R. L. HILDERBRANDT, A. I;. ANDREASSEN, AND S. H. BAUER
1586
An Electron Diffraction Investigation of Hexafluoroacetone, Hexafluoropropylimine, and Hexafluoroisobutene
by R. L. Hilderbrandt, A. L. Andreassen, and S. H. Bauer Department of Chemistry, Cornell University, Ithaca, Nezo York
14860
(Received September 9, 1969)
The structures of (CF&C=O, (CF&C=NH, and (CF&C=CH2 have been determined by gas-phase electron diffraction. Diffraction photographs for (CH&C=O, the structure of which is well known,lJ were analyzed concurrently to provide a check on the scale. It appears that feplacement of CH, by CF3 leads to a longer O C bond (by 0.03-0.04 b). Thus, C-C = 1.549 i: 0.008 A in (CF&C=O, 1.533 f 0.006 b in (CF3)2C=CHn, and 1.549 f 0.007 b in (CF&C=NH in contrast to 1.507 in acetone and 1.505 b in isobutene. This is accompanied by an appreciable lengthening of the C=X bond: C=O = 1.246 f 0.014 b and C=C = 1.373 A 0.013 b. The C-F bond distances obtained are in agreement with those found in previous studies: C-F = 1.335 i 0.002 b in (CF&C=O, 1.327 f 0.002 b in (CF3)2C=CH2, and 1.324 i 0.003 b in (CF3)ZC=NH. The C=N distance for (CF&C=NH (1.294 ri: 0.029 b) appears to be the first estimation of this bond length. The structural parameters determined for (CH&C=O are in excellent agreement with those obtained in recent microwave studies.liz
a
Introduction Because of its high electronegativity and small size, fluol'ine when substituted into various compounds leads to significant structural changes. The effects of fluorine substitution into ethane, for instance, have been studied in depth, and the trends are well established.a-6 In recently improved molecular orbital calculations,s fluorine substitution is used as a test of the theory, to check whether changes in molecular geometries are correctly predicted. It is therefore of interest to investigate structural trends in fluorinesubstituted compounds in order to provide the needed experimental data for correlation with theory and with their well-known chemical behavior. While the consequences of fluorine substitution on carbon-carbon single- and double-bond lengths have been the subject of numerous structural investigalittle work has been done on compounds tions, containing the system FaC-X=Y. Schwendeman's4 recent study of acetaldehyde and l,l,l-trifluoroacetaldehyde suggested that FsC-X=Y may prove to be an unusual grouping. The present investigation revealed that upon fluorine substitution the C-C bond length increased, contrary to most previously observed trends. 8-817-10
Experimental Section The were Obtained from the SOUrCeS: acetone, from Fisher Scientific (Certified ACE Spectroanalyzed) ; perflUOrOaCetone, from Pierce Chemical Co., Rockford, Ill. ; hexafluoroacetonimine from the Hynes Chemical Research carp., ~ ~ N. c. The sample of hexafluoroisobutene Was graJ. Middleton ciously supplied by c. G. Krespan and of E. I. du Pont de Nemours and GO. Gas chro-
w.
The Journal of Physical Chemistry
matographic analysis of these samples indicated better than 95% purity for all of them, and therefore no further purification was necessary. Sectored diffraction photographs were taken with the dual-mode instrument,l' in the convergent-beam mode. The sector was cut to level the background scattering produced by atomic carbon. Two sets of data were obtained for each compound , corresponding to the following conditions: 62 kV, at 253-mm nozzleplate distance, covered the range from p = 5 to 65 A-l, and 62 kV, at 124 mm, convered the range p = 15 to 125 8-l. Exposures ranged from 30 to 120 sec, at a beam current of 0.3 PA, for sample inlet pressures of 5-10 Torr. Kodak Process plates were used. The fluorine-substituted samples , gaseous at room temperature, were introduced into the diffraction chamber through a double-needle valve regulator. The acetone sample was cooled to -35" with a Dry Iceethanol bath to provide an equilibrium pressure between 5 and 10 Torr. A magnesium oxide powder sample situated above the nozzle tip was introduced to establish the p scale for the resulting patterns. (1) R.Nelson and L. Pierce, J . Mol. Spectrosc., 18, 344 (1965). (2) J. D.Swalen and C. C. Costain, J. Chem. Phys., 31, 1562 (1959). (3) K . Kuchitsu, ibid., 49, 4456 (1968). (4) R. Schwendeman, Thesis, University of Michigan, 1956. (5) D. A. Swick and I. L. Karle, J . Chem. Phys., 23, 1499 (1955). (6) M. s. Gordon and J. A. Pople, ibid., 49, 4643 (1968). (7) B. Bak, D. Christensen, L. Hansen-Nygard, and J. RastrupAndersen, Spectrochim. Acta, l3) 120 (lg5@. (8) J. K. Tyler and J. Sheridan, TTana. Faraday SOCV 79, 2391 ~ h ~ ~ , (1957). (9) v. w. Laurie and D. T. Pence, J . Chem. Phys., 38, 2693 (1963). (10) I. L. Karle and J. Karle, ibid., 18, 963 (1950). (11) H. Bauer and K. Kimura, J . Phys. 8oc. Jup., 17, 300 (1962)
s.
I
1587
ELECTRON DIFFRACTION OF (CF3)2C0,(CF3)2CNH,AND (CF3)&CH2 For each operating condition pairs of light and dark plates were selected and microphotometered on the modified double-beam Jarrel Ash densitometer, which is equipped with a rotating stage driven at 600 rpm.12 The analog signal from the densitometer was recorded on a Bristol strip chart recorder. The transmittances were read at 0.25-mm intervals, and the intensities were corrected for flatness and saturation,13 in the usual manner. The experimental relative intensities , interpolated at integral q [=(40/X) sin (0/2)] values over the range 6-125, have been deposited with NAPS, as document NAPS-00777. The data were analyzed in the conventional manner using the radial distribution functions and least-squares fitting of the intensity patterns. The elastic and inelastic form factors of Tavard, et a1.,14were used in con junction with the Ibers and Hoerni16 phase-shift approximation. The radial distribution curves were used primarily for refinement of the pattern background in the manner described by the Karles,lBwhile the final parameters and error estimates were obtained by the least-squares fitting of the experimental sM(s) curve.
Analysis Since the parameters obtained from the analysis are subject to the constraints imposed upon the molecular geometry, it is necessary at the outset to make explicit the assumptions concerning the structures. In all cases, the three carbon atoms and the atom attached to the central carbon were assumed to lie in a plane. While this is consistent with the widely accepted condition for sp2 hybridization, it must be viewed objectively as an imposed constraint. The hydrogens attached to the nitrogsn of the imine and to the carbon in isobutene were also confined to the plane of the carbon atoms. The perfluoromethyl groups were required to maintain Cavsymmetry about the C-C bond axis; i.e., F2 was obtained from Fl by a 120" rotation about the C-C axis, and F3 was obtained from F2 by a further 120' rotation. This is equivalent to constraining the three C-C-F angles to be equal; similar conditions hold for the three F-C-F angles. Finally, the molecules were assumed to have overall CZvsymmetry, with the exception of the hydrogen attached to nitrogen in the imine. Had it been found during the course of the analysis that these restricted models were inadequate to account for the recorded features of the intensity and radial distribution curves, some of these constraints might have been dropped. I n light of the excellent agreement obtained under the above assumptions, this was not necessary. Coordinates for the various models tested were calculated with a recently written algorithm for converting internal coordinates, such as bond distances, valence angles, and dihedral angles, into Cartesian co-
Figure 1. Minimum energy conformation for (CFa)&O. View along CI-CS line.
ordinates. l7 This innovation greatly increased the speed and simplicity of the computations. Subject to the above mentioned constraints, various models were tested for different combinations of the C-C-C angle and the rotational conformation of the methyl groups about the C-C axis. The model which was found to give a minimum in the residuals in the least-squares computation was one in which the fluorines were staggered ( T = 36") when viewed along an axis drawn through the CB' .C3 atoms. This staggering is shown in Figure 1 for perfluoroacetone. Similar results were obtained for acetone although the angle was slightly smaller (T = 33"), but the uncertainty was appreciably greater. Clearly the nonbonded repulsions play an important role in determining the equilibrium structure. To justify the approximation in using models with no extensive interval rotations, it is necessary to estimate the barriers to rotation of the methyl groups. Berneyl8 and Plaush and PaceIe reported that the barrier height in hexafluoroacetone is 2800 and 1470 cal/mol, respectively. The case for acetone is not as convincing, however. Swalen and Costain2 estimated the barrier height to be in the vicinity of 770 cal/mol. The rigid-molecule assumption is still justifiable in the latter case as long as it is qualified by including large mean-square amplitudes of motion for the various nonbonded distances. The radial distribution curves show (Figure 2) that the dominant scattering contribution in the bonded region is due to the six C-F pairs. This outweighs the C-X contributions by a factor of 6, and the C-C contribution by a factor of 3. In addition, the near equality of the C-F and the C=X bond lengths
-
(12) S. H . Bauer, R. Jenkins, and R. L. Hilderbrandt, in preparation. (13) J. L. Hencher and S. H.Bauer, J . Arner. Chern. Soc., 89, 5527 (1967). (14) C . Tavard, D.Nicholas, and M. Rouault, J . Chem. Phys., 64, 540 (1967). (15) J. A. Ibers and J. A. Hoerni, Acta Cryst., 7, 405 (1954). (16) I. L.Karle and J. Karle, J . Chem. Phys., 17, 1052 (1949). (17) R. L.Hilderbrandt, ibid., 51, 1654 (1969). (18) C.V. Berney, Spectrochim. Acta, 21, 1809 (1965). (19) A. C.Plaush and E. L. Pace, J. Chem. Phys., 47, 44 (1987). Volume 7 4 , Number 7
April 8, 1970
R. L. HILDERBRAWDT, A. L. ANDREASSEN,AND S. H. BAUER
1588
I
I
I
0.5
I
I
I
I
I
l
l
I
l
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 55
1.0
l
60
I
6!5
0.5
I
l
1.0 1.5
l
l
l
2.0 2.5 3.0
I , I
I
I
I
l
l
l
l
1
1
1
3.5 4.0 4.5 50
I
I
1
1
5.5 60
65
r(h-
r(ii)I
-
---
b -
I
I
I
I
I
I
1
1
1
1
1
1
1
(cF,),c=N~~
(CFJ,C = CH,
--
--.
L
r
u-
I
0.5
l
l
1.5
1.0
l
l
2.0 2.5
l
e ,
l
l
l
l
3.0 3,5 4.0 4.5 5.0
l
55
l
6.0 6.5
I
I
I
0.5
ID
1.5 2.0 2.5 3.0 3.5
J
I
I
I
l
1
1
1
l
4.0 4.5 5.0 5.5 6.0
5
Table I : Least-Squares Values for (FaC)2C=Y
A
F-C,
c-c, ii
C=Y, ii Y-HI FCC, deg CCC, deg CYH, deg Torsion, deg IC-c, IC-F,
IC-Y,
A A
ii
~Y-H, I C s . aF,
A
1 , ~ .. . F .A U
(FsC)zC=O
(FsC)zC=NH
(FaC)zC=CHz
1.335 f 0.002 1.549 i0.008 1.246 f 0.014
1.324 f 0.003 1.549 1 0 . 0 0 7 1.294 & 0.029 1.02 f 0.05 110.0 f 0.4 121.6 f 0.4 110.0 f 9 35.1 f 1 . 0 0.051 f 0.008 0.052f0.004 0.051 f 0.024 0.050f0.034 0.068f0.005 0.063 f 0.004 0.02684
1.327 f 0.002 1.533 f 0.006 1.373 f 0.013 1.07 f 0.04 110.5 f 0.2 123.6 f 0.3 111.0 i4 36.9 f 0 . 7 0.054f0.007 0.052 f 0 . 0 0 5 (0.049) assumed 0.067 zk 0.034 0.066f0.003 0.062f0.004 0.02116
110.3 f 0.3 121.4 i 0 . 4 36.6 i 1.1 0.457 i 0.013 0.052 f 0.003 (0,049) assumed 0.088 i 0.005 0.062 f 0.003 0.03278
leads to the rather large uncertainties obtained for the C=X distances (Table I). In the case of acetone, this complication toes not arise and the C=O uncertainty is *0.003 A. However, in spite of these large uncertainties, the fact that the C-C and C=X bonds are longer in the fluorine- than in the hydrogensubstituted compounds is apparent. Initially the magnitude of this effect was suspected as arising from an unknown systematic error, and for this reason The Journal of Physical Chentastry
another set of data was obtained for each compound along with photographs for acetone. The latter data were taken under identical conditions and analyzed by the same procedures used for the other compounds. It is believed that any systematic errors such as scale factors and program errors in the analysis would thus have become apparent. For acetone, the parameters thus evaluated reproduced reasonably well those obtained in the microwave studies1t2 (Table 11). The
1589
ELECTRON DIFFRACTION OF (CF3)&0, (CF3)2CNH,AND (CFa)&CH2 Table I1 : Parameters for Acetone
H-C, C-C, C=O,
Nelson and Piercea
Swalen and Costainb
(pw,’’ 1965)
(pw,” 1959)
1.085 f 0.007 1.507 f 0.003 1.222i0.003
1.086 =k 0.010 1.515 f 0.005 1.215i0.005 (assumed) 110.3 116.1 49.7315 59.3676 102.9563 Axis of CHa group up 1.5’ from C-C axis
R b
110.1 117.2 =k 0 . 3 49.7296 59.3698 102.9559 Axis of CHI group up 1.5’ from C-C axis
HCC, deg CCC, deg I A , amu i2 amu ba amu bz Remarks
IB, IC,
a R. Nelson and L. Pierce, J . Mol. Spectrosc., 18, 344 (1965). Microwave. Electron diffraction.
This work 1969)
1.076 f 0.006 1.507 i0.002 1.210i0.003 111.7 i 1.5 116.7 f 0 . 3 49.12 59.26 102 * 35 Torsion 33 f 6’
J. D. Swalen and C. C. Costain, J . Chem. Phys., 31, 1562 (1959).
(CH,),C
-
v
W
I ~
I.
I
I
I
I
I
I
4.
I
4
1
-
5.0
r(X)
Figure 3. Comparison of the experimental radial distribution curve for (CFa)&O with those calculated for Boulet’s model and for the structure deduced in this investigation.
largest discrepancies appear in the C=O and C-H distances, Perhaps if the C-H distance were corrected for anharmonicity this difference would be reduced. The moments of inertia listed in the last column of Table I1 were calculated for the electron diffraction inodel, assuming a rigid structure, while the entries in the other two columns were obtained
b
I
IO
I
20
1
1
30
40
4
q
=0
I
l
l
I
I
60
70
80
90
100
-
(I-’)
Figure 4. Comparisons of the experimental and calculated intensity curves. For each pair, the dotted curves show the differences between them.
from the observed rotational constants. The agreement is fairly good considering the unavoidable approximations. Supporting evidence for the observed parameters in these molecules is the fact that although the C-X and C-C distances appear to be appreciably longer than anticipated, the C-F bond lengths are in excellent agreement with all other previously reported investigations. The structure of perfluoroaceVolume 74, Number 7 April 8, 1070
1590
R. L. HILDERBRANDT, A. L. ANDREASSEE,AND S. H. BAUER
r(O-CI-CZ-FII
T(C4- CI-C2-FI
i
36.6'
I ~36.9'
Figure 5. Minimum energy conformations and least-squares calculated parameters.
tone has previously been studied20 by electron diffraction. The quoted bond lengths (C-F 1.321 8, C=O 1.185 A, and C-C 1.527 8) are considerably shorter than those determined in this work, while the angular parameters agree quite well with those deduced in the present analysis. I n Figure 3 the radial distribution curves calculated for our model and for Boulet's model are compared with the experimental radial distribution curve based on our data. While most of the observed features are reproduced by Boulet's model, there are noticeable discrepancies possibly arisingkom an error in the scale factor. During the course of the analysis it was not possible to vary all of the mean-square amplitudes of vibration. The bonded ll.,'s were varied with the exception of C=O and C=C in perfluoroacetone and hexafluoroisobutene, respectively. Attempts to vary these parameters produced large correlations among the other parameters, particularly those distances under the bonded peak of the radial distribution curve. The Id;s used in these cases were estimated from corresponding unsubstituted species. The two nonbonded Ll)s The Journal of Physical Chemistry
-
which could be varied were those for C1.. F and F F on the same carbon, both with rather low uncertainties. The remaining ljl)s were either assumed or obtained by visual matching with the experimental radial distribution curve. The final theoretical and experimental intensity curves for the best fitted least-squares models are shown in Figure 4. The errors quoted for the parameters listed in Table I are three times the standard deviations obtained in the least-squares calculations. As previously shown21 this always places the quoted errors outside of the experimental errors estimated as due to calibrations and reading of the plates. The distances quoted are the rg(l) parameters defined by Bartell.22 No attempt has been made to correct these values for anharmonicities or shrinkage. Also no attempt was made to take into account correlations +
(20) G. A. Boulet, Thesis, University of Michigan, Dissertation Ab&., 25, 3283 (1964). (21) R. L. Hilderbrandt and S. H. Bauer, J. Mol. Struct., 3 , 326 (1969). (22) L. S. Bartell, J. Chem, Phys., 23, 1269 (1985).
ELECTRON DIFFRACTION OF (CF&CO, (CFa)zCNH, AND (CF3)zCCH,
1591
Table I11 : Dimensional Changes Due to F-H Substitution
c-x bond length,
X
Hac-CHa FaC-CHa FaC-CFa HzC= CHz FzC= CHZ FzC= CFz H-CH F-CH Hac-NHz HaC-NF2 HCO FHCO FzCO
-CHz -CFs =CHg =CF2 =CH -NF, =O =O
A
1.5319 1.512 1.56 1.334 1.315 1.313 1.205 1.198 1.465 1.449 1.230 1.181 1.174
Change in bond length,
A
Ref -0.02 +0.02
Ref -0.019 -0.021
Ref -0.007
Ref -0.016
Ref -0.049 -0.056
Reported by
Kuchitsu,s E D Schwendeman,4 ED Swick and Karle,6 E D Bartell and Bonham,O E D Laurie and Pence,g pw Karle and Karle,lo E D Christensen, Eaton, Green, and Thompson,b ir Tyler and Sheridan,8 pw Higgeiibotham and Bartell,c E D Pierce, Hayes, and Bucher,d pw Davidson, Stoicheff , and Berstein; pw Miller and Curl,' pw Laurie and Pence,Qpw
L. S. Bartell and R. H. Bonham, J. Chem. Phys., 27, 1414 (1957).
M. T. Christensen, et al., Proc. Roy. Xoc., A238, 15 (1956). L. Pierce, R. G. Hayes, and J. F. Bucher, ibid., 46, 4352 R.F. Miller and R. F. Curl, Jr., ibid., 34, (1967). e D. W. Davidson, B. P. Stoicheff, and H. J. Bernstein, ibid., 22, 289 (1959). 1847 (1961). a
' H. K. Higgenbotham and L. S. Bartell, J . Chem. Phys., 42, 1131 (1965).
'
of adjacent data points; however, the p / h parameter suggested by Morino, Kuchitsu, and MurataZ3 was calculated and found to be approximately 0.30, indicating a relative off-diagonal weight of -0.30 would be appropriate. It is felt that multiplication of the standard deviations by a factor of 3 more than compensates for the neglect of the nondiagonal elements.
distance is only 0.017 less than in the hydrogen analog. The longer C=X separations observed in these compounds might indeed be due to the presence of CFI groups but additional work is needed to substantiate this. In particular, the structures of (CF,) (CH3)CO and (CFa)(CH,)CCHZ may reveal C=X bond lengths intermediate between the unsubstituted and hexasubstituted species. Discussion Although the INDO calculations by Gordon and The results of this investigation (Figure 5 ) are conPople6 are not sufficiently precise to check directly sistent with the results reported by S c h ~ e n d e m a n . ~ against the observed interatomic distances, onemay have Three fluorine atoms substituted for hydrogens in a anticipated that their predicted differences due to H-F methyl group attached to a carbon which is in turn substitution on adjacent bond lengths would be of the doubly bonded to a third species increase the length correct sign and (hopefully) magnitude. I n eight of the C-C bond by 0.03-0.04 A. Further inspection cases, three check fully, two show the indicated trend of fluorine-substituted compounds for which structures but the magnitudes are wrong, and for the remaining have been determined revealed that, in general, two three cases, the directions of the increments are in error. types of bonding exhibit two distinct trends. Hcpd). For the Deviations are defined as ( F c p d Case 1 is schematically represented by F3C-X, FZC= C-C bond length in CzH6us. C2F6, d(C-C) = +Ob03 8; X, or FC-X, where X is N, C, 0, or a halogen but is for N-N in NzH4vs. NzF4,6(N-N) = +0.03 A; for not double bonded to a third atom. In this case fluorine C=N in HCN us. FCN, d(C=N) = 0 (calculated for hydrogen substitution decreases the CX bond length, values = observed values). For C-F, in the sequence with one notable exception: perfluoroethane.s Evi- (C-F),=3 = CH,F,-,, they predicted (C-F).,o dently, the nonbonded fluorine-fluorine repulsions in -0.01 A, while the observed magnitude is -0.06 A; this molecule dominate and the C-C bond is lengthened also 6(N-F)calcd = 0 for the pair NHFz vs. NF3 while to 1.56 A. d(N-F)obsd = -0.03 A. Finally, for the pair CzHd Case 2 can be represented by F3C-X=Y, where X us. C S F ~ { (C=?)E - (C=C)H)calcd = +0.02 8, while is C or N, and Y is C, N, or 0. In this case the obser6obsd = -0.02 A; for C=O in HzCO VS. FzCO, bcalcd = vations indicate a lengthening of the CX bond. Table 0 while &bsd = -0.06 8; for 0-0 in HzOz V S . FzOz, I11 and Table IV illustrate these two cases. The longer doalcd = $0.01 B while 8obsd = -0.27 A. Clearly some than anticipated C-X (X = N, C , 0) separations found in the present investigation appear to be distinctive of this type of molecule. In 1,1,l-triflu~roacetaldehyde~ (23) Y. Murata and Y . Morino, Acta Cryst., 20, 605 (1966). the C=O distance is almost the same as in the parent (24) C . H. Chang, R. F. Porter, and 8. H. Bauer, private oomcompound, while in hexafluoroa~omethane~~ the N=N munioation.
-
Volume 74, Number 7 April I,1970
R. L. HILDERBRANDT, A. L. ANDREASSES, AND S. H. BAUER
1592 Table IV : Dimensional Changes Due to F-H Substitution
c-c or C-N bond lengths,
Change in bond lengths,
Y
A
A
Ref $0.036 Ref $0.042 Ref { $0.041 Ref
H&HC=O F,CHC=O (H&)zC=O (FaC)zC=O { (HIC)~C=NH} (FaC)tC=NH H3CN=NCH3 (trans)
=O =O =O =NH =NH =NCHs
1.504 1.540 1.507 1,540 11.511 1.549 1.474
F&N=NCHa (trans) F&N=NCF3 (cis) H~CCECH F~CCECH
=NCHs =NCF3 =CH =CH
1.476 1.490 1.459 1 464
$0.002 $0.016 Ref $0.005
H~CCECCH~ FsC@CCHa FsCCzCCFs (HaC)z=CHz (F&)Z=CH2
ECCH~ ECCH~ ECCF~ = CHz == CH2
1.467 1.464 1.475 1,505 1.533
Ref -0.003 $0.008 Ref +0.028
-0
I
Reported by
Schaendeman,4 ED Ref 4 Kelson and Pierce,' pw This work, ED Estimated This work, E l l Chang, Porter, aiid B a ~ e r , ~ ~ E Il Ref 24 Ref 24 Costain," p~ Shoolery, Shulmaii, Sheehan, Schomaker, aiid Yost,* E D and p w Chang and Bauer,d E D T, W.Laurie,' pw Chaiig and BauerJdE D Bartell and BoiihamjeE D This work, E D
G. Shulman, W. F. Sheehau, 1'. Schomaker, and I). 31.Yost, a C. C. Costain, J . Chem. Phys., 29, 864 (1958). * J. N. Shoolery, 1%. Unpublished data. e L. S. Bartell and R. A. Bonham, J. ibid., 19, 1364 (1951). ' V. W. Laurie, J . Chem. Phys., 30, 1101 (1959). Chem. Phys., 32, 824 (1960).
significant interactions were neglected in these calculations. It is interesting to note that their predictions of bond angles proved more successful than of bond lengths. The conformations of the CFa groups around the C-C bonds deduced in this investigation indicate that the minima in potential energy are not a t the most symmetric positions. We believe this to be a real effect, and not merely an artifact of large amplitudes of torsional vibrations. Indeed, the helical twist in the C-C backbone of perfluoropolyethylene chainsZ5J6
The Journal of Physical Chemistry
is of a magnitude which checks closely with that expected for T = 35".
Acknowledgments, The authors wish to thank the Material Science Center of Cornel1 Universit). (XSC ARPA SD-68) for their financial support, and we also t,hank Drs. Krespan and 1Iiddleton for supplying us with a sample of hexafluoroisobutene.
c.
(26) l l r . B~~~ and E. R, ~ ~ ~xatzLre, ~ 174, ~ 549 l (1954). l ~ , (26) E. s. Clark and L. T. MUUS, 2. K ~ y s t . ,117, 119 (1962).
