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Emission studies of the mechanism of gaseous biacetyl photolysis at 3450, 3650, 3880, and 4358 a and 28°C
Author(s) -
Horowitz Abraham,
Calvert Jack G.
Publication year - 1972
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550040208
Subject(s) - chemistry , phosphorescence , reaction rate constant , photodissociation , excited state , molecule , photochemistry , kinetics , singlet state , singlet oxygen , decomposition , activation energy , oxygen , fluorescence , atomic physics , organic chemistry , physics , quantum mechanics
The quantum yields of phosphorescence (Φ p ) of biacetyl have been determined in pure biacetyl, biacetyl‐SO 2 , and biacetyl‐ c ‐C 6 H 12 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 Φ p with [Ac 2 ], 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 ( 1 Ac 2 ‡): 1 Ac 2 ‡ → products (1), 1 Ac 2 ‡ + Ac 2 → 1 Ac 2 + Ac 2 (2); the minimum energy necessary in 1 Ac 2 ‡ for reaction (1) to occur is estimated to be about 72.8 kcal/mole above the ground state of biacetyl: k 1 / k 2 = (4.3 ± 0.1) × 10 −3 M at 3450 Å, (4.07 ± 0.04) × 10 −4 M at 3650 Å, and (5.6 ± 0.4) × 10 −5 M 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 ( 3 Ac 2 ) rate constants were determined by measurements of Φ p in O 2 and NO‐containing mixtures: 3 Ac 2 + S → (Ac 2 –S, products) (8); for O 2 = S, k 8 = (5.76 ± 0.40) × 10 8 (3650 Å experiments), (5.76 ± 0.27) × 10 8 (4358 Å); for NO = S, k 8 = (3.34 ± 0.20) × 10 9 (3650 Å), (3.33 ± 0.18) × 10 9 1./mole‐sec (4358 Å). A comparison between these and previous findings of the SO 2 triplet ( 3 SO 2 )‐sensitized excitation of biacetyl [5,6] show that the decomposition of the initial 3 Ac 2 product of the exothermic energy transfer reaction 3 SO 2 + Ac 2 → SO 2 + 3 Ac 2 is unimportant.