The kinetics of elementary reactions involving the oxides of sulphur III. The chemiluminescent reaction between sulphur monoxide and ozone

Abstract

The chemiluminescent reaction between sulphur monoxide (SO) and ozone has been studied in a fast flow system at pressures between 0·3 and 3·0 mmHg, These species undergo a rapid bimolecular reation (1) SO + O₃ = SO₂ + O₂ + 106 kcal/mole (1) to yield ground state products, where k₁ = 1·5 x 10¹² exp ( –2100/RT) cm³ mole^-1 s^-1. This reaction also yields electronically excited SO₂ molecules in the ¹B and ³B₁ states. The ¹BSO₂ molecules are produced with up to 16 kcal/mole vibrational energy. Emission from the longer lived ³B₁ state is vibrationally relaxed and provides no information about the initial energy distribution. Comparison with fluorescence studies shows that the ³B₁SO₂ molecules are produced mainly by collisional quenching of SO₂ molecules formed in the ¹B state. The formation of electronically excited SO₂ is also a simple bimolecular process, but it involves a higher energy barrier than formation of ground state SO₂. Our measurements on the chemiluminescence, when combined with data on the quenching of the SO₂ fluorescence, yield the rate constants k_1a = 10¹¹exp ( – 4200/RT) and k_lb ≯ 3 x 10¹⁰exp ( –3900/RT) cm³ mole^-1 s^-1 for the bimolecular reactions SO + O₃ = SO₂(¹B) + O₂ + 21 kcal/mole, (1a) SO + O₃ = SO₂(³B₁) + O₂ + 35 kcal/mole (1b) which form electronically excited SO₂. No electronically excited O₂ appears to be formed. It is deduced that electronically excited SO₂ is produced by crossing to a separate potential surface at or near the transition state rather than by the formation of a highly vibrationally excited SO₂ molecule which crosses to the excited electronic state.