J . Phys. Chem. 1987, 91, 2235-2236
2235
Radical Pairs in Urea Channels‘ H. L. Casal, D. Griller,* F. W. Hartstock, R. Kolt, D. J. Nortbcott, J. M. Park, and D. D. M. Wayner Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada, K l A OR6 (Received: February 3, 1987)
Photolysis of a single crystal of a decanoyl peroxide-urea complex at 10 K produced pairs of nonyl radicals that were arranged lengthwise in the urea channels. The radical centers were separated by two molecules of carbon dioxide. A combination of electron paramagnetic resonance, EPR, and infrared spectra gave considerable information about the freedom of motion of the radicals within the channels. Warming the crystal to 155 K caused the radical centers to move apart and cooling caused a partial return. EPR spectra showed that the radicals rotated with reasonable freedom about their long axes.
Introduction In a series of elegant investigations, McBride and his colleagues have shown that the development of local stress in crystals can yield important insights into molecular motion within solid^.^-^ These investigators have generally used partial photolysis of single crystals of diacyl peroxides to generate stress centers (eq l ) , which
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were then investigated by monitoring the behavior of the resulting radical pairs by electron paramagnetic resonance, EPR, spectroscopy. The generation of four molecules from one often creates local pressures equivalent to thousands of atmosphere^.^ We have adopted this approach to investigate molecular motion in urea inclusion complexes and have found that it provides some fascinating insights into the nature of urea as a host matrix.
Experimental Section Materials. All chemical reagents were commercially available and were used as received. Complex Preparation. The inclusion complex of decanoyl peroxide in urea was prepared by slow addition of a solution of the peroxide (0.1 g) in methanol ( 5 mL) to one of urea (0.5 g) in 2-propanol (25 mL). The complex crystallized as long colorless rods when the solvent was allowed to evaporate slowly. These crystals were used in single crystal EPR studies. Small crystals were obtained by rapid cooling of the solution and served as a source of powders for infrared and Raman experiments. Spectra. The urea inclusion complex was studied by three spectroscopic techniques. Raman spectroscopy was used to confirm the integrity of the inclusion complex of decanoyl peroxide in urea. EPR and infrared spectroscopy were used to investigate the stress centers that were generated by UV photolysis (Hg/Xe arc lamp) of the complex. For infrared work, powders of the complex mixed with KBr were compressed into transparent disks which were mounted in an Air Products Displex Model 202 cryostat, and transmission spectra were obtained with a Nicolet 2ODXB FTIR spectrometer. EPR spectra were taken of single crystals which were mounted in a two-circle goniometer. The EPR (1) Issued as NRCC publication No. 27498. (2) McBride, J. M.; Gisler, M. R. Mol. Cryst. Liq. Cryst. 1979, 52, 425. (3) Vary, M. W.; McBride, J. M. Mol. Cryst. Liq. Cryst. 1979, 52, 437. (4) Walter, D. W.; McBride, J. M. J . Am. Chem. SOC.1981, 103, 7069. (5) Walter, D. W.; McBride, J. M. J . Am. Chem. SOC.1981, 103, 7074. (6) McBride, J. M.; Vary, M. Tetrahedron 1982, 38, 765. (7) McBride, J. M. Acc. Chem. Res. 1983, 16, 304. (8) McBride, J. M. Mol. Cryst. Liq. Cryst. 1983, 96, 19. (9) Hollingsworth, M. D.; McBride, J. M. J. Am. Chem. SOC.1985, 107, 1792.
0022-3654/87/2091-2235$01.50/0
and Raman spectrometers have been described in detail elsewhere.10,11,15-17
Results and Discussion It has been known for a considerable time that urea forms inclusion compounds with a variety of roughly linear organic molecule^.^^^^^ In the presence of these compounds, urea crystallizes forming very long channels, with a 5.5-A-diameter cross ~ecti0n.l~The included molecules are packed end to end in these pipelike structures. The channels generally constrain the motion of the included molecules, permitting only rotation about their long axes, so that the spectroscopic properties of the included compounds are dramatically different from their solution properties.l5 The freedom of torsional and rotational motions of molecules included in urea is much greater than that which is observed in crystals of the pure material.l6,” Lateral interchain interactions in the urea complexes are insignificant, since the interchannel distance is -8.2 A. In fact, the included chains interact mainly at their terminii, where a certain degree of overlap o c ~ u r s . ~ ~ J ~ Raman spectra of the crystals obtained from decanoyl peroxide and urea showed that a true inclusion compound had been formed.15 For example, the CN stretching and NCO skeletal bands of the host (1024 and 532, 61 1 cm-l) were quite different from those observed for pure urea (1010 and 556, 570 cm-’, respectively). The spectra also confirmed that the crystals contained
