Detection of transient infrared absorption of SO₃ and 1,3,2-dioxathietane-2,2-dioxide [cyc-(CH₂)O(SO₂)O] in the...

We recorded time-resolved infrared absorption spectra of transient species produced on irradiation at 308 nm of a flowing mixture of CH₂I₂/O₂/N₂/SO₂ at 298 K. Bands of CH₂OO were observed initially upon irradiation; their decrease in intensity was accompanied by the appearance of an intense band at 1391.5 cm^-1 that is associated with the degenerate SO-stretching mode of SO₃, two major bands of HCHO at 1502 and 1745 cm^-1, and five new bands near >1340, 1225, 1100, 940, and 880 cm^-1. The band near 1340 cm^-1 was interfered by absorption of SO₂ and SO₃, so its band maximum might be greater than 1340 cm^-1. SO₃ in its internally excited states was produced initially and became thermalized at a later period. The rotational contour of the band of thermalized SO₃ agrees satisfactorily with the reported spectrum of SO₃. These five new bands are tentatively assigned to an intermediate 1,3,2-dioxathietane-2,2-dioxide [cyc-(CH₂)O(SO₂)O] according to comparison with anharmonic vibrational wavenumbers and relative IR intensities predicted for this intermediate. Observation of a small amount of cyc-(CH₂)O(SO₂)O is consistent with the expected reaction according to the potential energy scheme predicted previously. SO₃+HCHO are the major products of the title reaction. The other predicted product channel HCOOH+SO₂ was unobserved and its branching ratio was estimated to be <5%.