NOTES Irradiated Single Crystal Clathrate of Dianin’s : Compound and 1,Z-Dibromo-1,l-difluoroethane
Electron Spin Resonance Observation of the Brz- Radical’”
by Lowell D. Kispert” and Jane PearsonIb Department of Chemirtry, University of Alabama, University, Alabama 56486 (Received J u n e 7 , 3971)
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Publication costs assisted by the 77. S. Atomic Energy Commission
133 and ethanol from which single crystals were grown.? The resulting crystals were X-irradiated at 77”K, and mounted on quartz rods for electron spin resonance (esr) investigation at that temperature. The esr spectrum of the 77°K irradiated crystals resulted in a spectrum (Figure 1) of 7 groups of lines spread over 2600 G at g = 1.9980 when the crystals were mounted with the magnetic field parallel to the 2 hexagonal crystal axis, an axis coincident with the long direction of the hourglass cavity. On rotation towards the X crystal direction, a large anisotropy in g and A was noted. Comparison of the g value and the spectral width in the Z direction with that found for the 81Br2-radical in irradiated KBr,8 or in irradiated lithium halide g l a s ~ e s indicated ,~ that the Brzradical had been trapped in Dianin’s compound. A calculation based on the A and g values given in Table I for 81Brz- trapped in Dianin’s compound and the quadrupole coupling constant reported by Liicken and Mazelinelo for the (CH3)2SBr radical was carried out using the computer program M A G N S P E C . ~ ~ The results indicated that the outside high- and low-field esr lines were due to 81Br2-, the middle line of the outside group was due to 81Br70Br- and 70Br81Br-, and the inside line of the outside group was due to 79Bi-z-. The other groups of lines showed large secondorder effects as indicated by the theoretical stick diagram calculated from the RiAGNSPEC program. The fact that the Brz- radical lies parallel to the long direction of the hourglass rather than across the hourglass cavity like included ethanol also explains the presence of only one radical site per unit cell. Upon
During the past few years, several examples of irradiated single crystals of organic inclusion compounds have been r e p ~ r t e d . ~ -Examination ~ of these examples indicates that a clathrate host lattice can serve as a convenient means of stabilizing a free radical derived from compounds of more than three carbons normally found in the liquid state, permitting the study of new radicals,4 potential barrier eff ects,2band orientat i ~ n . ~ ~ ~ ~ We wish to report the use of the clathrate 4-phydroxyphenyl-2,2,4-trimethylchroman(Dianin’s compound) in trapping and orienting the Brz- radical at 77°K. The Br2- radical is believed to have been formed from the reaction of included 1,2-dibromo-l,ldifluoroethane and ionizing radiation yielding 1,ldifluoroethene and the Brz- radical. The fact that the Brz- radical was found oriented in the Dianin’s clathrate suggests the possibility that the anisotropic esr hyperfine parameters and g tensors of radicals composed of two or three atoms may be studied by (1) (a) This research was partially supported by the U. S. Atomic choosing the appropriate radical precursors to be Energy Commission under Contract No. At-(40-1)-4062 and this is AEC Document No. ORO-4062-2. (b) NSF Undergraduate Parincluded in the Dianin’s clathrate. ticipant, 1969-1970. Flippen and his coworkers6 have shown that the (2) (a) 0. H. Griffith and A. L. Kwiram, J . A m e r . Chem. Soc., 86, cavity in Dianin’s compound is formed by the hydrogen 3937 (1964); T. Ichikawa, M . Iwasaki, and K . Kuwata, J. Chem. Phys., 44, 2979 (1966); 0. H. Griffith, Proc. Natl. Acad. Sci. bonding of the OH groups from 6 different molecules. U . S., 54, 1296 (1965); C. Corvaya, J. Chem. Phys., 44, 1958 Since there is only one OH group available for bonding (1966); (b) 0. H. Griffith, J . Chem. Phgs., 41, 1093; 42, 2651 (1965); 0. H. Griffith and H. M. McConnell, Proc. Natl. Acad. from each molecule, there are separate columns o,f Sci. U . S., 48, 1877 (1962). independent cages. Each cage is 11 A long, 6.2 A (3) H. Ohigashi and Y . Kurita, J . Chem. SOC.Jap., 40, 704 (1967). wide at the point of maximum extension, and 4.2 (4) G. A. Helcke and R. F. Euratom, Mol. Phys., 18, 1 (1970); D. E. Wood and R . V. Lloyd, J. Chem. Phys., 52, 3840 (1970); wide where the methyl groups protrude into the cage D. E. Wood, R. V. Lloyd, and D. W. Pratt, J . A m e r . Chem. SOC., to give its hourglass shape. The 2 direction lies 92, 4115 (1970). along the needle axis of the crystal, and the cage size (5) J. L. Flippen, J. Karle, and I. L. Karle, ibid., 92, 3750 (1970). can vary for shapes that differ from cylindrical sym(6) W. Baker, A. J. Floyd, J. F. W. McOmie, G. Pope, A. S. Weaving, and J. H. Wild, J . Chem. Soc., 2010 (1956). metry. It has also been determined by crystallog(7) W. Baker and J. F. W. McOmie, Chem. I n d . (London), 256 raphy that when ethanol is included in Dianin’s (1955). compound, it lies across the hourglass cavity a t its (8) T. G. Castner and W. Kanzig, J . P h y s . Chem. Solids, 3, 178 widest point, thus compounds with one or two carbons (1957). (9) E. B. Zvi, R. A. Beaudet, and W. K. Wilmarth, J . Chem. Phys., can be included. 51, 4166 (1969). The unsolva,ted Dianin’s compound was prepared (10) E. A. C. Liicken and C. Mazeline, ibid., 48, 1942 (1968). and carefully purified by methods of Baker, et aL6 (11) M. Kapp and J. H. Mackey, J . Comput. Phys., 3, 539 (1969); Dianin’s compound was found to be insoluble in 1’2J. H. Mackey, M. Kapp, and E. C . Tynan in “Electron Spin Resonance of Metal Complexes,” T. F. Yen, Ed., Plenum, New York, dibromo-1,l-difluoroethane; however, it was found E. Y., 1969. An expanded and revised version has been written soluble in a mixture of ~,2-dibromo-l,l-difluoroethane for the Univac 1108 a t the University of Alabama. T h e Journal of Physical Chemistry, Vole76,NO.1, 1978
134
NOTES
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CF2BrCH2Br, OIANIN’S
COMPOUND
Figure 1. Esr spectrum of irradiated Dianin’s compound with 1,2-dibromo-l,l-difluoroethane included. axis is parallel to t h e magnetic field. T h e theoretical stick diagram calculated for each Brz- isotope is reconstructed below the spectrum.
Table I:
g and
A Values Found for the *lBrz- Radical
----
-----&--Matrix
g
A
BrzBrz-
KBr CFzBrCHzBrin Dianin’s
I . 980
Brz-
compound Lithium halide glasses
Radical
T h e hexagonal 2
y
I -
A
g
A
1.998
455 450
2.179 2.151
80 90
2.175 2.16
80 90
1.969
459
2.112
50-70
2.112
50-70
warming to 110°K the spectrum decays irreversibly, not unexpected from the large degree of motional freedom present. Additional lines (indicated by arrows in Figure 1) spread over 500 G were also observed when the crystals was mounted with Z / / H . The large spectral width suggested either the presence of a biradical or a fluorine containing radical; however, it appeared that these lines were dependent on the purity of the CF2BrCBrHz and, therefore, were ignored. The large central line (Figure 1) observed for all irradiated samples, including an irradiated powder sample of pure Dianin’s compound, could be due to a radical with the odd electron centered on a nucleus of zero spin such as the substituted acetyl radical12 or from a color center such as a trapped electron. The possibility still exists that the Brz- radical was due to a trace of molecular bromine rather than the product of ionized CF2BrCH2Br. However, carefully purified CFzBrCHzBrincluded in Dianin’s compound The Journal of Physical Chemistry, Vol. 76, No. 1, 1972
_____
X----.
g
showed no change in the Br2- radical esr spectral intensity over that of a sample containing a slight trace of Br2. On the other hand, an irradiated frozen sample of very carefully purified CBrF2CBrH2 showed no Br2- radical. However, if the reasonably weak esr signals of Brz- found in the Dianin’s clathrate were averaged over a powder spectrum, it is doubtful whether they would be observed. Acknozoledgnzent. The authors wish to acknowledge the University of Alabama’s Research Committee, Project 600 for support of this work, an NSF departmental matching grant which provided for the Varian E-12 esr spectrometer, the Physics Department a t the University of Alabama for use of their X-ray machine, and the TJniversitg of Alabama computer center for providing a generous amount of computer time. (12) R . C. McCalley and A . L. Kwiram, J . Amer. Chem. Soc., 9 2 , 1441 (1970).
