Primary process in the photolysis of hexafluoroacetone in the presence of mercury vapor

Abstract

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...

Excited state kinetics of the π*←n photochemistry of hexafluoroacetone

“…The studies of Kutschke and co-workers [2,6,13,21] support the weak collision hypothesis, with multistage collisional relaxation of the initially formed molecules in the IA* state. The data of Porter and coworkers [8,23] at 25°C point to the opposite conclusion, that there is strong collisional single-stage deactivation to the lA0 state.…”

“…This can be achieved experimentally by photolyzing in the presence of sufficient triplet quencher. From an analysis of the Stern-Volmer type (S-V) plots for photochemical decomposition versus collisional quenching, deductions can be made as to the nature of the dissociating states, and the mechanism by which collisional relaxation occurs [2,6,[13][14][15]. All materials were completely degassed and distilled on the vacuum line, and the middle portion of each was retained.…”

The stationary-state approximation in the photolysis of acetone

“…CD measured rates at two temperatures to give k, = 2.5 x 1016 exp (-70 700/RT) and k6 = 2.5 x 10" exp (-26 800/RT) and get a good fit to their data using reactions [1]- [8]. If the additional reactions of CH,CO are included, reactions [9], [12], [13], [14], [17], [18], [19], and [20], a best fit is obtained with k, = 1.8 x 1016 exp (-7 1 000/RT) and k6 = 1.25 x 10'' exp (-24 000/RT). However, the rate constants used by CD result in a sharp decrease in Qco with acetone pressure such,that calculation for the experimental conditions of Pearson give values of both Qco and QCzH6 that are 50% too low.…”

“…For example, CD ignore reactions [12]- [20] (6) and Howland and Noyes (29). At 25"C, p = 50 Torr and I = 2 x 10"'q cm-' s-I it can be calculated from the mechanism of the present paper that the reactions omitted by CD only contribute 1.5% to the removal of acetyl radicals: Qco calculated by the steady-state approximation [ii] is 95% of the value obtained froni the full calculation.…”

“…Ausloos and Steacie (19) showed that the low temperature deviation from the straight line can be partially accounted for by reaction [18]. This is arbitrarily assigned a rate constant one tenth of that of reaction [12]. This gives the curved portion of Fig.…”