3982
J. Phys. Chem. 1984, 88, 3982-3986
The Triplet State of 2,2’-Biquinoxaline Investigated through ODMR Line-Shape Simulations K. Vinodgopal, Stephen H. Fleischman, and Willem R. Leenstra* Department of Chemistry, University of Vermont, Burlington, Vermont 05405 (Received: November 28, 1983; In Final Form: March 7, 1984)
The lowest excited triplet state of 2,2’-biquinoxaline has been investigated by low-temperature phosphorescenceand zero-field optically detected magnetic resonance spectroscopy (ODMR) at C2 K in the single-crystal matrix of benzoic acid and in the pseudocrystalline Shpolskii matrix of n-decane. The phosphorescence spectrum in both matrices is well resolved and can be readily analyzed by using the characteristic Raman vibrations of similar molecules. The ODMR studies reveal two close-lying excited triplet states, which are unresolved in the optical spectrum. These two states can be distinguished on the basis of their associated hyperfine and quadrupolar splittings, arising from the presence of the 14Nnuclei in the molecule. A computer simulation of the 2E ODMR transitions in the two different triplet states in each matrix shows reasonable agreement with experimental line-shape patterns. The results of this analysis also reveal differences in the magnitudes of the spin Hamiltonian parameters, which indicate that one of the triplet states behaves as a localized quinoxaline-like state, whereas the other triplet state exhibits a delocalized character, with electron spin density distributed evenly over both halves of the molecule.
Introduction The double molecule, 2,2’-biquinoxaline (Figure 1) possesses a lowest photoexcited triplet state that warrants investigation for several reasons. Excited-state properties of dimeric systems are of general interest with ramifications in, for example, photosynthesis.’ More specifically, however, the nature of the triplet state of dimers is a field of study being actively p u r s ~ e d . ~For , ~ example, several interesting features have recently been observed in the triplet states of the similar molecule, 2,2’-biq~inoline.~We have chosen to investigate 2,2’-biquinoxaline as it presents an opportunity to analyze quadrupolar splittings and hyperfine couplings of four 14N nuclei, superimposed on an electronic triplet-state zero-field splitting (ZFS). Consequently, we have examined in detail the lowest triplet state of 2,2’-biquinoxaline on the basis of its low-temperature phosphorescence and zero-field optically detected magnetic resonance (ODMR) spectra in the pseudocrystalline Shpolskii matrix, ndecane, and as a guest molecule doped into single crystals of benzoic acid (BZA). This study involves a comparison of the experimentally obtained ODMR line shape with computer simulations via a numerical diagonalization of the triplet-state spin Hamiltonian matrix. It will be shown that such a comparison enables us to determine the hyperfine coupling constants of the triplet states in question. The analysis reveals that biquinoxaline behaves much in the same way as 2,2’-biq~inoline,~ with two close-lying photoexcited triplet states, one corresponding to a pseudolocalized quinoxaline-like state and the other arising from a delocalization of the triplet-state excitation over the entire molecule. We chose to investigate two dissimilar matrices to substantiate the generality of our observations. Experimental Section Synthesis of 2,2’-biquinoxaline followed the method of Chupakhin et ala4 The product was purified by recrystallization and column chromatography. Purity was ascertained by absorption, IR, and proton N M R spectro~copy.~Gold label n-decane obtained from Aldrich Chemicals was purified by distillation, followed by passing the hydrocarbon several times through columns of activated silica gel. Samples of biquinoxaline in decane were (1) J. R. Norris, R. A. Uphaus, and J. J. Katz, Chem. Phys. Lett., 31, 157 (1974);R. H. Clarke, D. R. Hobart, and W. R. Leenstra, J. Am. Chem. Soc., 101,2416 (1979). (2) H. C. Brenner in “Triplet State ODMR Spectroscopy”, R. H. Clarke, Ed., Wiley-Interscience, New York, 1982;D. M. Burland and A. H. Zewail, Adv. Chem. Phys., 40,369 (1979). (3) R. H. Clarke, P. Mitra, and K. Vinodgopal, J. Chem. Phys., 77, 5288 ( 1982). (4) 0.N.Chupakhin, E. 0. Sidorov, S. M. Shein, and I. I. Vilkis, J. Org. Chem. USSR (Engl. Transl.), 12, 2384 (1976). (5) Z.Gregorowicz, I. Baranowska, W. Karminski, and R. Baranowski, Mikrochim. Acta, 4, 503 (1972).
0022-3654/84/2088-3982$01.50/0
-
made up to a concentration of M and quickly frozen in a quartz sample tube. Mixed single crystals of biquinoxaline in zone-refined BZA were grown in a Bridgman furnace with a guest/host concentration of 1% mol/mol. The phosphorescence spectra were obtained at 1.9 K with a 0.5-m Jarrell-Ash monochromator via the filtered output of a Hg/Xe lamp excitation source.6 For the ODMR experiments the sample was situated inside a helical microwave antenna and suspended in a Janis Model 10 DT liquid-helium Dewar which was pumped on to achieve a temperature of 1.8 K. Excitation was effected by a PRA model P500 Xe arc lamp system accompanied by appropriate solution and glass filters. The molecular emission was detected, through a Jarrell-Ash 0.25-111 monochromator, by an EM1 95584 phototube located in a Products For Research Model TE- 104RF thermoelectrically cooled housing. Microwave power was delivered by a Hewlett-Packard Model 8620C sweeper. Changes in the phosphorescence intensity, constituting the ODMR signal, were additively collected by a Tracor-Northern Model TN- 1505 signal averager. Calculations An analysis of the ODMR spectra can be made using a suitably shortened triplet spin Hamiltonian in which only the out-of-plane component of the 14Nhyperfine tensor is included; x and y define the in-plane axes, while z defines the out-of-plane axis in accordance with magnetic resonance convention. The use of such a truncated Hamiltonian is justified, since it has been shown to yield satisfactory numerical results in the case of phenazine and In other planar azaaromatic molecules with 3 ( ~ - 7 r * ) all of these cases, the values of the in-plane components of the hyperfine tensor, i.e. A,, and A,, are much smaller. The spin Hamiltonian then becomes 7f = H,, Hq Hhf
+
+
where H,, = -XS; - YS: - ZS,Z Hq = k=x,y,z
6k(Ilk2
+ IZkZ + 13k2 + 1 4 k 2 )
Hhf = A z ~ s z ( ~ + l z I22 + I 3 2 + I4z) denoting respectively the triplet electron spin dipolar, the 14N (6) We would like to thank Prof. Clarke for making available his laboratory to perform the high-resolution optical experiments and some exploratory ODMR experiments. (7) K. P. Dinse and C. J. Wilscom, J . Chem. Phys., 68, 1337 (1978). (8)W. Frohling, C. J. Winscom, K. P. Dinse, and K. Mobius, Chem. Phys., 51, 369 (1980). (9) C. B. Harris and M. J. Buckley in “Advances in Nuclear Quadrupolar Resonance”, Vol. 2,J. A. S.Smith, Ed., Heyden, London, 1975.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 3983
Triplet State of 2,2/-Biquinoxaline
Figure 1. The 2,2’-biquinoxaline molecule.
I
510
520
530
540
550
560
WAVELENGTH I nm
Figure 3. Phosphorescence of 2,2’-biquinoxaline in n-decane at 1.9 K.
I
I
I
I
I
I
520
530
540
550
560
J
WAVELENGTH / nrn
Figure 2. Phosphorescence of 2,2’-biquinoxaline in BZA at 1.9 K.
quadrupolar, and the triplet electron spin-I4N nuclear hyperfine interactions. It is also assumed that the principal axis systems of the three tensor components of the spin Hamiltonian are collinear. Such an assumption is reasonable, since it has been shown to be justified in the case of the planar azaaromatic quinoxaline.I0 ODMR line shapes were simulated by a numerical diagonalization procedure using the above spin Hamiltonian and the 162 possible zero-order eigenfunctions of H, + Ifg. The eigenfunctions used in the numerical procedure were simple product functions where 7 = T,, Tyand 5, = x , y, z are the of the type I ?E,&&&,) triplet electron spin and nuclear quadrupolar states, respectively. Dinse and Winscorn” have carried out such exact diagonalizations on phenazine (with two I4N nuclei), to illustrate the advantages of such a procedure over a simple second-order perturbation treatment. We shall focus our analysis on the 2E transition since this transition is most dominantly affected by the hyperfine tensor element AZr9The procedure diagonalizes the matrix using suitable values for the input parameters X , Y, Z , ,E e,, cz, and A,,, which yields the energy eigenvalues and the corresponding eigenstates of the above Hamiltonian. To illustrate, for a four-nitrogen case, the state IXzyzy) evolves into 0.9912lXzyzy) 0.12361Xzxzx) O.O129(IYzxzy) (Yzyzy)] ... upon inclusion of the hyperfine Hamiltonian. Subsequently, all transition probabilities between manifold T, and T,, are calculated from the standard magnetic dipole operator V = hy,H1 C,=x,y,z Sle9These transitions were then folded with a Gaussian line-shape function of appropriate line width and additively combined to produce the theoretical spectra. The reliability of the program was tested against a similarly generated spectrum of the 2E transition in phenazine reported by Frohling et a1.* These simulated spectra thus produced do not distinguish between transitions that are allowed in zero order and those allowed only in higher orders (the so-called forbidden transitions), since the program carries out an exact diagonalization.
+
+
+
+
Results and Discussion The low-temperature phosphorescence spectra of biquinoxaline in BZA and in n-decane are shown in Figures 2 and 3. In both (10) I. Y . Chan and J. H. van der Waals, Chem. Phys. Lett., 20, 157 (1973). (11) K. P. Dinse and C. J. Winscom in “Triplet State ODMR Spectroscopy”, R. H. Clarke, Ed., Wiley-Interscience, New York, 1982.
TABLE I: ODMR Data for 2,2’-Biquinoxaline multiplets of low-freq matrix Xdet, nm transn state, MHz BZA 513.5 2E 556.0 557.0 557.8 558.8 559.8 D-E 2239.0 2241.7 2244.2 2247.6 2250.5 n-decane
512.0
2E
D-E
570.0 570.9 571.9 572.8 573.9 2035.6 2036.9 2038.1 2039.6 2041 .O
multiplets of high-freq state, MHz 836.1 837.4 838.5 840.9 2273.3 2276.0 2278.4 2281.5 2285.2 851.5 859.0 860.5 862.5 2253.2 2356.1 2361.6 2364.3 2367.0
cases, the emission is highly structured and well resolved. A detailed vibronic analysis using the well-characterized vibrations of the similar molecule 2,2/-biquinoline3 shows that the n-decane spectrum can be resolved into emissions arising from just two major sites. Such an analysis can also be carried out for the BZA-doped single-crystal spectrum, which confirms the existence of only a single major site in the matrix. In BZA the origin at 19 474 cm-’ is the most intense line in the spectrum. The same is true for the two sites in n-decane with 0-0 bands at 19 532 and 19 512 cm-’. This indicates that in both hosts the biquinoxaline molecule does not undergo any drastic structural changes in the emitting triplet state as compared to the ground state. Both spectra reveal major vibronic bands at 275, 686, and 1358 cm-I, which correlate well with the Raman active a, vibrations at 273 and 1365 cm-’ and the b, vibration at 710 cm-I observed in biq~inoline.~ Furthermore, the vibronic analysis of the phosphorescence shows no nontotally symmetric vibrations, indicating a center of inversion (thus planarity) in the ground state. The emission origin of biquinoxaline in our matrices is red-shifted by approximately 1760 cm-’ relative to that observed for quinoxaline in biphenyl.’* A shift of this magnitude in going from the single to the double molecule is consistent with similar observations for other double molecules such as biquinoline and 2,2’-binaphthyL3J3 The zero-field ODMR spectral data for biquinoxaline in both BZA and n-decane are summarized in Table I. Hyperfine and (12) S. M. Ziegler and M. A. El-Sayed, J . Chem. Phys., 52, 3257 (1970). (13) P. Mitra, Ph.D. Thesis, Boston University, 1982.
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
Vinodgopal et al.
TABLE 11: Zero-Field Splitting, Quadrupolar, A,, and Gaussian Line-Width Parameters Used To Obtain Best-Fit Simulated Spectra“
matrix BZA n-decane a
transn low-frea 2E high-friq 2E low-freq 2E high-freq 2E
X 1121.3 1317.9 1061.5 1359.8
Y
z
563.8 481.8 489.9 502.3
-1685.1 -1797.8 -1551.4 -1862.1
Gaussian
. %
6.V
CY
2.69 2.69 2.69 2.69
1.67 -0.36 1.67 -0.36
-4.36 -2.33 -4.36 -2.33
-
A,, (ratio)
Av (ratio)
9.6 23.0 9.2 23.8
0.19 0.45 0.15 0.42
(1) (2.4) (1) (2.6)
(1) (2.4) (1) (2.8)
All values are reported in MHz. I
I
0 2a 4a
I
0a
d
FREQUENCY I MHr
I
la
da
Oa
I
I
I
I
058.9
050.6
080.3
882.0
FREQUENCY I MHz
Figure 4. Experimental (continuous line) and simulated (dotted line) ODMR spectra of the high-frequency 2E transition of biquinoxaline in BZA.
Figure 5. Experimental (continuous line) and simulated (dotted line) ODMR spectra of the high-frequency2E transition of biquinoxaline in
quadrupolar satellites due to the I4N nuclei are seen in all of the ODMR spectra. In each matrix four transitions were observed. Identical results are obtained when monitoring the vibronic bands belonging to the emission origins reported in Table I. Also, the other emitting site in n-decane (at 19512 crn-’) showed analogous behavior but is not described here. If one assumes triplet sublevel behavior similar to that of quinoxaline, the four transitions observed can be considered as a set of two 2E transitions and another set of two D-E transitions. The D+E transition was not observed in biquinoxaline, as was the case in biquinoline, indicating that the triplet sublevel’dpa@cs in both molecules follow similar patterns. It must be mentioned that double-resonance experiments done on the two sets of 2E and D-E transitions in biquinoline3 indicate two D+E transitions, one corresponding to the low-frequency set of the 2E and D-E transitions and the other corresponding to the high-frequency set of the above. To facilitate our analysis and discussion of the ODMR spectra, we shall henceforth label the triplet states giving rise to the two sets of transitions as high-frequency and low-frequency states, respectively. We shall focus our attention separately on the two states, beginning with the high-frequency state. High-Frequency State. As can be seen from Table I, the ZFS parameters 1 0 1 and IEI arising from the high-frequency transitions are reasonably close to those of quinoxaline,14 indicating that the high-frequency transitions originate from a triplet state restricted to a smaller molecular skeleton,15 i.e. only half of the double molecule. Figures 4 and 5 show the experimentally obtained ODMR spectra for the high-frequency 2E transition in both matrices. The
dotted curve is the theoretical spectrum resulting from the calculations described above, in which interactions with only two of the four I4N nuclei were considered. The optimal values of the parameters used in the calculated spectra are given in Table 11. Since data on the quadrupolar I4N nuclei are not available in either the ground or excited states for biquinoxaline, values for the quadrupolar constants ex, e,,, and ez were obtained from 1,8-diazanaphthalene.I6 These values are respectively 2.69, -0.36, and 2.33 MHz. In any case, variations in these quadrupolar constants while keeping A,, constant did not severely affect the simulated results. As can be seen from Figure 5 , the numerically simulated and the experimentally obtained high-frequency transitions fit very well. In particular, the spectra follow the expected hyperfine pattern for a simple two-nitrogen case with transitions located at vo, vo + 2a, vo + 4a, and vo 8a, with vo = X - Y and a = Az~/vo.8,10,16 Such an analysis on the experimental high-frequency spectra gives A, values of 23.0 and 23.8 MHz in BZA and decane, respectively. When these values of A,, are used in the theoretical simulations, excellent fits are obtained. It should be mentioned that additional forbidden satellites are predicted in the simulated spectrum. Although experiments at higher microwave power exhibit some extra satellites, a correlation with theory was not carried out due to complexities arising from transferred hyperfine interaction^'^ from the other half of the molecule. Low-Frequency State. The Z’FS parameters 1 0 1 and IEI for the low-frequency triplet state in biquinoxaline are respectively 21% and 51% lower than those for the high-frequency state and following the arguments used in biquinoline can be interpreted as arising from triplet spin density spread over the entire double molecule. Thus, for the low-frequency 2E transition, whose
n-decane.
+
(14) J. Schmidt, D. A. Antheunis, and J. H. van der Waals, Mol. Phys., 22, l(1971).
(15) S.P. McGlynn, T. Azumi, and M. Kinoshita, “The Triplet State”, Prentice-Hall, Englewood Cliffs, NJ, 1969.
(16) L. W. Dennis and D. S. Tinti, J . Chem. Phys., 62, 2015 (1975). (17) M. D. Fayer, C. B. Harris, and D. A. Yuen, J . Chem. Phys., 53,4719 (1970).
The Journal of Physical Chemistry, Vol. 88, No. 18. 1984 3985
Triplet State of 2,2’-Biquinoxaline
1
I
1
I
I
~~
I
I
I
tl
I
I
58 1.4
558.0
1
+b
6177eb4a
m 0 2 4 6 8 14a
I
I
-b
I
b -b-
I
572.0
568.8
FREQUENCY I MHZ
I
575.0
FREQUENCY I MHZ
Figure 6. Experimental (continuous line) and simulated (dotted line) ODMR spectra of the low-frequency 2E transition of biquinoxaline in BZA.
Figure 7. Experimental (continuous line) and simulated (dotted line) ODMR spectra of the low-frequency 2E transition of biquinoxaline in
quadrupolar and hyperfine satellites can be considered to arise from all four I4N nuclei, our numerical diagonalizations indicate the presence of six “allowed” transitions at approximately vo, vo 2a, vo 4a, vo 6a, vo 8a, and vo + 14a. The vo 2a and yo 14a transitions correspond to the strongest and weakest in intensity, respectively. The numerical procedures also indicate the presence of additional, largely forbidden transitions, two of which can be correlated with similar transitions observed at vo 4a b in the two-nitrogen case,* where b = [4(t, - cy)* 16a2]1/2. As can be seen in Figures 6 and 7, the experimental spectrum for the low-frequency 2E transition in both matrices is a five-line spectrum. At higher applied microwave powers, the spectrum in n-decane remained the same, although line widths of individual satellites were considerably broader; in the BZA host, similar experiments resulted in line broadening to give a single peak and no evidence of any quadrupolar and hyperfine structure. At lower microwave powers, only a single satellite was observed, corresponding to the most intense satellite in Figures 6 and 7. In order to obtain the best fit with experiment, several simulations were carried out by varying the values of vo, A,,, and the quadrupolar constants. They are enumerated below in order of complexity: (i) When the lowest frequency experimentally observed satellite is taken to be equal to vo, i.e. X - Y, and A,, values were varied from 9 to 18 MHz, the simulations produced were widely different from experiment, both in intensity and positions of the satellites. Essentially, this is because the most intense line must occur next to vo, at vo + 2a; clearly, the experimental spectrum contradicts this. (ii) When the second low-frequency experimental satellite was chosen to be equal to vo and simulations were again carried out with different values of A,,, poor fits were obtained. With A,, values greater than 11 MHz, the most intense experimental peak does correspond to vo 2a, but under this interpretation, the predicted transition at vo + 14a is not observed experimentally. With A,, values lower than 11 MHz, the numerical simulations are widely different in intensity, when compared with those from experiment; in particular, the most intense experimental satellite observed does not correspond to the most intense transition predicted numerically at vo 2a. (iii) The most intense and the highest frequency experimental satellites observed were taken to be equal to vo 2a and vo 14a,
respectively. When values for A,, (-9.5 MHz) and vo so obtained were used in the simulations, reasonable fits were obtained. It was found that the transitions predicted numerically at vo, vo + 2a, and vo 4a would be buried under the line width of the most intense experimental satellite, and given the experimental line widths, these three transitions would then not be distinguishable. However, such a choice of vo and A,, also implies that the two low-frequency experimental satellites must correspond to forbidden transitions. These forbidden transitions would be expected to be sensitive to variations in quadrupolar parameters. Thus, the lowest frequency experimental satellite (in each matrix) was taken to be equal to vo 4a - b and was used to compute appropriate values for the quadrupolar constants. When these values were used along with the above deduced values for A,, and vo, good fits were obtained. In the BZA host, the best fit was obtained with a value of A,, equal to 9.5 M E z and in n-decane with an A,, value of 9.2 MHz. All the transitions predicted by the numerical procedure are indicated in a stick diagram in Figures 6 and 7. These transitions were then folded as before with an appropriate Gaussian line-shape function to give the best fit with experiment and are shown as dotted lines in Figures 6 and 7 corresponding to the BZA and the n-decane matrix, respectively. As can be seen from the figures, there are still some differences between the simulations and the experimental spectra. Only two of the predicted forbidden transitions are observed in the experimental spectrum (at 556.0 and 557.0 MHz in BZA; at 570.0 and 570.9 MHz in n-decane), while the corresponding high-frequency analogues are absent in experiment. A possible reason for such a variance with experiment is our neglect of proton hyperfine effects in the Hamiltonian used in our numerical calculations. Such an effect could be potentially large since biquinoxaline contains ten protons, of which at least five are magnetically distinct. However, no attempt was made in the course of this study to include these effects since this would make the resulting eigenvalue matrix problem intractable. Both Frohling et a1.* and Dennis and Tinti16 have shown that these proton hyperfine effects can lead to significant intensity contributions.
+
+
+
+
+
+
+
+
+
+
+
+
+
n-decane.
+
+
Conclusion Our experimental evidence clearly indicates the presence of two distinct triplet states. The observed Franck-Condon progression and the absence of nontotally symmetric vibrations in the phos-
3986
J. Phys. Chem. 1984,88, 3986-3989
phorescence spectra together imply planarity in the emitting triplet state, allowing us to assume A,, >> A,.., A,. This assumption of A,, >> A, Ayycertainly holds for the high-frequency localized state, regardless of the relative geometry of the quinoxaline moieties. We have presented evidence to substantiate our interpretations of the two observed triplet states in biquinoxaline. For the high-frequency state the ZFS parameters are in good agreement with that of quinoxaline, indicative of a localized state. The A,, values of 23.0 and 23.8 M H z are also fairly close to those of quinoxaline (28 MHz). The slight reduction of A,, values between those of quinoxaline and the high-frequency triplet state in biquinoxaline suggests that the latter is not entirely localized on half of the molecule but may have some residual spin density on the other half as well. Furthermore, the pattern of satellite intensities follows that predicted for a photoexcited triplet split by two nitrogens. For the low-frequency state, the ZFS parameters are considerably less than that of the localized state, indicating a delocalization of electron spin density over the entire double molecule. Secondly, the simulated intensity pattern agrees well with an interpretation of a triplet state split by four nitrogens. Similar observations have been reported on the analogous system 2,2’biq~inoline.~ Thirdly, our simulations indicate A,, values close to 9.5 M H z for this state, which is almost half that observed in the localized state. Such a reduction is consistent with theoretical arguments forwarded by Hutchison and King in their work on naphthalene dimers.18 Finally, the residual linewidth indicated as Av in Table I1 can be seen to be reduced in going from the high- to the low-frequency (18) C. A.
Hutchison and J. S. King, J . Chem. Phys., 58, 392 (1973).
state. This reduction is to be expected since the principal contributions to the satellite inhomogeneous line width are the proton hyperfine interactions, which like their nitrogen counterparts should similarly be reduced by approximately a factor of 2. In fact, the ratio of line widths between localized and delocalized states (-2.6) is in very good agreement with the ratio obtained through a comparison of nitrogen A,, values (-2.5). This thus provides additional evidence for a localized/delocalized model. The results and analysis presented above corroborate those reported by Clarke et al. for 2,2’-biq~inoline.~Whereas in the case of biquinoline, the characterization of the triplet states was arrived at via a second-order perturbation treatment, our work on biquinoxaline utilized an exact diagonalization procedure and line-shape simulation. The similarity of the observations, i.e. a localized and a delocalized photoexcited triplet state, in both double molecules strongly suggests that these constitute a general feature of 2,2‘ systems. We have begun further studies using deuterated biquinoxaline in an attempt to reduce the line widths of the observed ODMR spectra and thus make a more rigorous assignment of the hyperfine and quadrupolar satellites. The precise energy and nature of the high-frequency localized state also remains to be determined. With this in mind we have begun calculations designed to give us information about the geometry, energy, and vibrational frequencies of the ground and excited triplet states. Acknowledgment. This work was supported by grants from the Research Corporation (9326), the donors of the Petroleum Research Fund, administered by the American Chemical Society (13295-G6), and the University of Vermont (UVM PS-11). Registry No. BZA,65-85-0;2,2’-biquinoxaline, 27739-37-3;n-decane, 124-18-5.
Electron Spin Resonance Characterization of the Dynamical Properties of 2-Methyloctadecane-Urea Adduct Using Peroxy Spin Probes Walee Chamulitrat and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: December 2, 1983)
The motional effects of 2-methyloctadecane-urea adduct have been investigated by using a peroxy spin probe employing electron spin resonance (ESR). Temperature variation in the ESR spectra of the 2-methyloctadecane-urea adduct were measured in the temperature range from 125 to 284 K. The ESR spectra at 125 K reveal a rigid limit anisotropic g tensor with values of gl = 2.035, gz = 2.014, and g3 = 2.002. At temperatures above 284 K the g tensor is motionally averaged to give the new values of g, = 2.002. At intermediate temperatures the ESR spectra reveal a single extra feature between the gll and g, regions which is the characteristic of peroxy radical motional effects. The spectral line shape dependence with temperature has been investigated by simulations based on the modified Bloch equations for different specific motional models. The most satisfactory fit is achieved by a chain-axis rotational model with 90’ jumps over the entire temperature range. The fit gives an Arrhenius activation energy of 11.3 kJ mol-’ which shows only slight rotational hindrance compared to n-alkane-urea adducts.
Introduction The adducts of long-chain alkanes in urea have been extensively Urea crystallizes with studied by a variety of (1) Redlich, 0.; Gable, C. M.; Dunlop, A. K.; Millar, R. W. J . Am. Chem. SOC.1950, 72, 4153. (2) Gilson, D. F. R.; McDowell, C. A. Mol. Phys. 1961, 4, 125. (3) Griffith, 0.H. J. Chem. Phys. 1964, 41, 1093. (4) (a) Hori, Y . ;Shimada, S.; Kashiwabara, H. Polymer 1977.18, 1143: (b) J . Chem. Phys. 1981, 75, 1582.
(5) Hoffman, J. D. “Molecular Relaxation Processes”;The Chemical Society of London: London, 1966; Vol. 20, p 47. (6) Bell, J. D.; Richards, R. E. Trans. Faraday Soc. 1969, 65, 2529. (7) Umemoto, K.; Danyluk, S. S. J . Phys. Chem. 1967, 71, 3757.
0022-3654/84/2088-3986$01.50/0
hexagonal cylindrical channels in which “guest” long-chain molecules can be entrapped with their longitudinal axes parallel to the c axis of the urea crystalline structure.8 The motional properties of such urea adducts have been studied by dielectric r e l a ~ a t i o n nuclear ,~ magnetic r e ~ o n a n c e and ,~~~~~ electron spin r e s ~ n a n c e . ~ The , ~ , ~results suggest that the guest molecules fit only loosely in the cylindrical channels of the area. Hori et al.4 reported the motional process in n-tetradecane-urea adduct to be chain-axis rotation but the specific motional (8) Smith, A. E. J . Chem.Phys. 1950, 18, 150. (9) (a) Schlick, S.; Kevan, L. J. Am. Chem. Soc. 1980, 102, 4622; (b) J . Phys. Chem. 1979.83, 3424: (c) J . Chem. Phys. 1980, 72, 784.
0 1984 American Chemical Society
