3457
RADIOLYSIS OF LIQUIDAND SOLIDDIMETHYLMERCURY
Radiolysis of Liquid and Solid Dimethylmercury' by Clarence J. Wolf and John Q. Walker Research Division, McDonnell Douglas Corporation, St. Louis, Missouri
68166
(Received March $1, 1968)
The cobalt-60 y-ray radiolysis of liquid and solid dimethylmercury was investigated as a function of dose and irradiation temperature and in the presence of radical scavengers. The 100-eV yields of methane, ethane, ethylene, propylene, propane, elemental mercury, and methylethy'lmercury were determined at 28, 0, -21, -78, and -196'. The mechanism of the radiolysis reactions is discussed and it is compared with photolysis and thermal decomposition.
Introduction The photolysis and thermal decomposition of dimethylmercury has been studied by a large number of investigators. 2-15 However, with a single exception, lo the work has been concerned with the kinetics and mechanism of the gas-phase reactions. Ethane and methane are the primary products formed when dimethylmercury vapor is photolyzed. The relative ratio of CH, to CzH6 is temperature dependent, the ratio increasing with temperature.za Methane is most probably produced by an abstraction reaction involving a methyl radical and the m o n ~ m e r . ~Cunning~~~'~ ham and Taylorzb noted that polymer was produced during photolysis, but its rate of production was inhibited by the presence of Hz. The mechanism by which C2H6 is formed from photolyzed or pyrolyzed (CH3)2Hgvapor is not as well established. The quantum yield for ethane formation increases slightly with temperatures and at high temperatures exceeds unitya2b This suggests that ethane may be produced, at least in part, in a chain reaction. Rebbert and Steacie6suggested that the temperature effect on ethane arises from increased light absorption a t higher temperatures rather than a chain reaction. They felt, as did others,2,10that the ethane was formed by the direct combination of two methyl radicals. Gomer and Noyes4 studied the photolysis in detail and concluded that at high light intensities CZH6 was formed by a reaction other than direct recombination of two methyl radicals . Kallend and Purnell13 studied the thermal decomposition of (CH3),Hg vapor and found CzH4, C3H6, and C3Hs as well as CH, and C2H6.13 Waring and Pellinls also studied the thermal decomposition, but they observed only traces of C2H4,CaH6,and C3Hs. I n addition, Waring and PellinI6 found that NO reduced the rate of decomposition of CH3HgCH3, while Iiallend and Purnell13 observed no such decrease. Both groups did, however, find that the rate of the production of C2H6 was greatly reduced when NO was present. They both concluded that CH3 was not the precursor of CZH6 and that other radicals were responsible for its formation.
Rebbert and A u s l o ~ sstudied ~~ the photolysis of liquid and solid (CH&Hg and concluded that in the condensed phase C2Hs was primarily formed via a cage-recombination reaction and that thermal radicals were not responsible for its production. It is of interest to compare the photolysis and pyrolysis of dimethylmercury, which have been extensively investigated, with the radiolysis studies reported here. While all three decomposition methods initiate reaction by the same process, scission of an Hg-C bond to form HgCH3 and CHBradicals, the subsequent chemistry of the radicals depends strongly on the temperature and phase of the surrounding medium. Since all the pyrolytic and most of the photolytic studies were carried out with Hg(CH& vapor, this phase dependence introduces important differences between those results and the results of our radiolytic study which was confined to liquid and solid Hg(CH3)2.
(1)This research was conducted under the McDonnell Douglas Independent Research and Development Program. (2) (a) H.W.Thompson and J. W. Linnett, Trans. Faraday Soc., 33, 501, 874 (1937); (b) J. P. Cunningham and H. 8. Taylor, J . Chem. Phys., 6 , 359 (1938). (3) M. K. Phibbs and B. deB. Darwent, Trans. Faraday SOC.,45, 541 (1949). (4) R. Gomer and W. A. Noyes, Jr., J . Amer. Chem. Soc., 71, 3390 (1949). (5) R. E. Rebbert and E. W. R. Steacie, Can. J . Chem., 31, 631 (1953). (6) R. B. Martin and W. A. Noyes, Jr., J . Amer. Chem. SOC., 75, 1583 (1953). (7) R. A. Holroyd and W. A. Noyes, Jr., ibid., 76, 1583 (1954). (8) D. H.Derbyshire and E. W. R. Steacie, Can. J . Chem., 32, 457 (1954). (9) H. G. Oswin, R. Rebbert, and E. W. R. Steacie, ibid., 32, 472 (1955). (10) R. E.Rebbert and P. Ausloos, ibid., 85, 3086 (1963); 86, 2068 (1964). (11) B. G. Gowenlock, J. C. Polauyi, and E. Washurst, Proc. Rog. SOC.,A218, 269 (1953). (12) M.Kreck and 8. J. Price, Can. J . Chem., 41, 224 (1964). (13) A. S. Kallend and J. H. Purnell, Trans. Faraday SOC.,60, 93, 103 (1964). (14) K. B. Yerrich and M. E. Russell, J . Phys. Chem., 68, 3752 (1964). (15) C. E.Waring and R. Pellin, ibid., 71, 2044 (1967). Volume 72,Number 10 October 1968
3458
Experimental Section The dimethylmercury was obtained from two sources; some was from Distillation Products and some was prepared in our laboratory via a Grignard reaction from CHI3 and HgC12.16 All samples were distilIed in an efficient distillation column after drying 48 hr over PZO5,and only the center third of the distillate was retained for further use. The distilled sample was analyzed with an F & M 5750 gas chromatograph equipped with a thermal conductivity detector employing He carrier gas. The analysis column was 25 ft (0.125-in. o.d.), was packed with 1%SE62 on Chromasorb G, and was operated isothermally at 80". The analysis showed the material to be at least 99.9% pure. The dimethylmercury was loaded into an all-glass vacuum system, was deaerated by the freeze-thaw-pump technique, and was slowly distilled into glass ampoules which were sealed off under vacuum. To several ampoules containing approximately 0.2 g of dimethylmercury 414 torr of oxygen was also added. The ampoules were sealed while the dimethylmercury was frozen. The solubility of oxygen in dimethylmercury was determined by measuring the oxygen concentration in a saturated solution when the pressure of oxygen above the solution was known. The ratio of the oxygen concentration in the liquid to that in the vapor was 0.105. Therefore, the initial concentration of oxygen in the radical-scavenging experiments was 2.3 x 10-3 A[. The samples were irradiated in a 3000-Ci cobalt-60 irradiator. The dose rate in the center of the source was 4.75 X 1019 eV/g hr, measured by means of a ferrous sulfate d0simeter.l' The energy absorbed per unit mass of different materials in the same radiation field is directly proportional to the energy mass absorption Coefficient. The absorption coefficient for a compound can be calculated by the absorption coefficient of each element multiplied by its fraction of the total mass. Extensive tabulations of absorption coefficients are given in the report by Grodstein.'* The ratio of the absorption coefficient of (CH&Hg to that of HZO was calculated by assuming that the energy of the y rays from cobalt-60 was 1.26 MeV and that Hg has the same absorp-tion coefficient as does Pb. Thus the energy absorption rate in (CH&Hg was obtained from the product of the absorption coefficient ratio times the dose rate in the dosimeter. The low-temperature irradiations were performed while the ampoule was immersed in an appropriate cold liquid. The irradiated samples were warmed to room temperature and were opened in the vacuum system, and, with the aid of a Toepler pump, the product gases were passed through a Dry Ice trap into a calibrated gas buret. The gases were admitted directly into a gas chromatograph for analysis. The gaseous products, Hz, CH4, GHe, CZH4, C3H0, and C3Hs, were determined with a micro thermal conThe Journal of Physical Chemistry
CLARENCE J. WOLFAND JOHN Q. WALKER ductivity detector using argon carrier gas (35 ml/min) with a Porapak Q column heated to 75". Elemental mercury appeared as a small ball at the bottom of the irradiation vessel and its quantity was determined gravimetrically. When oxygen was present during irradiation, a dark gray powder appeared at the bottom of the vessel. The amount insoluble in benzene was presumed to be a 50:50 mixture of HgO and Hg. Several small ampoules were irradiated, and the liquid was analyzed with an F & h/r Model 5750 chromatograph employing a thermal conductivity detector. The 1% SE52 column was used for these analyses. The only identifiable product in the liquid was methylethylmercury and its identity was confirmed mass spectrometrically. With samples containing oxygen, the product gases were analyzed chromatographically with a 25-ft (0.125-in. 0.d.) Teflon column packed with 40-80 mesh Polypack-2 (Hewlett-Packard F & M Scientific Division, Avondale, Pa.) held a t 0" employing an argon sweep and a micro thermal conductivity detector (Carle Instruments Inc., Fullerton, Calif.). Polymer was formed when dimethylmercury was irradiated at 28". The polymer was not appreciably soluble in the monomer and appeared as a grayish white residue at the bottom of ampoule. However, the polymer was relatively soluble in benzene. Attempts at characterizing the polymer chromatographically were unsuccessful. The amount of mercury incorporated into the polymer was not measured, and a material balance was not obtained whenever an appreciable amount of polymer was formed. The total radical yield, G(R), was estimated by measuring the decrease in diphenylpicrylhydrazyl concentration (DPPH) with irradiation. The solutions were 1.11 X M with respect to DPPH, and the concentrations were measured at 520 mp with a Beckman DK-2 spectrophotometer. This method indicates a total radical yield of 15 molecules/100 eV. Several experiments were performed in which cyclohexene was used as a radical scavenger. Cyclohexene (Phillips Petroleum Co. research grade) was used without further purification, and solutions 0.172 M were prepared. The solutions were degassed by the freezepump-thaw technique, were irradiated, and were analyzed.
Results and Discussion The 100-eV yields of Hz, CHI, Ci", CzH4, C3H6, C3H8, Hg, and CH3HgCzH5were determined as a function of dose when dimethylmercury was irradiated as a (16) H. Gilman and R. E. Brown, J. Amer. Chem. SOC.,52, 3314 (1930). (17) 8. C. Lind, "Radiation Chemistry of Gases," Reinhold Pub lishing Gorp., New York, N. Y.,1961,p 59. (18) G. W.Grodstein, U. 8. National Bureau of Standards Circular No. 583, U. 8. Government Printing Ofice, Washington, D. C., 1957.
3459
RADIOLYSIS OF LIQUID AND SOLIDDIMETHYLMERCURY Table I: Products Formed from the Radiolysis of (CH8)zHg as a Function of Temperature
______-----_ Compd
28"
14.4 f 0 . 5 12.6 f 0 . 6 5.8 f0.1 1 . 6 f0 . 2 0.48 f 0.06 0.21 f 0.02 0.23 f 0.07 0.04 f 0.02 a
Liquid ph~~~-----------00
9.4 f0.5 11.4 f 0.5 5 . 8 f0 . 1 1 . 9 f 0.2 0.37 f 0.03 0.18 f 0.02 0.09 f 0.03 0.03 4 0.02
Solid phase------
-210
- 780
- 196'
7.0 f 0 . 6 8.1 f 0 . 6 5 . 4 f 0.2 1.0 f 0 . 4 0.30 f 0.04 0.16 f 0.03
1 . 6 =t0.1 3 . 5 f0.4 2.6 f 0 . 2 0.32 f 0 . 1 0.28 zk 0.02 0.17 f 0.02 0.09 =!= 0.03 0.02 f 0.01
1 . 2 f 0.1 2.9 f 0 . 4 1 . 8 f 0.2 0 . 3 f 0.1 0.20 f 0.02 0.10 f 0.02 0.03 f 0.02 0.02 f 0.01
...
...
Molecules per 100 eV absorbed.
solid at -196 and -78" and as a liquid a t -21, 0, and 25". The data are summarized in Table I. I n all cases the G values shown in Table I were obtained from the slope of the curve in which the amount produced was plotted against dose. Within experimental error, all major products were formed as a linear function of dose in the range (1-6) X lozoeV/g. The error shown in the table represents the larger of either the standard deviation of the repeatability or a value estimated on the basis of the signal-to-noise ratio. The material balance for the conversion of (CH3)zHg into radiolysis products at different temperatures is summarized in Table 11. The number of molecules of (CH3)zHgdecomposed is the sum of the Hg and CHsHgC2H6yields. The calculated number of Hg, C, and H atoms was determined directly from the yields listed in Table I. The molecular formula represents a ratio of the product atoms to Hg. The error shown in the formula was determined from the individual errors listed in Table I. The material balance for C atoms is relatively good; only at 28" is it apparently outside the limit of experimental error. The material balance for hydrogen is at best fair. This suggests that a substantial amount of product which is relatively hydrogen poor has not been considered. At 25" this material is most likely the uncharacterized polymer which is expected to be Hg and carbon rich. The reason for the large deviation in hydrogen from the expected ratio of 6: 1 at -21" is unknown. The high yields of the products formed in the radiolysis of dimethylmercury together with a radical yield estimated to be about 15 suggests that either the mechanism producing products involves chains or that dimethylmercury is very sensitive to radiation. The yield for the decrease in 0 2 as a function of dose is shown in Figure 1. The number of molecules of O2 consumed per 100 eV absorbed varies from 9.8 to 5.5 for doses of (0.40--1.50) X lozo eV/g. The decrease in yield with dose is not unexpected, since the O2 concentration decreases during the irradiation. An extrapolation of the yield curve back to zero dose (ie., this corresponds to maximum oxygen concentration)
Table I1 : Material Balance for the Conversion of (CHa)zHg into Radiolysis Products Molecules of (CHa)nHg decomposeda
Temp,b
14.2 13.4 9.2 3.8 3.2
28 0 - 21 78 - 196
O C
-
Calcd molecular formula of productsC
HgCz.zr*o.~4H~.~6*o.rc HgCz.os~o.ilHe.7rio.3e HgCz .I 9*0.22H7.60+0.7e HgCz ,1110.zaHs.9 0 1 0 .74 HgCl .s6~o.zclHS.aa*o.67
a Molecules per 100 eV absorbed. * Irradiation temperature. Molecular formula calculated from the sum of all C and H atoms present in the products. The error is determined from the individual errors on each product listed in Table I.
gives an initial yield of oxygen consumption of 13 molecules/100 eV. The nature of the reactions with the scavenger is not known and even speculation concerning a mechanism is not warranted. The yields of the various products observed from the radiolysis of oxygen-dimethylmercury solutions (2.3 X M ) are summarized in Table 111. Note that the yields of Hg, CzHe, Hz, and CzH4 are essentially the same as those in the deaerated solutions (see Table I). This indicates that these products are formed by some process which is unaffected by the radical scavenger 02. However, the yields of both CH3HgC2Hsand CH, are considerably less than observed in the deaerated solutions. This observation indicates that both products are formed by some process which is inhibited by oxygen. The production of both CH, and CzH6at 28 and 0" in the oxygenated solutions as a function of dose is shown in Figure' 2 . The yield per 100 eV of ethane is independent of both temperature (in the range 28-0') and of dose. However, the production of CH, is considerably less at 0" than at 28") in addition to being dependent on dose. The slope of the methane production curve in the range from 0 to 1.8 X lozoeV/g varies from 2.0 to 4.2 molecules/100 eV and from 0.6 to 1.4 molecules/lOO eV for the 28 and 0" irradiations, respectively. The temperature dependence again sugVolume W 9Number 10 October 1968
3460
CLARENCE J. WOLFAND JOHNQ. WALKER
Table 111: Yields from Irradiated Oxygen-Dimethylmercury Solutions (2.3 x 10-3 M ) Produot
CHI" Hgb CZH6
______
G------
280
00
0.6-1.4 9.4 5.5
2.04.2
Ha CzH4 CHaHgC&
9.5 5.5 0.44 0.19
