R. E. REBBERTAND P. AWSLOOS
3086 [CONTRIBUTION FROM
THE
NATIONAL BUREAUOF STANDARDS, WASHINGTON 25, D.
Vol. 85
c.]
The Reactions of Methyl Radicals in the Solid-, Liquid-, and Gas-Phase Photolysis of Dimethylmercury BY R . E. REBBERT A N D P. AUSLOOS RECEIVED JUNE 6, 1963 The gas-phase photolysis o! CDICOCDI has been investigated in the presence of CH3HgCH3 from 376 to 453°K. From the isotopic distributions of the methane and ethane fractions, evidence was obtained for the occurrence of the reaction: CD3 CH3HgCHa -+ CDaHgCHa f CH3 (.& = 12.6 kcal./mole). This reaction is also postulated to occur in the liquid-phase photolysis of dimethylmercury. From the isotopic distribution of the ethane produced in the liquid- and solid-phase photolysis of CH3HgCHa-CD3HgCDI mixtures, it was concluded that cage recombination of methyl radicals does take place. Contrary to the conclusions reached in an earlier work by Derbyshire and Steacie, no evidence could be obtained for hot methyl radical effects in the liquid-phase photolysis of dimethylmercury.
+
Introduction Earlier' work on the gas-phase photolysis of dimethylmercury had indicated that the primary process is essentially
+ hv --+ Hg + 2CHa Hg(CH3)t + hv -+ HgCH3 + CHS Hg(CH3)z
(1)
This may be written as a sequence of two steps HgCHs +Hg
+ CH3
(Ia) (Ib)
However, the methylmercury radical is known to be very unstable (-6 kcal.2) and may be expected to decompose readily. This is substantiated by the quantum yield close to unity reported for the photochemical decomposition a t 30°.3 Evidence was also presented' to show that methane is formed only by a hydrogen abstraction reaction and that ethane is formed only by a recombination of two methyl radicals. However, various experimentors have suggested a second ethane formation step in addition to the recombination of two methyl radicals. Gomer and Noyes4 suggested the occurrence of the following steps, especially a t high intensities CHB
+ Hg(CH3)3 --+ CzHe + (CH3 + Hg + CHa)
Oswin, et ~ l .reported , ~ appreciable quantities of ethaned3 in the photolysis of acetone-de a t 3130 A. with dimethylmercury added. A methyl abstraction process, such as CDj
+ CHaHgCH3 +CD3CHa + Hg + CHB
(A)
was proposed to explain these observations. In order to obtain more detailed information concerning the reactions of methyl radicals with dimethylmercury, the photolysis of CI33COCD3 in the presence of CH3HgCH3has been reinvestigated. Furthermore, the photolysis of CH3HgCH3-CD3HgCD3 mixtures has been studied in the liquid and solid phases in order to obtain information about (a) hot methyl radical effects which have been proposed6 to occur in the liquidphase photolysis of dimethylmercury-n-heptane mixtures and (b) cage recombination of methyl radicals. Experimental Apparatus.-The solid-phase photolysis experiments a t 4 and 77'K. were performed in a stainless steel low-temperature dewar previously described.' The flow rate for deposition of the sample varied from about 1.3 cc. ( S T P ) per minute for just dimethylmercury t o about 10 cc. ( S T P ) per minute for the carbon dioxide(1) R . E. Rebbert and E. W. R Steacie, C o n . J C h e m . , 31, 631 (1953). ( 2 ) B. G Gowenlock, J. C . Polanyi, and E . Warhurst, Proc. Roy. SOL. (London), 8218, 269 (1953). (3) R . A . Holroyd and W. A . Noyes, J r . , J . A m Chem. Sac., 7 6 , 1583 (1954). (4) R . Gomer and W . A Noyes, J r . , ibid., 71, 3390 (1949). ( 5 ) H. G. Oswin, R . Rebbert, and E. W. R . Steacie, Can. J C h e m . , 33, 472 (1955). (6) D. H . Derbyshire and E. W.R . Steacie, ibid., 33, 457 (1954). (7) R . E. Rebbert and P. Ausloos, J . Phys. C h e m . , 66, 2253 (1962).
dimethylmercury mixture. In most experiments, about 26 cc. ( S T P ) of gas was deposited. The unfiltered light from a n Hanovia SH-100 lamp was used. For the liquid-phase photolysis, a quartz cell (about 0.05 cm. in depth, 2.8 cm. diameter, and 0.4 cc. volune) was used. This cell, which was closed by means of a breakseal, was immersed in a Pyrex dewar flask with double-quartz windows. The light source was a n Osram-100 lamp and, in a few cases, an Hanovia SH-100. In most experiments, either a diphenylbutadiene solution* or a 2637 A . Baird-Atomic interference filter (Type .4-31) was used. For the liquid-phase photolysis of azomethane in the presence of deuterated dinethylmercury, a Corning 3-75 filter was used. In the gas-phase experiments, a cylindrical quartz cell (10 cm. long, 5 cm. diameter, and approximately 180 cc. volume) was used in conjunction with a n Hanovia SH-100 lamp and a Corning 0-54 filter. The cell was situated in an aluminum-block furnace 30 cm. long with quartz windows. The maximum temperature variation during an experi nent was f1'. Conversions .-In the liquid-phase photolysis, conversions were kept below O , l c L . In the solid and gas phases, conversions were usually about 0.55&. Materials.-Dimethylmercury, dimethyl nercury-&, azomethane, and acetone46 were obtained from Merck Sharp and Dohme of Canada. Mass spectrometric analysis showed 3.8y6 CDXOCDzH in tbe acetone-& and about 2% CDBHgCDTH in the dirnethylmercury-d6. 2,3-DiLnethylbutanewas a standard sample irom the National Bureau of Standards with 0.11 i 0.06 mole yoimpurity. Analysis.-The analytical system consisted of a series of traps, a modified Ward still, and a Toepler pump-gas buret. Nitrogen, carbon monoxide, and methane were removed at - 19B0, while ethane was removed a t - 175'. These fractions were analyzed on the mass spectrometer. Standard mixtures of CzHs, CzD!, and CH3CD3were run on the same mass spectrometer for calibration.
Results Solid and Liquid Phases.-Table I gives the results for the photolysis of approximately equimolar mixtures of CH3HpCH3and CD3HgCD3. In the solid phase, CH3CD3is a minor product a t both 4 and 77'K. Moreover, the addition of ethanol or carbon dioxide decreases the percentage of CHsCD3 formed. Methane is present in surprisingly large amounts. In the liquid phase, the distribution of the ethanes does not seem to follow any pattern. In these experiments, the same mixture was reused in order to conserve dimethylmercury. The percentage of CH3CD3 roughly follows the order in which the experiments were performed rather than the variation i n . temperature. Mass spectrographic analysis of the undecomposed dimethylmercury, after these experiments, showed essentially a statistical distribution of CHsHgCH3, CH3HgCD3, and CD3HgCDa in the ratio of 1 : 2 : 1 . The ratio CD1/CD3H is roughly equal to the ratio CHaD/CH4 and both increase with an increase in temperature. I t should be noted that the CDJCD3H ratio could be determined with a better accuracy than the CH3D/CH4 ratio. The Arrhenius plot of the log C D I / C D ~ H vs. 1!T gave an activation energy of 1.4 f 0.4 kcal./ mole. The quantum yield (ethane plus 0.5 methane) for dimethylmercury decomposition in the equimolar (8) M. Kasha, J Opt Soc. A m . , 38, 929 (1948)
METHYLRADICALS IN PHOTOLYSIS OF DIMETHYLMERCURY
Oct. 20, 1963
3087
TABLE I
-
SOLID-ASD LIQUID-PHASE PHOTOLYSIS OF CH3HgCH3-CD3HgCD3MIXTURES (1 :1) Temp.,
Rethane
7-Distribution, C2De CHaCDa
Rmethane
moles set.-' X 1O1O
O K .
4 77
12.1 6.68 3.84 7.60
--a i l
-4 i i
6.09 5.23 3.64
36.7 39.5 38.8 42.2
..
%-CzHe
-Distribution, CDaH CHzD2
CD4
Solid phase 48.9 6 2 31.2 45.0 11.6 39.0 53.6 1.3 32.3 54.4 .. ..
14.4 15.4 7.6 3.3
Liquid phase 2 73 1.91 1.96 25.3 26.1 48.6 2.2 309 1.63 6.41 21.6 29.2 49.3 2.5 0 37 3.91 22.3 17.6 60.1 3.7 338 338 5 91 15.3 30.6 44.8 24.7 3 2 338.5 0.76 7.13 27.7 17.2 55.1 3.1 1.00 21 8 30.2 28.7 41.2 3.9 364 a Ethanol-dimethylmercury (10: 1). * Carbon dioxide-dimethylmercury
CHaD
CHr
CDdCDaH
CHaD/CHI
0.43 ,38 ,049
4.1 1.7
18.8 12.4 3.0
43.8 32.9 61.7
0.20 ,30 040
...
..
..
...
1.5 2.1
2.8 3.1
53.1 51.6 51.5 51.9 51.9 50.7
0.055 ,063 ,083 080 ,078 097
...
40.3 40.5 44.7 40.1 40.0 40.0 (22: 1).
%
...
1.2 1.1 1.1
3.7 3.9 4.2
.
.
0.053 0.061 ...
0 .O i l 075 ,083
wave length limit of -2800 A., while the Corning 3-75 filter has a short wave length limit of -3600 A. LIQUID-PHASE PHOTOLYSIS OF CH8HgCHa-CD3HgCD3-2,3-D~Gas Phase.-The results for the photolysis of CD3METHYLBUTANE (1: 1 : 2) COCD3 in the presence of CH3HgCH3, using a Corning Temp Rethane Rmethane ---Distribution, %--Run "K moles sec -1 X 1011 C2D6 CHaCDa CzH6 CD4/CDyH 0-54 filter (short wave length limit -3000 A.), are given in Tables I11 and TV. Blank experiments 46 1 0 007 33 2 20 7 23 8 3 226 5 21 (without CD3COCD3) showed that less than 3% of 50 8 007 0 9 35 8 48 3 8 49 1 273 the total product could be accounted for by the thermal 005 5 1 59 1 46 7 35 8 2 325 7 82 TABLE I1
I
TABLE I11 GAS-PHASE PHOTOLYSIS OF CDlCOCD3 IN Temp., Run
OK.
10 11 12 13 11 15 16 17 18 19 20 21 22
376.5 399 400 415 425 425 425 425.5 435.5 440 440 440 453
-
-Pressure, mm CDa CH3 COCDa HgCH3
37 39 39 40 41 41 39 41 43 87 43 23 44
5 5 0 0 0 5 5 5 5 5 5 0 5
37 40 39 41 42 42 39 42 43 43 22 43 44
5 0 5 5 0 5 0 5 5 5 5 5 5
Rethane
ðane
moles liter-1 sec
7 71 7 67 8 7 30 7 1 1 7 10 5 3 5
20 75 3 68 62 14 02 1 72 63 34
1 3 3 5 14 7 3 2 9 16 6 5 12
57 33 42 58 5 00 09 53 40 4 87 93 4
RCO -1
X 1010
8 59 9 37 11 7 11 0 38 4 10 7 2 81 2 06 11 0 17 1 8 91 5 39 8 80
mixture a t 2537 A. is about 1.1 a t 364'K. and 0.3 a t 273°K. The ethane quantum yield is 0.1 a t 364°K. and 0.2 a t 273°K. These quantum yield measurements are based on the assumption that the quantum yield for carbon monoxide formation from diethyl ketone in the liquid phase a t 367°K. is 0.87.' In Table 11, the results are given for the photolysis of a mixture of CH3HgCH3, CD3HgCD3, and 2,3dimethylbutene (1 : 1 :2). Again the sample was reused. I t is even more obvious that the percentage of CH3CDafollows the order in which the experiments were performed rather than the temperature variation. The intensity ( I , = l O I 5 quanta per cc.-sec.) was kept essentially constant for these three runs. Thus, the relative quantum yield of decomposition (ethane plus 0.5 methane) increases with temperature. The photolysis of CH3N2CH3, in the presence of CD3HgCD3in the liquid phase a t 0' using a Corning 3-75 filter, showed the presence of 12.5% CD3H in the methane fraction but no detectable amount (
