COMMUNICATIONS TO THE EDITOR
of a series of investigations involving the reactions of thermal hydrogen atoms with a variety of organic solids, as well as with irradiated ethanol glass, in the manner of Klein and Sheer,G we have made the following observations. (1) Thermal hydrogen atoms under these conditions undergo no appreciable reaction with solid ethanol. This is based on the observation that there is no pressure change observed during H atom bombardment and that no products detectable by flame ionization gas chromatography were observed. (2) When a sample of solid ethanol at 77°K which has previously received a total dose of 4.2 X 1020 ev/g of 3-Mev-peak X-rays was subjected, before melting, to H atom bombardment, G(glyco1) was reduced from 0.94 to 0.02 while G(a1dehyde) was increased from 2.64 to 4.70. Assuming that two a-ethanol radicals are required to produce one glycol molecule, this corresponds to a loss of 2 X 0.92 = 1.84 radicals, which is nearly equivalent to the increase in G(a1dehyde). These observations suggest that the following processes are significant in the radiolysis of ethanol glasses. Those H atoms which are thermalized diffuse freely through the matrix, but can react only by combination or with species other than the substrate ethanol; thus cyclic photochemical reactions resulting from the reaction of hydrogen atoms produced by the photolysis of trapped free radicals as suggested by Dainton’ are possible only when the radiation employed is energetic enough to produce hot hydrogen atoms. This point is currently under investigation, and preliminary results suggest that in methanol, light of wavelengths less than 3300 A is necessary to achieve this type of cyclic process, while light of wavelengths as long as 5400 A is capable of photolyzing certain of these trapped radicals. Glycols are produced by the combination of a-ethanol radicals subsequent to the softening of the matrix rather than by radical diffusion at 77°K. Otherwise, the bombardment by H atoms would not be likely to affect the glycol yield so drastically. H atoms react with a-ethanol radicals mainly by disproportionation rather than combination, which is attested to by the appearance of acetaldehyde in an amount essentially equivalent to the reduction in the glycol yield. The aldehyde which is produced directly by the radiation (G = 2.64) is not affected by the thermal H atoms at this low temperature. This suggests that this yield of aldehyde does not result from the disproportionation of ethanol free radicals but is rather the result of an ionic reaction or a unimolecular dissociation. This is in agreement with the observations of Myron and Freeman8 that scavengers do not lower the yield of aldehyde in the liquid state.
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These observations are a part of a general study of the reactions of thermal hydrogen atoms with the aliphatic alcohols which will be reported on in the near future. Acknowledgment. This work was supported in part by the U. S. Atomic Energy Commission under Contract AT- (40-1)-200 1. (4) J. Teply, A. Habersbergerova, and K. Vacek, Collection Czech. Chem. Commun., 30,793 (1965). (5) J. A. Leone and W. Koski, J. Am. Chem. SOC.,88,224 (1966). (6) R. Klein and M. D. Sheer, ibid.,80,1007 (1958). (7) F. S. Dainton, G. A. Salmon, and J. Teply, Trans. Faraday Soc., 61,27 (1965). (8) J. J. J. Myron and G. R. Freeman, Can. J . Chem., 43,381 (1965).
R. H. JOHNSEN A. K. E. HAGOPIAN H. B. YUN
DEPARTMENT OF CHEMISTRY FLORIDA STATEUNIVERSITY TALLAHASSEE, FLORIDA 32306 RECEIVED MAY25, 1966
The Radiolysis of Pure Decaborane-14l Sir: Preliminary results are herewith reported on the Corn y-ray radiolysis of pure decaborane in the solid state. This work was carried out using standard vacuum2 and gas chromatographic technique^.^^^ The results of this study indicate that the products of the radiolysis are hydrogen, diborane, pentaborane-9, and a polymeric substance. The polymeric substance has not yet been identified except that it is not the icosaborane-26 reported by Hall and Koski4 in their deuteron irradiation of decaborane-14. The yields of the gaseous products at 35” as a function of total dose are shown in Figure 1. The initial product yields in the linear region of these curves have G values for hydrogen, diborane, and pentaborane of 0.84, 1.70, and 0.15, respectively. The results shown in Figure 1 are typically what is to be expected from radiolysis as a function of dose except in the case of the diborane yield. It is felt that this unusual behavior of the diborane yield may be due to either radiolysis of the polymer produced or interconversion reactions between the diborane and unstable boranes such as tetraborane between the time of radiolysis and analysis of the ss~mples.~ (1) This research was supported in part by the Oak Ridge Associated Universities Inc. (2) S. Dushman, “Scientific Foundations of Vacuum Techniques,” John Wiley and Sons, Inc., New York, N. Y., 1962. (3) G. F. Shipman, Anal. Chem., 34,877 (1962). (4) L. H. Hall and W. S. Koski, J. Am. Chem. Soc., 84,4205 (1962). (5) The time here was of the order of 3-5 days a t a temperature of about 30’.
Volume 70, Number 7 J u l y 1966
COMMUNICATIONS TO THE EDITOR
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54
8'
2 42 X
I
B
4 30 a
L(
8
n
3 18
z
-180
-140
6
10
30 50 Total dose X lozo,ev.
70
Figure 1. Radiolysis of B10H14 a t 35" as a function of total dose. Product yields: +, Hz; A, B2Ha; 0, BhHs.
The radiolysis of decaborane-14 was also carried out as a function of temperature a t total doses which were in the linear region of all of the curves shown in Figure 1. The results are shown in Figure 2. It appears from these data that hydrogen, diborane, and pentaborane-9 are produced in the region between -196 and -80" by a temperature-independent mechanism, such as molecular detachment or hot radicals. The observed G values at - 196" are 0.59, 0.15, and 0.05 for hydrogen, diborane, and pentaborane, respectively.
The Journal of Phyekal Che?n&ry
-100 -60 Temperature, >C.
-20
20
Figure 2. Radiolysis of BloHlras a function of temperature. Product yields: Hz; A, B2Hs; and 0, BsHg.
+,
It appears from the data in Figure 2 that there is the onset of a temperature-dependent set of reactions in the temperature range between -80 and 0" which gives rise to only diborane and pentaborane as gaseous products. At 0" the yield of hydrogen begins to rise rapidly, indicating the onset of a second set of temperature-dependent reactions producing all three observed gaseous products. The polymeric solid is observed over the entire temperature range. DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF MISSISSIPPI UNIVERSITY, MISSISSIPPI 38677 RECEIVED MAY31, 1.966
T. J. KLINOEN J. M. O'NEAL
