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

The quantum yields of the sulfur dioxide triplet ( 3 SO 2 )‐sensitized phosphorescence of biacetyl (Φ sens ) were determined in experiments with N 2 –SO 2 –Ac 2 and c ‐C 6 H 12 –SO 2 –Ac 2 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 [SO 2 ]/[M] and [SO 2 ]/[Ac 2 ] ratios but at varied total pressure. A kinetic treatment of the Φ sens results and singlet sulfur dioxide ( 1 SO 2 ) quenching rate constant data gave the following new kinetic estimates: 1 SO 2 + M → (SO 2 –M) (1b) 1 SO 2 + M → 3 SO 2 + M (2b); for 1 SO 2 –N 2 collisions, k 2b /( k 1b + k 2b ) = 0.033 ± 0.008; for 1 SO 2 – c ‐C 6 H 12 collisions, k 2b /( k 1b ± k 2b ) = 0.073 ± 0.024; previous studies have shown this ratio to be 0.095 ± 0.005 for 1 SO 2 –SO 2 collisions. It was concluded that the inter‐system crossing ratio in 1 SO 2 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 1 SO 2 ( k 2b ) and the total 1 SO 2 ‐quenching constants ( k 1b + k 2b ) are quite sensitive to the nature of M: k 2b / k 2a varies from 0.10 ± 0.03 for M = N 2 to 1.11 ± 0.37 for M = c ‐C 6 H 12 , and ( k 1b + k 2b )/( k 1a + k 2a ) varies from 0.29 for M = N 2 to 1.44 for M = c ‐C 6 H 12 ; k 1a and k 1b are the rate constants for the reactions 1 SO 2 ‐ SO 2 → (2SO 2 ) (1a) and 1 SO 2 + SO 2 → 3 SO 2 + SO 2 (2a), respectively.