The radiation chemistry of methyl iodide in the gas phase

Abstract

The 60Co-gamma radiolysis of pure methyl iodide vapor at 300 torr (40 kN m −2) and 25°C has been studied. Major products found and their respective G-values are: I 2, 0·16; HI, 0·13; H 2, 0·55; CH 2I 2, 1·4; CH 4, 2·9; C 2H 2, 0·12; C 2H 4, 0·065. Addition of I 2 as a radical scavenger reduced the initial G value for methane production to 1·4. The mechanism for production of CH 2I 2 and molecular CH 4 was assumed to involve hot methyl radicals. Iodine scavenging did not alter G(H 2), G(CH 2I 2), G(C 2H 2) or G(C 2H 4), indicating that molecular processes may account for as much as 70 per cent of all observed product formation. Ethane was found to be a minor product; the initial 100 eV yield of about 0·07 was reduced to 0·0055 by scavengers added initially or produced in the radiolysis. Use of HI as a radical scavenger resulted in a low initial G(C 2H 2) which increased with dose to its value in pure CH 3I. The C 2H 4 yield was nearly twice that observed in the pure system. The rate of methane production showed an initial maximum of about 25 molecules/100 eV, and the G-value decreased to about 2·6 with a dose of 1 × 10 20 eV. A chain mechanism is proposed to account for the high CH 4 yields in the presence of excess HI, and some of the G(CH 4) observed in pure CH 3I.