July, 1963
CORIMUNICATIOW TO THE EDITOR
static charge on the sample tube due to the very intense radiation field (- 3 X l O I 4 disintegrations/min.), nonuniform packing, and, of course, possible ferromagnetic contamination of the sample are possible experimental errors. Since promethium oxide may not be magnetically dilute, there may be an appreciable TNeiss constant which makes the observed value lower than that calculated.
5 i=i
1569
Ei(ick,T) = constant (1 QAT)
+
and
HAXFORD LABORATORIES J. C. SHEPPARD The first equation specifies that the molar absorptivjCOMFANY E. J. WHEELWRIGHT ties of all absorbing species in the solution must have GENERAL ELECTRIC RICHLAND, WASHINGTON F. P. ROBERTS the same temperature dependence as the volume of the RECE~VED MARCH 4, 1963
solution a t all S isosbestic points. The probability of such an occurrence for n > 1and S > 1is nil.
ISOSBESTIC POINTS I N ABSORBANCE SPECTRA
Si?-: With reference to my recent article on the occurrence of isosbestic points1 It has been kindly drawn to my attention by Dr. K. Buijs that isosbestic points can conceivably occur in closed systems consisting of three variable absorbing species. It was argued in the paper that the occurrence of more than two absorbing species was highly unlikely sirice the equation
must hold a t each isogbestic point, requiring in general that all molar absorptivities be equal. Systems which provide constant values of dCi/dCj, or derivatives all of which show the same dependence on the variable j , were overlooked. Under these conditions closed systems containing not only three but several absorbing species may give rise to isosbestic points, providing the species can be grouped into not more than two groups wherein the ratios of concentrations of the species within the group are constant. Such systems fall into the category described by Cohen and Fischer2 wherein the deDonder-Van Rysselberghe parameter can be successfully defined as a system parameter (linearly related systems). My original conclusions that occurrence of isosbestic points in closed, temperature dependent systems indicates only one absorbing species are still valid, even for linearly related systems. This is directly shown by development of Cohen’s and Fischer’s equation in extended version with consideration of time, concentration of a j t h species, and temperature as independent variables. The equation is given as follows. The reader is referred to the papers cited for definition of symbols.
In a closed system with equilibrium a t each temperature, this equation is reduced to
For a system OS n absorbing species to produce spectra with S wave lengths (&) of temperature invariant absorbance. two conditions must be obeyed, namely (1) J. R. RIorrey, J . Phys. Chem., 66, 2169 (1962). ( 2 ) X D. Cohsn a n d E. Fischer, J . Chem. Soc., 3044 (1962).
LABORATORIES GEXERAL ELIBCTRIC COMPANY RICHLAND, WASHIXGTON RECEIVED MAY2, 1963 HANFORD
J. R. MORRI~Y
THE CRYSTAL STRUCTURE OF THE MOLECULBR ADDITION COMPOUND XENOJS DIFLUORIDE-XENON TETRAFLUORIDE
Sir: The existence of the crystalline phase whose structure is reported here was noted in the earliest examinations1f2 of the xenon fluorides. Because it could be crystallized from vapor having primarily the infrared spectrum of XeF4, the substance was reported3 to be a polymorph of XeF4. From this assumed composition and the b = 7.33 8.) c = crystallographic data,3 a = 6.64 8., 6.40 A., p = 92” 40’, 2 = 4, it was deduced that the density was 10% higher than that of the other form; hence it has been referred to in the literature as “the highdensity form of XeF4.” We have shown, by crystal structure analysis, that it is in reality a distinct compound with the composition XeFz.XeF4. The true X-ray density is 4.02 g. ~ m . - - ~ . The preparation of this compound from the elements was described previously,3 but it should be added that the results of the crystal structure analysis indicate that some XeF2must have been present in the predominantly XeF4 preparation, either by incomplete reaction4 or by decomposition of XeF4. Further work is being carried out to prepare 1arge.r quantities of XeFz.XeF4by combining the components. The diffraction intensities from a single crystal of XeFz* XeF4 were measured by use of ill0 K a X-rays, ab goniostat,, and a scintillation counter detector. A total of 574 independent reflections was recorded, which included virtually all having detectable intensity. The specimen grew in size during the data collection, and a normalizat’ion factor, derived from repeated measurements of a reference reflection, was applied. The approximate shape O F the crystal was determined, making it possible t.0 calculat,e an absorption correction for each reflection.6 The mean diameter of the crystal was about 0.015 cm. ; the value of the absorption coefficient used was 119.5 cm.-l. (1) C. L. Chernick, et al., Science, 188, 136 (19132). (2) 8. Siege1 and E. Gebert, J . A m . Chem. SOC.,85, 240 (1963). (3) J. H. Burns, J . Phys. Chenz., 67, 536 (1963). (4) D. F. Smith, J . Chem. Phys., 38, 270 (1963). ( 5 ) D. J. Wehe, W. R. Busing, and H. A. Levy, “ORABS, A Fortran Program for Calculating Single Crystal Absorption Corrections,” ORNL TX-229, 1962.
COMMUNICATIONS T O THE EDITOR
1570
A trial structure (and the formula) was determined by the Fourier synthesis method. Previous work3 gave the space group, P21/c, and showed that the Xe atoms are in a face-centered arrangenient. Xenon atoms in these sites dominate the scattering and determine the signs of all structure factors, F(hkt), for which h, k , I are unmixed (all odd or all even). Electron density maps computed with unmixed-index reflections only should show the structure, but with false symmetry. Such maps mere made, but no reasonable assignment of fluorine positions was deduced from them. However, the inclusion in the Fourier series of five of the strongest mixed-index reflections, with signs determined by Sayre’s squaring method16yielded an interpretable map. A pair of ceiitrosyinmetrically equivalent fluorine atoms was found to be bonded to each Xe atom in positions 2(a) : 0, 0,O; 0, l/2, l / 2 ; and two pairs of fluorine atoms were found to be bonded to each Xe atom in positions 2(d): I/*, 0, l/2; l / 2 , l/2, 0. Attempts to interpret a Fourier map with Xe atoms in positions 4(e) with x = x = I/d, y = 0, which also gives a face-centered arrangement, did not prove fruitful. Structure-factor calculations indicated the correctness of the model. It was refined by iterative least-squares procedures using a full-matrix program for the IBM 7090.’ As the refinement proceeded, it became apparent that the 35 strongest reflections were affected by extinction; consequently they were omitted from the refinement. The agreement factor
R
= 2
/Firlow
- F 2 c a i c d / / z FZ0bsd
reached 0.050. The Xe positions are given above, and the F atoms are in three sets of equivalent positions, x ) , with the following 4(e): f(x,y, x; x, - y, parameters. Least-squares standard errors of the above
+
W1) F(2) F(3)
5
Y
2
0.168
-0.187 .O79 .lo9
0.153 ,212 .516
.505 .240
parameters are all 0.001. Anisotropic temperature factors were determined and mill be given in a more detailed discussion later. The structures of the XeFz and XeF4 molecules in (6) D. Sayre, Acta Cryst., 6, 60 (1952). (7) W. R. Busing, K. 0. Martin, and H. A. Levy, “ORFLS, 4 Fortran Crystallographic Least-Squares Program, ORNL TRI-305, 1962.
Vol. 67
their respective pure phases were determined previously by neutron diffractions and by X-ray diffraction s t u d i e ~ . I~n~XeFz ~ the Xe-F bond lengths, correctedl0 for thermal motion on the assumption that the fluorine atoins “ride” on xenon, are 2.00 A. ; in XeF4the square planar molecule has Xe-F bond lengths, corrected for thermal motion, of 1.95 A. and the F-Xe-F angle is 90.0’. The corresponding molecular parameters found in this study of the XeFz.XeF4crystal are 2.01 (u = 0.01) 8. for the XeFz moiety and two values, 1.94 A. and 1.965 A.(each with (r = 0.01), for the XeF4moiety with an F(2)-Xe-F(3) angle of 89.0’ (U = 0.4’). (The distances before thermal correction are 2.00, 1.91, and 1.935.) The errors quoted are least squares measures of precision. We estimate the corresponding measures of accuracy to be about twice as large. The hypothesis that the XeF4 molecule is square-planar is thus easily consistent with this study, and the individual molecular geometries are retained in this compound with very little change. I n crystalline XeF2, the closest intermolecular fluorine-to-xenon approach is 3.41 8.,agd in XeF4there are two such contacts at 3.22 and 3.25 A. The XeFz.XeF4 crystal has close intermolecular fluoriqe-to-xenon distances of 3.28,3.35,3.35,3.37, and 3.42 A.; Le., a t about the same average separations as in the two components. The minimum intermolecular fluorine-to-ffuorine contact is a little shorter (2.87 A.) in XeF2.XeF4but this distance is not unusual for non-bonded fluorine atoms. Hence there is no structural evidence for the formation of any strong bonds between molecules, and this phase appears to be appropriately described as a molecular addition compound. (8) H. A. Levy and P. A. Agron, J . Am. Chenz. Sac., 85, 241 (1963); J. H. Burns, P. A. Agron, and H. A. Levy, Science, 189, 1208 (1963). (9) J. A. Ibersand W. C. Hamilton, ibid., 139, 106 (1963); D. H. TempleJ. Am. Chem. Soc., 85, 242 (1963). ton, et d., (10) W. R. Busing and H. A. Levy, paper submitted t o Acta Cryst.: W. R . Busing and H. A. Levy, “A Crystallographic Function and Error Program for the I B M 704,” Rept. No. 59-12-3, Oak Ridge National Laboratory (1959); see also D. W. J. Cruickshank, Acta Cryst., 9, 767 (1956). (11) Operated for the Atomic Energy Commission by Union Carbide Corporation.
REACTOR CHEMISTRY AND CHEMISTRY DIVISIONS OAKRIDGENATIONAL LABORATORY~~ OAKRIDGE,TENXESSEE RECEIVED MAY20, 1963
J. H. BURNS R. D. ELLISON H. A. LEVY
