1018
NOTES
Vol. 62
!I 2.400
f
2.200 2.000
1.800
4 1.600 1 u
3 1.400
> %
5 1.200 s
.r(
8
1.000 0.800 0.600
0.400
0.200 I
I
I
I
1
L
I
I
I
-0.200 0.400 0.600 0.800 1.000 -0.200 0.400 0.600 0.800
I
I
I
I
J
1.000 1.200 1.400 1.600
Set (b) Set (a) Potential against Hg/HgzSOd, 0.2 M NaZSOd, v. Fig. 1.-Differential capacity curves for various adsorbate concn. C’ of n-pentanoic acid: set (a) in 0.1 M HClOl alone; set (b) in 0.1 M HC104, 0.001 M KCI, C”: 0, 0; X, 0.026; 0,0.150; O,0.300; A, 0.400. polarizat.ions the level of the mercury meniscus in the cathode tube was adjusted to the cross-hair of a fixed travelling microscope by means of a mercury levelling device. The final adjustments were always made when the bridge was nearly accurately balanced. Perchloric acid was Analar “Carlo Erba” product which was used as supplied with no further purification. “Prolabo” n-pentanoic acid was distilled once, and the fraction boiling a t the temperature range 186-187’ was used. Deaeration of solutions was affected with purified hydrogen gas.
cause of a trace of 4,4’-dibromoazobenzene, were purified by repeated crystallizations from organic solvents and the pure compound was separated in the form of thin straw-yellow,leaves, strongly pleochroic, which melted at 169-170.5’. A mixed m.p. determination, using crystals obtained by the two methods, also gave the above value, and chemical analysis showed that the crystals had the theoretical composition. Large crystals, suitable for CRYSTAL STRUCTURE OF X-ray work, finally were obtained by slow cooling of hot saturated solutions in alcohol. 4,4 ’-DIBROMOAZOXYBENZENE X-Ray Data.--Oscillation, rotation and equiBY ARRIQO ADDAMIANO inclination Weissenberg photographs up to the Lamp Research Laboratory, Lamp Division, General Electric Co., Nela second level about the a- and b-axis, using CuK, Park, CEeveEand, Ohio radiation (A = 1.540 A,) and a cylindrical camera Received December 1I , 1067 57.3 mm. nominal diameter, have shown the crysIn an attempt to elucidate the configuration of tals to be monoclinic with a = 4.01 A., b = 6.13 A., the azoxy group, crystals of 4,4‘-dibromoazoxyben- c = 26.g8A.,0 = l l l ” ,V = 619.1 A.a. Assuming zene were prepared and used in an X-ray diffrac- for the crystals a density close to that of 4,4’-dition investigation. and with a molecular weight, , The substance was obtained by reduction of p - bromoazobenzene12 M = 356.0 g., the number of molecules per cell is bromonitrobenzene with zinc dust in alcoholic solu- two. Thence F(000) = 344 electrons; d, = 1.909 tion in presence of NH&l and also by reduction of p-bromonitrobenzene with dextrose.’ The prod- g./cc.; p = 86.0 cm.-’. A careful examination of the Weissenberg photoucts of the reactions, originally orange-yellow begraphs, obtained from fairly large crystals and a (1) A. H.Biatt (editor), “Organic Syntheses,” Vol. 11, Reinhola Publ. Corp., New York, N. Y., 1943, p. 58.
(2) A. Addamiano, Suppl. Ric. Bci., 2 2 , 53 (1952).
1
w
1019
NOTES
August, 1958
TABLE I N(Z) FOR THE Okl-REFLECTIONS OF 4,4‘-DIBROMOAZOXYBENZENE z 0.5 0.6 0.7 0.8 0.2 0.3 0.4 ,582 .527 .554 .527 .500 .338 .419 .631 .590 .631 .501 .390 ,450 .330 .606 .558 .592 .513 .405 .475 .334 .62 .56 .59 .52 .41 .47 .34
VALUES OF
sin 0 range 0.2-0.5 0.5-0.7 Mean Theor. values
0.1 .162 .170 .166 .24
copper tube running at 40 KVP and 20 ma., the exposure times being up to three days, shows that the systematic absences include Ok0-reflections with k odd, and h01 reflections with E odd. On the basis of the systematic absences, therefore, the space group of the substance is c 5 2 h - P21/c, which requires four asymmetric units per cell. With two molecules per cell, however, this is only possible if the molecules themselves are centrosymmetrical. As the asymmetry of the azoxy group has clearly been demonstrated,a the presence of a true center of symmetry in 4,4’-dibromoazoxybenzene cannot be accepted. If the space group is truly P21/c, an apparent center of symmetry in the molecules can be originated from lattice disorder. On the other hand, one may suspect that some h0Z reflection with 1 odd, or OkO reflection with k odd, though very weak, be actually present and the space group be either P21 or Pc, which do not require molecular symmetry. In space group P21, a large number of h01 reflections with 1 odd can be theoretically recorded. As no such reflection appears, the first possibility is ruled out. On the other hand, only three OkO reflections with k odd fall into the copper sphere and the fact that they are not observed may be due to their being very weak. It has been suggested4s5that the distribution of intensities be used as a space group criterion in those cases where doubts exist or other methods fail. Statistical tests are particularly useful when a random distribution of equal atoms exists. In a case like ours, however, where a heavy atom is contained in the molecules, the validity of statistical tests is greatly impaired and did not warrant the collection of full three-dimensional data. The method, in the form suggested by Howells and other^,^ was therefore applied only to h01 and Okl reflections. It was found that the values of the intensities for h01 reflections did not follow a “normal” distribution curve, but were intermediate between the ‘centric and acentric distribution for low values of z, and were in fair fit with the centric distribution for intermediate values of 2, thereafter growing faster than the theoretical values. The OLE reflections, instead, fit perfectly the normal distribution curve for T. The numerical data for Okl reflections are collected in Table I. The fact that the distribution of intensities for OkZ reflections is so much closer to the “normal” distribution for i than found for h01 reflections is probably due to the atoms being completely re(3) A. Angeli, ffaxr. chirn. ital.. 46, [II]67 (1916). (4) A. J. C. Wilson, Acta C ~ y s t . ,2, 318 (1949).
(6) E. R. Howeh, D. C . Phillips and D. Rogers, {bid., 8 , 210 (1960).
, I
0.9 .609 .641 .625 .65
1.0 .675 .e61 .668 .68
“E c
Fig. 1.--a-Axis Patterson projection. Arbitrary intervals, tripled at the origin. The lowest level drawn is dashed. c sin 6 -
c
Fig. 2.-a-Axis Fourier projection. Origin a t 7. Contours at intervals of 1 electron per A.a. The dotted contour is the 2 electrons per A.zline. On the bromine atom the interval is 4 electrons per A.2.
solved in the a-projection but not quite so in the bprojection. Fourier Analysis.-Out of some 200 independent Okl reflections contained in the copper sphere, 177 were actually recorded. The intensities were estimated by the multiple film technique6 with the aid of calibration scales prepared from a crystal of 4,4‘-dibromoazoxybenzene. After correction for (6) J,
M, Robertson, J . Sei. Xnatr., PO,
176 (1943).
1020
NOTES
Lorentz and polarization factors, t o get a set of values proportional to the F2’s, the Patterson series (Fig. 1) was calculated with the aid of BeeversLipson strips.’ In this projection, as well as in the electron density maps below, values were calculated at intervals of c/120 along c, and of b/30 along b. From the Patterson projection the Coordinates of the Br atom were estimated t o be, approximately, y/b = 0.033 and x/c = 0.206. These coordinates were used to determine the signs of most of the strong reflections, and a first Fourier series was calculated. After two more refinements only the signs of a few weak reflections were still uncertain, and the signs of all the strong or moderately strong reflections were known with certainty. Figure 2 shows the third refinement, calculated with all but nine of the available terms. Using the atomic coordinates obtained from this projection by analytical interpolation,* the reliability index for the observed Okl reflections is R = 0.29, when an artificial isotropic temperature factor exp [- B(sin O/h)2],with B = 5.17 X 10-ls cm.2,is introduced and the contribution of the oxygen to the reflections is calculated as one-half of that corresponding t o a regular oxygen atom.g Discussion I n the Fourier projection, all the atoms are resolved and have about the expected electron content, except the oxygen atoms, which do not appear a t centers of symmetry, but are linked to the nitrogen atoms, in the number of two per molecule, each with approximately one-half the correct number of electrons. If the crystallographic evidence in favor of space-group P21/c corresponds to the truth, the apparent center of symmetry in the molecules, and the halving of the heights corresponding to the oxygen atoms in the a-projection, call for a disordered molecular arrangement, whose statistical effect is t o increase the true symmetry of the molecules. There is no evidence of diffuseness in the reflections recorded, so that our case recalls very closely the structure of pchlorobromobenzene, where, according to investigations of Hendricks’O and Klug,’l two “centrosymmetrica1;j”molecules are contained in a unit cell requiring four asymmetric units. Likewise, in the structure of 2-amino-4methyl-6-~hloropyridine,’~the maxima corresponding to chlorine atoms and methyl groups were found t o be identical, with an electron content intermediate between that of the halogen and the methyl group. Some similarities among these structures do exist. I n each case the molecules are approximately planar, and are almost perpendicular to the shortest cell axis. The space group is also the same, and Acta Cvyet., 2, 131 (1949). (8) A. D. Booth, “Fourier Technique in X-Ray Organic Structure Analysis.” Cambridge, 1948, pp. 62-65. , (7)
(9) A list of observed and calculated amplitudes of OM-reflections has been deposited as Document number 5591 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Waahington 25, D. C. A copy may be secured by citing the Document number and by remitting $1.26 for photoprints, or $1.25 for 3.5 mm. microfilm in advance b y check or money order payable to: Chief, Photoduplication Service, Library of Congress. (10) B. Hendrioks, 2. Kriet.. 84, I ( 1 9 3 3 ) . (11) A. Klug, Nature, 160, 670 (1947). (12) C. J. B. Clews and W. Coahran. Acta Cwrt., 1, 4 (1048).
Vol. 62
the statistical symmetry acquired by the molecules through disorder brings about a somewhat closer packing than would be achieved in an ordered structure. As far as 4,4’-dibromoazoxybenzene is concerned, it should also be noted that azoxy compounds are known to form liquid crystals, indeed more than one liquid crystal form has been observed for some azoxy derivatives.13 It is possible that the presence of disorder in crystalline 4,4‘-dibromoazoxybenzene bears some relation to the liquid crystal state. Finally, a word should be said about the reliability index. I n view of the apparent simplicity of the structure the value of R = 0.29 is fairly high, even if we take into account the fact that no correction for spot-shape, absorption and the like were introduced, and individual anisotropic temperature factors were not used in the computation of the structure amplitudes. Comparable data are lacking. Klugll does not give information as to the sort of agreement obtained for p-chlorobromobenzene but explains some false details appearing in the Fourier projections as due to poor intensity data. Similarly, Clews and Cochran12do not give any R value for 2-amino-4-methyl-6-chloropyrimidine. The R value for the b-projection of 2-amino-l, 6-dichloropyrimidine, isomorphous to it, is given as R = 0.24. It is likely that a somewhat higher value was found for 2-amino-4-methyl-6-chloropyrimidine, probably of the same order that we find for 4,4‘-dibromoazoxybenzene. In conclusion, I wish to thank Mr. R. Levetan, of our laboratory, for supplying the crystals of 4,4‘dibromoazoxybenzene prepared by the dextrose reduction method. (13) D. VorlBnder, Trans.Faraday Xoc., 29, 913 (1933).
PRODUCTS OF THE RADIOLYSIS OF WATER CONTAINING BENZENE AND AIR BY JEROME GOOD MAN^ AND JOSEPH STEIQMAN* Department of Chemistry Polytechnic Institute of Brooklyn. Brdoklyn, N. Y . Received December 16, 1Qb7
I n the high-energy irradiation of water saturated with benzene and containing dissolved air, phenol and hydrogen peroxide are formed until oxygen is depleted; phenol is then produced at a reduced rate, and biphenyl appears in increasing yield.a Reported values for G(pheno1) (Le., molecules of phenol formed per 100 electron volts absorbed) vary from 1.32 to 3.05, with most values falling between 2.2 and 2.7. They are reported in Table I. We have found that a previously unreported substance, which we have not succeeded in identifying, is also formed. This material behaves like phenol in most of the analytical methods employed by previous investigators. As a consequence, where its presence is not recognized, the reported (1) In partial fulfillment of the requirements for the degree of Doator of Philosophy in Chemistry at the Polytechnic Institute of Brooklyn. (2) To whom inquiries should be addressed. (3) E. Colliion and A. J. Swallow, Chsm. Rats., 86, 605 (1956).
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