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Figure 4. Structure and an ortep view of 6. 9 O field chemical shift of the ortho protons may reflect the combined anisotropic effects of the central benzene ring and an adjacent aniline ring.Another interesting example of stereochemical switching was found during the N-methylation of ureas. Aromatic ureas exist in transoid structures.* 123456 However, the cis preference of Nmethyl-MN-diphenylureas7 and the bis-cis preference of N,N'dimethyl-MA^'-diphenylurea8 also seem to be intrinsic. Two examples, IV,IV'Af'W",-tetramethyl-1,5-bis( 1 -naphthylureido)naphthalene ( 6) and an Mmethyl derivative of a poly(phenyleneureido)benzene ( 7) may be of interest.The bis-cis conformation of the ureido structure of 6 is retained in both the ureido structures (Figure 4). An NMR study on 6 was not very informative, but definitely showed the presence of at least partial aromatic-aromatic interactions. The structure of the five-layered urea derivative ( 7) is beautifully shown in Figure 5. The angle between the first and the second benzene-ring planes is 38.5® and that between the second and the third ring planes is 46.7®. The angle between the first and the third is 12.1®, close to parallel, and the distance between the parallel second and fourth planes is 7.29 A. The distances between aromatic carbon atoms bearing a nitrogen atom are 3.31 A (the first and second) and 2.96 A (the second and third). The solution stereochemistry is (5)

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Xenon and fluorine combine readily. Xenon tetrafluoride is a colorless crystalline material, stable at room remperature. The existence of at least one other fluoride and two oxyfluorides has been demonstrated. The heaviest "inert gas," radon, also reacts with fluorine, yielding a compound less volatile than xenon tetrafluoride.

Xenon tetroxide is a yellow solid at low temperature. It has a vapor pressure of about 25 mm-Hg at 0 degrees C. It is unstable at room temperature. The infrared spectrum of the vapor shows it to have tetrahedral symmetry.

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The infrared spectrum of VOF3 has been studied and interpreted in terms of a C3v molecular symmetry. All six fundamentals have been observed: the a1 frequencies are 1057.8, 721.5, and 257.8 cm−1 and the e frequencies are 806, 308, and 204.3 cm−1. The infrared spectrum of POF3 has also been studied and the lowest e fundamental, earlier observed in the Raman spectrum, has been confirmed.