Acetone over a range of pressures and temperatures (1.2ndash;15 cm Hg; 126ndash;295deg;C) was photolyzed with light of 3130 A in the presence of small amounts of hydrogen iodide. The primary process was found to be the Csngbnd;C split to form acetyl and methyl radicals. Hydrogen iodide acted as an excellent radical ``trap'' for these radicals. No spontaneous decomposition of acetyl radicals was observed. Relative rates of methyl radical reactions with iodine and hydrogen iodide were measured. Complications due to surface effects in these reactions are considered.Kinetic constants for the reactions CH3+HIrarr;CH4+I(1); CH3+I2rarr;CH3I+I(2) areE1mdash;E2=1.3plusmn;0.5 kcal,A2/A1=2.4, and log(k1/k2) = mdash;0.38ndash;1300/4.575T.Above 200deg;C the acetyl radical decomposition is measurable and the rate constant is observed to be pressure dependent. The pressure dependence is treated by an approximate method related to the Hinshelwoodhyphen;Lindemann equation and the highhyphen; and lowhyphen;pressure limiting rate constants are given by logkinfin;=10.3ndash;15 000/4.575Tsecmdash;1, logk0=11.5ndash;12 000/4.575T(l/molehyphen;sec). The number of vibrational modesScontributing to the decomposition is 4.5 and the lifetime of the excited acetyl radical istsim;10mdash;8sec. The abnormally low frequency factor of the highhyphen;pressure rate constant implies a nonadiabatic decomposition of the acetyl radical. From the kinetic constants, the heat of formation of the acetyl radical is found to be Dgr;Hf0(CH3CO)mdash;6.2 kcal and the values of the appropriate bond dissociation energies in a series of acetyl derivatives are tabulated.
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