NOTES hydrogen cyanide which is about twice too large. Similar anomalously small coupling constants involving 16Nhave previously been r e p ~ r t e d ;examples ~ include (C~HF,)~C="NH, C6H5CH='5N'3CH3, C6H513CH= 15iSCH3,and CH3l3C=l6N. The common denominator for these cases seems to be a carbon-nitrogen multiple bond. It will be interesting to see whether these anomalies can be rationalized on the basis of the Fermi contact mechanism for nuclear spin-spin coupling. The commonly assumed linear relationship6 between the per cent of s character of carbon orbitals and coupling constants between directly bonded 13Cand H, s = O . ~ O J % H , predicts 55% s character for the carbonhydrogen bond of hydrogen cyanide, slightly more than in acetylene. This conclusion is in qualitative agreement with the idea that the p character of the C=N bond should be greater than for the C s C bond of acetylene because nitrogen is more electronegative than carbon. This would mean more s character for the CH bond in hydrogen cyanide. (6) N. Muller and D. E. Pritchard. J . Chem. Phvs.. 31. 768 (1959): N: Muller, ibid., 36,359 (1962) ; C.' Juan and H. S , Gutowsky, ibid.; 37,2198 (1962).
The Crystal Structure of Benzophenone by Everly B. Fleischer,' Nako Sung, and Stuart Hawkinson Department of Chemistry, University of Chicago, Chicago, Illinois 60637 (Received December 81,1967)
Benzophenone is used in many types of organic and physical-organic experiments.2 We report here the crystal structure of benzophenone determined by threedimensional X-ray diffraction. The X-ray data were collected on a benzophenone crystal grown from hexane by slow evaporation. The cell constants ofobenzophenone are a = 10.30, b = 12.15, c = 8.00 A and the space group is P212121 with four molecules per unit cell (doalCd = 1.208 g/cm3; dobsd = 1.20 g/cm3). The 866 independent reflections were collected employing Cu K a radiation with a GE XRD5 diffractometer.3 The structure was solved by using the orientation of the molecule as determined from magnetic measurements2 and translating the molecule in the xy plane until a minimum R was determined for the Fhao's. The molecular translation in the direction followed directly from packing considerations. The structure was refined by full-matrix least-squares and Fourier difference techniques. The final weighted R factor [R = (zwlFo- Fc(2/zWFo2)'/2] for all refleetions is 5.3%. The weighting scheme used was w =
4311
11.23
A
Figure 1. Bond parameters for benzophenone. Average standard deviations in bond distances are 0.01 A ; in bond angle 0.5". The bond angles in benzene rings are 120 1".
1.0 for all Fhkz> 4.5 and w = 0.10 for FhRz< 4.5, where on this scale 2.0 is the smallest Fhk,considered to have a measurable value. The refinement was anisotropic and included hydrogen atoms. The coordinates of benzophenone are given in Table I, and Figure 1 gives the numbering system and important bond parameters of the molecule. The bond distances are as expected in the molecule and no special comment about them is necessary. The benzene rings are both planar. The dihedral angle between the two benzene rings is 56". Table I : Fractional Coordinates for Benzophenone Structure' Atom
z
2/
E
C(1) C(2) C(3) C(4) C(5) C(6) (37) C(8) C(9) CUO)
0.2147 ( 5 ) 0.1170 (5) 0,0109 (6) -0.0011 (6) 0.0945 (5) 0.2036 (4) 0.3003 ( 5 ) 0.2648 (3) 0.4402 (5) 0.5104 (6) 0.6439 (6) 0.7034 (6) 0.6352 (6) 0.5018 (5)
0.0282 (3) -0.0512 (5) -0.0312 (5) 0.0667 (6) 0.1499 (5) 0.1283 (4) 0.2176 (4) 0.3141 (3) 0.1935 (4) 0.2713 (5) 0.2526 (5) 0.1621 (5) 0.0866 (5) 0.1010 (4)
0.2165 (8) 0.2344 (8) 0.3315 (8) 0.4172 (9) 0.4001 (7) 0.3019 (6) 0.2786 (7) 0.2891 (6) 0.2442 (6) 0.1516 (7) 0.1263 (9) 0.1930 (9) 0.2812 (9) 0.3082 (7)
Wl) W2) (313) C(14)
a Standard deviations of coordinates in parentheses. gen atoms are not included.
Hydro-
(1) Alfred P. Sloan Fellow. (2) For example, R. W. Brandon, G. L. Class, C. E. Davoust, C. A. Hutchinson, Jr., B. E. Kohler, and It. Silbey, J. Chem. P h y s . , 43, 2006 (1965). (3) The observed and calculated structure factors for benzophenone have been deposited as Document No. NAPS-00082 with the ASIS National Auxiliary Publication Service, C/O CCM Information Sciences, Inc., 22 West 34th St., New York, N. Y. 10001. A copy may be secured by citing the document number and remitting $1.00 for microfiche or $3.00 for photocopies. Advance payment is required. Make checks or money orders payable to: ASIS-NAPS. T h e hydrogen positions and temperature factors are also with these data,
Volume 7 2 , Number 12
iVoaember 1968
4312
NOTES
The orientation of the molecule in the crystal is close to the one determined by Brandon, et aL12the CO lies close to the ab plane of the crystal as predicted by ?1/I~Clure,~ and the structure is consistent with the magnetic properties of the molecule measured by Banerjee.6 The recent X-ray structure of benzophenone by the Russian workers is consistent with our work6
Acknowledgment. This research was supported by grants from the KSF and ARPA. (4) D. S. McClure and P. L. Hanst, J . Chem. Phys., 23, 1772 (1955). (5) K. Banerjee and A. Haque, I n d i a n J . Phys., 12, 87 (1938). (6) E. B. Vu1 and G. M. Lobanova, Soviet Phys. Cryst., 12, 355 (1967).
Determination of Critical Temperature by Differential Thermal Analysis
by Horst W. Hoyer, Angelo V. Santoro, and Edward J. Barrett Department of Chemistry, Hunter College of the City Unizlersity of N e w York, N e w York, N e w York 10021 (Received J a n u a r y 2, 1968)
In the course of our inve~tjgations’-~of differential thermal analysis techniques utilizing sealed cells it became evident that one could use this technique to determine critical temperature. It has been predicted6 and experimentally verifiede,’ that C , should show a discontinuity similar to most order-disorder transitions as one passes through the coexistence curve. In a dta thermogram such a discontinuity would be evident as a sigmoid curve representing the rapid change in Cv . If a sealed tube containing liquid and its vapor is heated uniformly, then one of three changes may occur. (Refer to Figure 1.) If the over-all density (01) is less than the critical density (0,)) the volume of the liquid decreases until at a point A, corresponding to temperature 7’1, all of the liquid has been converted to vapor. Point A will be evident in the thermogram as a discontinuity at TI. The second possibility is that if the density (Ds)exceeds the critical value (De) then the volume of liquid increases upon heating until at point B ( T z )no vapor remains. This point will also be evident as a discontinuity in the thermogram at Tz. Finally, if the density is equal to the critical value ( D z ) ,the volume of liquid and vapor remains essentially constant and at T , (the critical temperature) a discontinuity in the thermogram occurs corresponding to the point where the liquid and vapor become identical. To determine T , it would be necessary only to plot the density as a function of the discontinuity T h e Journal of Physical Chemistry
Figure 1. Density us. temperature coexistence curve.
temperature; the maximum temperature would then correspond to the critical temperature. We have found, in practice, a more precise indication of the discontinuity is obtained by cooling rather than heating the sample through the coexistence curve. A plot of these discontinuity temperatures us. density, however, does not yield a clear maximum but rather st broad plateau, as shown in Figure 2. Such behavior has been observed by ,Ifa.asss-lo for ethylene and ethane and was later attributed by Schneider and others11-16 to the existence of a large density gradient within the vertical tube. Owing t o the infinite compressibility of a substance at its critical point, the earth’s gravitational force, although small in magnitude, can produce large density gradients over a distance of only a few centimeters. Assuming that such a density gradient does exist, we would predict that the plateau temperature must correspond to the critical temperature. (1) A. V. Santoro, E. J. Barrett, and H. W. Hoyer, J . A m e r . Chem. SOC.,89, 4545 (1967). (2) E. J. Sarrett, H. W. Hoyer, and A. V. Santoro, Tetrahedron Lett., 5, 603 (1968). (3) H. W.Hoyer, A. V. Santoro, and E. J. Barrett, J . Polym. Sci., P a r t A - I , 6, 1033 (1968). (4) A. V. Santoro, E. J. Barrett, and H. W. Hoyer, Tetrahedron Lett., 19, 2297 (1968). (5) J. 5. Rowlinson, “Liquids and Liquid Mixtures,” Butterworth and Co. Ltd., London, 1959, pp 95-101. (6) D. B. Pall, J. W.Broughton, and 0. Maass, Can. J . Rev., B16, 230, 499 (1938). (7) A. Michels and J. Strijland, Physica, 18, 613 (1952). (8) 0. Maass, Chem. Rea., 23, 17 (1938). (9) S. G. Mason, S. TV. Naldrett, and 0. Maass, Can. J . Rev., B18, 103 (1940). (10) S. N. Naldrett and 0. Maass, ibid., B18, 118 (1940). (11) W.G. Schneider, Compt. Rend. R e u n i o n Ann. C o m m . Thermod y n a m . , U n i o n I n t e r n . P h y s . (Paris), 69 (1952). (12) D. Atack and W.G. Schneider, J . P h y s . Chem., 5 4 , I323 (1950); 5 5 , 532 (1951). (13) K. E. MacCormack and W. G. Schneider, Can. J . Chem., 29, 699 (1951). (14) M. A. Weinberger and W. G. Schneider, ibid., 30, 422, 847 (1952). (15) W. G. Schneider and H. W.Babgood, J . Chem. P h y s . , 21, 2080 (1953). (16) H. W. Habgood and W. G. Schneider, Can. J . Chem., 32, 98, 164 (1954).
