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A B S T R A C TI t has been confirmed that the apparent rate constant for the reaction between methyl radicals, produced by the photolysis of deuterated acetone, and isobutane increases with decreasing isobutane pressure. A n explanation is proposed to account for this observation suggesting that the production of methane by disproportionation between methyl and t-butyl radicals was not negligible as has been assumed previously. I N T R O D U C T I O NObservations made recently in this laboratory indicated that values of the apparent rate constants for the abstraction of hydrogen by methyl radicals, calculated using conventional methods, depended on the pressure of the added hydrogen donor. The reaction of methyl radicals with isobutane using the pyrolysis of di-t-butyl peroxide a s a source of methyl radicals is an exanlple of such a system (I). When mixtures of deuterated acetone and hydrogen were photolyzed, the rate constant for the reaction of methyl radicals with hydrogen was found to be a function of the hydrogen pressure (2). In view of the apparent similarities between the two systems, an investigation of the photolysis of deuterated acetone -isobutane ~llixtures was made. E X P E R I M E N T A LThe apparatus and methods of analysis were similar to those employed in the photolysis of the deuterated acetone -hydrogen mixtures (2, 3). The deuterated acetone consisted of 98.2% acetone-ds and 1.8% acetone-d5. Phillips Research Grade isobutane was outgassed by bulb-to-bulb distillations a t -160" C. The temperature of the reaction cell was 471% 0.5" K for all experiments. R E S U L T S A N D DISCUSSIONIn the photolysis of mixtures of deuterated acetone and isobutane, it is usually assumed that methane and ethane are formed b y the following reactions:When the deuterated acetone was photolyzed alone ( 2 ) , small amounts of CD3H were formed due to the presence of acetone-d5 in the sample, and its yield was given bywhere [ A ] is the acetone concentration in molecules/cc. Allowance for this CDjH was made when calculating values of k,/kalt2.

The photolysis of azoethane a t X 3660 A has been reinvestigated. The quantum yield of nitrogen formation was found to be dependent on the azoethane pressure and the temperature, indicating collisional deactivation of excited azoethane molecules.'I'he results conhrm the mechanism proposed by Ausloos and Stcacie (1). For the activatiorl energy of the addition reaction C?HS+C.'H~X~CYH;, a value of 6 . 0 f 0.3 kcal./nlole has been obtained, assunling a ~legligible acti\.ation energy for the combillation reaction of two ethyl radicals. INTRODUCTIOX

Studies have been made of the variation with ketone pressure of the quantum yield for the decomposition of hexafluoroacetone, excited by 3130 A, at 25 and 78 "C, and of the relative yield of the total emission at several temperatures. These are described in considerable detail with a view to discerning possible sources of systematic error. No large systematic error could be identified except that of decreased yields caused by deactivation by mercury vapor. The decomposition data support the weak collision mechanism for vibrational relaxation in the excited singlet state unless an as yet unknown source of large systematic error can be identified.A description is offered of the path of the decomposition in terms of simplified diatomic representations of the nr* singlet and triplet states and of a repulsive state arising from the radicals CF3 and CF3C0 in their ground electronic states. On the basis of that description an activation energy >, 11 kcal mole-1, derived from the combined decomposition and emission data, is to be ascribed to the first-order decomposition of triplet state molecules in vibrational equilibrium with their surroundings.Canadian Journal of Chemistry. 46, 175 (1968) Introduction An earlier publication (1) offered a reinterpretation of literature data as they pertain to the mechanism of the primary process in the photolysis of hexafluoroacetone following excitation by the 3130 group of the mercury arc. The present paper describes in detail the new data cited there, reports additional new data bearing on the primary photodecomposition, and describes a study of some aspects of the emission from the excited states of this ketone. Two brief studies of other aspects of the overall investigation of the primary process have been reported recently (2,3).Near the com~letion of the oresent work the investigations of Bowers and Porter, which are now published (4, 5), came to our attention. There are two important quantitative discrepancies between those results and those reported here. One of these discrepancies can be attributed to the quenching of the triplet state of this ketone by mercury vapor, which was reported recently (3) and since has been confirmed (6, 7). It seems probable that the method used by Bowers and Porter (8) also eliminated a substantial fraction of the mercury from their reactant. Thus their result that 4, -0.2 at room temperature is closer to the unquenched value than that found here (-0.025). The discrepancy (9) regarding the shape of plots of 114 vs. pressure at low pressures cannot be explained as readily.Because of the unexplained discrepailcies the experimental work is described in greater detail here than is usually the case and a discussion is given of possible sources of systematic errors. This description is divided into two sections of which the first describes the experiments on the photodecomposition and the second those on the emission. Experimental ( I ) P/~oton%cotnpositior~The cylindrical quartz photolysis cell (10 crn long, 5 cm diameter) was housed in a cyli...

The pyrolysis of di-t-butyl peroxide has been reinvestigated and used as a source of methyl radicals to study the abstraction reaction between methyl radicals and formaldehyde. At low [IHCIlO]/[peroxidc] ratios the system was simple enough for kinetic analysis, and a value of 6.6 Iccal/mole was obtained for the activation energy. At higher [HCHOl/[peroxide] ratios the system became very complicated, possibly due to the increased inlportance of addition reactions. INTRODUCTIONThe reaction between methyl radicals and formaldehyde has been previously studied by Kodama and Taltezalti (1) and by Toby and ICutschke (2), who used the photolysis of azomethane as a source of inethyl radicals. They obtained values of 5.6 and 6.2 kcal/ mole, respectively, for the activation energy of the process. I t seemed desirable to reinvestigate the reaction using the pyrolysis of di-t-butyl peroxide as a source of methyl radicals. This source possesses the advantage that there is little abstraction reaction between methyl radicals and the n~olecule which produces the radicals.Methyl radicals were produced by the pyrolysis of di-t-butyl peroxide in a Pyrex glass reaction vessel (volume = 550 cm3) the temperature of which was controlled to within 0.1" C. T o start the reaction the reactants were expanded into the cell from a preheater maintained a t 80" C, a t which temperature no measurable decomposition of the peroxide took place in a period of 30 minutes. The reaction was stopped by expansion into a liquid-nitrogen-cooled trap. The products were analyzed by conventional methods. CHI and CO were separated from the remaining products a t -210" C, and the CO measured as C 0 2 after oxidation in a copper oxide furnace a t 260' C. Methane was removed directly from the furnace and measured. Ethane was distilled from Ward stills a t -180" C. The purity of each fraction was checked lnass spectrolnetrically in some experiments. The remailling lnixture of liquid products and unreacted starting materials was removed for further analysis. Liquid product analyses for acetone, t-butanol, and acetaldehyde mere carried out gas chro~natographically on a 10-ft column consisting of 30% nonyl-phthalate on firebrick, with a small proportion of polyethylene glycol added to the phthalate to cut down tailing of the peaks. No substances were detected other than the residual reactants and those products for which analyses are given.Di-t-butyl peroxide, fro111 the Novadel-Agene Corporation, contained traces of acetone and methanol and approximately 1/2% t-butanol. The first two were entirely removed by chromatographic purification; the t-butanol content was reduced to < 0.1% by this treatment. Isobutane mas Phillips Research Grade. Formaldehyde was prepared by heating a-polyoxy~nethyle~le followed by distillation in vacz~o. All these materials were thoroughly outgassed before use.lA!fanz~script recei-ded M a y 14, 1969.