A study of the intersystem crossing reaction induced in gaseous sulfur dioxide molecules by collisions with nitrogen and cyclohexane at 27°C

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Abstract
The quantum yields of the sulfur dioxide triplet (3SO2)‐sensitized phosphorescence of biacetyl (Φsens) were determined in experiments with N2–SO2–Ac2 and c‐C6H12–SO2–Ac2 mixtures excited at 2875 Å at 27°C. The fraction of the biacetyl triplets which reacts homogeneously by radiative or nonradiative decay reactions was determined in a series of runs at constant [SO2]/[M] and [SO2]/[Ac2] ratios but at varied total pressure. A kinetic treatment of the Φsens results and singlet sulfur dioxide (1SO2) quenching rate constant data gave the following new kinetic estimates: 1SO2 + M → (SO2–M) (1b) 1SO2 + M → 3SO2 + M (2b); for 1SO2–N2 collisions, k2b/(k1b + k2b) = 0.033 ± 0.008; for 1SO2–c‐C6H12 collisions, k2b/(k1b ± k2b) = 0.073 ± 0.024; previous studies have shown this ratio to be 0.095 ± 0.005 for 1SO2–SO2 collisions. It was concluded that the inter‐system crossing ratio in 1SO2 induced by collision is relatively insensitive to the nature of the collision partner M. However, the individual rate constants for the collision‐induced spin inversion of 1SO2 (k2b) and the total 1SO2‐quenching constants (k1b + k2b) are quite sensitive to the nature of M: k2b/k2a varies from 0.10 ± 0.03 for M = N2 to 1.11 ± 0.37 for M = c‐C6H12, and (k1b + k2b)/(k1a + k2a) varies from 0.29 for M = N2 to 1.44 for M = c‐C6H12; k1a and k1b are the rate constants for the reactions 1SO2 ‐ SO2 → (2SO2) (1a) and 1SO2 + SO2 → 3SO2 + SO2 (2a), respectively.