THECRYSTAL STRUCTURE OF BIPHENYLENE
Dec., 1944
equation of the straight line representing these data, obtained by the method of least squares, is CpO= 3.32
+ 0.516T
The values obtained with this equation are given in the last column of Table 11. By comparison of the results attained with air and n-butane with the apparatus and technique employed, i t is believed that the over-all accuracy of the 1,3-butadiene heat capacities is better than 1 part in 100. Acknowledgment.-The authors wish to thank Dr. C. S. Cragoe of the National Bureau of Standards for his interest in the problem and for
[CONTRIBUTION KO.975 FROM
THE
2035
his efforts in securing the sample of butadiene. They also wish t o thank the Phillips Petroleum Company of Bartlesville, Oklahoma, for the liberal sample of highly purified butadiene used in this infestigation. Summary 1. The heat capacities of n-butane and of air were determined, for calibration purposes, a t 25’. 2. Six values of the molar heat capacities of 1,3-butadiene were determined over the range 5 to 105’. AUSTIN,TEXAS
RECEIVED AUGUST,21, 1944
GATESAND CRELLINLABORATORIES OF CHEMISTRY, CALIFORNIA INSTITUTE OF TECHNOLOGY]
The Crystal Structure of Biphenylene BY JURG WASERAND CHIA-SI’Lu In an earlier paper,‘ which reported on the electron dzraction of biphenylene molecules, C12H8, the configuration of the carbon atoms in these molecules, as suggested by Lothrop’s synthesis,2 was confirmed, and values for various interatomic distances were assigned. In addition resonance energy and bond strengths were calculated. The present investigation deals with the crystal structure of biphenylene, and provides further proof that the compound investigated is indeed dibenzcyclobutadiene. Unit Cell and Space Group It was found that sublimation of biphenylene under controlled conditions resulted in sharp needles, which were, however, much too thin for X-ray work. Recrystallization from n-propyl alcohol gave satisfactory crystals. They were prisms, 0.25-0.5 mm. thick and 2-3 mm. long, with side faces belonging predominantly to the forms { 110), {310},{loo}and { O l O ) . Thestraw-colored crystals had no distinct cleavage and showed no abnormal birefringence. Due to their appreciable vapor pressure a t room temperature, their faces disappeared within a few hours on standing in open air, and the crystals evaporated completely within a few days. All X-ray photographs showed a rather large temperature factor. Rotation and Weissenberg photographs about the three crystallographic axes led to the following dimensions of the monoclinic unit cell: a0 = 19.(jO * 0.03 A., bu = 10.50 * 0.02 k., cu = 5.84 0.02 k., /3 = Y1°20/ 20’. The absence of (h01) reflections with odd It and of (040) reflections with odd k indicates c&P2Ja as the probable space group. Rough density measurements by flotation in an aqueous f
f
J. Waser and V Schomaker, THISJOURNAL, 66, 1451 (1943) (2) W C Lothrop, tbrd , 69,1187 (1941); 64,1698 (1942). (1)
solution of potassium iodide gave p = 1.24 g./cc. Hence there are six molecules per unit cell, the calculated density being p = 1.25 g./cc. Intensities of (hkO) and (h01) reflections were estimated from Weissenberg photographs taken with unfiltered Cu K radiation, using the multiple film t e c h n i q ~ e . ~The specimens chosen for diffraction work were small enough to make absorption corrections unnecessary. It was very difficult to obtain satisfactory visual intensity correlations between different (h01) reflections due to their varying sizes, as for these reflections the axis of rotation of the crystal was perpendicular to the needle axis. I t was not possible to cut a crystal sufficiently short so that its length would approximate its thickness Since there is no similar difficulty for (hkO) reflections, the intensity values obtained for them should be much more reliable than the ones for (h01) reflections. No quantitative intensity data for (Okl) reflections were collected, since a Fourier projection along the a axis was not expected to show any resolution. The relative intensities obtained were corrected for the Lorentz and polarization factors with Lu’s chart4 and the scale of their square roots was adjusted to approximately absolute scale by comparison with the values calculated from the final structure. These experimental structure factors are recorded in the second. columns of Tables I and 11 in the order of decreasing spacing, except that the values for (h01) and (h01) have been grouped together. Determination of the Structure 111 the space group c i h - P&/a there are four sets of two-fold positions with the point symnietry (3) J J D e Lange. J M Robertson and I Woodward, P i o ~R o y JOG(London), A171, 398 (1939) (4) C S Lu, Rev Scr I n s f , 14,331 (1943)
2036
Vol. 66
LTJ
JURG WASER AND CHIA-SI
TABLE I (hkO)
IFoba.i Fc22d,
301 020
320 600 610 330 620 040 340 630 910 640 920 350
930 650
060
360 940 ;2.0.0 12.1.0 12.2.0 660
950 12.30 370 12.4.0 960
34 51
40 87 62 99 37 -34 38 -37 50 69 0 7 11 11 19 -23 0 - 2 0 4 0 - 8 31 31 23 13 35 -20 32 14 41 40 0 1 29 27 30 -25 0 - 4 15 - S 32 -32 15 -10 13 9 0 8 16 16 0 0 0
-
1
- 8 670 - 8 080 0 0 15 1.0 8 12 5.0 12 0 - 5 380 1 15.2.0 0
TABLE I1 (11
(hkO)
IFah..l F&d. 970 0 - 8
15.3.0 680 12.6.0 15.4.0 390 980 15.5.0 12.70
22 0
19 0
0 20 0 0 690 0 18 0 . 0 35 18 1.0 14 18 2.0 16 0 10.0 14 15 6 0 0 18 3 . 0 0 3.10 0 33 990 0 12.8.0 0 18.4 0 0 6 10.0 0 15.7.0 0 18.5.0 0 12.9.0 0 9.10.0 12 3.11.0 0 21.1.0 21 15.8.0 0 18.6.0 11 21.2.0 18 6.11.0 0 21.3.0 8 12.10.0 0 18.7.0 0
-22 - 4 22 - 8 5 -21 4 - 7 1 -39 14 -14 22 0
Wo)
(1)
IFoba.1 Fc,lod.
21.4.0 0 0.12.0 21 9.11 0 0 15.9 0 0 3.12.0 0 21.5.0 0 6.12.0 0 18.8.0 0 12.11.0 0 21.6.0 0 15.10.0 0 24.0.0 10 24.1.0 0 9.12.0 0 2-1.2.0 0 3.13.0 0 18.9.0 0 24.3.0 0 21.7.0 0
26
aoi
6
7 1 5 1
201 400 4oi 401 600
37 5 18 19 37
002
0
GOT
26
5 15
so1 202
32 10 38 42 13
-6
- 1
-
1 2 9 7 3 - 3
4 -24 - 2 9 - 6 - 2 2 -6 - 5 -11 - 2 IFm/ observed for h # 3n‘) -19 4 -12 220 7 -19 230 9 -11 - 5 420 5 - 6 520 6 530 6 3
Cf.also Table 11.
Ci: 000, 440; $00,030; 004, $43; and $04, 033; and sets of four-fold general positions: *(xyz, 3 x i - yz). As there are six molecules in the unit cell, a t least two must have centers of symmetry with these centers placed in the set of twofold special positions. The special positions 000, 330 may be chosen without loss of generality. The considerations of the next paragraph rule out special positions for the other four molecules, which accordingly are placed in the set of general positions. All atoms will thus in general occupy four-fold positions. The reflections (hkO) show a very distinctive feature. Except for a few, all of which are very weak, only reflections (hkO) with h = 3n appear. The reflections (h01) show no similar regularity. Assuming in a first approximation that all reflections (hkO) vanish unless h = 3n (“h = 3n rule”), atoms have to be grouped in triplets, the partner in each triplet occupying the following 4 yzz, x.+ 9 yzs. The set of positions: xyz1,x part of the structure factor due to each triplet then becomes zero unless h = 3n. The seventy-two carbon atoms (and the fortyeight hydrogen atoms, which, however, will be neglected) thus fall into groups of twelve atoms, the positions of which are interrelated either by space-group operations or by the requirements of the “ h = 3n rule.” The coordinates of such a set of positions are: x1 yl zltl5171 21, $ x1 x1 y1 7 z:, t - Y1 u”1, 1 - x’1 3 y1 21; z
+
+
+
+
+ +
(h01)
200 001
4 29 4 -2 2
-
(hO1)
202 402 40‘‘
-28 2 44 0
27 15 -34 1 29 -33 13 -32 23 18 0
-31 - 5 47
1* 19
14 -40 0 23 -33 9 -34 32 12 0
800
0
8oi 801
0
0
0
3 11 SO2 8 602 10.0.0 6 5 003 13 203 11 203 5 802 44 802 5 10.0 i 6 10.0.1 34 403 28 403 0 SO3 17 603 12.0.0 30 10.0.2 43 4 10.0.2 7 i2.o.i 6 12.0.1 7 803 15 803 0 004 13 2OZ 204 13 0 12.0 2 4 g0.2 A.O.o 0 404 12 402 10 10.0.3 0 7 10.0.3 3 14 0.i 14 14 0.1 23 60z 18 604 14.0.2 8 3 14.0.2
4 4 1 0
7 5 6 1*
802
S O ! 12.0.3 12.0.3 16.0.0 16.o.i 113.0.1 10.0.3 10.0.4 005 205 205 14.0.3 14.0.3 408 405
8 12 15 -10 42 2 8 -25 -20 0
-12 -25 -41 2 6
- 1 - 5 -14
8 16 12 -6 38 -10 18 -31 -23 - 2 -14 -27 -33 - 6* 2 4
-7 -12
0
0
- 7 7
- 9 13
5
1
3 0 7 12 7 13 0
0 - 2 9 8 7 10 4 -10 22 -19
-
8
0 0
17 -18 - 4 - 9 -14 -7 2 3
0
0
0
13 12
-11 - 1 - 5
-12 - 8
0
17
0 0
27 23 6
7
16.0.1
11 17 30 14
16 0.2
11
- I -26 27 -6 6 15
15 -31 16 -10
- 6
- 7 - 8 -12 3 0
- 8 8
-32 22 - 7 7 15 16
-32 14
- 9
IFo~m.1
606 7 605 8 18.0.0 35 12.0.3 8 4 12.0.4 i8.o.i 13 18.0.1 32 14 SOB 4 805 16.0.3 9 4 16.0.3 18.0.2 0 18.0.2 5 9 14.0.Z 14.0.4 13 0 10.0.5 7 10.0.5 7 20.0.0 006 0 206 0 0 206 2o.o.i 3 7 20.0.1 5 406 5 406 0 18.0.3 0 18.0.3 12.0.8 0 12.0.5 3 0 16.0.z 0 16.0.4 606
606 20.0.2 20.0.2 806 806
14.0.5 14.0.5 22.0.0
zz.0.i
0 0 0
3 -1
4 9 4 3 0 0
22.0.1 20.0.3 20.0.3 18.0.Z 18.0.4 1o.o.e
6 11 4 0
1o.o.s
0
22.0.2 22.0.3 16.0.3 16.0.5 12.O.F 12.0.6 007 207 207 407 407 24.0.0 20.0.z 20.0.4 22.0.3 22.0 3 24.0.i 24.01
5
607
607 14.0.6 14.0.6 18 0.8 18.05 809 807
* Omitted in calculation of P ( X . 2 ) .
6
0 0
4 5 5 3 0
2
3
6 -89 7 - 9 -11 20 10 -6 3 - 8 - 4 - 3 - 4 -11 1 - 9 0
11 -31 8
-6 -13 25 11 - 5
6 - 5 - 5 - 5 - 8 -16 3 - 8 - 5 0
- 8 1 - 2 - 5 -12
0 - 2 -4 -10
4
6
3
7
0
0
- 2 4 5 2 - 2 1
1 3 5 1 1 1
0
1 1 - 1 - 2 10 - 7 0 - 6 -6 - 2 - 8 -5 7 2 1 - 4 - 1 - 4 - 8 6 - 1 6 1 - 1 1
- 1 2 1 - 4 - 3 12 - 5 1 - 4 - 1 - 2 - 8 -11 9 1 0 - 5 3 - 1 -6 5 - 5 - 1* 1
o* - 3
0
0
2
10 10 4
14
0
15 9 2 1 6 2 - 3 4 9 - 1
0
- 1
5 3
3 10 4 - 7
0
3 2 .5
4 4
0
0
9
5 0
5 6 - 6
7 4 0
.1 4 6
4 - 6
Dec. , 1944
THECRYSTAL STRUCTURE OF BIPHENYLENE
+
+
$ - x1 7; r - zl; 3 x1 Yl f zi, 3 - x1 5-1 7 - z:;,+xl$-ylP+S:, f-x1$+y1 ?--si; i+xl& -ylr+zi1f-x1++y1 r - z:, with r = t ( z z - z3), zi = 3 (22 sa). These positions can be grouped, as has been done above, into pairs about six centers of symmetry with the parameters 000, ff0,f 0 r, p 0 ?, t 4 i;, f f r. Since the magnitudes of the x parameters and of the y parameters relative to these centers are, respectively, the same, these centers must be the centers of the six molecules in the unit cell. Assuming that the molecules in the general positions have the same size and shape as the ones in the special position, there are two possibilities I. with 21: (a-) the molecules at 000 left to relate $ and $ 0 r are parallel to each other, thus z: = 21; (b) the orientation of the molecule a t $ 0 r results from the orientation of the molecule at 000 by a mirror operation by a plane perpendicular to the c axis. Then z: = -zl, since the c axis is almost perpendicular to the a axis. In the present approximation the orientations of all six molecules are interrelated. This makes possible the application of the method of the rnolecular structure f a ~ t o r . ~In this method, the size and the shape of the molecule are assumed and its Fourier transform is constructed. If the orientations of all molecules in the unit cell are interrelated, then the molecular structure factors, read off from a chart of the Fourier transform, are related to the structure factors of the whole unit cell. The problem is then to orient the reciprocal lattice of the crystal with respect to the system of axes of the Fourier transform in such a way as to account for the observed structure factors. In the case of biphenylene the Fourier transform has an especially useful form. Since the molecule may be assumed to have a center of symmetry and to be planar, the transform is real and essentially two-dimensional. The model chosen for the calculation was suggested by the electron diffraction investigation.' It consists of two regular (a = 120') coplanar hexagons of carbon atoms with bond lengths a = 1.40 Hi., joined by two bridges, in ortho positions, of lengths b = 1.46 k. (Fig. 1). The use of the Fourier transform reduces the original fifty-four parameter problem, involving the placement of
+
t
1"
Fig. 1. (6) G . Knott, I'ruc. Phys. SOC.(London), 62, 220 (1940).
2037
eighteen carbon atoms, to a four parameter problem, in which three parameters characterize the orientation of the molecule and the fourth one is the parameter r. Reflections (ItkO) are especially suited to determine the orientation of the molecules since in this case the relationship between molecular and crystal structure factors does not involve the parameter r, and does not distinguish between the possibilities (a) and (b). After a few trials, general agreement between calculated and observed structure factors was obtained, resulting in the determination of most of the phase constants. These phase constants cornlined with the observed structure factors were used in the calculation of the Fourier projection p(X,y)
= CffFnm
COS
2k(kX -k by)
This calculation was carried out with use of Lipson-Beevers stripsa at intervals of ao/180 and bo/60. The plot of the final projection obtained with the signs calculated from the final choice of parameters (Table 111, assignment No. 1) is shown in Fig. 2. Due to the approximation involved, the length of the a axis of the unit cell of the projection is equal to Qao,and the projection shown corresponds to a superposition of the molecules a t 000, 4 0 I , 3 0 ?. Figure 3 shows the actual unit cell of the crystal viewed along the c axis and its relation to Fig. 2.
-
I
116 0 1/6 Fig. 2.-Fourier projection p ( x , y ) . Dots (0) indicate parameters of assignment No. 1.
The knowledge of the approximate orientations of the molecules and the observed values of / F ~ owere ~ I used with the Fourier transform to decide between the alternatives (a) and (b) and to determine the parameter r. It was found that the arrangement (a) could not account for the observed (h01) intensities. For testing the arrangement (b) it proved helpful to determine the probable range of the parameter r in the following manner. Paper models of the xz projections of the molecules were constructed to scale ( 6 ) H.Lipson nnd C. A. Beevers, ibid., 48, 772 (193W.
JURG WASER AND CIIIA-SI
,OI 6 7
LU
, 0.5
0.0
Vol. 66
08 33
a, Fig. 3.-View of xy projection of unit cell of crystal (parameter assignment No. 1). Numbers given t o three decimal places are z parameters of center and of highest atom for each molecule. Section enclosed within broken lines indicates area represented in Fig. 2.
tained with the final choice of signs explained later. Twenty-six of the thirty-one peaks are resolved, although not all completely. The x parameters of the various resolved peaks agree well with the x parameters of the peaks in p(x,y) (Fig. 2). Figure 5 , a diagram of two unit cells of the crystal viewed along the b axis, explains the relationship between the peaks of p(x,z). An interpretation of the peaks of p(x,z) was sought in the following manner. The two crystalp ( U , Z ) = CZZFnni COS ‘7a(h.X + / Z ) hi lographically ‘different kinds of molecules were aswas calculated. The summations involved were sumed to have the dimensions suggested by the evaluated with the aid of International Business electron diffraction work,l a coplanar molecule Machine Corporation machines and a set of with a = 1.40 A., a = 121°, and b = 1.46 A. (Fig. 1). The orientations of the molecules 0 relative to each other, and the parameters of the centers of the molecules in the general positions (par and space group equivalents) were Chosen so as to agree with the requirements of the “ h = 3n rule” ( p = +; q = 0). Parameter values for all carbon atoms were then found by matching these molecules (by ’/Z trial and error), as closely as possible to the maxima of p ( x , y ) and of p(x,z). The final choice for the parameters is recorded in Table I11 (Assignment No. 1) and dots in Figs. 2 and 4 indicate their positions with respect to 1 the maxima of p ( x , y ) and of p(x,z). Figures /2 3 and 5 were drawn with these parameters, which were also used for the calculation of Fig 4 -Fourier projectioii p j ~ , z ) Black dots ( 0 )iridicatc structure factors FQo and Fkj. These strucparameters of assignment N o 1, crosses ( f ) represent pa ture factors are recorded in the third columns rameters of assignment hTo 2 of Tables I and 11. Robertson’s atornib scatterpunched cards,’ sixty-six lines parallel to the G ing factors for hydrocarbons* were used in these axis and eighty-six lines parallel to the a axis calculations. The agreement between 1 Fcalcd 1 and being chosen. Figure 4 shows a plot of P ( X , Z ) ob- lFobs 1 is reasonably good, but not entirely satisfactory. Attempts to Orient the molecules in the (7) P A bhaffer J r Ph D 7 hest& California t n z t i t u t e of Terh
and manipulated over a plot of several unit cells. The packing was tested for each value of r chosen by estimating the intramolecular distances. General agreement between observed structure factors and the values calculated from the Fourier transform was obtained for r = 0.43, which was in the expected range. The signs of the F h o i obtained in this manner were combined with the experimental / F h o l l and a Fourier projection
nolnh)
1942
R) I hl R o b e r t s o n Prilc R o y 5oc ( L o n d o n ) 8160, 10b (I9’3i)
THECRYSTAL STRUCTURE OF BIPHENYLENE
Dec., 1944
general positions in a different way (abandoning the “h = 3n rule”) failed to give improvement. TABLE 111 Parameters Assignment No. 1 U
Molecule with center at (000) :
v
( m y ): Center (Par):
V
U
0.029 0.011 -0,140 0.029 (-0.003) ,077 ,003 - ,314 ,073 ( - ,026) ,127 - ,094 - ,294 ,122 ( - ,119) ,128 - ,179 - ,107 ,126 ( - ,189) ,080 - ,171 .067 ,082 ( - ,166) ,030 - ,074 .047 ,033 ( - ,073) ,363 - ,074 413 - ,171 ,461 - ,179 ,460 - . o w
Molecule with center at
Assignment No. 2 W
,410 .3W
.OOH .(I1 1 ,011
,380 ,367 ,547 .734 ,747 , ,567 .27!)
.366 ( ,416 ( 463 ( 460 ,410
,363 ,308 ,2fil
((-
( ( ( ( ( ( (
,304 ,250 - ,003 ,206 ,094 ,205 ,179 ,253 ,171 ,303 ,074
,112 ,299 ,479 ,466
,255 ,305
,333
,423
,3355 (
,000
.ow
,211 ,208
-
,069) ,181) ,183)
W
-0.147 - ,319 - ,294 - ,097 ,075
+
,050
,370 ,348 ,526
,111) .nio) . 003)
,724 ,746
003) ,019)
,069)
,278 ,100 ,122 ,320 ,498 ,476
,000)
.423
,111) ,183) ,161)
,568
I n principle the projection p ( x , z ) can be used to discard the “ h = 3n rule.” The reasonable assumption could be made that the molecules have the symmetry D2h-mmm. The molecules in the general positions would have their centers a t pqr and corresponding positions (with p = 4,
2039
0), and in general would have size and shape different from the molecule with center a t 000. Parameters xyz could be determined (except for the small additional constant q in the parameters y) by a least-squares fit of such molecules to the maxima of p(x,z), using the symmetry conditions as auxiliary conditions. These parameters could then be used to determine the signs of all Fhko and a complete projection p(x,y) could be constructed. An attempt to obtain refinement along these lines was made. For simplicity, i t was further assumed that the six-rings of biphenylene (Fig. 1) are regular hexagons. Although these assumptions are probably not strictly valid, they can be used as a basis for a second approximation. The two kinds of biphenylene molecules were fitted to the maxima of p(x,z) by trial and error, rather than by a laborious least-squares treatment. The resulting parameters are recorded in Table I11 (Assignment No. 2), and the crosses in Fig. 4 indicate their relationship with the peaks of p ( x , z ) , which they approach more closely than do the parameters of assignment No. 1. It is seen that the y parameters are not in good agreement with the y parameters of assignment No. 1 corresponding to the maxima of p ( x , y ) . The C-C bond distances obtained for the two kinds of molecules for assignment No. 2 do not agree well with each other nor with the electron diffraction result.’ (The values are a = 1.37 A., b = 1.58 A. q.
2040
JURC
WASERAND CHIA-SILu
for the molecule with center at 000, and a = 1.39 k., b = 1.44 tf. for the molecule with center a t pqr.) Nevertheless the parameters obtained were used to calculate a set of structure factors F& which are recorded in the fourth column of Table 11. Some improvement over the values of Fi$ is noted. The Fourier projection shown in Fig. 4 was made using the signs of the Fii', (omitting a few terms with uncertain signs), since the parameters used to calculate the FG{ correspond more closely to the maxima of p(x,z) than do the parameters used in the calculation of the FA!:. In most instances the signs of Ffi] and F&\are the same. With few exceptions jF:i{l
3 8 0
A. A.
30 24 2 4
31 21 3 6
Ai. A.
6
Y
The calculated packing distances in general agree with van der Waals distances found for crystals of other aromatic hydrocarbon^.^ Some distances are somewhat short, the two worst kin$ being carbon-hydrggen distances of about 2.65 A. Both these contacts could be improved by bending the C-H bonds in question by a few degrees in the plane of the molecule. A change in the carbon parameters would, of course, change these distances also, so that no real significance can be attached to their shortness. Of interest is the overlapping of the six-rings of molecules IV’ and VI a t B. The distance between the planes of the two molecules is 3.55 A. which may be compared with the value of 3.66 A. found for hexamethylbenzene”; the interplanar (10) I,. 0. Brockway and J. M. Robertson, J . Chem. .TOG., 1324 (1939).
distance in the latter case is partly determined by the size of the methyl groups. The distances involved in the interactions between hydrogen atoms and benzene rings of the type D, E, F and N are quite short. For example the interaction a t N involves C . . . H distances of 3.1, 3.0, 2.9, 2.9, 2.9 and 3.0 A. while the distance between the c e n t p of the six-ring and the hydrogen atomis 2.6 A. This distance is short enough to show that the van der Waals representation of benzene rings by domes, as suggested by Mack,” is inappropriate. A crater-like characterization as used in Fig. 6 represents the actual conditions much better. The closely knit packing of the biphenylene crystal accounts satisfactorily for the fact that no pronounced cleavages were observed. The packing is different from that characteristic of naphthalene and anthracene.12 We wish to express our thanks to Professor L. Pauling for suggesting this problem; to Dr. V. Schomaker for discussion and valuable suggestions; and to him and Mr. J. Donohue for the tracing of the Fourier projections (Figs. 2 and 4). We are indebted to Dr. W. C. Lothrop for the sample of biphenylene.
Summary The crystal structure of biphenylene is based on a monoclinic unit cell with dimensions a0 = 19.60 * 0.03 A., bo = 10.50 * 0.02 A., GO = 5.84 * 0.02 A., /3 = 91’20’ * 20‘, containing six molecules. The probable space group is c&- P21/a. The centers of two of the six molecules are space group centers. Approximate values of the fiftyfour parameters for the carbon atoms are evaluated for planar molecules of dimensions suggested by an electron diffraction investigation. The packing of the molecules is discussed. PASADENA, CALIFORNIA
RECEIVED JULY 18, 1944
(11) E. Mack, Jr., THIS JOURNAL, 54,2141 (1932); J . Phys. Chem., 41, 221 (1937). 112) J. M. Robertson, Proc. Roy. SOC.(London), 8140,79 (19333. For packing drawings cf. R. W,G . Wyckoff, “The Structure of Crystals.” Supplement 1930-1934, New York, N.V., 1935, p 154-13.5.
