Photochemistry of the SO2, 2-pentene system at 3660 Å

The photolysis of SO₂ in the presence of cis- and trans-2-pentene has been investigated at 3660 Å and 22°C. Quantum yield measurements of the SO₂ photosensitized conversion of one isomer into the other are consistent with a mechanism in which the only participating excited electronic state of SO₂ is the SO₂(³B₁) state. Quantum yield measurements were made for a variation in P_SO2/P_isomer reactant ratios of 4.01–283 and 57.5–351 for the cis and trans isomers, respectively. The data are consistent with a mechanism in which a (SO₂-olefin)³ collision intermediate is the precursor to the photosensitized isomeric products. The intermediate undergoes unimolecular decay to yield the cis and trans isomers with probabilities of 0.26 ± 0.05 and 0.69 ± 0.04, respectively. Estimates of the quenching rate constants at 22°C for removal of SO₂(³B₁) molecules by cis- and trans-2-pentene are (0.633 ± 0.125) × 10¹¹ l./mole/sec and (1.00 ± 0.27) × 10¹¹ l./mole/sec, respectively. An experimentally determined photostationary composition, [trans-2-pentene]/[cis-2-pentene] = 2.3 ± 0.1 was in fair agreement with that of 1.7 ± 0.7 as predicted from kinetic data derived in this study.