cycle of refinement were 0.27 and 1.69, respectively. An analysis of the difference map showed no peak greater than 0.15e Å-3. Tables 1 and 2 list the final atomic parameters. 2. Discussion On account of the biological importance of urea, methods to determine it have received much attention. Three methods are in general use: in one method, urea is hydrolyzed to ammonia and carbon dioxide using urease. The ammonia is then titrated with standard acid [8]. In another method, small amounts of urea are determined spectrophotometrically (or colorimetrically) using either Nessler's reagent (alkaline mercury potassium iodide) or following the reac tion with diacetyl monoxime [3, 10, 11], diacetyl [9], or cyclohexane-1,2-dione dioxime (fluorescence spectra) [12]: strong acid media. In a third method, urea forms the very insoluble dixanthylurea when treated with a solution of xa5thydrol (9H-xanthen-9-ol) in methyl alcohol in the presence of glacial acetic acid [4]. The insoluble precipitate may be estimated gravimetrically [4], colorimetrically [1] or as microcapillary columns of the dixanthylurea sediment [13] A possible new method for urea determination, based on dixanthylurea isotope-dilution mass spectrometry, is being evaluated at NBS in the Organic Analytical Research Div sion. In work on this method, it was found necessary to accurately characterize, by single crystal X-ray diffraction, the molecular parameters of dixanthylurea. RI TABLE 1. Positional (× 10′) and thermal (Åa × 10′) parameters of the non-hydrogen atoms. The form of the thermal correction is: T = exp [−2m2(a*2h2U11 + b*2k2U22 + c*2l2U33 + 2a*b*hkU12 + 2a*c*hlU13 + 2b*c*kl U23)] nd distances and angles for dixanthylurea are given in e 1. A stereoview of the molecule is shown in figure 2. e is a considerable difference between the nitrogen to on bond lengths. For example, N(1)–C(1) is 1.355(6)Å N(1)-C(2) is 1.475(5)Å. A similar difference was rved in monomethylurea [6]. These bond distances ince that partial double bond character exists between C(I) and N(2)–C(I) with resonance in the planar ty. 0 1: N(1)-C(1)÷N(2). the molecule of dixanthylurea there are a number of - planar components. To assist in forming a visual picof the molecule it is helpful to consider the angles that e planes make with one another. Table 3 shows that the benzenoid rings and the urea group are planar and 4 shows the angles that the various planes make with other. There is no crystallographic symmetry element in the molecule of dixanthylurea. In fact, from the pus figures (especially fig. 4), it can be clearly seen that e is considerable deviation from any possible mirror symmetry that could relate the left and the right side of the molecule. The packing in the structure is illustrated in figure 3. Molecules of dixanthylurea are hydrogen bonded one to another o form infinite chains along the a-axis. The hydrogen. bonding within each chain is illustrated in figure 4 which shows that the oxygen atom of one molecule is hydrogen bonded to two nitrogen hydrogen atoms of an adjacent molecule. The O(1). . .N(1) and the O(1). . .N(2) oxygen to nitrogen distances are 2.957 and 2.928Å, respectively. A similar type of hydrogen bonding and N. . .O distances were found in the packing of monomethylurea [6] and in phenylurea [7]. In addition to the 2:1 complex dixanthylurea, the 1:1 complex has been prepared. We thank Vicky Himes for technical help. The figures were drawn using a local version of the ORTEP program of C. K. Johnson. The X-ray structure determination described in this paper was supported by the Food and Drug Administration, FDA-NBS Interagency Agreement 224758253. * For rings R1, R2, R3, and R4, the least-squares plane is calculated through the six ring atoms. See figure 1 for atom and ring numbering. 3. References [1] Allen, F. W., Luck, J. M., J. Biol. Chem. 82, 693 (1929). [2] Cromer, D. T. and Mann, J. B., Acta. Cryst. A24, 321–324 (1968). [3] Fearon, W. R., Biochem. J. 33, 902 (1939). [4] Fosse, M. R., Compt. Rend. 154, 1187 (1912); Ibid. 157, 948 (1913); Ibid., 158, 1076, 1588 (1914), Ibid; 159, 253 (1914). [5] Germain, G., Main, P., and Woolfson, M. M., Acta Cryst. A27, 368376. (1971). [6] Huiszoon, C. and Tiemessen, G. W. M., Acta Cryst. B32, 1604-1606. (1976) [7] Kashino, S. and Haisa, M., Acta Cryst. B33, 855-860. (1977). [8] Marshall, E. K., J. Biol. Chem. 15, 487-495. (1913). [9] Natelson, S., Scott, M. L., Beffa, C., Amer. J. Clinic. Path. 21, 2 (1951). [10] Ormsby, A. A., J. Biol. Chem. 146, 595. (1942). [11] Osinski, T. and Sosnowski, J., Chem. Anal. (Warsaw) 12, 1191 (196) Chem. Abstr., 68, 66268 (1968). [12] Panek, E., Siest, G., Bull. Soc. Pharm Nancy., 37-40 (1967) Cher Abstr., 69, 6847 (1968). [13] Searcy, R. L., Korotzer, J. L., Douglas, G. L., Berquist, L. M. C Chem., 10, 128, (1964). [14] Stewart, J. M., Machin, P. A., Dickinson, C. W., Ammon H. L., H H., and Flack, H., The XRAY System-Version of March 197 Tech. Rep. TR-446, Computer Science Center, University Maryland, College Park, Maryland (1976). [15] Stewart, R. F., Davidson, E. R. and Simpson, W. T., J. Chem P 42, 3175-3187. (1965). |