Emission studies of the mechanism of gaseous biacetyl photolysis at 3450, 3650, 3880, and 4358 a and 28°C

The quantum yields of phosphorescence (Φp) of biacetyl have been determined in pure biacetyl, biacetyl‐SO2, and biacetyl‐c‐C6H12 mixtures in experiments using bands of radiation centered at 3450, 3650, 3880, and 4348 Å. It has been shown that the unexpected effect of gas concentration on the quantum yields of the sulfur dioxide triplet‐sensitized phosphorescence of biacetyl resulted largely from the significant destruction of biacetyl triplets at the wall of the cell. The kinetics of the variation of Φp with [Ac2], wavelength of the absorbed light, and added gases provide new estimates of the energy relations and the rate constants for the decomposition reaction of vibrationally rich biacetyl molecules in the first excited singlet state (1Ac2‡): 1Ac2‡ → products (1), 1Ac2‡ + Ac2 → 1Ac2 + Ac2 (2); the minimum energy necessary in 1Ac2‡ for reaction (1) to occur is estimated to be about 72.8 kcal/mole above the ground state of biacetyl: k1/k2 = (4.3 ± 0.1) × 10−3M at 3450 Å, (4.07 ± 0.04) × 10−4M at 3650 Å, and (5.6 ± 0.4) × 10−5M at about 3800 Å. The variation of the rate constant ratio is shown to be consistent with the expectations of the simple theory of excited molecule decomposition. Biacetyl triplet (3Ac2) rate constants were determined by measurements of Φp in O2 and NO‐containing mixtures: 3Ac2 + S → (Ac2–S, products) (8); for O2 = S, k8 = (5.76 ± 0.40) × 108 (3650 Å experiments), (5.76 ± 0.27) × 108 (4358 Å); for NO = S, k8 = (3.34 ± 0.20) × 109 (3650 Å), (3.33 ± 0.18) × 109 1./mole‐sec (4358 Å). A comparison between these and previous findings of the SO2 triplet (3SO2)‐sensitized excitation of biacetyl [5,6] show that the decomposition of the initial 3Ac2 product of the exothermic energy transfer reaction 3SO2 + Ac2 → SO2 + 3Ac2 is unimportant.