules per 100 eV radiation energy absorbed) of 5.6.’ The ESR spectra were recorded at 77 K on a Varian Model E-109 X-band spectrometer with an incident microwave of 0.1 mW. The ESE measurements were made at 77 K on a homemade spectrometer with a two-pulse 9Oo-rl 80’ sequence.8 Transverse
~~
( 1 ) Ichikawa, T.; Yoshida, H. J . Phys. Chem. 1984, 88, 3199. (2) Ichikawa, T.; Wakasugi, S.; Yoshida, H. J . Phys. Chem. 1985, 89, 3583. (3) Ichikawa, T.; Kawahara, S.;Yoshida, H. Radiat. Phys. Chem. 1985, 26. 73 I .
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(4) Ichikawa, T. J . Phys. Chem. 1988, 92, 1431. (5) Ichikawa, T.; Yoshida, H. J . Phys. Chem. 1988, 92, 5684. (6) Webster, 0.W.; Mahler, W.; Benson, R. E. J . Am. Chem. SOC.1962, 84, 3678. (7) Shida, T.; Imamura, M . J . Phys. Chem. 1974, 78, 232.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
Ichikawa and Yoshida
1
CW ESR
h
Figure 1. CW and 1,-dependentESR spectra of tetracyanoethylene anion radical in glassy ethanol. The arrow indicates the spectral position for relaxation measurement. Figure 3. CW and t,-dependent ESR spectra of hydroxyethyl radical in
y-irradiated glassy ethanol.
Figure 2. Transverse relaxation curve of tetracyanoethylene anion radical in y-irradiated glassy ethanol. The concentration of hydroxyethyl radical is (-) 0, (- -) 2, (-.) 4, 6 , and 8 mmol/dm'. The concentration of tetracyanoethylene anion radical is fixed at 2 mmol/dm3. (-e-)
(-e.-)
relaxation was measured by recording the ESE intensity as a function of time t2 = 27. For observation of longitudinal relaxation, the total magnetization of on-resonant spins was first detected by the two-pulse method with a fixed T of 0.5 p s . The ESE intensity thus detected is equal to the amount of transversally unrelaxed spins at t 2 = 1 p s after the 90" pulse. After time to, which was enough for complete relaxation of the spin system, a 90" saturation pulse was applied to saturate the spins. Then, after time t , , the total magnetization was detected by the two-pulse method. The difference in the ESE intensity with and without the saturation pulse, E , ( t l ) , is the amount of the longitudinally unrelaxed spins at t , after the saturation pulse. An ESE-detected ESR spectrum was obtained by detecting the E , ( t , ) component as the external magnetic field was swept. The spectrum thus obtained will be designated as a t,-dependent ESR spectrum. The microwave pulses with the widths of 120 ns-120 ns-240 ns and the incident power of 7.5 W were used throughout the experiment. The pulse width of more than 120 ns is wide enough to suppress the nuclear modulation by protons. Results and Discussion Paramagnetic Relaxation of Tetracyanoethylene Anion Radical. Figure 1 compares the C W and tl-dependent ESR spectra of tetracyanoethylene anion radicals (TCNP-) before y-irradiation. The ?,-dependent spectrum at tl = 0 ms is actually the t2-dependent ESR spectrum at t2 = I ps. The t,-dependent ESR spectra are not the same as the C W ESR spectrum and show better spectral resolution. Figure 2 shows the ESE envelope decay of TCNE'- recorded at the peak of the ESR spectrum. The shallow nuclear modulation arises from nuclear quadrupole in-
teractions with 14N n ~ c l e i . Kevan ~ et al. have reported that an ESE-detected ESR spectrum differs from the corresponding C W ESR spectrum when deep nuclear modulation is present, because the change of the external magnetic field causes the change of the modulation frequency and therefore the change of the ESE amplitude.1° The modulation depth for TCNE'- radicals is, however, too shallow to cause the distortion of the ESR spectrum. The distortion on the t,-dependent ESR spectra arises from the field-dependent or, more strictly, spectral position-dependent relaxation rate of TCNE'- radicals. The resolution of the t,dependent spectra is better than the CW ESR spectrum because paramagnetic relaxation is slower near the peak of each hyperfine line. Paramagnetic relaxation is generally slower near the peak of each hyperfine line when the spectral broadening of the hyperfine line is caused by the time fluctuation of the hyperfine field due to the thermal motion of the paramagnetic specie^.^ As is seen in Figure 1, the relative intensity of the central hyperfine component corresponding to the I4N nuclear spin state of (0, 0, 0,O) increases with increasing t,, because the thermal motion of the I4N nuclei with m, = 0 does not cause the fluctuation of the hyperfine field. The rate of longitudinal relaxation does not depend on the concentration of TCNE' radicals up to 2 mmol/dm3, which indicates that the cross relaxation between TCNE'- radicals is negligibly slow. Paramagnetic Relaxation of Hydroxyethyl Radicals. Figure 3 compares the CW and t,-dependent ESR spectra of hydroxyethyl radicals (R') in y-irradiated neat ethanol. The relative shape of the t2-dependent spectrum is scarcely changed with t2 and is the same as that of the t,-dependent spectrum at tl = 0 ps (tZ= 1 ps). The tl-dependent spectrum is totally different from the C W spectrum. The CW spectrum is composed of five equispaced lines with the hyperfine coupling constant (hfcc) of 2.2 mT, which indicates that the methyl groups on R' radicals are quickly rotated and the four protons (three P protons and an a proton) give approximately the same hfcc. The t,-dependent spectrum is composed of a doublet-doublet with the hyperfine coupling constants of 6.6 and 1.9 mT, respectively. A weak signal at the center of the spectrum is due to the color center of the y-irradiated quartz tube. The isotropic hfcc of R' radicals in liquid are 1.54 mT ( a proton) and 2.22 mT (three methyl p protons)." Since the splitting of 1.9 mT is close to the isotropic hfcc of the a proton, it is concluded that the hyperfine splitting of 1.9 mT is due to the isotropic and anisotropic interactions by the a proton. Because of the reason hereinafter mentioned, the hfcc of 6.6 mT is assigned as due to two equivalent methyl protons with the nuclear spin states As is well-known, the hfcc of a of (I/*, I / * ) and (9) Ichikawa, T. J . Chem. Phys. 1985, 83, 3790.
(8) Ichikawa, T. J . Magn. Reson. 1986, 70. 280
(IO) Goldfarb, D.; Kevan, L. J . Mugn. Reson. 1988, 76, 276. ( 1 1) Livingston, R.; Zerdes, H . J . Chem. Phys. 1966, 44, 1245.
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 951
Cross Relaxation in y-Irradiated TCNE'-
'.
3
6 9 CONC./m m ~ k d m - ~ Figure 5. Longitudinal relaxation rate of tetracyanoethyleneanion radicals plotted as a function of the concentration of hydroxyethyl radical.
0
5, the rate constant b of the longitudinal relaxation linearly increases with the increasing concentration of R' radicals, C,as b = 0.029
+ 0.012C
(3)
where C is in mmol/dm3 and t l is in microseconds. The linear concentration dependency indicates that the cross relaxation between TCNE'- radicals and the surrounding R' radicals accelerates the longitudinal relaxation. On the other hand, as shown in Figure 2, the transverse relaxation of T C N P - radicals is less affected by R' radicals. The transverse relaxation of TCNE'radicals is mainly caused by the fluctuation of the hyperfine and superhyperfine fields due to the thermal motion of TCNE'radicals and the surrounding ethanol molecules and to the temperature-independent nuclear spin flip-flop of ethanol protons. The efficient cross relaxation between TCNE'- and R' radicals might be induced by the spectral diffusion of off-resonant R' radicals into the on-resonant spectral region of the TCNE'radicals. The diffused R' radicals with a low spin temperature accept the excess energy of the TCNE'- radicals by resonant energy transfer and then move out of the on-resonant spectral region. The excess energy carried out by the R' radicals is dissipated to other radicals through the cross relaxation, so that the spin temperature of the R' radicals moving into the on-resonant spectral region is always cold enough to lower the spin temperature of the TCNE'- radicals very quickly. On the other hand, the cross relaxation between TCNE'- radicals is slow because the rate of spectral diffusion is much slower than that of R' radicals. The above assumption can be justified by comparing the observed cross relaxation rate with the theoretical one. The rate of cross relaxation between two electron spins by flip-flop-type resonant energy transfer is given byI4 k(7) = ( i ~ / 1 6 ) r ~ h ~ { (31COS2 o)/r3l2SA(W) = ( ~ / 1 6 ) y ~ h ~- (3 (cos2 1 O)/r3)2SA(Ij)
(4)
where y is the magnetogyric ratio, IF1 = r is the distance from the energy donor spin to the acceptor spin, 0 is the angle that the vector i: makes with the static magnetic field, w is the angular velocity of spin precession, and S,(H) is the spectral intensity of the acceptor at the magnetic field H o f the on-resonant donor spin. The apparent spectral intensity of R' radicals at the on-resonant field is obtained by normalizing the observed CW ESR spectrum with the entire concentration of R', as S(H) = 0.20/mT
(5)
When the energy donor spin is surrounded by the number M of (12) Heller, C.; McConnell, H. M . J . Chem. Phys. 1960, 32, 1535. (13) Miyagawa, I.; Itoh, K . J . Chem. Phys. 1962, 36, 2157.
(14) Bowman, M. K.; Norris, J. R. J . Phys. Chem. 1982, 86, 3385.
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The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
acceptor spins, the cross relaxation rate is expressed by
Ichikawa and Yoshida and
M
k r = xk(Fi) i= 1
where Pi is the location of the ith acceptor spin. The cross relaxation kinetics is expressed by E(ti) = exp(-krti)
(7)
The R' radicals are known to be formed pairwise by y-irraThe diation with the average intrapair distance of 4.5 cross relaxation kinetics of TCNE'- radicals including the spatial correlation between R' radicals and their pairs is given by averaging eq 7 for all possible locations of R' radicals, as m
U t l ) = exp{-0.5nf(tI)l
n = 6N/16
(6)
"
9
I
where N is the entire number density of R' radicals. On the other hand, if we assume that the contribution of the 1/2)8(1/2)a]radicals to the C W ESR intensity of R' radicals at the on-resonant position is the same as that of the 1 / 2 ) s ( - ' / 2 ) u ] radicals, we obtain
S*(H) = (16/12)S(H)
(14)
5
(8)
and
n = 12N/16
(15)
Substituting eq 12 and 13 or eq 14 and 15 into eq 10 or 11, we obtain
where f(t,) =
I[
1 - exp{-k(?)t,]
j+(P-P') exp{-k(P')tl) df'] dP (9)
n is the number density of R' radicals acting as energy acceptors and +(F.Tq is the probability that a R' radical at P finds the paired
E,(t,) = e~p(-O.0147Ct~'/~)
(16)
as the fastest cross relaxation, and E,(t,) = e ~ p ( - 0 . 0 0 7 3 C t ~ l ~ ~ )
(17)
as the slowest cross relaxation. The observed cross relaxation of e ~ p ( - 0 . 0 1 2 C t ~ ~lies / ~ between ) the above two functions, which certifies that the cross relaxation is induced by the spectral diffusion of R' radicals into the on-resonant spectral region of TCNE'- radicals followed by resonant energy transfer between the R' and the TCNE'- radicals. The cross relaxation between on-resonant TCNE'- radicals is E,(?,) = e ~ p [ - ( 4 ~ ~ h y ~ ~ ~ n / 3 ~ ~ ~ ) ( S , ( H (10) ) t l J ' ~ ~neglected ] in the present analysis. Although the cross relaxation between on-resonant T C N F - radicals does not cause longitudinal On the other hand, when the bulk concentration is much lower relaxation under the presence of no R' radical, if the cross rethan 18 mmol/dm3 and the average distance between the onlaxation rate between on-resonant T C N F - radicals is comparable resonant TCNE' and R' radicals is much longer than the intrapair to that between T C N P - and R' radicals, it affects the longitudinal distance, the TCNE'- radical regards the paired radicals as two relaxation through the cross relaxation with R' radicals. Suppose radicals located at the same position. The relaxation kinetics is there are two groups of on-resonant TCNE- radicals in the system; then given by One group cross-relaxes faster with R' radicals than the other group so that the spin temperature decreases faster. Then the E,(t,) = e~p[-(2a~hy~/~n/3~/~)(2S~(H)r,1]/~] (1 1) excess energy of the latter group is transferred to R' radicals through the cross relaxation with the former TCNE'- radicals The a- and &proton spin states of R' radicals with rotating followed by the cross relaxation between the former TCNE'methyl groups are expressed by 1/2),J(k1/2)a (total radicals and R' radicals. In this case, the exponent o f t , becomes weight = 4/16), f [ ( ' / 2 , -'/2, '/2),d-1/2)a1 (total weight = 6/16) larger than 1 / 2 , and the cross relaxation rate depends on the and f [ ( l / 2 , I/Z),&I/~)~] (total weight = 6/16). The 0-proton concentration of TCNE'- radicals. In the present study, the cross spin states of -1!2)8 and 1/2)8 are equivalent relaxation rate between TCNE'- and R' radicals is found to be 1/2)8. As IS shown in Figure 3, the to independent on the concentration of TCNE'- radicals up to 2 (fl/2)a radicals cannot diffuse into the on-resonant spectral mmol/dm3, so that the cross relaxation between T C N F - radicals position at the center of the ESR spectrum. On the other hand, may not play an important role in the longitudinal relaxation. 1/2)p(-1/2)a]radicals can diffuse into the onthe Under the present experimental condition, we observe the resonant spectral position. The hyperfine field of, for example, longitudinal relaxation of on-resonant TCNE'- radicals that are 1/2)8(-1/2)a radicals changes from (6.6 - 1.9)/2 the transversally unrelaxed within t 2 = 1 ps. Since the transverse to -1.9/2 mT following the change of the @proton spin state to relaxation rate of TCNE'- radicals is not much affected by R' 1/2)&-1/2)a radicals with the initial I/*),+ The radicals, the effect of transverse relaxation on the observed spectral location at 2.35 mT from the spectral center therefore longitudinal relaxation curve is neglected in the present analysis. pass through the on-resonant spectral position and then go into The effect of transverse relaxation cannot be neglected if the the spectral position of -0.95 mT. magnetic interaction between T C N F - and R' radicals significantly It is difficult to estimate the contribution of the contributes to the transverse relaxation of TCNE'- radicals. In 1 / 2 ) a ( ' / 2 ) a ]radicals to the cross relaxation. Due to the rotation such a case, the TCNE'- radicals with higher transverse relaxation of the methyl /3 protons, the spectral position of these radicals rates also have higher cross relaxation rates, so that a part of fluctuate between -0.95 and -4.25 mT and between 0.95 and 4.25 TCNE'- radicals with higher cross relaxation rates cannot be mT. If the rotation of the methyl groups does not induce the detected by the ESE method. The observed cross relaxation rate change of the a-proton spin states and/or the width of these at shorter t , then becomes lower than the theoretical value. f0.95-mT lines is not enough to cover the on-resonant spectral position, only the f [ ( l / 2 , 1 / 2 ) 8 ( - 1 / 2 ) a ]radicals contribute Concluding Remarks to the cross relaxation. The spectral intensity and the concentration of the R' radicals acting as energy acceptors are then given The ESE-detected ESR spectrum of paramagnetic species is significantly affected by spectral diffusion or time fluctuation of by the local magnetic field, so that the spectral shape is generally = ('%)s(H) (12) different from the corresponding CW ESR spectrum. An extreme case is the spectrum of organic radicals with rotating methyl @ protons, on which only the radicals with the @-protonspin states ( 1 5 ) The intrapair distances given in Table I of ref 1 are a little too long. of are observed. Spectral diffusion also opens an The correct values can be obtained by multiplying the distances by a factor of 0.9. efficient path for the cross relaxation between on-resonant and radical at P'. The spatial correlation does not affect much when the average interpair distance is much shorter than 9 nm or the bulk concentration of R' is much higher than 18 mmol/dm3. In this case, the TCNE'- radical is regarded to be surrounded by randomly distributed R' radicals and the relaxation kinetics is given by
953
J . Phys. Chem. 1990, 94, 953-958 off-resonant electron spins. Paramagnetic species with high spectral diffusion rates act as an energy carrier or spin coolant for on-resonant spins. Acknowledgment. This work was supported in part by a Grant-in-Aid for Developmental Scientific Research from the
Japanese Ministry of Education, Science and Culture under Contract No. 62840012. Registry No. TCNE (radical anion), 345 12-48-6; TCNE, 670-54-2; ethanol, 64-17-5; lithium iodide, 10377-51-2; hydroxyethyl radical, 2348-46-1.
Nuclear Relaxation and Fractal Structure of a Cross-Linked Polymer J.-P. Korb,* B. Sapoval, Laboratoire de Physique de la Matibre Condensee, U.R.A. 1254 du Centre National de la Recherche Scientifque, Ecole Polytechnique, 91 128 Palaiseau, France
C . Chachaty, and A. M. Tistchenko CEAIIRDI, Dgpartement d'gtude des lasers et de la Physico-Chimie, DLPCIBP 121, CEN de Saclay, 91 191 Gif sur Yvette, France (Received: February 13, 1989; In Final Form: May 26, 1989)
We have investigated the longitudinal 2H and 13Crelaxations of methanol and nitromethane adsorbed on poly(4-vinylpyridine) (P4VP) cross-linked by paramagnetic V 0 2 + and Cuz+ ions as well as by diamagnetic Cd2+ ion. The aim of the proposed method is to gain dynamical as well as structural information on a disordered organic system. Electron spin resonance, scanning electron microscopy, and small-angle X-ray scattering give complementary information on the rigid network (resin) whose nodes are the paramagnetic ions. The nonexponential decay of the longitudinal nuclear magnetization observed in paramagnetic systems has been modeled, following a theory of Mendelson, by a superposition of exponential decays associated to a fractal distribution of quasi disconnected spherical pores. The power law -t3+f found in this theory at short time has allowed a determination of the fractal dimension dl of the surface as well as the surface contribution of the spin-lattice relaxation rate. The frequency and temperature dependences of the surface relaxation of methanol are explained by a reorientation of the adsorbed molecule in the coordination sphere of the paramagnetic ion. The bulk relaxation is due to a translational diffusion of the solvent relaxed by a distribution of paramagnetic probes.
1. Introduction Nuclear proton relaxation has been recently used for the measurements of pore size distribution',2 and permeability3 in various fully or partially saturated inorganic porous media. These studies generally imply that the proton relaxation inside a pore is an average of bulk and surface contributions, the latter being dependent on the surface to volume ratio. As a consequence the pore size dispersity results in a distribution of spin-lattice relaxation rates that can be potentially extracted from the longitudinal magnetization decay.4 In principle the proton relaxation may be used to study organic materials like polymer resins. This would be useful in gaining an insight into their possible structure. However this technique is not well suited owing to the difficulty in separating properly signals and relaxation due to the solvent and the polymer. This difficulty is not encountered in the 13Cor 2H relaxation of small molecules diffusing in such matrices. But the relaxation of these nuclei is essentially intramolecular and governed by rotational motions which do not give any information on the structure of the matrix. A simple possibility is then to incorporate some paramagnetic probes in the matrix. In that case an intermolecular contribution to the relaxation occurs, which is dependent on the translational diffusion through the porous solid and thus provides information on the topology of the matrix. Such systems can be obtained with a polymer whose the functional groups are able to give stable complexes with divalent or tervalent ions in order to form three-dimensional cross-linked networks. This is the case of poly(4-vinylpyridine) (P4VP) whose copper 11 complexes have been subjected to several works dealing with its catalytic activity in redox In this work, we have investigated the 2H and I3C relaxations of methanol and nitromethane adsorbed on P4VP cross-linked by
* Author
to whom correspondence should be addressed.
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paramagnetic VOz+and Cu2+ions as well as by diamagnetic Cd2+ ion. The latter samples were used to give an estimate of the contribution of relaxation mechanisms other than paramagnetic. As the samples cross-linked by V 0 2 + and Cu2+ yield nearly the same results, we report mainly the experiments with V 0 2 + . The choice of the two solvents has been motivated by their similar sizes and their very different behaviors toward divalent ions.
2. Experimental Methods The cross-linked polymer, denoted hereafter as resin, was prepared from 0.5 M (in monomer unit) solutions in methanol or ethanol of a 90% 4-vinylpyridine-l0% styrene copolymer from Aldrich Chemicals. The degree of polymerization of this copolymer is 6000 as determined by light scattering. Solutions 0.01-0.1 M of VOS04, 5 H z 0 , CuCI2, or CdCI, in the same solvents were slowly poured under stirring in an equivalent volume of polymer solution. After an induction period of a few tens of seconds, a compact gel is formed from which the solvent is partially expelled by syneresis. This gel was isolated by filtering on a sintered glass and was washed repetitively to remove the noncross-linked polymer; it was then dried overnight under vacuum until a constant weight was obtained. The properties of the resins were found to be independent of the solvent (methanol or ethanol) used in their preparation. In particular they are insoluble in the usual solvents of P4VP and not swollen by them. The concen(1) Lipsicas, M.; Banavar, J. R.;Willemsen, J. Appl. Phys. Lett. 1986,48, 1544.
( 2 ) Gallegos, D. P.; Smith, D. M.; Brinker,
C. J. J . Colloid Interface Sci.
1988, 124, 186. (3) Banavar, J. R.; Schwartz, L. M. Phys. Reu. Lett. 1987, 58, 1411. (4) Mendelson, K. S.; Phys. Reu. B 1986, 34, 6503. (5) Tsuchida, E.; Nishide, H.; Nishiyana, T. J . Polym. Sci. 1974,47C, 35. (6) Nishide, H.; Tsuchida, T. Makromol. Chem. 1976, 177, 2295. (7) Kirsh, Yu. E.; Kovner, V. Ya.; Kokorin, A. I.; Zamaraev, K. I.; Chernyak, V. Ya.; Kabanov, V . A. Eur. Polym. J . 1974, 10, 671.
0 1990 American Chemical Society
