June 20, 1954
NOTES
X-Ray Studies of Three Crystalline Fluorocarbons Showing Rotational Disorder
checks of the X-ray symmetry class. Except for the Laue photographs, characteristic CuKm radiation filtered through nickel foil was used. As the cylindrical specimens proved to be either optically isotropic or uniaxial, grew in the tubes without obviously preferred orientations, and displaved no developed faces, the problem of accurately orienting specimens had to be solved by X-ray methods alone. A rather efficient procedure which appears to have some new features was developed to this end and will be reported elsewhere. Qualitative flotation experiments to set upper and lower limits to the density were carried out for each compound, thereby facilitating the determination of the number of molecules within the unit cell. We did not attempt to determine highly accurate values for the lattice constants of the cells given by the X-ray patterns. However, we did make some very long exposures in a search for reflections which might require laiger cells or which might be significant in selecting the most probable space group.
B Y P. J. SCHAPIRO AND J. L. HOARD RECEIVED JANUARY 14, 1954
We have carried out X-ray investigations of three fluorocarbons believed to contain cyclobutane ring systems and find in each case that "molecular rotation" exists in the crystalline phase stable just below the melting point. Professor W. T. Miller kindly supplied samples of four related fluorocarbons, C&F4, C6Fl~,CsC14F8 and CeF,,. Conditions of synthesis suggest that CsFlo' is a bicyclohexane and that C8C14Fs2 and C8F1z3 are tricyclooctanes as indicated by the formulas
3347
Results and Discussion Some general results applying to all three coin1:- 1 I F- ' I i pounds are given first. Intensities of Rragg refleci i l l tions fall off extremely rapidly with increasing aniP i gle of scattering. Despite very long exposures the F-1 1Z-i l-F I , 1--I' maximum values of the Bragg angle 0 correspondF F F F C1 C1 F ing to recorded reflections are: CsC14F8, 40'; The C4CI4F4is known to be 1,2,3,4-tetrachlorotet- C6Fl0, 30'; and C4C14F4, 25". The Bragg reflecrafluorocyclobutane, but may contain one or tions appear against unusually strong backgrounds more of the four possible isomers. The high tem- of diffuse scattering which tend to develop into peratures used during synthesis4may have induced definite patterns. For C4C14F4 and C6Fll) espemolecular rearrangement (if required) to give pre- cially, diffuse spots accompany most or all of the dominantly the most stable isomer, presumably the Bragg reflections of fair intensity. For the latter isomer in which two halogen atoms of the same kind compound certain reciprocal lattice points appear attached to any two adjacent carbon atoms are to be joined as surfaces of diffuse scattering, altrans to one another. Of the four compounds though the intensity remains larger in the neighlisted only CsFlz does not show rotational disorder borhood of a Laue maximum. Three such surin the crystal. A one-molecule triclinic unit of faces, reflecting the hexagonal symmetry of the space group P i or P1 has been established by other crystal, were recognized. A further description of workers in this Laboratory, but attempts to deter- kirc thermaly diffuse scarrering wiil not be atmine the atomic arrangement are thus far unsuc- tempted as details of these patterns are significant only when recorded with an experimental refinecessful. A t room temperature C4C14F4, C6F10 and C8C14Fs ment beyond that we have used. Data characteristic of the individual comare clear, colorless, volatile solids, the first two being especially soft materials commonly formed pounds follow. C4C14F4.-The unit cell of C4C14Fd is body-centered into adhesive masses. At one atmosphere the liquid ranges given for C4C14F4 and CBFlo are, re- cubic with a = 7.95 *0.10 A.,and contains two molecules to give a calculated density of 1.75 g./cc. spectively, 42.3 0.3 to 130.1 & 0.2", and 40-41'. The triple point of C8C14Fsseems- to he above one The Laue symmetry class is the highest ~ Q S S ~ & atmosphere; a sublimation vapor pressure of 740 Oh-m3m, and the probable space group5is restricted mm. a t 164" is given. All three materials behave to the set without special criteria, Im3m, I432 and chemically as saturated compounds of relatively I?3m, with, in our case, the respective minimum high thermal stability. molecular symmetries of m3m, 43 and 33m. C&Fs.-The unit cell of CgC14Fs is face-centered Experimental 0.10 A.,and contains four Samples of all three materials were introduced into thin- cubic with a = 11.07 molecules to give a calculated density of 1.92 g./ walled Pyrex capillary tubes by sublimation under vacuum, the tubes then being sealed off. By choosing for each sub- cc. The Laue symmetry class is nib, and the stance an appropriate working temperature range within probable space group5 is limited to one of the set which a temperature gradient along the tube was maintained; a single crystal completely filling one end of the tube FmSm, F432 and FZSm with required molecular could. be grow-n-by sublimation. Cylindrical- specimens of. symmetries (one molecule per lattke point) of the desired radii and of more than adequate length could be m3m. 43. a m The gnup F&n might be considgrown in this way. It WXS possible also t o prepare a satisfactory powder specimen of the least volatile compound, ered less probable as freely grown crystals are CsCldF8. octahedra of quite uniform face development. Our data were derived mostly from single crystal photoCBFm-The unit cell of CsFlo is hexagonal with a graphs, the Weissenberg equi-inclination technique playing = 6.95 f 0.10, c = 11.2 f O . l O A . , ( G / U = 1.61 as the major role. For each substance photographs were taken with two or more orientations of the crystal, and Laue compared with 1.63 for hexagonal closest packing of and precession techniques also were employed in careful spheres), and contains two molecules t o give a calculated density of 1.86 g./cc. The Laue sym(1) A. H. Fainberg a n d W. T. Miller, private communication. metry is 6/mmm, and the probable space group5 is (2) F. W. McLafferty, Doctoral Thesis, Cornell University, 1950. F
F
F
F
e1
c1
F
~
,
~
*
(3) M. Prober a n d W. T. Miller, THISJ O U R N A L , 1 1 , 598 (1949). (4) R.
T.Carroll, Doctoral Thesis, Cornell University, 1952.
(5) "International Tables f o r X - R a y Crystallography,' Kynoch Press, Birmingham, England, 1952, Vol I
The
3348
NOTES
Vol. 76
one of the set Pci2c, P6smc and PB~/mmcrequiring there are two alternative configurations to be conthe glide plane vanishings 1 hh.1) for 1odd. No such sidered. The four chlorine and the four fluorine reflections were observed, but a total of only 21 atoms give two bisphenoids having a corntiion cenforms of all types could be recorded. However, a ter, the one elongated the other flattened along the threefold axis (and additional symmetry) will be unique axis. Probably there are two not very different minima in the niolecular energy correspondrequired of the molecule in any case. Rigorous application of space group theory re- ing to the assignment of chlorine atonis to o m or the quires every atom to have a single equilibrium po- other bisphenoid. The packing shapes of the two sition; assuming further only that discrete mole- configurations, as formulated by analogy with cules of the assigned composition exist in the crys- c4c&and C4F8, are quite different. Either gives tals we arrive a t the following conclusions. No dis- easy packing relations within the unit cul-e for crete molecule having just four atoms of one kind, many but not for random orientations of adjacent e.g., 4C1 in C4C14F4 or C8C14F8,can have the symme- molecules. Perhaps inversion of moleciilar contry of m3m or 43. Further, no molecule C4CI4F4or figuration between the two extremes plays an itnC&&F8 of chemically reasonable nature can meet portant role in the mechanism of “rotation” in the the requirements of 43m; in particular, neither a crystal; a further study utilizing data from the cyclobutane nor a tricyclooctane framework, how- purified isomer might be quite profitable. Based upon the formulas with condensed rings ever distorted, can do so. Nor does it seem possible to devise a configuration with threefold sym- for C6F10and CsC14Fs, packing models having an asmetry which is also chemically plausible for a mole- sumed angle of folding of 120’ between rings sharing cule C6FlO. Certainly the bicyclohexane franie- an edge (with or without puckering of the individual rings) appear to give packing relations within the work cannot have a threefold axis. We conclude that for all three substances the respective cells much like the case of C4C14F4; cermolecules in the crystal must have sufficient rota- tain molecular orientations give easy packing relational mobility to give the observed diffraction tions, but reorientation must involve a codperativc symmetries as statistical averages over groups of mechanism. We may conclude that the bond diacells containing molecules in various orientations. grams suggested by the methods of synthesis are Previously cited data on the Bragg intensities, the not inherently improbable in terms of our strucdiffuse background scattering, and the physical tural data. properties of the crystals support this conclusion. We wish to thank the Atomic Energy CommisHowever, the “molecular rotation” cannot be sion for support of this w o r k u n d e r Contract No. unhindered as is shown by direct experimental evi- AT(30-1)-578 with Cornel1 University. dence of the following sort. Were the rotation BAKERLABORATORY OF CHEMISTRY entirely “free” the effective electron density in the CORNELL UNIVERSITY NEW YORK molecule would be independent of angular vari- ITHACA, ables, ;.e., be spherically symmetric, and the reflection intensity would be determined by the value Studies in Low Concentration Chemistry. VIII. of the quadratic form. But, for example, in c&&of Tracer Yttrium, Antimony and Fs (333) has several times the intensity of (511), Some PropertiesSilver in Solution and (551) is far more intense than (711). A slightly BY GEORGE K. SCHWELTZEFAND W. MORRISON JACKSON different analysis applies to the several possibilities for placing two molecules of C6F10 within the hexRECEIVED JANUARY 8, 1954 agonal unit. Unrestricted rotation of the molecule The purpose of these experiments was to investiabout the threefold axis (cylindrically symmetric gate the radiocolloidal properties of several ions in electron density) would in every case require a characteristic extinction of certain reflections which very low concentration solutions. Yttrium-00, are experimentally recorded. The existence of a antimony-125 and silver-11 1 were used as tracers. Experimental restricting potential of appropriate symmetry can Solutions.-Solutions of yttrium-90 0.01 N in hydroscarcely be doubted; the strong coupling of rotachloric acid and silver-111 0.01 N i n nitric acid were prepared tion with lattice vibrations is indicated. previously reported methods.’ A solution of antimonyIire have not attempted to calculate Bragg in- by 125 0.01 N in hydrochloric acid was prepared by dilution of tensities from an assumed model. Except possibly a stock solution obtained from the Oak Ridge National for C4C14F4,our lack of definite knowledge of the Laboratory. The concentrations of the yttrium and silver M and about were established as below equilibrium molecular configurations makes the solutions M by spectrographic analysis,’ and the concentration of the problem a poor choice for study of molecular rota- antimony was established a t about lo-’ A l by a microchemition; and, in contrast with the diffuse background, cal test by Feigl.3 Techniques.-Adjustments of pH, sample preparations, the Bragg intensities are not mficient!y sensitisre radioactivity measurements, as well as filtration, centrifuto details of the model to promise a strong case for gatiqn and adsorption techniques have been described in a a particular assumed configuration. The simplest previous paper.‘ The main difference in procedure is that case of C4CI4Fais probably quite complex. Assum(1) G. K.Schweitzer, B. R. Stein and W.M. Jackson, THIS JOURNAL, ing that we have the stable isomer and that the cy- 76,793 (1953); G. K. Schweitzer a n d J. W. Nehls, ibid.,74,6186 (1952). clobutane ring is markedly puckered as in C ~ C I S ~ (2) Private communications from J. H. Gillette, Oak Ridge National a n d T. DeVries, Purdue University. and C4F87to give a molecule of symmetry D 2 d , Laboratory, (3) F. Feigl, “Laboratory Manual of Spot Tests,” Academic Press, (6) T. B Owen a n d J L H o a r d , A c f a C r y s t , 4, 172 (1951) (7) H F. Lemaire a n d R L. Livingston. THISJ O U R N A, I74, 5732 1952).
I n c . , New York, N. Y., 1943. ( 4 ) G. K. Schweitzer and W. N. Bishop, THISJ O U R N A L . 75, 6330 (1953).
