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HC13C13H,13.69%; HC13C12H,23.90%; and IICl2C~ZH 62.117,. , The mercury-photosensitized experiments \yere performed at room temperature (23 f 1’) using a quartz cylindrical cell, 5 cni. in diameter and 10 cm. long. The light source employed was a low pressure mercury lamp described previously.6 A TTycor filter to absorb any 1849 8. radiation and a neutral density filter to reduce the intensity were inserted between the lamp and the reaction vessel. The incident intensity, as measured by X20-propane actinometry,’ was found to be 0.17 peinstein/min. under these conditions. Conversions were kept below 0.5yoin view of the large quenching cross section of benzenes8 The benzene product was recovered in a trap at -131’ (n-pentane slush) and analyzed by mass spectrometry, neglecting isotope effects in the niass spectral patterns of the benzenes. In the radiolysis experiments, Pyrex cells fitted with in-blown windows described previously5 were immersed up to the windows in a water bath at room temperature and irradiated by a 20-pa. beam of 5-AIev. electrons from a Van de Graaff accelerator. Bombardment times were kept low (5-10 sec.) to avoid secondary reactions. Conversions were lYc or less. The acetylene pressure used in both photolysis and radiolysis was 30 mm. Each experiment was performed in duplicate for both types of radiation.
Results and Discussion Table I compares the predicted and observed distribution of C13 in the benzene product for both photolysis and radiolysis. Keglecting for the moment the large apparent yields of C136Heand C136C12Ha, it may be readily seen that both the photolysis results and the radiolysis results conform very closely to that predicted for a mechanism involving no carbon-carbon bond cleavage. The deviations in the yields of benzene containing two or less C12 atoms are almost certainly due to small aniounts of other products, such as CCH8, which are also formed and which exhibit mass spectral patterns which overlap those of the benzene products. These minor products will not significantly affect the analysis of the benzenes obtained in larger yields and the agreement between the yields calculated, assuming no carbon-carbon cleavage, and the experimental results may be readily seen. The results reported here preclude any radiolysis mechanism resulting in randomization of C13 as might be expected from carbon-carbon triple bond cleavage. Any such process must play a minor role in the mechanism of benzene formation. T h e Journal of Physical Chemistry
NOTES
Table I : Relative Abundance of the Isotopic Benzenes in %
Benzene
Ci3&&
c l35C“He C134C’22H6 C’33C’ZaHe
C’3zC‘‘aHB C’3C’2~Hs
C”eH6
----Predicted---C E C bond KO C S C cleavage bond cleavage
0 0 3 13 30 34 16
03
49 56 89 16 97 90
0 26 1 35
5 76 13 61
26 66 27 99 24 37
----Observed----5.Mev. (e-) I-Ig 6 (IP1) irradiation sensitization
0 4 1 6
5 8 4
13 26 28 24
3 0
5
1 2 3 3 4 0 2 6
2 6 13 25 27 24
Acknowledgments. This research was supported by the U. S. Atomic Energy Commission under Contract Yo. AT(30-1)2007, and grateful acknowledgment is made thereto. The authors wish to thank *\h. S. Wrbican for careful determination of the mass spectra, and Dr. T. Hardwick, Gulf Research and Development Co., for the use of the Tan de Graaff accelerator. (7) Y. Rousseau and H. E . Cunning, Can. J . ChPm., 41, 465 (1963). (8) J. R. Bates, J . Am. Chem,. Soc., 54, 569 (1932).
Change i n t h e Heat Capacity of Boron Trioxide during t h e Glass Transformation by S.5. Chang and A. B. Bestul National Bureau of Standards, Washington, D. C. ZOZ?U+ (Received April 2, 1964)
For 41 glass-forming substances, Wunderlichl compiled values for the difference, AcP, in heat capacity a t the glass transformation temperature, T,, between the glass and the equilibrium supercooled liquid. These values should represent the configurational heat capacities of the equilibrium supercooled liquids. With the exception of two extreme variations, Wunderlich gives for Ac, the “universal” value of 2.7 i 0.5 cal. deg.-l per specially defined “bead.” For polymers, these beads are the equivalent of niolecular chain links. One of the exceptions was Bz03, for which a value of 0.55 cal. deg.-l was quoted. We have examined the original publication2 from which the values for B203 were taken. It appears that the values compiled for B203in ref. 1 should be multiplied by a factor of five. The heat capacity values in the original (1) B. Wunderlich, J Phys. Chem., 64, 1052 (1960). (2) 9. B. Thomas and G S. Parks, ibid., 35, 2091 (1931).
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SOTES
liquid should lose all its configurational entropy on reference were listed in one column in units of calories cooling in equilibrium. per (mean) gram-at,om, as was the fashion a t that time. Apparently, in computing the data in ref. 1 (3) J. E. Kunzler and W. F. Giauque, J . Am. Chem. Soc., 74, 797 the mean gram-aton1 mas taken for the gram molecular (1952). weight, which contains 3 mean g.-atoms. Application (4) J. H. Gibbs and E. A. Di-Maraio, private communication. of this factor gives a value of 2.75 cal. deg.-’ for AI;, per bead for Bz03. This value is well within the stated limit of uncertainty of Wunderlich’s “universal” value. In addition to any inherent variation between T h e Wetting of Gold Surfaces by Water‘ the configurational heat capacities for the diff ererit substances, the applicable uncertainty includes any contributions associated with any inadequacies in the by Malcolm L. White treatment of measurement and interpretation problems Bell Telephone Laboratories, Incorporated, Allentown, P e n n w l v a n i a such as: (1) heat eFfects arising from potentially iso(ReceGed M a y 7 , 1964) thermal configurational equilibration during the course of a heat capacity measurement, ( 2 ) the specification of the value of T , to which the heat capacity values are Past work on the wetting of metal surfaces by water extrapolated, and (3) the specification of the “bead” has yielded some apparently conflicting results. For unit in the case of molecules usually considered ais example, Harkins2 reported that water does not spread monomers. Without attention to such problems, the on clean mercury. Bartell and Smith,3however, found several different curves given in ref. 2 can lead to values that water had a low contact angle on vacuum-evapoof Acp for BzOa varying a t least from 2.3 to 4.4 cal. rated films of gold and silver. Trevoy and Johnson4 deg.-‘. The selection of the (here corrected) value in found that a number of different metal surfaces could ref. 1 represents the application of current understandbe made hydrophilic if properly cleaned, but this cleaning of the nature of the glass transformation process i n ing involved vigorous chemical and electrocheniical order to treat the above-mentioned problems in such oxidation, so that the metal surfaces dere undoubtedly a way as to evaluate a t T , the heat capacity of the equjcovered with an unknown amount of Dxide. Bewig and libriuni supercooled liquid (including configurational Zisman5 obtained low contact angles of water on gold heat capacity) and of the glass in a Configurational and platinum by an HK03-H2S04etching treatment state of fixed deviation from equilibrium (thus excluding which would result in oxide formation, even on these configurational heat capacity). The original curve noble metals. thus selected for use happens to be the same as that Sonie recent observations in this laboratory have indesignated (without detailed explanation) by Thomas dicated that the oxide on metal surfaces has a very and Parks as “the normal or standard values for a striking effect on the wettability of the metal by water, carefully annealed boron trioxide.” The above coran oxide-free metal surface being hydrophobic and the rection changes the conclusion from Wunderlich’~ oxidized surface hydrophilic. original compilation that B20a is an exceptional Gold is a metal of particular interest in surface studies, substance in the above respect. It is now shown to because it is the only metal which does not forin an behave exactly like other glass-forming substances oxide on heating in air or oxygen. This is apparent not investigated in this respect. only from thermodynamic data,6 but also from experiWe have also examined the original reference3 for mental studies in which gold was heated in air and oxyHSSOI.3H20, the other exception in Wunderlich’s gen to 900” with no evidence of oxide formation as deoriginal tabulation. No tabulation errors were found. termined by X-ray and electron diffraction In ref. 1 the universality of Ac, was discussed in __ terms of glass transformation based on the theory of (1) Presented a t the 145th National Meeting of the American Chemical Society, New York, N. Y., September, 1963. Hirai and Eyring for molecular rearrangement in (2) W. D . Harkins, “Physical Chemistry of Surface Films,’’Reinhold liquids, and reference was made to the desirability of a Publishing Co., New Pork, N.Y . , 1952, p. 12. solution to the heat capacity equation based on the (3) F. E. Bartell and J. T. Smith, J . Phys. Chem., 57, 165 (1953). second-order thermodynamic transition theory of (4) D. J. Trevoy and H. Johnson, Jr., ihid., 6 2 , 833 (1958). Gibbs and DiMarzie for glass transformation. Ac(5) K. W. Bewig and W. A. Zisman, Advances in Chemistry Series, No. 33, American Chemical Society, Washington D. C., 1961. cording to Gibbs and Di,llarzio,* this theory does also (6) A. Glassner, “The Thermodynamic Properties of the Oxides, lead to universality of Ac, a t T z , the temperature a t Fluorides and Chlorides to 2500°K.,” Argonne National Laboratory which they predict that an equilibrium supercooled Report ANL-5750. ~~
Volume 68, Number 10
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October, 1964
