4822
J. Phys. Chem. 1982, 86, 4822-4825
Electron Spin Resonance Characterization of the Dynamical Properties of Urea-n -Alkane Adducts Using Peroxy Spin Probes Darbha Suryanarayana, Walee Chamulltrat, and Larry Kevan’ Department of Chemistry, University of Houston, Houston, Texas 77004 (Received April 22, 1982; I n Final Form: July 19, 1982)
The motional effects in urea adducts of n-alkanes, of chain size C12H26 to C24Hm, have been investigated by using peroxy spin probes employing electron spin resonance (ESR). Temperature variations in the ESR spectra of the respective peroxy radicals of urea-n-alkanes were measured in the temperature range from 77 to 300 K. The ESR spectra at low temperatures (-100 K) reveal an anisotropic g tensor, gll = 2.037 and g, = 2.005, whereas, at higher temperatures (-200 K), the g tensor is averaged to a new set of values, g,,r= 2.002 and gLT = 2.021. The isotropic g value is constant for the entire temperature range. At intermediate temperatures the ESR spectra reveal “extra features”between the gll and g, regions which are characteristicof peroxy radical motional effects. The spectral dependence with temperature has been successfully simulated by using a chain-axis rotational model with 120° jumps for the entire temperature range employing the method of modified Bloch equations. The peroxy motion in urea-n-alkane adducts gives the following activation energies for the threefold ~ HE,~ ~= 10.4 kJ/mol for urea-n-CzOHlz. These values show the barrier: E, = 6.8 kJ/mol for ~ r e a - n - C ~and effect of chain length on the freedom of motion.
Introduction The adducts of long-chain alkanes including polyethylene in urea have been extensively studied by a variety of technique^.'-^ Urea crystallizes with hexagonal cylinders in which “guest” long-chain molecules are entrapped with their longitudinal axes parallel to the c axis of the urea crystalline This is shown schematically in Figure 1. The motional properties of urea adducts of n-alkanes have been studied by dielectric relaxation,’ nuclear magnetic resonance (NMR),4,a,gand electron spin resonance (ESR).5~6~12a The results suggest that the guest molecules fit only loosely in the cylindrical cavities of urea and can undergo a large degree of molecular motion about their longitudinal axes. Since the urea channel has sixfold symmetry (see Figure l ) , a sixfold potential barrier for molecular motion is expected for the guest molecules. Hori et al. have reported motional effects of peroxy radicals (R-O-0) in urea-polyethylene complex (UPEC)G“ using ESR. Schlick and Kevan12a have analyzed the spectral line shapes vs. temperature in terms of either a C-0 bond rotation or a chain-axis rotation; the limited data did not allow a clear distinction between these two motional models. In a later study Hori et aLeb studied single-crystal ESR spectra of urea-n-tetracosane adduct and concluded that chain-axis motion dominates. However, they were not able to determine the specific motional mechanism in terms of a large jump angle or a diffusive(1) 0. Redlich, C. M. Gable, A. K. Dunlop, and R. W. Millar, J . am. Chem. Soc., 72,4153 (1950). (2) K. Monobe and F. Yokoyama, J . Macromol. Sci., Phys., B8, 277 (1973). (3) (a) R. C. Pemberton and N. G. Parsonage, Trans. Faraday SOC., 61, 2112 (1965); (b) ibid., 62, 553 (1966). (4) D. F. R. Gilson and C. A. McDowell, Mol. Phys., 4, 125 (1961). (5)0. H. Griffith, J. Chem. Phys., 41, 1093 (1964). (6) (a) Y. Hori, S. Shimada, and H. Kashiwabara, Polymer, 18, 1143 (1977); (b) J. Chem. Phys., 75, 1582 (1981). (7) J. D. Hoffman, ‘Molecular Relaxation Processes”, Vol. 20, The Chemical Society of London, London, 1966, p 47. (8) J. D. Bell and R. E. Richards, Trans. Faraday SOC. 66,2529 (1969). (9) K. Umemoto and S. S. Danyluk, J. Phys. Chem. 71, 3757 (1967). (10) A. E. Smith, J. Chem. Phys., 18, 150 (1950). (11) Y. Chantani, Y. Taki, and H. Tadokoro, Acta Crystallogr., Sect. B , 33, 309 (1977). (12) (a) S. Schlick and L. Kevan, J. Am. Chem. Soc. 102,4622 (1980); (b) J. Phys. Chem. 83, 3424 (1979); (c) J . Chem. Phys., 72, 784 (1980). 0022-3654/82/2086-4822$01.25/0
type motion. In this investigation, we have studied peroxy radical motion in a series of urea adducts with four even n-alkanes n‘C24H50, n-C20H42, n-ClaH,s and n-C12H26.ESR spectra were recorded in these systems as a function of temperature. The peroxy radical motional effects were analyzed by using various specific motional models developed earlier.12-14 This analysis identifies chain-axis rotation with 120”jumps as the motional mechanism for peroxy radicals in urea-n-alkane adducts. It has also been possible to determine the energy of activation for this rotational motion for different alkane chain lengths. Experimental Section Commercial n-alkanes and powdered urea were obtained from Aldrich Chemical Co. and were used without purification. The urea-n-alkane complexes were obtained as f o l l ~ w s . ~ JA~ saturated urea solution is prepared by dissolving 5 g of urea in 20 mL of methanol. To this 1g of n-alkane is added, the mixture is warmed to 65 “C, and 2-propanol is added until the solution becomes clear. Upon slow cooling to a temperature slightly above the melting point of the n-alkane, crystals of the complex separate from the solution. The complex is filtered and dried. Powdered samples of the complex were ?-irradiated at room temperature in air. A typical dose of about 10 Mrd was used from a 6oCoGammacell-220 from Atomic Energy of Canada, Ltd. The ESR spectra were recorded from 77 to 300 K with a Varian ESR spectrometer operated at 9.2 GHz. The temperatures were measured to *2 “C by using a copper-constantan thermocouple with a digital readout. Computer simulations of the ESR spectra were performed on a Honeywell 66/60 computer and the spectra were plotted by using a Tektronix 4052 minicomputer with a 4662 digital plotter. Results Alkyl Radical Spectra. The radicals formed by y irradiation of urea-n-alkane adducts have been character(13) D. Suryanarayana, L. Kevan, and s. Schlick, J . Am. Chem. SOC.,
109, 668 (1982).
(14) D. Suryanarayana and L. Kevan, J. Phys. Chem. 86,2042 (1982). (15)C. McAdie, Can. J. Chem. 40, 2195 (1962).
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86,No. 24,
Dynamical Properties of Urea-n -Alkane Adducts
PEROXY RADICAL: UREA-n- EICOSANE 9.2GHz
n - A L K A N E / UREA COMPLEX
214K 196K
- 8 2 3 0 8 4
(a) I1 c-axis
(b) Ic-axis
Flgure 1. Schematic model of complexes of n-alkanes in the hexagonal structure of urea (a) viewed along the long c axls of the nalkane and (b) viewed perpendicular to the c axis. ALKYL RADICAL UREA-n-EICOSANE 9.2GHz
Flgure 3. ESR spectra at 9.2 GHz of the peroxy radical in urea-neicosane complexes measured at varlous temperatures. These ESR spectra were recorded at a microwave power of about 60 mW where the alkyl radical features are saturated.
ficient to show up the peroxy radical. When the microwave power is increased to -60 mW, the alkyl radical spectrum of Figure 2 saturates completely, leaving an observable peroxy radical spectrum shown in Figure 3. In order to explore the motional effects, we have recorded the ESR spectra of both the alkyl radical and the peroxy radical as a function of temperature. From Figure 2 one sees that the alkyl radical spectrum at 298 K shows narrower line widths as compared to the 77 K spectrum. This suggests the presence of motion in the urea-n-alkane complexes. However, the ESR spectra between 77 and 298 K do not reveal specific changes that can be interpreted as, for example, changes in the @-protonhyperfine constants. If the alkyl radical undergoes rotation ‘about the -C-C-C axis, the relative positions of the atoms with respect to the unpaired electron are fixed, causing negligible changes in the constituent nuclear hyperfine interactions. Also, if the proton hyperfine tensors are axially symmetric with their principal axes parallel to the alkyl chain axis, chain rotation will not affect the spectral shape much. Peroxy Radical Spectra. Upon examining the peroxy radical I1 ESR spectra shown in Figure 3, one sees that there is strong variation of the spectral line shapes with temperature. The ESR spectrum at 118 K and below exhibits g anisotropy, with the principal values of the g tensor measured as g, = 2.037, gr = 2.008, and g, = 2.002. These values were obtained from computer simulation of the ESR spectrum. These principal g values in this “rigid” limit case are close to those known for peroxy radicals.12-14 The largest principal g value (2.037) of the peroxy radical is known to be along the -0-0direction whereas the smallest value (2.002) is in the orbital of the terminal oxygen where the p z orbital is assumed to be parallel to the molecular chain axis.
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The Journal of Physical Chemistty, Vol. 86, No. 24, 7982 PEROXY RADICAL: UREA-n- TETRACOSANE 9.2 GHr
Suryanarayana et al. PEROXY RADICAL : UREA-n- DODECANE 9.2 GHz
188K 198K 178K 193K
I67 K I58 K
187K 183K
149K
178K
131 K
177K 121 K 163K 145K
IlOK
133K
121 K
v Figure 4. 9.2-GHz ESR spectra of the peroxy radical in urea-n-tetramsane complex measured as a function of temperature. Microwave power of about 60 mW was employed to saturate the alkyl radical features.
The ESR spectra of radical 11 measured at temperatures higher than 120 K reveal characteristic motional effects of the peroxy radical. Above 196 K the ESR spectrum reveals only partial g averaging due to motional effects. The new g values of the ESR spectrum at 196 K and above are measured as g,,I= 2.002 and gLr= 2.021. Upon comparison of the g values of the spectra at 118 and 196 K, one sees that g,1’ g,’
E
g,
k!, + g , ) / 2
where the superscript r indicates room temperature. The above agreement between the g values shows that the principal g value kZ), which is assumed to be parallel to the molecular chain axis, is unaltered by temperature variation. However, the g, and gy values are averaged to the g,’ value. This supports the interpretation that the molecular chains of n-eicosane rotate freely in the urea hexagonal cavities about their longitudinal axis. In order to investigate the generality of motional effects for n-alkanes doped in the urea lattice, we have produced similar peroxy radicals with different carbon chain sizes, such as urea-n-tetracosane as well as urea-n-dodecane complexes. These two complexes also show similar alkyl radical ESR spectra as shown in Figure 2. The corresponding peroxy radical spectra and their temperature variations are given in Figures 4 and 5, respectively. These two figures show practically the same g-value averaging that is seen in Figure 2. This implies that these n-alkane-urea complexes have the same motional dynamics. However, the temperature at which a given ESR spectrum is observed in an alkane-urea complex differs with the alkane chain length. In the urea-n-eicosane peroxy radical spectra, critical motional averaging occurs in the higher
Figure 5. 9.2-GHz ESR spectra of the peroxy radical in urea-ndodecane complex measured as a function of temperature. Microwave power of about 60 mW was employed to saturate the alkyl radical features.
temperature range of 164-179 K (see Figure 3). However, in the urea-n-dodecane complex critical motional averaging of the peroxy radical spectrum occurs between 131 and 158 K. Simulated Spectra. In order to interpret the temperature variations in the ESR spectra of the peroxy radicals, we have performed computer simulations using the modified Bloch equations incorporating the motional models developed earlier.12 The theoretical spectra have been calculated as a function of the jump correlation time, T,, which was varied between the “rigid” (7,N 1.0 ps) and the “fast” ( T N 0.0001 ps) limiting cases in appropriate intervals. In our previous papers, we have shown typical peroxy radical simulated spectra for the following motional models: (a) cubic-jump, (b) C-0 bond rotation with 90° and 120° jumps, and (c) chain-axis rotation with 90’ and 120’ jump^.'^-'^ Since the peroxy radicals in the present systems have a similar g tensor at 77 K, the simulated spectra using the above motional models are practically the same. The best fit to the present data occurs for chain-axis rotation with 120’ jumps which is shown in Figure 6. Comparison between the experimental spectra in Figures 3-5 and the simulated spectra in Figure 6 shows excellent agreement. Note that the two”extra” features, visible as weak shoulders on the g,, and g, components of the experimental spectra near the rigid limit, are very well reproduced in the simulated spectra using T , values of 0.1-0.03 ps. The other motional models given above yield quite different line shapes and do not fit the present data.
Discussion An important result of this investigation is that the midchain peroxy radical in urea-n-alkane complexes reveals partial g-tensor averaging near room temperature. Earlier we observed similar g-tensor averaging for the midchain peroxy radical in poly(tetrafluoroethy1ene) (PTFE).I3 In PTFE the ESR spectral variations with temperature show a single extra feature between the glland g , regions and a chain-axis motion with 90’ jumps was
The Journal of Physical Chemistry, Vol. 86, No. 24, 1982 4825
Dynamical Properties of Urea-n -Alkane Adducts PEROXY RADICAL CHAIN AXiS ROTATiON I2OoJUMPS
PEROXY RADICAL MOTION
0 UREA-n-EICOSANE 0 UREA-n- DODECANE -9
-
-7
-
-1
0.001
- 5L
0.004
I
0.006
0.008
0 01
I I T (K-' 1 Figure 7. Arrhenius plot of peroxy radical motion in urea-n-alkane complexes. The points are experimental and the lines are leastsquares fits.
v Flgure 6. Computer-simulated ESR spectra of peroxy radical motion using a chalnaxls rotation model with three 120' jumps. These spectra were calculated by using the "rigid" limit principal g values: g , = 2.034, g2 = 2.007, and g3 = 2.003 obtained from the 77 K ESR spectrum. The llnswidth parameter used in these simulations is 13 MHz.
predicted.', In the case of urea-n-alkane complexes two extra features are observed in the ESR spectra of the midchain peroxy radical. In urea-n-tetracosane, these features are clearly resolved in the 110-130 K spectra (see Figure 4) even at 9.0-GHz frequencies. These ESR spectra are fitted by using chain-axis rotation with 120' jumps. Thus,the simulation model can clearly distinguish between two- and three-jump mechanisms for the chain-axis rotation model. The 120' jump model deduced for the urean-alkane complexes is clearly consistent with the symmetry of the hexagonal channels in urea. Nuclear magnetic r e s o n a n ~ e and * ~ ~dielectric relaxation' results also suggest chain-axis motions in these complexes. For organic radicals terminating with CH3groups, both threefold (V,) and sixfold (v6)potential barriers have been reported.l& Recently, Suryanarayana and Sevilla16bhave explored the barriers in small organic systems such as acetic acid radicals, CH3-C(XlXz) using intermediate neglect of differential overlap (INDO) calculations with different X1and Xz side groups. Their results show that (16) (a) D. Suryanarayana and M. D. Sevilla, J.Phys. Chem., 83,1323 (1979); (b) zbzd., 84,3045 (1980).
acetic acid radicals prefer mostly pyramidal geometries where the CH3group exhibits a threefold potential barrier. Furthermore, the v6 barrier is found to be much smaller than the V3 barrier. Thus, in the hexagonal urea channels the experimental deduction of a threefold barrier seems reasonable. For n-alkane adducts in urea channels, Personage and Pemberton" have reported potential energy calculations on urea-alkane interactions and for their set I1 parameters have obtained sixfold potential barriers of 4.1, 5.1,5.7, and 7.7 kJ/mol for Cl0, CI2, c16, and Cz0carbon chain sizes. Smaller barriers are obtained for their set I parameters. However, for peroxy or alkyl radicals similar detailed calculations are not available. Experimental barrier heights from our spectral analyses are determined by Arrhenius plots of the kinetics of the peroxy radical motion. Results for urea-n-eicosane and urea-n-dodecane are given in Figure 7 along with the values of the activation energies. The activation energy for urea-n-tetracosane is nearly the same as that for urea-n-eicosane. These activation energies of 6.8 and 10.4 kJ/mol for the Clz and Cmcarbon chains are larger than the corresponding calculated values of 5.1 and 7.7 kJ/mol although the agreement is not that bad. This probably suggests that the peroxy substituent on the n-alkane peroxy radicals hinders the chain-axis motion in the urea channels. It is also noteworthy that the activation energy does increase with alkyl chain length up to about Cm This also supports the chain-axis rotation motional mechanism. Acknowledgment. This research was partially supported by the Army Research Office. (17) N.G.Parsonage and R. c. Pemberton, Trans. Faraday SOC.,63, 311 (1967).
